HTML5

None.

As the root element of a document.

Wherever a subdocument fragment is allowed in a compound document.

A head element followed by a body element.

manifest — Application cache manifest

An html element's start tag can be omitted if the first thing inside the html element is not a comment.

An html element's end tag can be omitted if the html element is not immediately followed by a comment.

Allowed ARIA role attribute values:

none

interface HTMLHtmlElement : HTMLElement {};

The html element represents the root of an HTML document.

Authors are encouraged to specify a lang attribute on the root html element, giving the document's language. This aids speech synthesis tools to determine what pronunciations to use, translation tools to determine what rules to use, and so forth.

The manifest attribute gives the address of the document's application cache manifest, if there is one. If the attribute is present, the attribute's value must be a valid non-empty URL potentially surrounded by spaces.

The manifest attribute only has an effect during the early stages of document load. Changing the attribute dynamically thus has no effect (and thus, no DOM API is provided for this attribute).

For the purposes of application cache selection, later base elements cannot affect the resolving of relative URLs in manifest attributes, as the attributes are processed before those elements are seen.

The window.applicationCache IDL attribute provides scripted access to the offline application cache mechanism.

The html element in the following example declares that the document's language is English.

<!DOCTYPE html>
<html lang="en">
<head>
<title>Swapping Songs</title>
</head>
<body>
<h1>Swapping Songs</h1>
<p>Tonight I swapped some of the songs I wrote with some friends, who
gave me some of the songs they wrote. I love sharing my music.</p>
</body>
</html>

4.2 Document metadata

4.2.1 The `head` element

None.

As the first element in an html element.

If the document is an iframe srcdoc document or if title information is available from a higher-level protocol: Zero or more elements of metadata content, of which no more than one is a title element and no more than one is a base element.

Otherwise: One or more elements of metadata content, of which exactly one is a title element and no more than one is a base element.

A head element's start tag may be omitted if the element is empty, or if the first thing inside the head element is an element.

A head element's end tag may be omitted if the head element is not immediately followed by a space character or a comment.

Allowed ARIA role attribute values:

none

interface HTMLHeadElement : HTMLElement {};

The head element represents a collection of metadata for the Document.

The collection of metadata in a head element can be large or small. Here is an example of a very short one:

<!doctype html>
<html>
 <head>
  <title>A document with a short head</title>
 </head>
 <body>
 ...

Here is an example of a longer one:

<!DOCTYPE HTML>
<HTML>
 <HEAD>
  <META CHARSET="UTF-8">
  <BASE HREF="http://www.example.com/">
  <TITLE>An application with a long head</TITLE>
  <LINK REL="STYLESHEET" HREF="default.css">
  <LINK REL="STYLESHEET ALTERNATE" HREF="big.css" TITLE="Big Text">
  <SCRIPT SRC="support.js"></SCRIPT>
  <META NAME="APPLICATION-NAME" CONTENT="Long headed application">
 </HEAD>
 <BODY>
 ...

The title element is a required child in most situations, but when a higher-level protocol provides title information, e.g. in the Subject line of an e-mail when HTML is used as an e-mail authoring format, the title element can be omitted.

4.2.2 The `title` element

In a head element containing no other title elements.

Text that is not inter-element whitespace.

Neither tag is omissible.

Allowed ARIA role attribute values:

none

interface HTMLTitleElement : HTMLElement {
           attribute DOMString text;
};

The title element represents the document's title or name. Authors should use titles that identify their documents even when they are used out of context, for example in a user's history or bookmarks, or in search results. The document's title is often different from its first heading, since the first heading does not have to stand alone when taken out of context.

There must be no more than one title element per document.

If it's reasonable for the Document to have no title, then the title element is probably not required. See the head element's content model for a description of when the element is required.

title . text [ = value ]

Returns the contents of the element, ignoring child nodes that aren't Text nodes.

Can be set, to replace the element's children with the given value.

The IDL attribute text must return a concatenation of the contents of all the Text nodes that are children of the title element (ignoring any other nodes such as comments or elements), in tree order. On setting, it must act the same way as the textContent IDL attribute.

Here are some examples of appropriate titles, contrasted with the top-level headings that might be used on those same pages.

  <title>Introduction to The Mating Rituals of Bees</title>
    ...
  <h1>Introduction</h1>
  <p>This companion guide to the highly successful
  <cite>Introduction to Medieval Bee-Keeping</cite> book is...

The next page might be a part of the same site. Note how the title describes the subject matter unambiguously, while the first heading assumes the reader knows what the context is and therefore won't wonder if the dances are Salsa or Waltz:

  <title>Dances used during bee mating rituals</title>
    ...
  <h1>The Dances</h1>

The string to use as the document's title is given by the document.title IDL attribute.

User agents should use the document's title when referring to the document in their user interface. When the contents of a title element are used in this way, the directionality of that title element should be used to set the directionality of the document's title in the user interface.

4.2.3 The `base` element

In a head element containing no other base elements.

Empty.

href — Document base URL

target — Default browsing context for hyperlink navigation and form submission

No end tag.

Allowed ARIA role attribute values:

none

Global aria-* attributes.

interface HTMLBaseElement : HTMLElement {
           attribute DOMString href;
           attribute DOMString target;
};

The base element allows authors to specify the document base URL for the purposes of resolving relative URLs, and the name of the default browsing context for the purposes of following hyperlinks. The element does not represent any content beyond this information.

There must be no more than one base element per document.

A base element must have either an href attribute, a target attribute, or both.

The href content attribute, if specified, must contain a valid URL potentially surrounded by spaces.

A base element, if it has an href attribute, must come before any other elements in the tree that have attributes defined as taking URLs, except the html element (its manifest attribute isn't affected by base elements).

If there are multiple base elements with href attributes, all but the first are ignored.

The target attribute, if specified, must contain a valid browsing context name or keyword, which specifies which browsing context is to be used as the default when hyperlinks and forms in the Document cause navigation.

A base element, if it has a target attribute, must come before any elements in the tree that represent hyperlinks.

If there are multiple base elements with target attributes, all but the first are ignored.

A base element that is the first base element with an href content attribute in a particular Document has a frozen base URL. The frozen base URL must be set, synchronously, whenever any of the following situations occur:

The base element becomes the first base element in tree order with an href content attribute in its Document.
The base element is the first base element in tree order with an href content attribute in its Document, and its href content attribute is changed.

To set the frozen base URL, resolve the value of the element's href content attribute relative to the Document's fallback base URL; if this is successful, set the frozen base URL to the resulting absolute URL, otherwise, set the frozen base URL to the fallback base URL.

The href IDL attribute, on getting, must return the result of running the following algorithm:

If the base element has no href content attribute, then return the document base URL and abort these steps.
Let fallback base url be the Document's fallback base URL.
Let url be the value of the href attribute of the base element.
Resolve url relative to fallback base url (thus, the base href attribute isn't affected by xml:base attributes or base elements).
If the previous step was successful, return the resulting absolute URL and abort these steps.
Otherwise, return the empty string.

The href IDL attribute, on setting, must set the href content attribute to the given new value.

The target IDL attribute must reflect the content attribute of the same name.

In this example, a base element is used to set the document base URL:

<!DOCTYPE html>
<html>
    <head>
        <title>This is an example for the &lt;base&gt; element</title>
        <base href="http://www.example.com/news/index.html">
    </head>
    <body>
        <p>Visit the <a href="archives.html">archives</a>.</p>
    </body>
</html>

The link in the above example would be a link to "http://www.example.com/news/archives.html".

4.2.4 The `link` element

Where metadata content is expected.

In a noscript element that is a child of a head element.

Empty.

href — Address of the hyperlink

crossorigin — How the element handles crossorigin requests

rel — Relationship between the document containing the hyperlink and the destination resource

media — Applicable media

hreflang — Language of the linked resource

type — Hint for the type of the referenced resource

sizes — Sizes of the icons (for rel="icon")

Also, the title attribute has special semantics on this element: Title of the link; alternative style sheet set name.

No end tag.

Allowed ARIA role attribute values:

link (default - do not set).

Any aria-* attributes applicable to the allowed roles.

For role value

interface HTMLLinkElement : HTMLElement {
           attribute boolean disabled;
           attribute DOMString href;
           attribute DOMString crossOrigin;
           attribute DOMString rel;
           attribute DOMString rev;
  readonly attribute DOMTokenList relList;
           attribute DOMString media;
           attribute DOMString hreflang;
           attribute DOMString type;
  [PutForwards=value] readonly attribute DOMSettableTokenList sizes;
};
HTMLLinkElement implements LinkStyle;

The link element allows authors to link their document to other resources.

The destination of the link(s) is given by the href attribute, which must be present and must contain a valid non-empty URL potentially surrounded by spaces. If the href attribute is absent, then the element does not define a link.

A link element must have a rel attribute.

If the rel attribute is used, the element is restricted to the head element.

The types of link indicated (the relationships) are given by the value of the rel attribute, which, if present, must have a value that is a set of space-separated tokens. The allowed keywords and their meanings are defined in a later section. If the rel attribute is absent, has no keywords, or if none of the keywords used are allowed according to the definitions in this specification, then the element does not create any links.

Two categories of links can be created using the link element: Links to external resources and hyperlinks. The link types section defines whether a particular link type is an external resource or a hyperlink. One link element can create multiple links (of which some might be external resource links and some might be hyperlinks); exactly which and how many links are created depends on the keywords given in the rel attribute. User agents must process the links on a per-link basis, not a per-element basis.

Each link created for a link element is handled separately. For instance, if there are two link elements with rel="stylesheet", they each count as a separate external resource, and each is affected by its own attributes independently. Similarly, if a single link element has a rel attribute with the value next stylesheet, it creates both a hyperlink (for the next keyword) and an external resource link (for the stylesheet keyword), and they are affected by other attributes (such as media or title) differently.

For example, the following link element creates two hyperlinks (to the same page):

<link rel="author license" href="/about">

The two links created by this element are one whose semantic is that the target page has information about the current page's author, and one whose semantic is that the target page has information regarding the license under which the current page is provided.

The crossorigin attribute is a CORS settings attribute. It is intended for use with external resource links.

The exact behavior for links to external resources depends on the exact relationship, as defined for the relevant link type. Some of the attributes control whether or not the external resource is to be applied (as defined below).

For external resources that are represented in the DOM (for example, style sheets), the DOM representation must be made available (modulo cross-origin restrictions) even if the resource is not applied. To obtain the resource, the user agent must run the following steps:

If the href attribute's value is the empty string, then abort these steps.
Resolve the URL given by the href attribute, relative to the element.
If the previous step fails, then abort these steps.
Do a potentially CORS-enabled fetch of the resulting absolute URL, with the mode being the current state of the element's crossorigin content attribute, the origin being the origin of the link element's Document, and the default origin behaviour set to taint.

The resource obtained in this fashion can be either CORS-same-origin or CORS-cross-origin.

User agents may opt to only try to obtain such resources when they are needed, instead of pro-actively fetching all the external resources that are not applied.

The semantics of the protocol used (e.g. HTTP) must be followed when fetching external resources. (For example, redirects will be followed and 404 responses will cause the external resource to not be applied.)

Once the attempts to obtain the resource and its critical subresources are complete, the user agent must, if the loads were successful, queue a task to fire a simple event named load at the link element, or, if the resource or one of its critical subresources failed to completely load for any reason (e.g. DNS error, HTTP 404 response, a connection being prematurely closed, unsupported Content-Type), queue a task to fire a simple event named error at the link element. Non-network errors in processing the resource or its subresources (e.g. CSS parse errors, PNG decoding errors) are not failures for the purposes of this paragraph.

The task source for these tasks is the DOM manipulation task source.

The element must delay the load event of the element's document until all the attempts to obtain the resource and its critical subresources are complete. (Resources that the user agent has not yet attempted to obtain, e.g. because it is waiting for the resource to be needed, do not delay the load event.)

Interactive user agents may provide users with a means to follow the hyperlinks created using the link element, somewhere within their user interface. The exact interface is not defined by this specification, but it could include the following information (obtained from the element's attributes, again as defined below), in some form or another (possibly simplified), for each hyperlink created with each link element in the document:

The relationship between this document and the resource (given by the rel attribute)
The title of the resource (given by the title attribute).
The address of the resource (given by the href attribute).
The language of the resource (given by the hreflang attribute).
The optimum media for the resource (given by the media attribute).

User agents could also include other information, such as the type of the resource (as given by the type attribute).

Hyperlinks created with the link element and its rel attribute apply to the whole page. This contrasts with the rel attribute of a and area elements, which indicates the type of a link whose context is given by the link's location within the document.

The media attribute says which media the resource applies to. The value must be a valid media query.

If the link is a hyperlink then the media attribute is purely advisory, and describes for which media the document in question was designed.

However, if the link is an external resource link, then the media attribute is prescriptive. The user agent must apply the external resource when the media attribute's value matches the environment and the other relevant conditions apply, and must not apply it otherwise.

The external resource might have further restrictions defined within that limit its applicability. For example, a CSS style sheet might have some @media blocks. This specification does not override such further restrictions or requirements.

The default, if the media attribute is omitted, is "all", meaning that by default links apply to all media.

The hreflang attribute on the link element has the same semantics as the hreflang attribute on a and area elements.

The type attribute gives the MIME type of the linked resource. It is purely advisory. The value must be a valid MIME type.

For external resource links, the type attribute is used as a hint to user agents so that they can avoid fetching resources they do not support. If the attribute is present, then the user agent must assume that the resource is of the given type (even if that is not a valid MIME type, e.g. the empty string). If the attribute is omitted, but the external resource link type has a default type defined, then the user agent must assume that the resource is of that type. If the UA does not support the given MIME type for the given link relationship, then the UA should not obtain the resource; if the UA does support the given MIME type for the given link relationship, then the UA should obtain the resource at the appropriate time as specified for the external resource link's particular type. If the attribute is omitted, and the external resource link type does not have a default type defined, but the user agent would obtain the resource if the type was known and supported, then the user agent should obtain the resource under the assumption that it will be supported.

User agents must not consider the type attribute authoritative — upon fetching the resource, user agents must not use the type attribute to determine its actual type. Only the actual type (as defined in the next paragraph) is used to determine whether to apply the resource, not the aforementioned assumed type.

If the external resource link type defines rules for processing the resource's Content-Type metadata, then those rules apply. Otherwise, if the resource is expected to be an image, user agents may apply the image sniffing rules, with the official type being the type determined from the resource's Content-Type metadata, and use the resulting sniffed type of the resource as if it was the actual type. Otherwise, if neither of these conditions apply or if the user agent opts not to apply the image sniffing rules, then the user agent must use the resource's Content-Type metadata to determine the type of the resource. If there is no type metadata, but the external resource link type has a default type defined, then the user agent must assume that the resource is of that type.

The stylesheet link type defines rules for processing the resource's Content-Type metadata.

Once the user agent has established the type of the resource, the user agent must apply the resource if it is of a supported type and the other relevant conditions apply, and must ignore the resource otherwise.

If a document contains style sheet links labeled as follows:

<link rel="stylesheet" href="A" type="text/plain">
<link rel="stylesheet" href="B" type="text/css">
<link rel="stylesheet" href="C">

...then a compliant UA that supported only CSS style sheets would fetch the B and C files, and skip the A file (since text/plain is not the MIME type for CSS style sheets).

For files B and C, it would then check the actual types returned by the server. For those that are sent as text/css, it would apply the styles, but for those labeled as text/plain, or any other type, it would not.

If one of the two files was returned without a Content-Type metadata, or with a syntactically incorrect type like Content-Type: "null", then the default type for stylesheet links would kick in. Since that default type is text/css, the style sheet would nonetheless be applied.

The title attribute gives the title of the link. With one exception, it is purely advisory. The value is text. The exception is for style sheet links, where the title attribute defines alternative style sheet sets.

The title attribute on link elements differs from the global title attribute of most other elements in that a link without a title does not inherit the title of the parent element: it merely has no title.

The sizes attribute is used with the icon link type. The attribute must not be specified on link elements that do not have a rel attribute that specifies the icon keyword.

The activation behavior of link elements that create hyperlinks is to run the following steps:

If the link element's Document is not fully active, then abort these steps.
Follow the hyperlink created by the link element.

HTTP Link: headers, if supported, must be assumed to come before any links in the document, in the order that they were given in the HTTP message. These headers are to be processed according to the rules given in the relevant specifications. [HTTP] [WEBLINK]

Registration of relation types in HTTP Link: headers is distinct from HTML link types, and thus their semantics can be different from same-named HTML types.

The IDL attributes href, rel, rev, media, hreflang, type, and sizes each must reflect the respective content attributes of the same name.

The crossOrigin IDL attribute must reflect the crossorigin content attribute, limited to only known values.

The IDL attribute relList must reflect the rel content attribute.

The IDL attribute disabled only applies to style sheet links. When the link element defines a style sheet link, then the disabled attribute behaves as defined for the alternative style sheets DOM. For all other link elements it always return false and does nothing on setting.

The LinkStyle interface is also implemented by this element; the styling processing model defines how. [CSSOM]

Here, a set of link elements provide some style sheets:

<!-- a persistent style sheet -->
<link rel="stylesheet" href="default.css">

<!-- the preferred alternate style sheet -->
<link rel="stylesheet" href="green.css" title="Green styles">

<!-- some alternate style sheets -->
<link rel="alternate stylesheet" href="contrast.css" title="High contrast">
<link rel="alternate stylesheet" href="big.css" title="Big fonts">
<link rel="alternate stylesheet" href="wide.css" title="Wide screen">

The following example shows how you can specify versions of the page that use alternative formats, are aimed at other languages, and that are intended for other media:

<link rel=alternate href="/en/html" hreflang=en type=text/html title="English HTML">
<link rel=alternate href="/fr/html" hreflang=fr type=text/html title="French HTML">
<link rel=alternate href="/en/html/print" hreflang=en type=text/html media=print title="English HTML (for printing)">
<link rel=alternate href="/fr/html/print" hreflang=fr type=text/html media=print title="French HTML (for printing)">
<link rel=alternate href="/en/pdf" hreflang=en type=application/pdf title="English PDF">
<link rel=alternate href="/fr/pdf" hreflang=fr type=application/pdf title="French PDF">

4.2.5 The `meta` element

If the charset attribute is present, or if the element's http-equiv attribute is in the encoding declaration state: in a head element.

If the http-equiv attribute is present but not in the encoding declaration state: in a head element.

If the http-equiv attribute is present but not in the encoding declaration state: in a noscript element that is a child of a head element.

If the name attribute is present: where metadata content is expected.

Empty.

name — Metadata name

http-equiv — Pragma directive

content — Value of the element

charset — Character encoding declaration

No end tag.

Allowed ARIA role attribute values:

none

interface HTMLMetaElement : HTMLElement {
           attribute DOMString name;
           attribute DOMString httpEquiv;
           attribute DOMString content;
};

The meta element represents various kinds of metadata that cannot be expressed using the title, base, link, style, and script elements.

The meta element can represent document-level metadata with the name attribute, pragma directives with the http-equiv attribute, and the file's character encoding declaration when an HTML document is serialized to string form (e.g. for transmission over the network or for disk storage) with the charset attribute.

Exactly one of the name, http-equiv, and charset, attributes must be specified.

If either name or http-equiv is specified, then the content attribute must also be specified. Otherwise, it must be omitted.

The charset attribute specifies the character encoding used by the document. This is a character encoding declaration. If the attribute is present in an XML document, its value must be an ASCII case-insensitive match for the string "UTF-8" (and the document is therefore forced to use UTF-8 as its encoding).

The charset attribute on the meta element has no effect in XML documents, and is only allowed in order to facilitate migration to and from XHTML.

There must not be more than one meta element with a charset attribute per document.

The content attribute gives the value of the document metadata or pragma directive when the element is used for those purposes. The allowed values depend on the exact context, as described in subsequent sections of this specification.

If a meta element has a name attribute, it sets document metadata. Document metadata is expressed in terms of name-value pairs, the name attribute on the meta element giving the name, and the content attribute on the same element giving the value. The name specifies what aspect of metadata is being set; valid names and the meaning of their values are described in the following sections. If a meta element has no content attribute, then the value part of the metadata name-value pair is the empty string.

The name and content IDL attributes must reflect the respective content attributes of the same name. The IDL attribute httpEquiv must reflect the content attribute http-equiv.

4.2.5.1 Standard metadata names

This specification defines a few names for the name attribute of the meta element.

Names are case-insensitive, and must be compared in an ASCII case-insensitive manner.

application-name

The value must be a short free-form string giving the name of the Web application that the page represents. If the page is not a Web application, the application-name metadata name must not be used. Translations of the Web application's name may be given, using the attribute to specify the language of each name.

There must not be more than one meta element with a given language and with its name attribute set to the value application-name per document.

User agents may use the application name in UI in preference to the page's title, since the title might include status messages and the like relevant to the status of the page at a particular moment in time instead of just being the name of the application.

To find the application name to use given an ordered list of languages (e.g. British English, American English, and English), user agents must run the following steps:

Let languages be the list of languages.
Let default language be the language of the Document's root element, if any, and if that language is not unknown.
If there is a default language, and if it is not the same language as any of the languages is languages, append it to languages.
Let winning language be the first language in languages for which there is a meta element in the Document that has its name attribute set to the value application-name and whose language is the language in question.

If none of the languages have such a meta element, then abort these steps; there's no given application name.
Return the value of the content attribute of the first meta element in the Document in tree order that has its name attribute set to the value application-name and whose language is winning language.

author

The value must be a free-form string giving the name of one of the page's authors.

description

The value must be a free-form string that describes the page. The value must be appropriate for use in a directory of pages, e.g. in a search engine. There must not be more than one meta element with its name attribute set to the value description per document.

generator

The value must be a free-form string that identifies one of the software packages used to generate the document. This value must not be used on pages whose markup is not generated by software, e.g. pages whose markup was written by a user in a text editor.

Here is what a tool called "Frontweaver" could include in its output, in the page's head element, to identify itself as the tool used to generate the page:

<meta name=generator content="Frontweaver 8.2">

keywords

The value must be a set of comma-separated tokens, each of which is a keyword relevant to the page.

This page about typefaces on British motorways uses a meta element to specify some keywords that users might use to look for the page:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Typefaces on UK motorways</title>
  <meta name="keywords" content="british,type face,font,fonts,highway,highways">
 </head>
 <body>
  ...

Many search engines do not consider such keywords, because this feature has historically been used unreliably and even misleadingly as a way to spam search engine results in a way that is not helpful for users.

To obtain the list of keywords that the author has specified as applicable to the page, the user agent must run the following steps:

Let keywords be an empty list.
For each meta element with a name attribute and a content attribute and whose name attribute's value is keywords, run the following substeps:
1. Split the value of the element's content attribute on commas.
2. Add the resulting tokens, if any, to keywords.
Remove any duplicates from keywords.
Return keywords. This is the list of keywords that the author has specified as applicable to the page.

User agents should not use this information when there is insufficient confidence in the reliability of the value.

For instance, it would be reasonable for a content management system to use the keyword information of pages within the system to populate the index of a site-specific search engine, but a large-scale content aggregator that used this information would likely find that certain users would try to game its ranking mechanism through the use of inappropriate keywords.

4.2.5.2 Other metadata names

Extensions to the predefined set of metadata names may be registered in the WHATWG Wiki MetaExtensions page. [WHATWGWIKI]

Anyone is free to edit the WHATWG Wiki MetaExtensions page at any time to add a type. These new names must be specified with the following information:

Keyword

The actual name being defined. The name should not be confusingly similar to any other defined name (e.g. differing only in case).

Brief description

A short non-normative description of what the metadata name's meaning is, including the format the value is required to be in.

Specification

A link to a more detailed description of the metadata name's semantics and requirements. It could be another page on the Wiki, or a link to an external page.

Synonyms

A list of other names that have exactly the same processing requirements. Authors should not use the names defined to be synonyms, they are only intended to allow user agents to support legacy content. Anyone may remove synonyms that are not used in practice; only names that need to be processed as synonyms for compatibility with legacy content are to be registered in this way.

Status

One of the following:

Proposed: The name has not received wide peer review and approval. Someone has proposed it and is, or soon will be, using it.
Ratified: The name has received wide peer review and approval. It has a specification that unambiguously defines how to handle pages that use the name, including when they use it in incorrect ways.
Discontinued: The metadata name has received wide peer review and it has been found wanting. Existing pages are using this metadata name, but new pages should avoid it. The "brief description" and "specification" entries will give details of what authors should use instead, if anything.

If a metadata name is found to be redundant with existing values, it should be removed and listed as a synonym for the existing value.

If a metadata name is registered in the "proposed" state for a period of a month or more without being used or specified, then it may be removed from the registry.

If a metadata name is added with the "proposed" status and found to be redundant with existing values, it should be removed and listed as a synonym for the existing value. If a metadata name is added with the "proposed" status and found to be harmful, then it should be changed to "discontinued" status.

Anyone can change the status at any time, but should only do so in accordance with the definitions above.

Conformance checkers may use the information given on the WHATWG Wiki MetaExtensions page to establish if a value is allowed or not: values defined in this specification or marked as "proposed" or "ratified" must be accepted, whereas values marked as "discontinued" or not listed in either this specification or on the aforementioned page must be reported as invalid. Conformance checkers may cache this information (e.g. for performance reasons or to avoid the use of unreliable network connectivity).

When an author uses a new metadata name not defined by either this specification or the Wiki page, conformance checkers should offer to add the value to the Wiki, with the details described above, with the "proposed" status.

Metadata names whose values are to be URLs must not be proposed or accepted. Links must be represented using the link element, not the meta element.

4.2.5.3 Pragma directives

When the http-equiv attribute is specified on a meta element, the element is a pragma directive.

The http-equiv attribute is an enumerated attribute. The following table lists the keywords defined for this attribute. The states given in the first cell of the rows with keywords give the states to which those keywords map. Some of the keywords are non-conforming, as noted in the last column.

State	Keyword	Notes
Content Language	`content-language`	Non-conforming
Encoding declaration	`content-type`
Default style	`default-style`
Refresh	`refresh`
Cookie setter	`set-cookie`	Non-conforming

When a meta element is inserted into the document, if its http-equiv attribute is present and represents one of the above states, then the user agent must run the algorithm appropriate for that state, as described in the following list:

Content language state (http-equiv="content-language")

This feature is non-conforming. Authors are encouraged to use the lang attribute instead.

This pragma sets the pragma-set default language. Until such a pragma is successfully processed, there is no pragma-set default language.

If the meta element has no content attribute, then abort these steps.
If the element's content attribute contains a "," (U+002C) character then abort these steps.
Let input be the value of the element's content attribute.
Let position point at the first character of input.
Skip whitespace.
Collect a sequence of characters that are not space characters.
Let candidate be the string that resulted from the previous step.
If candidate is the empty string, abort these steps.
Set the pragma-set default language to candidate.

This pragma is almost, but not quite, entirely unlike the HTTP Content-Language header of the same name. [HTTP]

encoding declaration state (http-equiv="content-type")

The encoding declaration state is just an alternative form of setting the charset attribute: it is a character encoding declaration. This state's user agent requirements are all handled by the parsing section of the specification.

For meta elements with an http-equiv attribute in the Encoding declaration state, the content attribute must have a value that is an ASCII case-insensitive match for a string that consists of: the literal string "text/html;", optionally followed by any number of space characters, followed by the literal string "charset=", followed by one of the labels of the character encoding of the character encoding declaration.

A document must not contain both a meta element with an http-equiv attribute in the encoding declaration state and a meta element with the charset attribute present.

The encoding declaration state may be used in HTML documents and in XML Documents. If the encoding declaration state is used in XML Documents, the name of the character encoding must be an ASCII case-insensitive match for the string "UTF-8" (and the document is therefore forced to use UTF-8 as its encoding).

The encoding declaration state has no effect in XML documents, and is only allowed in order to facilitate migration to and from XHTML.

Default style state (http-equiv="default-style")

This pragma sets the name of the default alternative style sheet set.

If the meta element has no content attribute, or if that attribute's value is the empty string, then abort these steps.
Set the preferred style sheet set to the value of the element's content attribute. [CSSOM]

Refresh state (http-equiv="refresh")

This pragma acts as timed redirect.

If another meta element with an http-equiv attribute in the Refresh state has already been successfully processed (i.e. when it was inserted the user agent processed it and reached the last step of this list of steps), then abort these steps.
If the meta element has no content attribute, or if that attribute's value is the empty string, then abort these steps.
Let input be the value of the element's content attribute.
Let position point at the first character of input.
Skip whitespace.
Collect a sequence of characters that are ASCII digits, and parse the resulting string using the rules for parsing non-negative integers. If the sequence of characters collected is the empty string, then no number will have been parsed; abort these steps. Otherwise, let time be the parsed number.
Collect a sequence of characters that are ASCII digits and "." (U+002E) characters. Ignore any collected characters.
Skip whitespace.
Let url be the address of the current page.
If the character in input pointed to by position is a ";" (U+003B) character or a "," (U+002C) character, then advance position to the next character. Otherwise, jump to the last step.
Skip whitespace.
If the character in input pointed to by position is a "U" (U+0055) character or a U+0075 LATIN SMALL LETTER U character (u), then advance position to the next character. Otherwise, jump to the last step.
If the character in input pointed to by position is a "R" (U+0052) character or a U+0072 LATIN SMALL LETTER R character (r), then advance position to the next character. Otherwise, jump to the last step.
If the character in input pointed to by position is s "L" (U+004C) character or a U+006C LATIN SMALL LETTER L character (l), then advance position to the next character. Otherwise, jump to the last step.
Skip whitespace.
If the character in input pointed to by position is a "=" (U+003D), then advance position to the next character. Otherwise, jump to the last step.
Skip whitespace.
If the character in input pointed to by position is either a "'" (U+0027) character or """ (U+0022) character, then let quote be that character, and advance position to the next character. Otherwise, let quote be the empty string.
Let url be equal to the substring of input from the character at position to the end of the string.
If quote is not the empty string, and there is a character in url equal to quote, then truncate url at that character, so that it and all subsequent characters are removed.
Strip any trailing space characters from the end of url.
Strip any "tab" (U+0009), "LF" (U+000A), and "CR" (U+000D) characters from url.
Resolve the url value to an absolute URL, relative to the meta element. If this fails, abort these steps.
Perform one or more of the following steps:
- After the refresh has come due (as defined below), if the user has not canceled the redirect and if the meta element's Document's active sandboxing flag set does not have the sandboxed automatic features browsing context flag set, navigate the Document's browsing context to url, with replacement enabled, and with the Document's browsing context as the source browsing context.
  
  For the purposes of the previous paragraph, a refresh is said to have come due as soon as the later of the following two conditions occurs:
  - At least time seconds have elapsed since the document has completely loaded, adjusted to take into account user or user agent preferences.
  - At least time seconds have elapsed since the meta element was inserted into the Document, adjusted to take into account user or user agent preferences.
- Provide the user with an interface that, when selected, navigates a browsing context to url, with the Document's browsing context as the source browsing context.
- Do nothing.
In addition, the user agent may, as with anything, inform the user of any and all aspects of its operation, including the state of any timers, the destinations of any timed redirects, and so forth.

For meta elements with an http-equiv attribute in the Refresh state, the content attribute must have a value consisting either of:

just a valid non-negative integer, or
a valid non-negative integer, followed by a U+003B SEMICOLON character (;), followed by one or more space characters, followed by a substring that is an ASCII case-insensitive match for the string "URL", followed by a "=" (U+003D) character, followed by a valid URL that does not start with a literal "'" (U+0027) or """ (U+0022) character.

In the former case, the integer represents a number of seconds before the page is to be reloaded; in the latter case the integer represents a number of seconds before the page is to be replaced by the page at the given URL.

A news organization's front page could include the following markup in the page's head element, to ensure that the page automatically reloads from the server every five minutes:

<meta http-equiv="Refresh" content="300">

A sequence of pages could be used as an automated slide show by making each page refresh to the next page in the sequence, using markup such as the following:

<meta http-equiv="Refresh" content="20; URL=page4.html">

Cookie setter (http-equiv="set-cookie")

This pragma sets an HTTP cookie. [COOKIES]

It is non-conforming. Real HTTP headers should be used instead.

If the meta element has no content attribute, or if that attribute's value is the empty string, then abort these steps.
Obtain the storage mutex.
Act as if receiving a set-cookie-string for the document's address via a "non-HTTP" API, consisting of the value of the element's content attribute encoded as UTF-8. [COOKIES] [RFC3629]

There must not be more than one meta element with any particular state in the document at a time.

4.2.5.4 Other pragma directives

Extensions to the predefined set of pragma directives may, under certain conditions, be registered in the WHATWG Wiki PragmaExtensions page. [WHATWGWIKI]

Such extensions must use a name that is identical to an HTTP header registered in the Permanent Message Header Field Registry, and must have behavior identical to that described for the HTTP header. [IANAPERMHEADERS]

Pragma directives corresponding to headers describing metadata, or not requiring specific user agent processing, must not be registered; instead, use metadata names. Pragma directives corresponding to headers that affect the HTTP processing model (e.g. caching) must not be registered, as they would result in HTTP-level behavior being different for user agents that implement HTML than for user agents that do not.

Anyone is free to edit the WHATWG Wiki PragmaExtensions page at any time to add a pragma directive satisfying these conditions. Such registrations must specify the following information:

Keyword: The actual name being defined. The name must match a previously-registered HTTP name with the same requirements.
Brief description: A short non-normative description of the purpose of the pragma directive.
Specification: A link to the specification defining the corresponding HTTP header.

Conformance checkers must use the information given on the WHATWG Wiki PragmaExtensions page to establish if a value is allowed or not: values defined in this specification or listed on the aforementioned page must be accepted, whereas values not listed in either this specification or on the aforementioned page must be rejected as invalid. Conformance checkers may cache this information (e.g. for performance reasons or to avoid the use of unreliable network connectivity).

4.2.5.5 Specifying the document's character encoding

A character encoding declaration is a mechanism by which the character encoding used to store or transmit a document is specified.

The following restrictions apply to character encoding declarations:

The character encoding name given must be an ASCII case-insensitive match for one of the labels of the character encoding used to serialize the file. [ENCODING]
The character encoding declaration must be serialized without the use of character references or character escapes of any kind.
The element containing the character encoding declaration must be serialized completely within the first 1024 bytes of the document.

In addition, due to a number of restrictions on meta elements, there can only be one meta-based character encoding declaration per document.

If an HTML document does not start with a BOM, and its encoding is not explicitly given by Content-Type metadata, and the document is not an iframe srcdoc document, then the character encoding used must be an ASCII-compatible character encoding, and the encoding must be specified using a meta element with a charset attribute or a meta element with an http-equiv attribute in the encoding declaration state.

A character encoding declaration is required (either in the Content-Type metadata or explicitly in the file) even if the encoding is US-ASCII, because a character encoding is needed to process non-ASCII characters entered by the user in forms, in URLs generated by scripts, and so forth.

If the document is an iframe srcdoc document, the document must not have a character encoding declaration. (In this case, the source is already decoded, since it is part of the document that contained the iframe.)

If an HTML document contains a meta element with a charset attribute or a meta element with an http-equiv attribute in the encoding declaration state, then the character encoding used must be an ASCII-compatible character encoding.

Authors should use UTF-8. Conformance checkers may advise authors against using legacy encodings. [RFC3629]

Authoring tools should default to using UTF-8 for newly-created documents. [RFC3629]

Encodings in which a series of bytes in the range 0x20 to 0x7E can encode characters other than the corresponding characters in the range U+0020 to U+007E represent a potential security vulnerability: a user agent that does not support the encoding (or does not support the label used to declare the encoding, or does not use the same mechanism to detect the encoding of unlabeled content as another user agent) might end up interpreting technically benign plain text content as HTML tags and JavaScript. Authors should therefore not use these encodings. For example, this applies to encodings in which the bytes corresponding to "<script>" in ASCII can encode a different string. Authors should not use such encodings, which are known to include JIS_C6226-1983, JIS_X0212-1990, HZ-GB-2312, JOHAB (Windows code page 1361), encodings based on ISO-2022, and encodings based on EBCDIC. Furthermore, authors must not use the CESU-8, UTF-7, BOCU-1 and SCSU encodings, which also fall into this category; these encodings were never intended for use for Web content. [RFC1345] [RFC1842] [RFC1468] [RFC2237] [RFC1554] [CP50220] [RFC1922] [RFC1557] [CESU8] [UTF7] [BOCU1] [SCSU]

Authors should not use UTF-32, as the encoding detection algorithms described in this specification intentionally do not distinguish it from UTF-16. [UNICODE]

Using non-UTF-8 encodings can have unexpected results on form submission and URL encodings, which use the document's character encoding by default.

In XHTML, the XML declaration should be used for inline character encoding information, if necessary.

In HTML, to declare that the character encoding is UTF-8, the author could include the following markup near the top of the document (in the head element):

<meta charset="utf-8">

In XML, the XML declaration would be used instead, at the very top of the markup:

<?xml version="1.0" encoding="utf-8"?>

4.2.6 The `style` element

Where metadata content is expected.

In a noscript element that is a child of a head element.

Depends on the value of the type attribute, but must match requirements described in prose below.

media — Applicable media

type — Type of embedded resource

Also, the title attribute has special semantics on this element: Alternative style sheet set name.

Neither tag is omissible.

Allowed ARIA role attribute values:

none

interface HTMLStyleElement : HTMLElement {
           attribute boolean disabled;
           attribute DOMString media;
           attribute DOMString type;
};
HTMLStyleElement implements LinkStyle;

The style element allows authors to embed style information in their documents. The style element is one of several inputs to the styling processing model. The element does not represent content for the user.

The type attribute gives the styling language. If the attribute is present, its value must be a valid MIME type that designates a styling language. The charset parameter must not be specified. The default value for the type attribute, which is used if the attribute is absent, is "text/css". [RFC2318]

When examining types to determine if they support the language, user agents must not ignore unknown MIME parameters — types with unknown parameters must be assumed to be unsupported. The charset parameter must be treated as an unknown parameter for the purpose of comparing MIME types here.

The media attribute says which media the styles apply to. The value must be a valid media query. The user agent must apply the styles when the media attribute's value matches the environment and the other relevant conditions apply, and must not apply them otherwise.

The styles might be further limited in scope, e.g. in CSS with the use of @media blocks. This specification does not override such further restrictions or requirements.

The default, if the media attribute is omitted, is "all", meaning that by default styles apply to all media.

The title attribute on style elements defines alternative style sheet sets. If the style element has no title attribute, then it has no title; the title attribute of ancestors does not apply to the style element. [CSSOM]

The title attribute on style elements, like the title attribute on link elements, differs from the global title attribute in that a style block without a title does not inherit the title of the parent element: it merely has no title.

The textContent of a style element must match the style production in the following ABNF, the character set for which is Unicode. [ABNF]

style         = no-c-start *( c-start no-c-end c-end no-c-start )
no-c-start    = < any string that doesn't contain a substring that matches c-start >
c-start       = "<!--"
no-c-end      = < any string that doesn't contain a substring that matches c-end >
c-end         = "-->"

All descendant elements must be processed, according to their semantics, before the style element itself is evaluated. For styling languages that consist of pure text (as opposed to XML), user agents must evaluate style elements by passing the concatenation of the contents of all the Text nodes that are children of the style element (not any other nodes such as comments or elements), in tree order, to the style system. For XML-based styling languages, user agents must pass all the child nodes of the style element to the style system.

All URLs found by the styling language's processor must be resolved, relative to the element (or as defined by the styling language), when the processor is invoked.

Once the attempts to obtain the style sheet's critical subresources, if any, are complete, or, if the style sheet has no critical subresources, once the style sheet has been parsed and processed, the user agent must, if the loads were successful or there were none, queue a task to fire a simple event named load at the style element, or, if one of the style sheet's critical subresources failed to completely load for any reason (e.g. DNS error, HTTP 404 response, a connection being prematurely closed, unsupported Content-Type), queue a task to fire a simple event named error at the style element. Non-network errors in processing the style sheet or its subresources (e.g. CSS parse errors, PNG decoding errors) are not failures for the purposes of this paragraph.

The task source for these tasks is the DOM manipulation task source.

The element must delay the load event of the element's document until all the attempts to obtain the style sheet's critical subresources, if any, are complete.

This specification does not specify a style system, but CSS is expected to be supported by most Web browsers. [CSS]

The media and type IDL attributes must reflect the respective content attributes of the same name.

The disabled IDL attribute behaves as defined for the alternative style sheets DOM.

The LinkStyle interface is also implemented by this element; the styling processing model defines how. [CSSOM]

The following document has its stress emphasis styled as bright red text rather than italics text, while leaving titles of works and Latin words in their default italics. It shows how using appropriate elements enables easier restyling of documents.

<!DOCTYPE html>
<html lang="en-US">
 <head>
  <title>My favorite book</title>
  <style>
   body { color: black; background: white; }
   em { font-style: normal; color: red; }
  </style>
 </head>
 <body>
  <p>My <em>favorite</em> book of all time has <em>got</em> to be
  <cite>A Cat's Life</cite>. It is a book by P. Rahmel that talks
  about the <i lang="la">Felis Catus</i> in modern human society.</p>
 </body>
</html>

4.2.7 Styling

The link and style elements can provide styling information for the user agent to use when rendering the document. The CSS and CSSOM specifications specify what styling information is to be used by the user agent and how it is to be used. [CSS] [CSSOM]

The style and link elements implement the LinkStyle interface. [CSSOM]

For style elements, if the user agent does not support the specified styling language, then the sheet attribute of the element's LinkStyle interface must return null. Similarly, link elements that do not represent external resource links that contribute to the styling processing model (i.e. that do not have a stylesheet keyword in their rel attribute), for which the link is an alternative stylesheet but whose title content attribute is absent or empty, or whose resource is CORS-cross-origin, must have their LinkStyle interface's sheet attribute return null.

Otherwise, the LinkStyle interface's sheet attribute must return null if the corresponding element is not in a Document, and otherwise must return a StyleSheet object with the following properties: [CSSOM]

The style sheet type: The style sheet type must be the same as the style's specified type. For style elements, this is the same as the type content attribute's value, or text/css if that is omitted. For link elements, this is the Content-Type metadata of the specified resource.
The style sheet location: For link elements, the location must be the result of resolving the URL given by the element's href content attribute, relative to the element, or the empty string if that fails. For style elements, there is no location.
The style sheet media: The media must be the same as the value of the element's media content attribute, or the empty string, if the attribute is omitted.
The style sheet title: The title must be the same as the value of the element's title content attribute, if the attribute is present and has a non-empty value. If the attribute is absent or its value is the empty string, then the style sheet does not have a title (it is the empty string). The title is used for defining alternative style sheet sets.
The style sheet alternate flag: For link elements, true if the link is an alternative stylesheet. In all other cases, false.

The same object must be returned each time.

The disabled IDL attribute on link and style elements must return false and do nothing on setting, if the sheet attribute of their LinkStyle interface is null. Otherwise, it must return the value of the StyleSheet interface's disabled attribute on getting, and forward the new value to that same attribute on setting.

The rules for handling alternative style sheets are defined in the CSS object model specification. [CSSOM]

Style sheets, whether added by a link element, a style element, an <?xml-stylesheet> PI, an HTTP Link: header, or some other mechanism, have a style sheet ready flag, which is initially unset.

When a style sheet is ready to be applied, its style sheet ready flag must be set. If the style sheet referenced no other resources (e.g. it was an internal style sheet given by a style element with no @import rules), then the style rules must be synchronously made available to script; otherwise, the style rules must only be made available to script once the event loop reaches its "update the rendering" step.

A style sheet in the context of the Document of an HTML parser or XML parser is said to be a style sheet that is blocking scripts if the element was created by that Document's parser, and the element is either a style element or a link element that was an external resource link that contributes to the styling processing model when the element was created by the parser, and the element's style sheet was enabled when the element was created by the parser, and the element's style sheet ready flag is not yet set, and, the last time the event loop reached step 1, the element was in that Document, and the user agent hasn't given up on that particular style sheet yet. A user agent may give up on a style sheet at any time.

Giving up on a style sheet before the style sheet loads, if the style sheet eventually does still load, means that the script might end up operating with incorrect information. For example, if a style sheet sets the color of an element to green, but a script that inspects the resulting style is executed before the sheet is loaded, the script will find that the element is black (or whatever the default color is), and might thus make poor choices (e.g. deciding to use black as the color elsewhere on the page, instead of green). Implementors have to balance the likelihood of a script using incorrect information with the performance impact of doing nothing while waiting for a slow network request to finish.

A Document has a style sheet that is blocking scripts if there is either a style sheet that is blocking scripts in the context of that Document, or if that Document is in a browsing context that has a parent browsing context, and the active document of that parent browsing context itself has a style sheet that is blocking scripts.

A Document has no style sheet that is blocking scripts if it does not have a style sheet that is blocking scripts as defined in the previous paragraph.

4.3 Sections

4.3.1 The `body` element

Sectioning root.

As the second element in an html element.

onafterprint

onbeforeprint

onbeforeunload

onhashchange

onmessage

onoffline

ononline

onpagehide

onpageshow

onpopstate

onstorage

onunload

A body element's start tag may be omitted if the element is empty, or if the first thing inside the body element is not a space character or a comment, except if the first thing inside the body element is a meta, link, script, style, or template element.

A body element's end tag may be omitted if the body element is not immediately followed by a comment.

Allowed ARIA role attribute values:

document role (default - do not set), application.

Allowed ARIA State and Property Attributes:

Any aria-* attributes applicable to the allowed roles.

interface HTMLBodyElement : HTMLElement {
};
HTMLBodyElement implements WindowEventHandlers;

The body element represents the content of the document.

In conforming documents, there is only one body element. The document.body IDL attribute provides scripts with easy access to a document's body element.

Some DOM operations are defined in terms of "the body element". This refers to a particular element in the DOM, as per the definition of the term, and not any arbitrary body element.

The body element exposes as event handler content attributes a number of the event handlers of the Window object. It also mirrors their event handler IDL attributes.

The onblur, onerror, onfocus, onload, onresize, and onscroll event handlers of the Window object, exposed on the body element, replace the generic event handlers with the same names normally supported by HTML elements.

Thus, for example, a bubbling error event dispatched on a child of the body element of a Document would first trigger the onerror event handler content attributes of that element, then that of the root html element, and only then would it trigger the onerror event handler content attribute on the body element. This is because the event would bubble from the target, to the body, to the html, to the Document, to the Window, and the event handler on the body is watching the Window not the body. A regular event listener attached to the body using addEventListener(), however, would be run when the event bubbled through the body and not when it reaches the Window object.

This page updates an indicator to show whether or not the user is online:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Online or offline?</title>
  <script>
   function update(online) {
     document.getElementById('status').textContent =
       online ? 'Online' : 'Offline';
   }
  </script>
 </head>
 <body ononline="update(true)"
       onoffline="update(false)"
       onload="update(navigator.onLine)">
  <p>You are: <span id="status">(Unknown)</span></p>
 </body>
</html>

4.3.2 The `article` element

Categories:: Flow content, but with no main element descendants.; Sectioning content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Flow content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: article (default - do not set), application, document or main.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The article element represents a complete, or self-contained, composition in a document, page, application, or site and that is, in principle, independently distributable or reusable, e.g. in syndication. This could be a forum post, a magazine or newspaper article, a blog entry, a user-submitted comment, an interactive widget or gadget, or any other independent item of content.

When article elements are nested, the inner article elements represent articles that are in principle related to the contents of the outer article. For instance, a blog entry on a site that accepts user-submitted comments could represent the comments as article elements nested within the article element for the blog entry.

Author information associated with an article element (q.v. the address element) does not apply to nested article elements.

When used specifically with content to be redistributed in syndication, the article element is similar in purpose to the entry element in Atom. [ATOM]

When the main content of the page (i.e. excluding footers, headers, navigation blocks, and sidebars) is all one single self-contained composition, the content should be marked up with a main element and the content may also be marked with an article, but it is technically redundant in this case (since it's self-evident that the page is a single composition, as it is a single document).

This example shows a blog post using the article element, with some schema.org annotations:

<article itemscope itemtype="http://schema.org/BlogPosting">
 <header>
  <h1 itemprop="headline">The Very First Rule of Life</h1>
  <p><time itemprop="datePublished" datetime="2009-10-09">3 days ago</time></p>
  <link itemprop="url" href="?comments=0">
 </header>
 <p>If there's a microphone anywhere near you, assume it's hot and
 sending whatever you're saying to the world. Seriously.</p>
 <p>...</p>
 <footer>
  <a itemprop="discussionUrl" href="?comments=1">Show comments...</a>
 </footer>
</article>

Here is that same blog post, but showing some of the comments:

<article itemscope itemtype="http://schema.org/BlogPosting">
 <header>
  <h1 itemprop="headline">The Very First Rule of Life</h1>
  <p><time itemprop="datePublished" datetime="2009-10-09">3 days ago</time></p>
  <link itemprop="url" href="?comments=0">
 </header>
 <p>If there's a microphone anywhere near you, assume it's hot and
 sending whatever you're saying to the world. Seriously.</p>
 <p>...</p>
 <section>
  <h1>Comments</h1>
  <article itemprop="comment" itemscope itemtype="http://schema.org/UserComments" id="c1">
   <link itemprop="url" href="#c1">
   <footer>
    <p>Posted by: <span itemprop="creator" itemscope itemtype="http://schema.org/Person">
     <span itemprop="name">George Washington</span>
    </span></p>
    <p><time itemprop="commentTime" datetime="2009-10-10">15 minutes ago</time></p>
   </footer>
   <p>Yeah! Especially when talking about your lobbyist friends!</p>
  </article>
  <article itemprop="comment" itemscope itemtype="http://schema.org/UserComments" id="c2">
   <link itemprop="url" href="#c2">
   <footer>
    <p>Posted by: <span itemprop="creator" itemscope itemtype="http://schema.org/Person">
     <span itemprop="name">George Hammond</span>
    </span></p>
    <p><time itemprop="commentTime" datetime="2009-10-10">5 minutes ago</time></p>
   </footer>
   <p>Hey, you have the same first name as me.</p>
  </article>
 </section>
</article>

Notice the use of footer to give the information for each comment (such as who wrote it and when): the footer element can appear at the start of its section when appropriate, such as in this case. (Using header in this case wouldn't be wrong either; it's mostly a matter of authoring preference.)

4.3.3 The `section` element

Categories:: Flow content.; Sectioning content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Flow content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: region role (default - do not set), alert, alertdialog, application, contentinfo, dialog, document, log, main, marquee, presentation, search or status.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The section element represents a generic section of a document or application. A section, in this context, is a thematic grouping of content. The theme of each section should be identified, typically by including a heading (h1-h6 element) as a child of the section element.

Examples of sections would be chapters, the various tabbed pages in a tabbed dialog box, or the numbered sections of a thesis. A Web site's home page could be split into sections for an introduction, news items, and contact information.

Authors are encouraged to use the article element instead of the section element when it would make sense to syndicate the contents of the element.

The section element is not a generic container element. When an element is needed only for styling purposes or as a convenience for scripting, authors are encouraged to use the div element instead. A general rule is that the section element is appropriate only if the element's contents would be listed explicitly in the document's outline.

In the following example, we see an article (part of a larger Web page) about apples, containing two short sections.

<article>
 <header>
  <h2>Apples</h2>
  <p>Tasty, delicious fruit!</p>
 </header>
 <p>The apple is the pomaceous fruit of the apple tree.</p>
 <section>
  <h3>Red Delicious</h3>
  <p>These bright red apples are the most common found in many
  supermarkets.</p>
 </section>
 <section>
  <h3>Granny Smith</h3>
  <p>These juicy, green apples make a great filling for
  apple pies.</p>
 </section>
</article>

Here is a graduation programme with two sections, one for the list of people graduating, and one for the description of the ceremony. (The markup in this example features an uncommon style sometimes used to minimize the amount of inter-element whitespace.)

<!DOCTYPE Html>
<Html
 ><Head
   ><Title
     >Graduation Ceremony Summer 2022</Title
   ></Head
 ><Body
   ><H1
     >Graduation</H1
   ><Section
     ><H2
       >Ceremony</H2
     ><P
       >Opening Procession</P
     ><P
       >Speech by Validactorian</P
     ><P
       >Speech by Class President</P
     ><P
       >Presentation of Diplomas</P
     ><P
       >Closing Speech by Headmaster</P
   ></Section
   ><Section
     ><H2
       >Graduates</H2
     ><Ul
       ><Li
         >Molly Carpenter</Li
       ><Li
         >Anastasia Luccio</Li
       ><Li
         >Ebenezar McCoy</Li
       ><Li
         >Karrin Murphy</Li
       ><Li
         >Thomas Raith</Li
       ><Li
         >Susan Rodriguez</Li
     ></Ul
   ></Section
 ></Body
></Html>

In this example, a book author has marked up some sections as chapters and some as appendices, and uses CSS to style the headers in these two classes of section differently. The whole book is wrapped in an article element as part of an even larger document containing other books.

<article class="book">
 <style>
  section { border: double medium; margin: 2em; }
  section.chapter h1 { font: 2em Roboto, Helvetica Neue, sans-serif; }
  section.appendix h1 { font: small-caps 2em Roboto, Helvetica Neue, sans-serif; }
 </style>
 <header>
   <h2>My Book</h2>
   <p>A sample with not much content</p>
  <p><small>Published by Dummy Publicorp Ltd.</small></p>
 </header>
  
 <section class="chapter">
  <h3>My First Chapter</h3>
  <p>This is the first of my chapters. It doesn't say much.</p>
  <p>But it has two paragraphs!</p>
 </section>
 <section class="chapter">
  <h3>It Continutes: The Second Chapter</h3>
  <p>Bla dee bla, dee bla dee bla. Boom.</p>
 </section>
 <section class="chapter">
  <h3>Chapter Three: A Further Example</h3>
  <p>It's not like a battle between brightness and earthtones would go
  unnoticed.</p>
  <p>But it might ruin my story.</p>
 </section>
 <section class="appendix">
  <h3>Appendix A: Overview of Examples</h3>
  <p>These are demonstrations.</p>
 </section>
 <section class="appendix">
  <h3>Appendix B: Some Closing Remarks</h3>
  <p>Hopefully this long example shows that you <em>can</em> style
  sections, so long as they are used to indicate actual sections.</p>
 </section>
</article>

4.3.4 The `nav` element

Categories:: Flow content.; Sectioning content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Flow content, but with no main element descendants.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: navigation role (default - do not set) or presentation.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The nav element represents a section of a page that links to other pages or to parts within the page: a section with navigation links.

In cases where the content of a nav element represents a list of items, use list markup to aid understanding and navigation.

Not all groups of links on a page need to be in a nav element — the element is primarily intended for sections that consist of major navigation blocks. In particular, it is common for footers to have a short list of links to various pages of a site, such as the terms of service, the home page, and a copyright page. The footer element alone is sufficient for such cases; while a nav element can be used in such cases, it is usually unnecessary.

User agents (such as screen readers) that are targeted at users who can benefit from navigation information being omitted in the initial rendering, or who can benefit from navigation information being immediately available, can use this element as a way to determine what content on the page to initially skip or provide on request (or both).

In the following example, the page has several places where links are present, but only one of those places is considered a navigation section.

<body itemscope itemtype="http://schema.org/Blog">
 <header>
  <h1>Wake up sheeple!</h1>
  <p><a href="news.html">News</a> -
     <a href="blog.html">Blog</a> -
     <a href="forums.html">Forums</a></p>
  <p>Last Modified: <span itemprop="dateModified">2009-04-01</span></p>
  <nav>
   <h1>Navigation</h1>
   <ul>
    <li><a href="articles.html">Index of all articles</a></li>
    <li><a href="today.html">Things sheeple need to wake up for today</a></li>
    <li><a href="successes.html">Sheeple we have managed to wake</a></li>
   </ul>
  </nav>
 </header>
 <main>
  <article itemprop="blogPosts" itemscope itemtype="http://schema.org/BlogPosting">
   <header>
    <h1 itemprop="headline">My Day at the Beach</h1>
   </header>
   <div itemprop="articleBody">
    <p>Today I went to the beach and had a lot of fun.</p>
    ...more content...
   </div>
   <footer>
    <p>Posted <time itemprop="datePublished" datetime="2009-10-10">Thursday</time>.</p>
   </footer>
  </article>
  ...more blog posts...
 </main>
 <footer>
  <p>Copyright ©
   <span itemprop="copyrightYear">2010</span>
   <span itemprop="copyrightHolder">The Example Company</span>
  </p>
  <p><a href="about.html">About</a> -
     <a href="policy.html">Privacy Policy</a> -
     <a href="contact.html">Contact Us</a></p>
 </footer>
</body>

Notice the main element being used to wrap the main content of the page. In this case, all content other than the page header and footer.

You can also see microdata annotations in the above example that use the schema.org vocabulary to provide the publication date and other metadata about the blog post.

In the following example, there are two nav elements, one for primary navigation around the site, and one for secondary navigation around the page itself.

<body>
 <h1>The Wiki Center Of Exampland</h1>
 <nav>
  <ul>
   <li><a href="/">Home</a></li>
   <li><a href="/events">Current Events</a></li>
   ...more...
  </ul>
 </nav>
 <main>
  <header>
   <h1>Demos in Exampland</h1>
   <p>Written by A. N. Other.</p>
  </header>
  <nav>
   <ul>
    <li><a href="#public">Public demonstrations</a></li>
    <li><a href="#destroy">Demolitions</a></li>
    ...more...
   </ul>
  </nav>
  <div>
   <section id="public">
    <h1>Public demonstrations</h1>
    <p>...more...</p>
   </section>
   <section id="destroy">
    <h1>Demolitions</h1>
    <p>...more...</p>
   </section>
   ...more...
  </div>
  <footer>
   <p><a href="?edit">Edit</a> | <a href="?delete">Delete</a> | <a href="?Rename">Rename</a></p>
  </footer>
 </main>
 <footer>
  <p><small>© copyright 1998 Exampland Emperor</small></p>
 </footer>
</body>

A nav element doesn't have to contain a list, it can contain other kinds of content as well. In this navigation block, links are provided in prose:

<nav>
 <h1>Navigation</h1>
 <p>You are on my home page. To the north lies <a href="/blog">my
 blog</a>, from whence the sounds of battle can be heard. To the east
 you can see a large mountain, upon which many <a
 href="/school">school papers</a> are littered. Far up thus mountain
 you can spy a little figure who appears to be me, desperately
 scribbling a <a href="/school/thesis">thesis</a>.</p>
 <p>To the west are several exits. One fun-looking exit is labeled <a
 href="http://games.example.com/">"games"</a>. Another more
 boring-looking exit is labeled <a
 href="http://isp.example.net/">ISP™</a>.</p>
 <p>To the south lies a dark and dank <a href="/about">contacts
 page</a>. Cobwebs cover its disused entrance, and at one point you
 see a rat run quickly out of the page.</p>
</nav>

4.3.5 The `aside` element

Categories:: Flow content.; Sectioning content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Flow content, but with no main element descendants.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: complementary role (default - do not set), note, search or presentation.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The aside element represents a section of a page that consists of content that is tangentially related to the content around the aside element, and which could be considered separate from that content. Such sections are often represented as sidebars in printed typography.

The element can be used for typographical effects like pull quotes or sidebars, for advertising, for groups of nav elements, and for other content that is considered separate from the main content of the page.

It's not appropriate to use the aside element just for parentheticals, since those are part of the main flow of the document.

The following example shows how an aside is used to mark up background material on Switzerland in a much longer news story on Europe.

<aside>
 <h1>Switzerland</h1>
 <p>Switzerland, a land-locked country in the middle of geographic
 Europe, has not joined the geopolitical European Union, though it is
 a signatory to a number of European treaties.</p>
</aside>

The following example shows how an aside is used to mark up a pull quote in a longer article.

...

<p>He later joined a large company, continuing on the same work.
<q>I love my job. People ask me what I do for fun when I'm not at
work. But I'm paid to do my hobby, so I never know what to
answer. Some people wonder what they would do if they didn't have to
work... but I know what I would do, because I was unemployed for a
year, and I filled that time doing exactly what I do now.</q></p>

<aside>
 <q> People ask me what I do for fun when I'm not at work. But I'm
 paid to do my hobby, so I never know what to answer. </q>
</aside>

<p>Of course his work — or should that be hobby? —
isn't his only passion. He also enjoys other pleasures.</p>

...

The following extract shows how aside can be used for blogrolls and other side content on a blog:

<body>
 <header>
  <h1>My wonderful blog</h1>
  <p>My tagline</p>
 </header>
 <aside>
  <!-- this aside contains two sections that are tangentially related
  to the page, namely, links to other blogs, and links to blog posts
  from this blog -->
  <nav>
   <h1>My blogroll</h1>
   <ul>
    <li><a href="http://blog.example.com/">Example Blog</a>
   </ul>
  </nav>
  <nav>
   <h1>Archives</h1>
   <ol reversed>
    <li><a href="/last-post">My last post</a>
    <li><a href="/first-post">My first post</a>
   </ol>
  </nav>
 </aside>
 <aside>
  <!-- this aside is tangentially related to the page also, it
  contains twitter messages from the blog author -->
  <h1>Twitter Feed</h1>
  <blockquote cite="http://twitter.example.net/t31351234">
   I'm on vacation, writing my blog.
  </blockquote>
  <blockquote cite="http://twitter.example.net/t31219752">
   I'm going to go on vacation soon.
  </blockquote>
 </aside>
 <article>
  <!-- this is a blog post -->
  <h1>My last post</h1>
  <p>This is my last post.</p>
  <footer>
   <p><a href="/last-post" rel=bookmark>Permalink</a>
  </footer>
 </article>
 <article>
  <!-- this is also a blog post -->
  <h1>My first post</h1>
  <p>This is my first post.</p>
  <aside>
   <!-- this aside is about the blog post, since it's inside the
   <article> element; it would be wrong, for instance, to put the
   blogroll here, since the blogroll isn't really related to this post
   specifically, only to the page as a whole -->
   <h1>Posting</h1>
   <p>While I'm thinking about it, I wanted to say something about
   posting. Posting is fun!</p>
  </aside>
  <footer>
   <p><a href="/first-post" rel=bookmark>Permalink</a>
  </footer>
 </article>
 <footer>
  <nav>
   <a href="/archives">Archives</a> —
   <a href="/about">About me</a> —
   <a href="/copyright">Copyright</a>
  </nav>
 </footer>
</body>

4.3.6 The `h1`, `h2`, `h3`, `h4`, `h5`, and `h6` elements

Where flow content is expected.

Neither tag is omissible

Allowed ARIA role attribute values:

heading role (default - do not set), tab or presentation.

Any aria-* attributes applicable to the allowed roles.

interface HTMLHeadingElement : HTMLElement {};

These elements represent headings for their sections.

The semantics and meaning of these elements are defined in the section on headings and sections.

These elements have a rank given by the number in their name. The h1 element is said to have the highest rank, the h6 element has the lowest rank, and two elements with the same name have equal rank.

h1–h6 elements must not be used to markup subheadings, subtitles, alternative titles and taglines unless intended to be the heading for a new section or subsection. Instead use the markup patterns in the Common idioms without dedicated elements section of the specification.

As far as their respective document outlines (their heading and section structures) are concerned, these two snippets are semantically equivalent:

<body>
<h1>Let's call it a draw(ing surface)</h1>
<h2>Diving in</h2>
<h2>Simple shapes</h2>
<h2>Canvas coordinates</h2>
<h3>Canvas coordinates diagram</h3>
<h2>Paths</h2>
</body>

<body>
 <h1>Let's call it a draw(ing surface)</h1>
 <section>
  <h1>Diving in</h1>
 </section>
 <section>
  <h1>Simple shapes</h1>
 </section>
 <section>
  <h1>Canvas coordinates</h1>
  <section>
   <h1>Canvas coordinates diagram</h1>
  </section>
 </section>
 <section>
  <h1>Paths</h1>
 </section>
</body>

Authors might prefer the former style for its terseness, or the latter style for its convenience in the face of heavy editing; which is best is purely an issue of preferred authoring style.

4.3.7 The `header` element

Categories:: Flow content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Flow content, but with no header, footer, or main element descendants.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: banner role (default - do not set) or presentation.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The header element represents introductory content for its nearest ancestor sectioning content or sectioning root element. A header typically contains a group of introductory or navigational aids.

When the nearest ancestor sectioning content or sectioning root element is the body element, then it applies to the whole page.

A header element is intended to usually contain the section's heading (an h1–h6 element), but this is not required. The header element can also be used to wrap a section's table of contents, a search form, or any relevant logos.

Here are some sample headers. This first one is for a game:

<header>
 <p>Welcome to...</p>
 <h1>Voidwars!</h1>
</header>

The following snippet shows how the element can be used to mark up a specification's header:

<header>
  <h1>Scalable Vector Graphics (SVG) 1.2</h1>
  <p>W3C Working Draft 27 October 2004</p>
 <dl>
  <dt>This version:</dt>
  <dd><a href="http://www.w3.org/TR/2004/WD-SVG12-20041027/">http://www.w3.org/TR/2004/WD-SVG12-20041027/</a></dd>
  <dt>Previous version:</dt>
  <dd><a href="http://www.w3.org/TR/2004/WD-SVG12-20040510/">http://www.w3.org/TR/2004/WD-SVG12-20040510/</a></dd>
  <dt>Latest version of SVG 1.2:</dt>
  <dd><a href="http://www.w3.org/TR/SVG12/">http://www.w3.org/TR/SVG12/</a></dd>
  <dt>Latest SVG Recommendation:</dt>
  <dd><a href="http://www.w3.org/TR/SVG/">http://www.w3.org/TR/SVG/</a></dd>
  <dt>Editor:</dt>
  <dd>Dean Jackson, W3C, <a href="mailto:dean@w3.org">dean@w3.org</a></dd>
  <dt>Authors:</dt>
  <dd>See <a href="#authors">Author List</a></dd>
 </dl>
 <p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notic ...
</header>

The header element is not sectioning content; it doesn't introduce a new section.

In this example, the page has a page heading given by the h1 element, and two subsections whose headings are given by h2 elements. The content after the header element is still part of the last subsection started in the header element, because the header element doesn't take part in the outline algorithm.

<body>
 <header>
  <h1>Little Green Guys With Guns</h1>
  <nav>
   <ul>
    <li><a href="/games">Games</a>
    <li><a href="/forum">Forum</a>
    <li><a href="/download">Download</a>
   </ul>
  </nav>
  <h2>Important News</h2> <!-- this starts a second subsection -->
  <!-- this is part of the subsection entitled "Important News" -->
  <p>To play today's games you will need to update your client.</p>
  <h2>Games</h2> <!-- this starts a third subsection -->
 </header>
 <p>You have three active games:</p>
 <!-- this is still part of the subsection entitled "Games" -->
 ...

Categories:: Flow content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Flow content, but with no header, footer, or main element descendants.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: contentinfo role (default - do not set) or presentation.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The footer element represents a footer for its nearest ancestor sectioning content or sectioning root element. A footer typically contains information about its section such as who wrote it, links to related documents, copyright data, and the like.

When the footer element contains entire sections, they represent appendices, indexes, long colophons, verbose license agreements, and other such content.

Contact information for the author or editor of a section belongs in an address element, possibly itself inside a footer. Bylines and other information that could be suitable for both a header or a footer can be placed in either (or neither). The primary purpose of these elements is merely to help the author write self-explanatory markup that is easy to maintain and style; they are not intended to impose specific structures on authors.

Footers don't necessarily have to appear at the end of a section, though they usually do.

When the nearest ancestor sectioning content or sectioning root element is the body element, then it applies to the whole page.

The footer element is not sectioning content; it doesn't introduce a new section.

Here is a page with two footers, one at the top and one at the bottom, with the same content:

<body>
 <footer><a href="../">Back to index...</a></footer>
 <div>
  <h1>Lorem ipsum</h1>
  <p>The ipsum of all lorems</p>
 </div>
 <p>A dolor sit amet, consectetur adipisicing elit, sed do eiusmod
 tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim
 veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex
 ea commodo consequat. Duis aute irure dolor in reprehenderit in
 voluptate velit esse cillum dolore eu fugiat nulla
 pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
 culpa qui officia deserunt mollit anim id est laborum.</p>
 <footer><a href="../">Back to index...</a></footer>
</body>

Here is an example which shows the footer element being used both for a site-wide footer and for a section footer.

<!DOCTYPE HTML>
<HTML><HEAD>
<TITLE>The Ramblings of a Scientist</TITLE>
<BODY>
<H1>The Ramblings of a Scientist</H1>
<MAIN>
 <ARTICLE>
 <H1>Episode 15</H1>
 <VIDEO SRC="/fm/015.ogv" CONTROLS PRELOAD>
  <P><A HREF="/fm/015.ogv">Download video</A>.</P>
 </VIDEO>
 <FOOTER> <!-- footer for article -->
  <P>Published <TIME DATETIME="2009-10-21T18:26-07:00">on 2009/10/21 at 6:26pm</TIME></P>
 </FOOTER>
</ARTICLE>
<ARTICLE>
 <H1>My Favorite Trains</H1>
 <P>I love my trains. My favorite train of all time is a Köf.</P>
 <P>It is fun to see them pull some coal cars because they look so
 dwarfed in comparison.</P>
 <FOOTER> <!-- footer for article -->
  <P>Published <TIME DATETIME="2009-09-15T14:54-07:00">on 2009/09/15 at 2:54pm</TIME></P>
 </FOOTER>
 </ARTICLE>
</MAIN>
<FOOTER> <!-- site wide footer -->
 <NAV>
  <P><A HREF="/credits.html">Credits</A> —
     <A HREF="/tos.html">Terms of Service</A> —
     <A HREF="/index.html">Blog Index</A></P>
 </NAV>
 <P>Copyright © 2009 Gordon Freeman</P>
</FOOTER>
</BODY>
</HTML>

Some site designs have what is sometimes referred to as "fat footers" — footers that contain a lot of material, including images, links to other articles, links to pages for sending feedback, special offers... in some ways, a whole "front page" in the footer.

This fragment shows the bottom of a page on a site with a "fat footer":

...
 <footer>
  <nav>
   <section>
    <h1>Articles</h1>
    <p><img src="images/somersaults.jpeg" alt=""> Go to the gym with
    our somersaults class! Our teacher Jim takes you through the paces
    in this two-part article. <a href="articles/somersaults/1">Part
    1</a> · <a href="articles/somersaults/2">Part 2</a></p>
    <p><img src="images/kindplus.jpeg"> Tired of walking on the edge of
    a clif<!-- sic -->? Our guest writer Lara shows you how to bumble
    your way through the bars. <a href="articles/kindplus/1">Read
    more...</a></p>
    <p><img src="images/crisps.jpeg"> The chips are down, now all
    that's left is a potato. What can you do with it? <a
    href="articles/crisps/1">Read more...</a></p>
   </section>
   <ul>
    <li> <a href="/about">About us...</a>
    <li> <a href="/feedback">Send feedback!</a>
    <li> <a href="/sitemap">Sitemap</a>
   </ul>
  </nav>
  <p><small>Copyright © 2015 The Snacker —
  <a href="/tos">Terms of Service</a></small></p>
 </footer>
</body>

4.3.9 The `address` element

Categories:: Flow content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Flow content, but with no heading content descendants, no sectioning content descendants, and no header, footer, or address element descendants.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: contentinfo role.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The address element represents the contact information for its nearest article or body element ancestor. If that is the body element, then the contact information applies to the document as a whole.

For example, a page at the W3C Web site related to HTML might include the following contact information:

<ADDRESS>
 <A href="../People/Raggett/">Dave Raggett</A>,
 <A href="../People/Arnaud/">Arnaud Le Hors</A>,
 contact persons for the <A href="Activity">W3C HTML Activity</A>
</ADDRESS>

The address element must not be used to represent arbitrary addresses (e.g. postal addresses), unless those addresses are in fact the relevant contact information. (The p element is the appropriate element for marking up postal addresses in general.)

The address element must not contain information other than contact information.

For example, the following is non-conforming use of the address element:

<ADDRESS>Last Modified: 1999/12/24 23:37:50</ADDRESS>

Typically, the address element would be included along with other information in a footer element.

The contact information for a node node is a collection of address elements defined by the first applicable entry from the following list:

If node is an article element
If node is a body element: The contact information consists of all the address elements that have node as an ancestor and do not have another body or article element ancestor that is a descendant of node.
If node has an ancestor element that is an article element
If node has an ancestor element that is a body element: The contact information of node is the same as the contact information of the nearest article or body element ancestor, whichever is nearest.
If node's Document has a body element: The contact information of node is the same as the contact information of the body element of the Document.
Otherwise: There is no contact information for node.

User agents may expose the contact information of a node to the user, or use it for other purposes, such as indexing sections based on the sections' contact information.

In this example the footer contains contact information and a copyright notice.

<footer>
 <address>
  For more details, contact
  <a href="mailto:js@example.com">John Smith</a>.
 </address>
 <p><small>© copyright 2038 Example Corp.</small></p>
</footer>

4.3.10 Headings and sections

The h1–h6 elements are headings.

The first element of heading content in an element of sectioning content represents the heading for that section. Subsequent headings of equal or higher rank start new (implied) sections, headings of lower rank start implied subsections that are part of the previous one. In both cases, the element represents the heading of the implied section.

Certain elements are said to be sectioning roots, including blockquote and td elements. These elements can have their own outlines, but the sections and headings inside these elements do not contribute to the outlines of their ancestors.

Sectioning content elements are always considered subsections of their nearest ancestor sectioning root or their nearest ancestor element of sectioning content, whichever is nearest, regardless of what implied sections other headings may have created.

For the following fragment:

<body>
 <h1>Foo</h1>
 <h2>Bar</h2>
 <blockquote>
  <h3>Bla</h3>
 </blockquote>
 <p>Baz</p>
 <h2>Quux</h2>
 <section>
  <h3>Thud</h3>
 </section>
 <p>Grunt</p>
</body>

...the structure would be:

Foo (heading of explicit body section, containing the "Grunt" paragraph)
1. Bar (heading starting implied section, containing a block quote and the "Baz" paragraph)
2. Quux (heading starting implied section with no content other than the heading itself)
3. Thud (heading of explicit section section)

Notice how the section ends the earlier implicit section so that a later paragraph ("Grunt") is back at the top level.

Sections may contain headings of any rank, and authors are strongly encouraged to use headings of the appropriate rank for the section's nesting level.

Authors are also encouraged to explicitly wrap sections in elements of sectioning content, instead of relying on the implicit sections generated by having multiple headings in one element of sectioning content.

For example, the following is correct:

<body>
 <h4>Apples</h4>
 <p>Apples are fruit.</p>
 <section>
  <h2>Taste</h2>
  <p>They taste lovely.</p>
  <h6>Sweet</h6>
  <p>Red apples are sweeter than green ones.</p>
  <h1>Color</h1>
  <p>Apples come in various colors.</p>
 </section>
</body>

However, the same document would be more clearly expressed as:

<body>
 <h1>Apples</h1>
 <p>Apples are fruit.</p>
 <section>
  <h2>Taste</h2>
  <p>They taste lovely.</p>
  <section>
   <h3>Sweet</h3>
   <p>Red apples are sweeter than green ones.</p>
  </section>
 </section>
 <section>
  <h2>Color</h2>
  <p>Apples come in various colors.</p>
 </section>
</body>

Both of the documents above are semantically identical and would produce the same outline in compliant user agents.

This third example is also semantically identical, and might be easier to maintain (e.g. if sections are often moved around in editing):

<body>
 <h1>Apples</h1>
 <p>Apples are fruit.</p>
 <section>
  <h1>Taste</h1>
  <p>They taste lovely.</p>
  <section>
   <h1>Sweet</h1>
   <p>Red apples are sweeter than green ones.</p>
  </section>
 </section>
 <section>
  <h1>Color</h1>
  <p>Apples come in various colors.</p>
 </section>
</body>

This final example would need explicit style rules to be rendered well in legacy browsers. Legacy browsers without CSS support would render all the headings as top-level headings.

4.3.10.1 Creating an outline

There are currently no known implementations of the outline algorithm in graphical browsers or assistive technology user agents, although the algorithm is implemented in other software such as conformance checkers. Therefore the outline algorithm cannot be relied upon to convey document structure to users. Authors are advised to use heading rank (h1-h6) to convey document structure.

This section defines an algorithm for creating an outline for a sectioning content element or a sectioning root element. It is defined in terms of a walk over the nodes of a DOM tree, in tree order, with each node being visited when it is entered and when it is exited during the walk.

The outline for a sectioning content element or a sectioning root element consists of a list of one or more potentially nested sections. A section is a container that corresponds to some nodes in the original DOM tree. Each section can have one heading associated with it, and can contain any number of further nested sections. The algorithm for the outline also associates each node in the DOM tree with a particular section and potentially a heading. (The sections in the outline aren't section elements, though some may correspond to such elements — they are merely conceptual sections.)

The following markup fragment:

<body>
 <h1>A</h1>
 <p>B</p>
 <h2>C</h2>
 <p>D</p>
 <h2>E</h2>
 <p>F</p>
</body>

...results in the following outline being created for the body node (and thus the entire document):

Section created for body node.

Associated with heading "A".

Also associated with paragraph "B".

Nested sections:
1. Section implied for first h2 element.
  
  Associated with heading "C".
  
  Also associated with paragraph "D".
  
  No nested sections.
2. Section implied for second h2 element.
  
  Associated with heading "E".
  
  Also associated with paragraph "F".
  
  No nested sections.

The algorithm that must be followed during a walk of a DOM subtree rooted at a sectioning content element or a sectioning root element to determine that element's outline is as follows:

Let current outline target be null. (It holds the element whose outline is being created.)
Let current section be null. (It holds a pointer to a section, so that elements in the DOM can all be associated with a section.)
Create a stack to hold elements, which is used to handle nesting. Initialize this stack to empty.
Walk over the DOM in tree order, starting with the sectioning content element or sectioning root element at the root of the subtree for which an outline is to be created, and trigger the first relevant step below for each element as the walk enters and exits it.
When exiting an element, if that element is the element at the top of the stack

The element being exited is a heading content element or an element with a hidden attribute.

Pop that element from the stack.

If the top of the stack is a heading content element or an element with a hidden attribute

Do nothing.

When entering an element with a hidden attribute

Push the element being entered onto the stack. (This causes the algorithm to skip that element and any descendants of the element.)

When entering a sectioning content element
Run these steps:
1. If current outline target is not null, run these substeps:
  1. If the current section has no heading, create an implied heading and let that be the heading for the current section.
  2. Push current outline target onto the stack.
2. Let current outline target be the element that is being entered.
3. Let current section be a newly created section for the current outline target element.
4. Associate current outline target with current section.
5. Let there be a new outline for the new current outline target, initialized with just the new current section as the only section in the outline.
When exiting a sectioning content element, if the stack is not empty
Run these steps:
1. If the current section has no heading, create an implied heading and let that be the heading for the current section.
2. Pop the top element from the stack, and let the current outline target be that element.
3. Let current section be the last section in the outline of the current outline target element.
4. Append the outline of the sectioning content element being exited to the current section. (This does not change which section is the last section in the outline.)
When entering a sectioning root element
Run these steps:
1. If current outline target is not null, push current outline target onto the stack.
2. Let current outline target be the element that is being entered.
3. Let current outline target's parent section be current section.
4. Let current section be a newly created section for the current outline target element.
5. Let there be a new outline for the new current outline target, initialized with just the new current section as the only section in the outline.
When exiting a sectioning root element, if the stack is not empty
Run these steps:
1. If the current section has no heading, create an implied heading and let that be the heading for the current section.
2. Let current section be current outline target's parent section.
3. Pop the top element from the stack, and let the current outline target be that element.
When exiting a sectioning content element or a sectioning root element (when the stack is empty)

The current outline target is the element being exited, and it is the sectioning content element or a sectioning root element at the root of the subtree for which an outline is being generated.

If the current section has no heading, create an implied heading and let that be the heading for the current section.

Skip to the next step in the overall set of steps. (The walk is over.)

When entering a heading content element
If the current section has no heading, let the element being entered be the heading for the current section.

Otherwise, if the element being entered has a rank equal to or higher than the heading of the last section of the outline of the current outline target, or if the heading of the last section of the outline of the current outline target is an implied heading, then create a new section and append it to the outline of the current outline target element, so that this new section is the new last section of that outline. Let current section be that new section. Let the element being entered be the new heading for the current section.

Otherwise, run these substeps:
1. Let candidate section be current section.
2. Heading loop: If the element being entered has a rank lower than the rank of the heading of the candidate section, then create a new section, and append it to candidate section. (This does not change which section is the last section in the outline.) Let current section be this new section. Let the element being entered be the new heading for the current section. Abort these substeps.
3. Let new candidate section be the section that contains candidate section in the outline of current outline target.
4. Let candidate section be new candidate section.
5. Return to the step labeled heading loop.
Push the element being entered onto the stack. (This causes the algorithm to skip any descendants of the element.)

Recall that h1 has the highest rank, and h6 has the lowest rank.
Otherwise

Do nothing.
In addition, whenever the walk exits a node, after doing the steps above, if the node is not associated with a section yet, associate the node with the section current section.
Associate all nodes with the heading of the section with which they are associated, if any.

The tree of sections created by the algorithm above, or a proper subset thereof, must be used when generating document outlines, for example when generating tables of contents.

The outline created for the body element of a Document is the outline of the entire document.

When creating an interactive table of contents, entries should jump the user to the relevant sectioning content element, if the section was created for a real element in the original document, or to the relevant heading content element, if the section in the tree was generated for a heading in the above process.

Selecting the first section of the document therefore always takes the user to the top of the document, regardless of where the first heading in the body is to be found.

The outline depth of a heading content element associated with a section section is the number of sections that are ancestors of section in the outermost outline that section finds itself in when the outlines of its Document's elements are created, plus 1. The outline depth of a heading content element not associated with a section is 1.

User agents should provide default headings for sections that do not have explicit section headings.

Consider the following snippet:

<body>
 <nav>
  <p><a href="/">Home</a></p>
 </nav>
 <p>Hello world.</p>
 <aside>
  <p>My cat is cute.</p>
 </aside>
</body>

Although it contains no headings, this snippet has three sections: a document (the body) with two subsections (a nav and an aside). A user agent could present the outline as follows:

Untitled document
1. Navigation
2. Sidebar

These default headings ("Untitled document", "Navigation", "Sidebar") are not specified by this specification, and might vary with the user's language, the page's language, the user's preferences, the user agent implementor's preferences, etc.

The following JavaScript function shows how the tree walk could be implemented. The root argument is the root of the tree to walk (either a sectioning content element or a sectioning root element), and the enter and exit arguments are callbacks that are called with the nodes as they are entered and exited. [ECMA262]

function (root, enter, exit) {
  var node = root;
  start: while (node) {
    enter(node);
    if (node.firstChild) {
      node = node.firstChild;
      continue start;
    }
    while (node) {
      exit(node);
      if (node == root) {
        node = null;
      } else if (node.nextSibling) {
        node = node.nextSibling;
        continue start;
      } else {
        node = node.parentNode;
      }
    }
  }
}

4.3.10.2 Sample outlines

This section is non-normative.

The following document shows a straight-forward application of the outline algorithm. First, here is the document, which is a book with very short chapters and subsections:

<!DOCTYPE HTML>
<title>The Tax Book (all in one page)</title>
<h1>The Tax Book</h1>
<h2>Earning money</h2>
<p>Earning money is good.</p>
<h3>Getting a job</h3>
<p>To earn money you typically need a job.</p>
<h2>Spending money</h2>
<p>Spending is what money is mainly used for.</p>
<h3>Cheap things</h3>
<p>Buying cheap things often not cost-effective.</p>
<h3>Expensive things</h3>
<p>The most expensive thing is often not the most cost-effective either.</p>
<h2>Investing money</h2>
<p>You can lend your money to other people.</p>
<h2>Losing money</h2>
<p>If you spend money or invest money, sooner or later you will lose money.
<h3>Poor judgement</h3>
<p>Usually if you lose money it's because you made a mistake.</p>

This book would form the following outline:

The Tax Book
1. Earning money
  1. Getting a job
2. Spending money
  1. Cheap things
  2. Expensive things
3. Investing money
4. Losing money
  1. Poor judgement

Notice that the title element does not participate in the outline.

Here is a similar document, but this time using section elements to get the same effect:

<!DOCTYPE HTML>
<title>The Tax Book (all in one page)</title>
<h1>The Tax Book</h1>
<section>
 <h1>Earning money</h1>
 <p>Earning money is good.</p>
 <section>
  <h1>Getting a job</h1>
  <p>To earn money you typically need a job.</p>
 </section>
</section>
<section>
 <h1>Spending money</h1>
 <p>Spending is what money is mainly used for.</p>
 <section>
  <h1>Cheap things</h1>
  <p>Buying cheap things often not cost-effective.</p>
 </section>
 <section>
  <h1>Expensive things</h1>
  <p>The most expensive thing is often not the most cost-effective either.</p>
 </section>
</section>
<section>
 <h1>Investing money</h1>
 <p>You can lend your money to other people.</p>
</section>
<section>
 <h1>Losing money</h1>
 <p>If you spend money or invest money, sooner or later you will lose money.
 <section>
  <h1>Poor judgement</h1>
  <p>Usually if you lose money it's because you made a mistake.</p>
 </section>
</section>

This book would form the same outline:

The Tax Book
1. Earning money
  1. Getting a job
2. Spending money
  1. Cheap things
  2. Expensive things
3. Investing money
4. Losing money
  1. Poor judgement

A document can contain multiple top-level headings:

<!DOCTYPE HTML>
<title>Alphabetic Fruit</title>
<h1>Apples</h1>
<p>Pomaceous.</p>
<h1>Bananas</h1>
<p>Edible.</p>
<h1>Carambola</h1>
<p>Star.</p>

This would form the following simple outline consisting of three top-level sections:

Apples
Bananas
Carambola

Effectively, the body element is split into three.

Mixing both the h1–h6 model and the section/h1 model can lead to some unintuitive results.

Consider for example the following, which is just the previous example but with the contents of the (implied) body wrapped in a section:

<!DOCTYPE HTML>
<title>Alphabetic Fruit</title>
<section>
 <h1>Apples</h1>
 <p>Pomaceous.</p>
 <h1>Bananas</h1>
 <p>Edible.</p>
 <h1>Carambola</h1>
 <p>Star.</p>
</section>

The resulting outline would be:

(untitled page)
1. Apples
2. Bananas
3. Carambola

This result is described as unintuitive because it results in three subsections even though there's only one section element. Effectively, the section is split into three, just like the implied body element in the previous example.

(In this example, "(untitled page)" is the implied heading for the body element, since it has no explicit heading.)

Headings never rise above other sections. Thus, in the following example, the first h1 does not actually describe the page header; it describes the header for the second half of the page:

<!DOCTYPE HTML>
<title>Feathers on The Site of Encyclopedic Knowledge</title>
<section>
 <h1>A plea from our caretakers</h1>
 <p>Please, we beg of you, send help! We're stuck in the server room!</p>
</section>
<h1>Feathers</h1>
<p>Epidermal growths.</p>

The resulting outline would be:

(untitled page)
1. A plea from our caretakers
Feathers

Thus, when an article element starts with a nav block and only later has its heading, the result is that the nav block is not part of the same section as the rest of the article in the outline. For instance, take this document:

<!DOCTYPE HTML>
<title>We're adopting a child! — Ray's blog</title>
...

<h1>Ray's blog</h1>

<main>
 <article>
<header>
  <nav>
   <a href="?t=-1d">Yesterday</a>;
   <a href="?t=-7d">Last week</a>;
   <a href="?t=-1m">Last month</a>
  </nav> 
</header>
  <h2>We're adopting a child!</h2>

  <p>As of today, Janine and I have signed the papers to become
  the proud parents of baby Diane! We've been looking forward to
  this day for weeks.</p>
</article> 
</main>
...

The resulting outline would be:

Ray's blog
1. Untitled article
  1. Untitled navigation section
2. We're adopting a child!

Also worthy of note in this example is that the header and main elements have no effect whatsoever on the document outline.

4.3.11 Usage summary

This section is non-normative.

Element	Purpose
Element	Example
`body`
`body`	<!DOCTYPE HTML> <html> <head> <title>Steve Hill's Home Page</title> </head> <body> <p>Hard Trance is My Life.</p> </body> </html>
`article`
`article`	<article> <img src="/tumblr_masqy2s5yn1rzfqbpo1_500.jpg" alt="Yellow smiley face with the caption 'masif'"> <p>My fave Masif tee so far!</p> <footer>Posted 2 days ago</footer> </article> <article> <img src="/tumblr_m9tf6wSr6W1rzfqbpo1_500.jpg" alt=""> <p>Happy 2nd birthday Masif Saturdays!!!</p> <footer>Posted 3 weeks ago</footer> </article>
`section`
`section`	<h1>Biography</h1> <section> <h1>The facts</h1> <p>1500+ shows, 14+ countries</p> </section> <section> <h1>2010/2011 figures per year</h1> <p>100+ shows, 8+ countries</p> </section>
`nav`
`nav`	<nav> <ul> <li><a href="/">Home</a> <li><a href="/biog.html">Bio</a> <li><a href="/discog.html">Discog</a> </ul> </nav>
`aside`
`aside`	<h1>Music</h1> <p>As any burner can tell you, the event has a lot of trance.</p> <aside>You can buy the music we played at our <a href="buy.html">playlist page</a>.</aside> <p>This year we played a kind of trance that originated in Belgium, Germany, and the Netherlands in the mid 90s.</p>
`h1`–`h6`	A section heading
`h1`–`h6`	<h1>The Guide To Music On The Playa</h1> <h2>The Main Stage</h2> <p>If you want to play on a stage, you should bring one.</p> <h2>Amplified Music</h2> <p>Amplifiers up to 300W or 90dB are welcome.</p>
`header`
`header`	<article> <header> <h1>Hard Trance is My Life</h1> <p>By DJ Steve Hill and Technikal</p> </header> <p>The album with the amusing punctuation has red artwork.</p> </article>
`footer`
`footer`	<article> <h1>Hard Trance is My Life</h1> <p>The album with the amusing punctuation has red artwork.</p> <footer> <p>Artists: DJ Steve Hill and Technikal</p> </footer> </article>

4.3.11.1 Article or section?

This section is non-normative.

A section forms part of something else. An article is its own thing. But how does one know which is which? Mostly the real answer is "it depends on author intent".

For example, one could imagine a book with a "Granny Smith" chapter that just said "These juicy, green apples make a great filling for apple pies."; that would be a section because there'd be lots of other chapters on (maybe) other kinds of apples.

On the other hand, one could imagine a tweet or reddit comment or tumblr post or newspaper classified ad that just said "Granny Smith. These juicy, green apples make a great filling for apple pies."; it would then be articles because that was the whole thing.

A comment on an article is not part of the article on which it is commenting, therefore it is its own article.

4.4 Grouping content

4.4.1 The `p` element

Where flow content is expected.

A p element's end tag may be omitted if the p element is immediately followed by an address, article, aside, blockquote, div, dl, fieldset, footer, form, h1, h2, h3, h4, h5, h6, header, hgroup, hr, main, nav, ol, p, pre, section, table, or ul, element, or if there is no more content in the parent element and the parent element is not an a element.

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLParagraphElement : HTMLElement {};

The p element represents a paragraph.

While paragraphs are usually represented in visual media by blocks of text that are physically separated from adjacent blocks through blank lines, a style sheet or user agent would be equally justified in presenting paragraph breaks in a different manner, for instance using inline pilcrows (¶).

The following examples are conforming HTML fragments:

<p>The little kitten gently seated himself on a piece of
carpet. Later in his life, this would be referred to as the time the
cat sat on the mat.</p>

<fieldset>
 <legend>Personal information</legend>
 <p>
   <label>Name: <input name="n"></label>
   <label><input name="anon" type="checkbox"> Hide from other users</label>
 </p>
 <p><label>Address: <textarea name="a"></textarea></label></p>
</fieldset>

<p>There was once an example from Femley,<br>
Whose markup was of dubious quality.<br>
The validator complained,<br>
So the author was pained,<br>
To move the error from the markup to the rhyming.</p>

The p element should not be used when a more specific element is more appropriate.

The following example is technically correct:

<section>
 <!-- ... -->
 <p>Last modified: 2001-04-23</p>
 <p>Author: fred@example.com</p>
</section>

However, it would be better marked-up as:

<section>
 <!-- ... -->
 <footer>Last modified: 2001-04-23</footer>
 <address>Author: fred@example.com</address>
</section>

Or:

<section>
 <!-- ... -->
 <footer>
  <p>Last modified: 2001-04-23</p>
  <address>Author: fred@example.com</address>
 </footer>
</section>

List elements (in particular, ol and ul elements) cannot be children of p elements. When a sentence contains a bulleted list, therefore, one might wonder how it should be marked up.

For instance, this fantastic sentence has bullets relating to

wizards,
faster-than-light travel, and
telepathy,

and is further discussed below.

The solution is to realise that a paragraph, in HTML terms, is not a logical concept, but a structural one. In the fantastic example above, there are actually five paragraphs as defined by this specification: one before the list, one for each bullet, and one after the list.

The markup for the above example could therefore be:

<p>For instance, this fantastic sentence has bullets relating to</p>
<ul>
 <li>wizards,
 <li>faster-than-light travel, and
 <li>telepathy,
</ul>
<p>and is further discussed below.</p>

Authors wishing to conveniently style such "logical" paragraphs consisting of multiple "structural" paragraphs can use the div element instead of the p element.

Thus for instance the above example could become the following:

<div>For instance, this fantastic sentence has bullets relating to
<ul>
 <li>wizards,
 <li>faster-than-light travel, and
 <li>telepathy,
</ul>
and is further discussed below.</div>

This example still has five structural paragraphs, but now the author can style just the div instead of having to consider each part of the example separately.

4.4.2 The `hr` element

Where flow content is expected.

Empty.

No end tag.

Allowed ARIA role attribute values:

separator (default - do not set) or presentation.

Any aria-* attributes applicable to the allowed roles.

interface HTMLHRElement : HTMLElement {};

The hr element represents a paragraph-level thematic break, e.g. a scene change in a story, or a transition to another topic within a section of a reference book.

The following fictional extract from a project manual shows two sections that use the hr element to separate topics within the section.

<section>
 <h1>Communication</h1>
 <p>There are various methods of communication. This section
 covers a few of the important ones used by the project.</p>
 <hr>
 <p>Communication stones seem to come in pairs and have mysterious
 properties:</p>
 <ul>
  <li>They can transfer thoughts in two directions once activated
  if used alone.</li>
  <li>If used with another device, they can transfer one's
  consciousness to another body.</li>
  <li>If both stones are used with another device, the
  consciousnesses switch bodies.</li>
 </ul>
 <hr>
 <p>Radios use the electromagnetic spectrum in the meter range and
 longer.</p>
 <hr>
 <p>Signal flares use the electromagnetic spectrum in the
 nanometer range.</p>
</section>
<section>
 <h1>Food</h1>
 <p>All food at the project is rationed:</p>
 <dl>
  <dt>Potatoes</dt>
  <dd>Two per day</dd>
  <dt>Soup</dt>
  <dd>One bowl per day</dd>
 </dl>
 <hr>
 <p>Cooking is done by the chefs on a set rotation.</p>
</section>

There is no need for an hr element between the sections themselves, since the section elements and the h1 elements imply thematic changes themselves.

The following extract from Pandora's Star by Peter F. Hamilton shows two paragraphs that precede a scene change and the paragraph that follows it. The scene change, represented in the printed book by a gap containing a solitary centered star between the second and third paragraphs, is here represented using the hr element.

<p>Dudley was ninety-two, in his second life, and fast approaching
time for another rejuvenation. Despite his body having the physical
age of a standard fifty-year-old, the prospect of a long degrading
campaign within academia was one he regarded with dread. For a
supposedly advanced civilization, the Intersolar Commonwealth could be
appallingly backward at times, not to mention cruel.</p>
<p><i>Maybe it won't be that bad</i>, he told himself. The lie was
comforting enough to get him through the rest of the night's
shift.</p>
<hr>
<p>The Carlton AllLander drove Dudley home just after dawn. Like the
astronomer, the vehicle was old and worn, but perfectly capable of
doing its job. It had a cheap diesel engine, common enough on a
semi-frontier world like Gralmond, although its drive array was a
thoroughly modern photoneural processor. With its high suspension and
deep-tread tyres it could plough along the dirt track to the
observatory in all weather and seasons, including the metre-deep snow
of Gralmond's winters.</p>

The hr element does not affect the document's outline.

4.4.3 The `pre` element

Where flow content is expected.

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLPreElement : HTMLElement {};

The pre element represents a block of preformatted text, in which structure is represented by typographic conventions rather than by elements.

In the HTML syntax, a leading newline character immediately following the pre element start tag is stripped.

Some examples of cases where the pre element could be used:

Including an e-mail, with paragraphs indicated by blank lines, lists indicated by lines prefixed with a bullet, and so on.
Including fragments of computer code, with structure indicated according to the conventions of that language.
Displaying ASCII art.

Authors are encouraged to consider how preformatted text will be experienced when the formatting is lost, as will be the case for users of speech synthesizers, braille displays, and the like. For cases like ASCII art, it is likely that an alternative presentation, such as a textual description, would be more universally accessible to the readers of the document.

To represent a block of computer code, the pre element can be used with a code element; to represent a block of computer output the pre element can be used with a samp element. Similarly, the kbd element can be used within a pre element to indicate text that the user is to enter.

This element has rendering requirements involving the bidirectional algorithm.

In the following snippet, a sample of computer code is presented.

<p>This is the <code>Panel</code> constructor:</p>
<pre><code>function Panel(element, canClose, closeHandler) {
  this.element = element;
  this.canClose = canClose;
  this.closeHandler = function () { if (closeHandler) closeHandler() };
}</code></pre>

In the following snippet, samp and kbd elements are mixed in the contents of a pre element to show a session of Zork I.

<pre><samp>You are in an open field west of a big white house with a boarded
front door.
There is a small mailbox here.

></samp> <kbd>open mailbox</kbd>

<samp>Opening the mailbox reveals:
A leaflet.

></samp></pre>

The following shows a contemporary poem that uses the pre element to preserve its unusual formatting, which forms an intrinsic part of the poem itself.

<pre>                maxling

it is with a          heart
               heavy

that i admit loss of a feline
        so           loved

a friend lost to the
        unknown
                                (night)

~cdr 11dec07</pre>

4.4.4 The `blockquote` element

Where flow content is expected.

cite - Link to the source of the quotation.

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLQuoteElement : HTMLElement {
           attribute DOMString cite;
};

The HTMLQuoteElement interface is also used by the q element.

The blockquote element represents content that is quoted from another source, optionally with a citation which must be within a footer or cite element, and optionally with in-line changes such as annotations and abbreviations.

Content inside a blockquote other than citations and in-line changes must be quoted from another source, whose address, if it has one, may be cited in the cite attribute.

In cases where a page contains contributions from multiple people, such as comments on a blog post, 'another source' can include text from the same page, written by another person.

If the cite attribute is present, it must be a valid URL potentially surrounded by spaces. To obtain the corresponding citation link, the value of the attribute must be resolved relative to the element. User agents may allow users to follow such citation links, but they are primarily intended for private use (e.g. by server-side scripts collecting statistics about a site's use of quotations), not for readers.

The cite IDL attribute must reflect the element's cite content attribute.

The content of a blockquote may be abbreviated, may have context added or may have annotations. Any such additions or changes to quoted text must be indicated in the text (at the text level). This may mean the use of notational conventions or explicit remarks, such as "emphasis mine".

For example, in English, abbreviations are traditionally identified using square brackets. Consider a page with the sentence "Fred ate the cracker. He then said he liked apples and fish."; it could be quoted as follows:

<blockquote>
 <p>[Fred] then said he liked [...] fish.</p>
</blockquote>

Quotation marks may be used to delineate between quoted text and annotations within a blockquote.

For example, an in-line note provided by the author:

 <figure>
 <blockquote>
 "That monster custom, who all sense doth eat
 Of habit's devil," <abbr title="et cetera">&c.</abbr> not in Folio
 
 "What a falling off was there !
 From me, whose love was of that dignity
 That it went hand in hand even with the vow
 I made to her in marriage, and to decline
 Upon a wretch."
 </blockquote>
 <footer>
 — <cite class="title">Shakespeare manual</cite> by <cite class="author">Frederick Gard Fleay</cite>, 
 p19 (in Google Books)
 </footer>
 </figure>

In the example above, the citation is contained within the footer of a figure element, this groups and associates the information, about the quote, with the quote. The figcaption element was not used, in this case, as a container for the citation as it is not a caption.

Attribution for the quotation, may be be placed inside the blockquote element, but must be within a cite element for in-text attributions or within a footer element.

For example, here the attribution is given in a footer after the quoted text, to clearly relate the quote to its attribution:

<blockquote>
 <p>I contend that we are both atheists. I just believe in one fewer
 god than you do. When you understand why you dismiss all the other
 possible gods, you will understand why I dismiss yours.</p>
 <footer>— <cite>Stephen Roberts</cite></footer>
 </blockquote>

Here the attribution is given in a cite element on the last line of the quoted text. Note that a link to the author is also included.

<blockquote>
 The people recognize themselves in their commodities; they find their 
 soul in their automobile, hi-fi set, split-level home, kitchen equipment. 
 — <cite><a href="http://en.wikipedia.org/wiki/Herbert_Marcuse">Herbert Marcuse</a></cite>
 </blockquote>

Here the attribution is given in a footer after the quoted text, and metadata about the reference has been added using the Microdata syntax (note it could have equally been marked up using RDFA Lite).

<blockquote>
 <p>... she said she would not sign any deposition containing the word "amorous" 
 instead of "advances". For her the difference was of crucial significance, 
 and one of the reasons she had separated from her husband was that he had never been 
 amorous but had consistently made advances.</p>
 
 <footer itemscope itemtype="http://schema.org/Book">
  <span itemprop="author">Heinrich Böll</span>,
  <span itemprop="name">The Lost Honor of Katharina Blum</span>, 
  <span itemprop="datePublished">January 1, 1974</span>
 </footer>
 </blockquote>

There is no formal method for indicating the markup in a blockquote is from a quoted source. It is suggested that if the footer or cite elements are included and these elements are also being used within a blockquote to identify citations, the elements from the quoted source could be annotated with metadata to identify their origin, for example by using the class attribute (a defined extensibility mechanism).

In this example the source of a quote includes a cite element, which is annotated using the class attribute:

  <blockquote>
  <p>My favorite book is <cite class="from-source">At Swim-Two-Birds</cite></p>
  <footer>- <cite>Mike[tm]Smith</cite></footer>
  </blockquote>

The other examples below show other ways of showing attribution.

Here a blockquote element is used in conjunction with a figure element and its figcaption:

<figure>
 <blockquote>
  <p>The truth may be puzzling. It may take some work to grapple with.
  It may be counterintuitive. It may contradict deeply held
  prejudices. It may not be consonant with what we desperately want to
  be true. But our preferences do not determine what's true. We have a
  method, and that method helps us to reach not absolute truth, only
  asymptotic approaches to the truth — never there, just closer
  and closer, always finding vast new oceans of undiscovered
  possibilities. Cleverly designed experiments are the key.</p>
 </blockquote>
 <figcaption><cite>Carl Sagan</cite>, in "<cite>Wonder and Skepticism</cite>", from
 the <cite>Skeptical Enquirer</cite> Volume 19, Issue 1 (January-February
 1995)</figcaption>
</figure>

This next example shows the use of cite alongside blockquote:

<p>His next piece was the aptly named <cite>Sonnet 130</cite>:</p>
<blockquote cite="http://quotes.example.org/s/sonnet130.html">
  <p>My mistress' eyes are nothing like the sun,<br>
  Coral is far more red, than her lips red,<br>
  ...

This example shows how a forum post could use blockquote to show what post a user is replying to. The article element is used for each post, to mark up the threading.

<article>
 <h1><a href="http://bacon.example.com/?blog=109431">Bacon on a crowbar</a></h1>
 <article>
  <header><strong>t3yw</strong> 12 points 1 hour ago</header>
  <p>I bet a narwhal would love that.</p>
  <footer><a href="?pid=29578">permalink</a></footer>
  <article>
   <header><strong>greg</strong> 8 points 1 hour ago</header>
   <blockquote><p>I bet a narwhal would love that.</p></blockquote>
   <p>Dude narwhals don't eat bacon.</p>
   <footer><a href="?pid=29579">permalink</a></footer>
   <article>
    <header><strong>t3yw</strong> 15 points 1 hour ago</header>
    <blockquote>
     <blockquote><p>I bet a narwhal would love that.</p></blockquote>
     <p>Dude narwhals don't eat bacon.</p>
    </blockquote>
    <p>Next thing you'll be saying they don't get capes and wizard
    hats either!</p>
    <footer><a href="?pid=29580">permalink</a></footer>
    <article>
     <article>
      <header><strong>boing</strong> -5 points 1 hour ago</header>
      <p>narwhals are worse than ceiling cat</p>
      <footer><a href="?pid=29581">permalink</a></footer>
     </article>
    </article>
   </article>
  </article>
  <article>
   <header><strong>fred</strong> 1 points 23 minutes ago</header>
   <blockquote><p>I bet a narwhal would love that.</p></blockquote>
   <p>I bet they'd love to peel a banana too.</p>
   <footer><a href="?pid=29582">permalink</a></footer>
  </article>
 </article>
</article>

This example shows the use of a blockquote for short snippets, demonstrating that one does not have to use p elements inside blockquote elements:

<p>He began his list of "lessons" with the following:</p>
<blockquote>One should never assume that his side of 
the issue will be recognized, let alone that it will 
be conceded to have merits.</blockquote>
<p>He continued with a number of similar points, ending with:</p>
<blockquote>Finally, one should be prepared for the threat 
of breakdown in negotiations at any given moment and not 
be cowed by the possibility.</blockquote>
<p>We shall now discuss these points...

Examples of how to represent a conversation are shown in a later section; it is not appropriate to use the cite and blockquote elements for this purpose.

4.4.5 The `ol` element

If the element's children include at least one li element: Palpable content.

Where flow content is expected.

Zero or more li and script-supporting elements.

reversed - Number the list backwards.

start - Ordinal value of the first item

type - Kind of list marker.

Neither tag is omissible

Allowed ARIA role attribute values:

list role (default - do not set), directory, listbox, menu, menubar, presentation, tablist, toolbar or tree.

Any aria-* attributes applicable to the allowed roles.

interface HTMLOListElement : HTMLElement {
           attribute boolean reversed;
           attribute long start;
           attribute DOMString type;
};

The ol element represents a list of items, where the items have been intentionally ordered, such that changing the order would change the meaning of the document.

The items of the list are the li element child nodes of the ol element, in tree order.

The reversed attribute is a boolean attribute. If present, it indicates that the list is a descending list (..., 3, 2, 1). If the attribute is omitted, the list is an ascending list (1, 2, 3, ...).

The start attribute, if present, must be a valid integer giving the ordinal value of the first list item.

If the start attribute is present, user agents must parse it as an integer, in order to determine the attribute's value. The default value, used if the attribute is missing or if the value cannot be converted to a number according to the referenced algorithm, is 1 if the element has no reversed attribute, and is the number of child li elements otherwise.

The first item in the list has the ordinal value given by the ol element's start attribute, unless that li element has a value attribute with a value that can be successfully parsed, in which case it has the ordinal value given by that value attribute.

Each subsequent item in the list has the ordinal value given by its value attribute, if it has one, or, if it doesn't, the ordinal value of the previous item, plus one if the reversed is absent, or minus one if it is present.

The type attribute can be used to specify the kind of marker to use in the list, in the cases where that matters (e.g. because items are to be referenced by their number/letter). The attribute, if specified, must have a value that is a case-sensitive match for one of the characters given in the first cell of one of the rows of the following table. The type attribute represents the state given in the cell in the second column of the row whose first cell matches the attribute's value; if none of the cells match, or if the attribute is omitted, then the attribute represents the decimal state.

Keyword	State	Description	Examples for values 1-3 and 3999-4001
`1` (U+0031)	decimal	Decimal numbers	`1.`	`2.`	`3.`	...	`3999.`	`4000.`	`4001.`	...
`a` (U+0061)	lower-alpha	Lowercase latin alphabet	`a.`	`b.`	`c.`	...	`ewu.`	`ewv.`	`eww.`	...
`A` (U+0041)	upper-alpha	Uppercase latin alphabet	`A.`	`B.`	`C.`	...	`EWU.`	`EWV.`	`EWW.`	...
`i` (U+0069)	lower-roman	Lowercase roman numerals	`i.`	`ii.`	`iii.`	...	`mmmcmxcix.`	`i̅v̅.`	`i̅v̅i.`	...
`I` (U+0049)	upper-roman	Uppercase roman numerals	`I.`	`II.`	`III.`	...	`MMMCMXCIX.`	`I̅V̅.`	`I̅V̅I.`	...

User agents should render the items of the list in a manner consistent with the state of the type attribute of the ol element. Numbers less than or equal to zero should always use the decimal system regardless of the type attribute.

For CSS user agents, a mapping for this attribute to the 'list-style-type' CSS property is given in the rendering section (the mapping is straightforward: the states above have the same names as their corresponding CSS values).

The reversed, start, and type IDL attributes must reflect the respective content attributes of the same name. The start IDL attribute has the same default as its content attribute.

The following markup shows a list where the order matters, and where the ol element is therefore appropriate. Compare this list to the equivalent list in the ul section to see an example of the same items using the ul element.

<p>I have lived in the following countries (given in the order of when
I first lived there):</p>
<ol>
 <li>Switzerland
 <li>United Kingdom
 <li>United States
 <li>Norway
</ol>

Note how changing the order of the list changes the meaning of the document. In the following example, changing the relative order of the first two items has changed the birthplace of the author:

<p>I have lived in the following countries (given in the order of when
I first lived there):</p>
<ol>
 <li>United Kingdom
 <li>Switzerland
 <li>United States
 <li>Norway
</ol>

4.4.6 The `ul` element

If the element's children include at least one li element: Palpable content.

Where flow content is expected.

Zero or more li and script-supporting elements.

Neither tag is omissible

Allowed ARIA role attribute values:

list role (default - do not set), directory, group, listbox, menu, menubar, presentation, tablist, toolbar or tree.

Any aria-* attributes applicable to the allowed roles.

interface HTMLUListElement : HTMLElement {};

The ul element represents a list of items, where the order of the items is not important — that is, where changing the order would not materially change the meaning of the document.

The items of the list are the li element child nodes of the ul element.

The following markup shows a list where the order does not matter, and where the ul element is therefore appropriate. Compare this list to the equivalent list in the ol section to see an example of the same items using the ol element.

<p>I have lived in the following countries:</p>
<ul>
 <li>Norway
 <li>Switzerland
 <li>United Kingdom
 <li>United States
</ul>

Note that changing the order of the list does not change the meaning of the document. The items in the snippet above are given in alphabetical order, but in the snippet below they are given in order of the size of their current account balance in 2007, without changing the meaning of the document whatsoever:

<p>I have lived in the following countries:</p>
<ul>
 <li>Switzerland
 <li>Norway
 <li>United Kingdom
 <li>United States
</ul>

4.4.7 The `li` element

None.

Inside ol elements.

Inside ul elements.

If the element is a child of an ol element: value - Ordinal value of the list item

An li element's end tag may be omitted if the li element is immediately followed by another li element or if there is no more content in the parent element.

Allowed ARIA role attribute values:

listitem role (default - do not set), menuitem, menuitemcheckbox, menuitemradio, option, tab, treeitem or presentation.

Any aria-* attributes applicable to the allowed roles.

interface HTMLLIElement : HTMLElement {
           attribute long value;
};

The li element represents a list item. If its parent element is an ol, or ul, then the element is an item of the parent element's list, as defined for those elements. Otherwise, the list item has no defined list-related relationship to any other li element.

If the parent element is an ol element, then the li element has an ordinal value.

The value attribute, if present, must be a valid integer giving the ordinal value of the list item.

If the value attribute is present, user agents must parse it as an integer, in order to determine the attribute's value. If the attribute's value cannot be converted to a number, the attribute must be treated as if it was absent. The attribute has no default value.

The value attribute is processed relative to the element's parent ol element (q.v.), if there is one. If there is not, the attribute has no effect.

The value IDL attribute must reflect the value of the value content attribute.

The following example, the top ten movies are listed (in reverse order). Note the way the list is given a title by using a figure element and its figcaption element.

<figure>
 <figcaption>The top 10 movies of all time</figcaption>
 <ol>
  <li value="10"><cite>Josie and the Pussycats</cite>, 2001</li>
  <li value="9"><cite lang="sh">Црна мачка, бели мачор</cite>, 1998</li>
  <li value="8"><cite>A Bug's Life</cite>, 1998</li>
  <li value="7"><cite>Toy Story</cite>, 1995</li>
  <li value="6"><cite>Monsters, Inc</cite>, 2001</li>
  <li value="5"><cite>Cars</cite>, 2006</li>
  <li value="4"><cite>Toy Story 2</cite>, 1999</li>
  <li value="3"><cite>Finding Nemo</cite>, 2003</li>
  <li value="2"><cite>The Incredibles</cite>, 2004</li>
  <li value="1"><cite>Ratatouille</cite>, 2007</li>
 </ol>
</figure>

The markup could also be written as follows, using the reversed attribute on the ol element:

<figure>
 <figcaption>The top 10 movies of all time</figcaption>
 <ol reversed>
  <li><cite>Josie and the Pussycats</cite>, 2001</li>
  <li><cite lang="sh">Црна мачка, бели мачор</cite>, 1998</li>
  <li><cite>A Bug's Life</cite>, 1998</li>
  <li><cite>Toy Story</cite>, 1995</li>
  <li><cite>Monsters, Inc</cite>, 2001</li>
  <li><cite>Cars</cite>, 2006</li>
  <li><cite>Toy Story 2</cite>, 1999</li>
  <li><cite>Finding Nemo</cite>, 2003</li>
  <li><cite>The Incredibles</cite>, 2004</li>
  <li><cite>Ratatouille</cite>, 2007</li>
 </ol>
</figure>

While it is conforming to include heading elements (e.g. h1) inside li elements, it likely does not convey the semantics that the author intended. A heading starts a new section, so a heading in a list implicitly splits the list into spanning multiple sections.

4.4.8 The `dl` element

If the element's children include at least one name-value group: Palpable content.

Where flow content is expected.

Zero or more groups each consisting of one or more dt elements followed by one or more dd elements, optionally intermixed with script-supporting elements.

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLDListElement : HTMLElement {};

The dl element represents an association list consisting of zero or more name-value groups (a description list). A name-value group consists of one or more names (dt elements) followed by one or more values (dd elements), ignoring any nodes other than dt and dd elements. Within a single dl element, there should not be more than one dt element for each name.

Name-value groups may be terms and definitions, metadata topics and values, questions and answers, or any other groups of name-value data.

The values within a group are alternatives; multiple paragraphs forming part of the same value must all be given within the same dd element.

The order of the list of groups, and of the names and values within each group, may be significant.

If a dl element is empty, it contains no groups.

If a dl element has one or more non-whitespace Text node children, or has child elements that are neither dt nor dd elements, all such Text nodes and elements, as well as their descendants (including any dt or dd elements), do not form part of any groups in that dl.

If a dl element has one or more dt element children but no dd element children, then it consists of one group with names but no values.

If a dl element has one or more dd element children but no dt element children, then it consists of one group with values but no names.

If a dl element's first dt or dd element child is a dd element, then the first group has no associated name.

If a dl element's last dt or dd element child is a dt element, then the last group has no associated value.

When a dl element doesn't match its content model, it is often due to accidentally using dd elements in the place of dt elements and vice versa. Conformance checkers can spot such mistakes and might be able to advise authors how to correctly use the markup.

In the following example, one entry ("Authors") is linked to two values ("John" and "Luke").

<dl>
 <dt> Authors
 <dd> John
 <dd> Luke
 <dt> Editor
 <dd> Frank
</dl>

In the following example, one definition is linked to two terms.

<dl>
 <dt lang="en-US"> <dfn>color</dfn> </dt>
 <dt lang="en-GB"> <dfn>colour</dfn> </dt>
 <dd> A sensation which (in humans) derives from the ability of
 the fine structure of the eye to distinguish three differently
 filtered analyses of a view. </dd>
</dl>

The following example illustrates the use of the dl element to mark up metadata of sorts. At the end of the example, one group has two metadata labels ("Authors" and "Editors") and two values ("Robert Rothman" and "Daniel Jackson").

<dl>
 <dt> Last modified time </dt>
 <dd> 2004-12-23T23:33Z </dd>
 <dt> Recommended update interval </dt>
 <dd> 60s </dd>
 <dt> Authors </dt>
 <dt> Editors </dt>
 <dd> Robert Rothman </dd>
 <dd> Daniel Jackson </dd>
</dl>

The following example shows the dl element used to give a set of instructions. The order of the instructions here is important (in the other examples, the order of the blocks was not important).

<p>Determine the victory points as follows (use the
first matching case):</p>
<dl>
 <dt> If you have exactly five gold coins </dt>
 <dd> You get five victory points </dd>
 <dt> If you have one or more gold coins, and you have one or more silver coins </dt>
 <dd> You get two victory points </dd>
 <dt> If you have one or more silver coins </dt>
 <dd> You get one victory point </dd>
 <dt> Otherwise </dt>
 <dd> You get no victory points </dd>
</dl>

The following snippet shows a dl element being used as a glossary. Note the use of dfn to indicate the word being defined.

<dl>
 <dt><dfn>Apartment</dfn>, n.</dt>
 <dd>An execution context grouping one or more threads with one or
 more COM objects.</dd>
 <dt><dfn>Flat</dfn>, n.</dt>
 <dd>A deflated tire.</dd>
 <dt><dfn>Home</dfn>, n.</dt>
 <dd>The user's login directory.</dd>
</dl>

The dl element is inappropriate for marking up dialogue. Examples of how to mark up dialogue are shown below.

4.4.9 The `dt` element

Categories:: None.
Contexts in which this element can be used:: Before dd or dt elements inside dl elements.
Content model:: Flow content, but with no header, footer, sectioning content, or heading content descendants.
Content attributes:: Global attributes
Tag omission in text/html:: A dt element's end tag may be omitted if the dt element is immediately followed by another dt element or a dd element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The dt element represents the term, or name, part of a term-description group in a description list (dl element).

The dt element itself, when used in a dl element, does not indicate that its contents are a term being defined, but this can be indicated using the dfn element.

This example shows a list of frequently asked questions (a FAQ) marked up using the dt element for questions and the dd element for answers.

<article>
 <h1>FAQ</h1>
 <dl>
  <dt>What do we want?</dt>
  <dd>Our data.</dd>
  <dt>When do we want it?</dt>
  <dd>Now.</dd>
  <dt>Where is it?</dt>
  <dd>We are not sure.</dd>
 </dl>
</article>

4.4.10 The `dd` element

Categories:: None.
Contexts in which this element can be used:: After dt or dd elements inside dl elements.
Content model:: Flow content.
Content attributes:: Global attributes
Tag omission in text/html:: A dd element's end tag may be omitted if the dd element is immediately followed by another dd element or a dt element, or if there is no more content in the parent element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The dd element represents the description, definition, or value, part of a term-description group in a description list (dl element).

A dl can be used to define a vocabulary list, like in a dictionary. In the following example, each entry, given by a dt with a dfn, has several dds, showing the various parts of the definition.

<dl>
 <dt><dfn>happiness</dfn></dt>
 <dd class="pronunciation">/'hæ p. nes/</dd>
 <dd class="part-of-speech"><i><abbr>n.</abbr></i></dd>
 <dd>The state of being happy.</dd>
 <dd>Good fortune; success. <q>Oh <b>happiness</b>! It worked!</q></dd>
 <dt><dfn>rejoice</dfn></dt>
 <dd class="pronunciation">/ri jois'/</dd>
 <dd><i class="part-of-speech"><abbr>v.intr.</abbr></i> To be delighted oneself.</dd>
 <dd><i class="part-of-speech"><abbr>v.tr.</abbr></i> To cause one to be delighted.</dd>
</dl>

4.4.11 The `figure` element

Categories:: Flow content.; Sectioning root.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected.
Content model:: Either: One figcaption element followed by flow content.; Or: Flow content followed by one figcaption element.; Or: Flow content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The figure element represents some flow content, optionally with a caption, that is self-contained (like a complete sentence) and is typically referenced as a single unit from the main flow of the document.

Self-contained in this context does not necessarily mean independent. For example, each sentence in a paragraph is self-contained; an image that is part of a sentence would be inappropriate for figure, but an entire sentence made of images would be fitting.

The element can thus be used to annotate illustrations, diagrams, photos, code listings, etc.

When a figure is referred to from the main content of the document by identifying it by its caption (e.g. by figure number), it enables such content to be easily moved away from that primary content, e.g. to the side of the page, to dedicated pages, or to an appendix, without affecting the flow of the document.

If a figure element is referenced by its relative position, e.g. "in the photograph above" or "as the next figure shows", then moving the figure would disrupt the page's meaning. Authors are encouraged to consider using labels to refer to figures, rather than using such relative references, so that the page can easily be restyled without affecting the page's meaning.

The first figcaption element child of the element, if any, represents the caption of the figure element's contents. If there is no child figcaption element, then there is no caption.

A figure element's contents are part of the surrounding flow. If the purpose of the page is to display the figure, for example a photograph on an image sharing site, the figure and figcaption elements can be used to explicitly provide a caption for that figure. For content that is only tangentially related, or that serves a separate purpose than the surrounding flow, the aside element should be used (and can itself wrap a figure). For example, a pull quote that repeats content from an article would be more appropriate in an aside than in a figure, because it isn't part of the content, it's a repetition of the content for the purposes of enticing readers or highlighting key topics.

This example shows the figure element to mark up a code listing.

<p>In <a href="#l4">listing 4</a> we see the primary core interface
API declaration.</p>
<figure id="l4">
 <figcaption>Listing 4. The primary core interface API declaration.</figcaption>
 <pre><code>interface PrimaryCore {
 boolean verifyDataLine();
 void sendData(in sequence&lt;byte> data);
 void initSelfDestruct();
}</code></pre>
</figure>
<p>The API is designed to use UTF-8.</p>

Here we see a figure element to mark up a photo that is the main content of the page (as in a gallery).

<!DOCTYPE HTML>
<title>Bubbles at work — My Gallery™</title>
<figure>
 <img src="bubbles-work.jpeg"
      alt="Bubbles, sitting in his office chair, works on his
           latest project intently.">
 <figcaption>Bubbles at work</figcaption>
</figure>
<nav><a href="19414.html">Prev</a> — <a href="19416.html">Next</a></nav>

In this example, we see an image that is not a figure, as well as an image and a video that are. The first image is literally part of the example's second sentence, so it's not a self-contained unit, and thus figure would be inappropriate.

<h2>Malinko's comics</h2>

<p>This case centered on some sort of "intellectual property"
infringement related to a comic (see Exhibit A). The suit started
after a trailer ending with these words:

<blockquote>
 <img src="promblem-packed-action.png" alt="ROUGH COPY! Promblem-Packed Action!">
</blockquote>

<p>...was aired. A lawyer, armed with a Bigger Notebook, launched a
preemptive strike using snowballs. A complete copy of the trailer is
included with Exhibit B.

<figure>
 <img src="ex-a.png" alt="Two squiggles on a dirty piece of paper.">
 <figcaption>Exhibit A. The alleged <cite>rough copy</cite> comic.</figcaption>
</figure>

<figure>
 <video src="ex-b.mov"></video>
 <figcaption>Exhibit B. The <cite>Rough Copy</cite> trailer.</figcaption>
</figure>

<p>The case was resolved out of court.

Here, a part of a poem is marked up using figure.

<figure>
 <p>'Twas brillig, and the slithy toves<br>
 Did gyre and gimble in the wabe;<br>
 All mimsy were the borogoves,<br>
 And the mome raths outgrabe.</p>
 <figcaption><cite>Jabberwocky</cite> (first verse). Lewis Carroll, 1832-98</figcaption>
</figure>

In this example, which could be part of a much larger work discussing a castle, nested figure elements are used to provide both a group caption and individual captions for each figure in the group:

<figure>
 <figcaption>The castle through the ages: 1423, 1858, and 1999 respectively.</figcaption>
 <figure>
  <figcaption>Etching. Anonymous, ca. 1423.</figcaption>
  <img src="castle1423.jpeg" alt="The castle has one tower, and a tall wall around it.">
 </figure>
 <figure>
  <figcaption>Oil-based paint on canvas. Maria Towle, 1858.</figcaption>
  <img src="castle1858.jpeg" alt="The castle now has two towers and two walls.">
 </figure>
 <figure>
  <figcaption>Film photograph. Peter Jankle, 1999.</figcaption>
  <img src="castle1999.jpeg" alt="The castle lies in ruins, the original tower all that remains in one piece.">
 </figure>
</figure>

The figure is sometimes referenced only implicitly from the content:

<article>
 <h1>Fiscal negotiations stumble in Congress as deadline nears</h1>
 <figure>
  <img src="obama-reid.jpeg" alt="Obama and Reid sit together smiling in the Oval Office.">
  <figcaption>Barrak Obama and Harry Reid. White House press photograph.</figcaption>
 </figure>
 <p>Negotiations in Congress to end the fiscal impasse sputtered on Tuesday, leaving both chambers
 grasping for a way to reopen the government and raise the country's borrowing authority with a
 Thursday deadline drawing near.</p>
 ...
</article>

4.4.12 The `figcaption` element

Categories:: None.
Contexts in which this element can be used:: As the first or last child of a figure element.
Content model:: Flow content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The figcaption element represents a caption or legend for the rest of the contents of the figcaption element's parent figure element, if any.

4.4.13 The `div` element

Where flow content is expected.

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLDivElement : HTMLElement {};

The div element has no special meaning at all. It represents its children. It can be used with the class, lang, and title attributes to mark up semantics common to a group of consecutive elements.

Authors are strongly encouraged to view the div element as an element of last resort, for when no other element is suitable. Use of more appropriate elements instead of the div element leads to better accessibility for readers and easier maintainability for authors.

For example, a blog post would be marked up using article, a chapter using section, a page's navigation aids using nav, and a group of form controls using fieldset.

On the other hand, div elements can be useful for stylistic purposes or to wrap multiple paragraphs within a section that are all to be annotated in a similar way. In the following example, we see div elements used as a way to set the language of two paragraphs at once, instead of setting the language on the two paragraph elements separately:

<article lang="en-US">
 <h1>My use of language and my cats</h1>
 <p>My cat's behavior hasn't changed much since her absence, except
 that she plays her new physique to the neighbors regularly, in an
 attempt to get pets.</p>
 <div lang="en-GB">
  <p>My other cat, coloured black and white, is a sweetie. He followed
  us to the pool today, walking down the pavement with us. Yesterday
  he apparently visited our neighbours. I wonder if he recognises that
  their flat is a mirror image of ours.</p>
  <p>Hm, I just noticed that in the last paragraph I used British
  English. But I'm supposed to write in American English. So I
  shouldn't say "pavement" or "flat" or "colour"...</p>
 </div>
 <p>I should say "sidewalk" and "apartment" and "color"!</p>
</article>

4.4.14 The `main` element

Categories:: Flow content.; Palpable content.
Contexts in which this element can be used:: Where flow content is expected, but with no article, aside, footer, header or nav element ancestors.
Content model:: Flow content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: main role (default - do not set) or presentation.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement

The main element represents the main content of the body of a document or application. The main content area consists of content that is directly related to or expands upon the central topic of a document or central functionality of an application.

The main element is not sectioning content and has no effect on the document outline

The main content area of a document includes content that is unique to that document and excludes content that is repeated across a set of documents such as site navigation links, copyright information, site logos and banners and search forms (unless the document or applications main function is that of a search form).

User agents that support keyboard navigation of content are strongly encouraged to provide a method to navigate to the main element and once navigated to, ensure the next element in the focus order is the first focusable element within the main element. This will provide a simple method for keyboard users to bypass blocks of content such as navigation links.

Authors must not include more than one main element in a document.

Authors must not include the main element as a descendant of an article, aside, footer, header or nav element.

The main element is not suitable for use to identify the main content areas of sub sections of a document or application. The simplest solution is to not mark up the main content of a sub section at all, and just leave it as implicit, but an author could use a grouping content or sectioning content element as appropriate.

Authors are advised to use ARIA role="main" attribute on the main element until user agents implement the required role mapping.

  <main role="main"> 
  ... 
  </main>

In the following example, we see 2 articles about skateboards (the main topic of a Web page) the main topic content is identified by the use of the main element.

<!-- other content -->

<main>

  <h1>Skateboards</h1>
  <p>The skateboard is the way cool kids get around</p>
  
  <article>
  <h2>Longboards</h2>
  <p>Longboards are a type of skateboard with a longer 
  wheelbase and larger, softer wheels.</p>
  <p>... </p>
  <p>... </p>
  </article>

  <article>
  <h2>Electric Skateboards</h2>
  <p>These no longer require the propelling of the skateboard
  by means of the feet; rather an electric motor propels the board, 
  fed by an electric battery.</p>
  <p>... </p>
  <p>... </p>
  </article>

</main>

  <!-- other content -->

Here is a graduation programme the main content section is defined by the use of the main element. Note in this example the main element contains a nav element consisting of links to sub sections of the main content.

<!DOCTYPE html>
  <html>
  <head>
<title>Graduation Ceremony Summer 2022</title>
  </head>
  <body>

  <header>The Lawson Academy:
  <nav>
  <ul>
  <li><a href="courses.html">Courses</a></li>
  <li><a href="fees.html">Fees</a></li>
  <li><a>Graduation</a></li>
  </ul>
  </nav> 
  </header>


<main>


  <h1>Graduation</h1>

  <nav>
  <ul>
  <li><a href="#ceremony">Ceremony</a></li>
  <li><a href="#graduates">Graduates</a></li>
  <li><a href="#awards">Awards</a></li>
  </ul>
  </nav>

  <H2 id="ceremony">Ceremony</H2>
  <p>Opening Procession</p>
  <p>Speech by Valedictorian</p>
  <p>Speech by Class President</p>
  <p>Presentation of Diplomas</p>
  <p>Closing Speech by Headmaster</p>

  <h2 id="graduates">Graduates</h2>
  <ul>
  <li>Eileen Williams</li>
  <li>Andy Maseyk</li>
  <li>Blanca Sainz Garcia</li>
  <li>Clara Faulkner</li>
  <li>Gez Lemon</li>
  <li>Eloisa Faulkner</li>
  </ul>

  <h2 id="awards">Awards</h2>
  <ul>
  <li>Clara Faulkner</li>
  <li>Eloisa Faulkner</li>
  <li>Blanca Sainz Garcia</li>
  </ul>

</main>

  <footer> Copyright 2012 B.lawson</footer>

  </body>
  </html>

4.5 Text-level semantics

4.5.1 The `a` element

Where phrasing content is expected.

Transparent, but there must be no interactive content descendant.

href - Address of the hyperlink

target - Default browsing context for hyperlink navigation and form submission

download - Whether to download the resource instead of navigating to it, and its file name if so

rel - Relationship between the document containing the hyperlink and the destination resource

hreflang - Language of the linked resource

type - Hint for the type of the referenced resource

Neither tag is omissible

Allowed ARIA role attribute values:

link (default - do not set), button, checkbox, menuitem, menuitemcheckbox, menuitemradio, tab or treeitem

Any aria-* attributes applicable to the allowed roles.

interface HTMLAnchorElement : HTMLElement {
           attribute DOMString target;
           attribute DOMString download;

           attribute DOMString rel;
           attribute DOMString rev;
  readonly attribute DOMTokenList relList;
           attribute DOMString hreflang;
           attribute DOMString type;

           attribute DOMString text;
};
HTMLAnchorElement implements URLUtils;

If the a element has an href attribute, then it represents a hyperlink (a hypertext anchor) labeled by its contents.

If the a element has no href attribute, then the element represents a placeholder for where a link might otherwise have been placed, if it had been relevant, consisting of just the element's contents.

The target, download, rel, hreflang, and type attributes must be omitted if the href attribute is not present.

If a site uses a consistent navigation toolbar on every page, then the link that would normally link to the page itself could be marked up using an a element:

<nav>
 <ul>
  <li> <a href="/">Home</a> </li>
  <li> <a href="/news">News</a> </li>
  <li> <a>Examples</a> </li>
  <li> <a href="/legal">Legal</a> </li>
 </ul>
</nav>

The href, target, download, and attributes affect what happens when users follow hyperlinks or download hyperlinks created using the a element. The rel, hreflang, and type attributes may be used to indicate to the user the likely nature of the target resource before the user follows the link.

The activation behavior of a elements that create hyperlinks is to run the following steps:

If the a element's Document is not fully active, then abort these steps.
If either the a element has a download attribute and the algorithm is not allowed to show a popup, or the element's target attribute is present and applying the rules for choosing a browsing context given a browsing context name, using the value of the target attribute as the browsing context name, would result in there not being a chosen browsing context, then run these substeps:
1. If there is an entry settings object, throw an InvalidAccessError exception.
2. Abort these steps without following the hyperlink.
If the target of the click event is an img element with an ismap attribute specified, then server-side image map processing must be performed, as follows:
1. If the click event was a real pointing-device-triggered click event on the img element, then let x be the distance in CSS pixels from the left edge of the image's left border, if it has one, or the left edge of the image otherwise, to the location of the click, and let y be the distance in CSS pixels from the top edge of the image's top border, if it has one, or the top edge of the image otherwise, to the location of the click. Otherwise, let x and y be zero.
2. Let the hyperlink suffix be a U+003F QUESTION MARK character, the value of x expressed as a base-ten integer using ASCII digits, a "," (U+002C) character, and the value of y expressed as a base-ten integer using ASCII digits.
Finally, the user agent must follow the hyperlink or download the hyperlink created by the a element, as determined by the download attribute and any expressed user preference. If the steps above defined a hyperlink suffix, then take that into account when following or downloading the hyperlink.

a . text: Same as textContent.

The IDL attributes download, target, rel, rev, hreflang, and type, must reflect the respective content attributes of the same name.

The IDL attribute relList must reflect the rel content attribute.

The text IDL attribute, on getting, must return the same value as the textContent IDL attribute on the element, and on setting, must act as if the textContent IDL attribute on the element had been set to the new value.

The a element also supports the URLUtils interface. [URL]

When the element is created, and whenever the element's href content attribute is set, changed, or removed, the user agent must invoke the element's URLUtils interface's set the input algorithm with the value of the href content attribute, if any, or the empty string otherwise, as the given value.

The element's URLUtils interface's get the base algorithm must simply return the element's base URL.

The element's URLUtils interface's query encoding is the document's character encoding.

When the element's URLUtils interface invokes its update steps with a string value, the user agent must set the element's href content attribute to the string value.

The a element may be wrapped around entire paragraphs, lists, tables, and so forth, even entire sections, so long as there is no interactive content within (e.g. buttons or other links). This example shows how this can be used to make an entire advertising block into a link:

<aside class="advertising">
 <h1>Advertising</h1>
 <a href="http://ad.example.com/?adid=1929&amp;pubid=1422">
  <section>
   <h1>Mellblomatic 9000!</h1>
   <p>Turn all your widgets into mellbloms!</p>
   <p>Only $9.99 plus shipping and handling.</p>
  </section>
 </a>
 <a href="http://ad.example.com/?adid=375&amp;pubid=1422">
  <section>
   <h1>The Mellblom Browser</h1>
   <p>Web browsing at the speed of light.</p>
   <p>No other browser goes faster!</p>
  </section>
 </a>
</aside>

4.5.2 The `em` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The em element represents stress emphasis of its contents.

The level of stress that a particular piece of content has is given by its number of ancestor em elements.

The placement of stress emphasis changes the meaning of the sentence. The element thus forms an integral part of the content. The precise way in which stress is used in this way depends on the language.

These examples show how changing the stress emphasis changes the meaning. First, a general statement of fact, with no stress:

<p>Cats are cute animals.</p>

By emphasizing the first word, the statement implies that the kind of animal under discussion is in question (maybe someone is asserting that dogs are cute):

<p><em>Cats</em> are cute animals.</p>

Moving the stress to the verb, one highlights that the truth of the entire sentence is in question (maybe someone is saying cats are not cute):

<p>Cats <em>are</em> cute animals.</p>

By moving it to the adjective, the exact nature of the cats is reasserted (maybe someone suggested cats were mean animals):

<p>Cats are <em>cute</em> animals.</p>

Similarly, if someone asserted that cats were vegetables, someone correcting this might emphasize the last word:

<p>Cats are cute <em>animals</em>.</p>

By emphasizing the entire sentence, it becomes clear that the speaker is fighting hard to get the point across. This kind of stress emphasis also typically affects the punctuation, hence the exclamation mark here.

<p><em>Cats are cute animals!</em></p>

Anger mixed with emphasizing the cuteness could lead to markup such as:

<p><em>Cats are <em>cute</em> animals!</em></p>

The em element isn't a generic "italics" element. Sometimes, text is intended to stand out from the rest of the paragraph, as if it was in a different mood or voice. For this, the i element is more appropriate.

The em element also isn't intended to convey importance; for that purpose, the strong element is more appropriate.

4.5.3 The `strong` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The strong element represents strong importance, seriousness, or urgency for its contents.

Importance: The strong element can be used in a heading, caption, or paragraph to distinguish the part that really matters from other parts of the that might be more detailed, more jovial, or merely boilerplate.

For example, the first word of the previous paragraph is marked up with strong to distinguish it from the more detailed text in the rest of the paragraph.

Seriousness: The strong element can be used to mark up a warning or caution notice.

Urgency: The strong element can be used to denote contents that the user needs to see sooner than other parts of the document.

The relative level of importance of a piece of content is given by its number of ancestor strong elements; each strong element increases the importance of its contents.

Changing the importance of a piece of text with the strong element does not change the meaning of the sentence.

Here, the word "chapter" and the actual chapter number are mere boilerplate, and the actual name of the chapter is marked up with strong:

<h1>Chapter 1: <strong>The Praxis</strong></h1>

In the following example, the name of the diagram in the caption is marked up with strong, to distinguish it from boilerplate text (before) and the description (after):

<figcaption>Figure 1. <strong>Ant colony dynamics</strong>. The ants in this colony are
affected by the heat source (upper left) and the food source (lower right).</figcaption>

In this example, the heading is really "Flowers, Bees, and Honey", but the author has added a light-hearted addition to the heading. The strong element is thus used to mark up the first part to distinguish it from the latter part.

<h1><strong>Flowers, Bees, and Honey</strong> and other things I don't understand</h1>

Here is an example of a warning notice in a game, with the various parts marked up according to how important they are:

<p><strong>Warning.</strong> This dungeon is dangerous.
<strong>Avoid the ducks.</strong> Take any gold you find.
<strong><strong>Do not take any of the diamonds</strong>,
they are explosive and <strong>will destroy anything within
ten meters.</strong></strong> You have been warned.</p>

In this example, the strong element is used to denote the part of the text that the user is intended to read first.

<p>Welcome to Remy, the reminder system.</p>
<p>Your tasks for today:</p>
<ul>
 <li><p><strong>Turn off the oven.</strong></p></li>
 <li><p>Put out the trash.</p></li>
 <li><p>Do the laundry.</p></li>
</ul>

4.5.4 The `small` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The small element represents side comments such as small print.

Small print typically features disclaimers, caveats, legal restrictions, or copyrights. Small print is also sometimes used for attribution, or for satisfying licensing requirements.

The small element does not "de-emphasize" or lower the importance of text emphasized by the em element or marked as important with the strong element. To mark text as not emphasized or important, simply do not mark it up with the em or strong elements respectively.

The small element should not be used for extended spans of text, such as multiple paragraphs, lists, or sections of text. It is only intended for short runs of text. The text of a page listing terms of use, for instance, would not be a suitable candidate for the small element: in such a case, the text is not a side comment, it is the main content of the page.

In this example, the small element is used to indicate that value-added tax is not included in a price of a hotel room:

<dl>
 <dt>Single room
 <dd>199 € <small>breakfast included, VAT not included</small>
 <dt>Double room
 <dd>239 € <small>breakfast included, VAT not included</small>
</dl>

In this second example, the small element is used for a side comment in an article.

<p>Example Corp today announced record profits for the
second quarter <small>(Full Disclosure: Foo News is a subsidiary of
Example Corp)</small>, leading to speculation about a third quarter
merger with Demo Group.</p>

This is distinct from a sidebar, which might be multiple paragraphs long and is removed from the main flow of text. In the following example, we see a sidebar from the same article. This sidebar also has small print, indicating the source of the information in the sidebar.

<aside>
 <h1>Example Corp</h1>
 <p>This company mostly creates small software and Web
 sites.</p>
 <p>The Example Corp company mission is "To provide entertainment
 and news on a sample basis".</p>
 <p><small>Information obtained from <a
 href="http://example.com/about.html">example.com</a> home
 page.</small></p>
</aside>

In this last example, the small element is marked as being important small print.

<p><strong><small>Continued use of this service will result in a kiss.</small></strong></p>

4.5.5 The `s` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The s element represents contents that are no longer accurate or no longer relevant.

The s element is not appropriate when indicating document edits; to mark a span of text as having been removed from a document, use the del element.

In this example a recommended retail price has been marked as no longer relevant as the product in question has a new sale price.

<p>Buy our Iced Tea and Lemonade!</p>
<p><s>Recommended retail price: $3.99 per bottle</s></p>
<p><strong>Now selling for just $2.99 a bottle!</strong></p>

4.5.6 The `cite` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The cite element represents a reference to a creative work. It must include the title of the work or the name of the author(person, people or organization) or an URL reference, which may be in an abbreviated form as per the conventions used for the addition of citation metadata.

Creative works include a book, a paper, an essay, a poem, a score, a song, a script, a film, a TV show, a game, a sculpture, a painting, a theatre production, a play, an opera, a musical, an exhibition, a legal case report, a web site, a web page, a blog post or comment, a forum post or comment, a tweet, a written or oral statement, etc.

Here is an example of the author of a quote referenced using the cite element:

  
  <p>In the words of <cite>Charles Bukowski</cite> -  
  <q>An intellectual says a simple thing in a hard way. An artist says a hard thing in a simple way.</q></p>

This second example identifies the author of a tweet by referencing the authors name using the cite element:

  
  <blockquote class="twitter-tweet">
  <p>♥ Bukowski in <a href="https://twitter.com/search?q=%23HTML5&src=hash">#HTML5</a> spec examples
  <a href="http://t.co/0FIEiYN1pC">http://t.co/0FIEiYN1pC</a></p><cite>— karl dubost (@karlpro) 
  <a href="https://twitter.com/karlpro/statuses/370905307293442048">August 23, 2013</a></cite>
  </blockquote>

In this example the cite element is used to reference the title of a work in a bibliography:

<p><cite>Universal Declaration of Human Rights</cite>, United Nations,
December 1948. Adopted by General Assembly resolution 217 A (III).</p>

In this example the cite element is used to reference the title of a television show:

<p>Who is your favorite doctor (in <cite>Doctor Who</cite>)?</p>

A very common use for the cite element is to identify the author of a comment in a blog post or forum, as in this example:

  <article id="comment-1"> 
  Comment by <cite><a href="http://oli.jp">Oli Studholme</a></cite> 
  <time datetime="2013-08-19T16:01">August 19th, 2013 at 4:01 pm</time>
  <p>Unfortunately I don't think adding names back into the definition of <code>cite</code>
  solves the problem: of the 12 blockquote examples in
  <a href="http://oli.jp/example/blockquote-metadata/">Examples of block quote metadata</a>,
  there's not even one that's <em>just</em> a person’s name.</p>
  <p>A subset of the problem, maybe…</p>
  </article>

Another common use for the cite element is to reference the URL of a search result, as in this example:

  <div id="resultStats">About 416,000,000 results 0.33 seconds) </div>
  ...
  <p><a href="http://www.w3.org/html/wg/">W3C <i>HTML Working Group</i></a></p>
  <p><cite>www.w3.org/<b>html</b>/wg/</cite></p>
  <p>15 Apr 2013 - The <i>HTML Working Group</i> is currently chartered to continue its 
  work through 31 December 2014. A Plan 2014 document published by the...</p>
  ...

A citation is not a quote (for which the q element is appropriate).

This is incorrect usage, because cite is not for quotes:

<p><cite>This is wrong!, said Hillary.</cite> is a quote from the 
   popular daytime TV drama When Ian became Hillary.</p>

This is an example of the correct usage:

<p><q>This is correct, said Hillary.</q> is a quote from the 
   popular daytime TV drama <cite>When Ian became Hillary</cite>.</p>

4.5.7 The `q` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes; cite - Link to the source of the quotation or more information about the edit
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLQuoteElement.

The q element represents some phrasing content quoted from another source.

Quotation punctuation (such as quotation marks) that is quoting the contents of the element must not appear immediately before, after, or inside q elements; they will be inserted into the rendering by the user agent.

Content inside a q element must be quoted from another source, whose address, if it has one, may be cited in the cite attribute. The source may be fictional, as when quoting characters in a novel or screenplay.

The q element must not be used in place of quotation marks that do not represent quotes; for example, it is inappropriate to use the q element for marking up sarcastic statements.

The use of q elements to mark up quotations is entirely optional; using explicit quotation punctuation without q elements is just as correct.

Here is a simple example of the use of the q element:

<p>The man said <q>Things that are impossible just take
longer</q>. I disagreed with him.</p>

Here is an example with both an explicit citation link in the q element, and an explicit citation outside:

<p>The W3C page <cite>About W3C</cite> says the W3C's
mission is <q cite="http://www.w3.org/Consortium/">To lead the
World Wide Web to its full potential by developing protocols and
guidelines that ensure long-term growth for the Web</q>. I
disagree with this mission.</p>

In the following example, the quotation itself contains a quotation:

<p>In <cite>Example One</cite>, he writes <q>The man
said <q>Things that are impossible just take longer</q>. I
disagreed with him</q>. Well, I disagree even more!</p>

In the following example, quotation marks are used instead of the q element:

<p>His best argument was ❝I disagree❞, which
I thought was laughable.</p>

In the following example, there is no quote — the quotation marks are used to name a word. Use of the q element in this case would be inappropriate.

<p>The word "ineffable" could have been used to describe the disaster
resulting from the campaign's mismanagement.</p>

4.5.8 The `dfn` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content, but there must be no dfn element descendants.
Content attributes:: Global attributes; Also, the title attribute has special semantics on this element.
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The dfn element represents the defining instance of a term. The paragraph, description list group, or section that is the nearest ancestor of the dfn element must also contain the definition(s) for the term given by the dfn element.

Defining term: If the dfn element has a title attribute, then the exact value of that attribute is the term being defined. Otherwise, if it contains exactly one element child node and no child Text nodes, and that child element is an abbr element with a title attribute, then the exact value of that attribute is the term being defined. Otherwise, it is the exact textContent of the dfn element that gives the term being defined.

If the title attribute of the dfn element is present, then it must contain only the term being defined.

The title attribute of ancestor elements does not affect dfn elements.

An a element that links to a dfn element represents an instance of the term defined by the dfn element.

In the following fragment, the term "Garage Door Opener" is first defined in the first paragraph, then used in the second. In both cases, its abbreviation is what is actually displayed.

<p>The <dfn><abbr title="Garage Door Opener">GDO</abbr></dfn>
is a device that allows off-world teams to open the iris.</p>
<!-- ... later in the document: -->
<p>Teal'c activated his <abbr title="Garage Door Opener">GDO</abbr>
and so Hammond ordered the iris to be opened.</p>

With the addition of an a element, the reference can be made explicit:

<p>The <dfn id=gdo><abbr title="Garage Door Opener">GDO</abbr></dfn>
is a device that allows off-world teams to open the iris.</p>
<!-- ... later in the document: -->
<p>Teal'c activated his <a href=#gdo><abbr title="Garage Door Opener">GDO</abbr></a>
and so Hammond ordered the iris to be opened.</p>

4.5.9 The `abbr` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes; Also, the title attribute has special semantics on this element.
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The abbr element represents an abbreviation or acronym, optionally with its expansion. The title attribute may be used to provide an expansion of the abbreviation. The attribute, if specified, must contain an expansion of the abbreviation, and nothing else.

The paragraph below contains an abbreviation marked up with the abbr element. This paragraph defines the term "Web Hypertext Application Technology Working Group".

<p>The <dfn id=whatwg><abbr
title="Web Hypertext Application Technology Working Group">WHATWG</abbr></dfn>
is a loose unofficial collaboration of Web browser manufacturers and
interested parties who wish to develop new technologies designed to
allow authors to write and deploy Applications over the World Wide
Web.</p>

An alternative way to write this would be:

<p>The <dfn id=whatwg>Web Hypertext Application Technology
Working Group</dfn> (<abbr
title="Web Hypertext Application Technology Working Group">WHATWG</abbr>)
is a loose unofficial collaboration of Web browser manufacturers and
interested parties who wish to develop new technologies designed to
allow authors to write and deploy Applications over the World Wide
Web.</p>

This paragraph has two abbreviations. Notice how only one is defined; the other, with no expansion associated with it, does not use the abbr element.

<p>The
<abbr title="Web Hypertext Application Technology Working Group">WHATWG</abbr>
started working on HTML5 in 2004.</p>

This paragraph links an abbreviation to its definition.

<p>The <a href="#whatwg"><abbr
title="Web Hypertext Application Technology Working Group">WHATWG</abbr></a>
community does not have much representation from Asia.</p>

This paragraph marks up an abbreviation without giving an expansion, possibly as a hook to apply styles for abbreviations (e.g. smallcaps).

<p>Philip` and Dashiva both denied that they were going to
get the issue counts from past revisions of the specification to
backfill the <abbr>WHATWG</abbr> issue graph.</p>

If an abbreviation is pluralized, the expansion's grammatical number (plural vs singular) must match the grammatical number of the contents of the element.

Here the plural is outside the element, so the expansion is in the singular:

<p>Two <abbr title="Working Group">WG</abbr>s worked on
this specification: the <abbr>WHATWG</abbr> and the
<abbr>HTMLWG</abbr>.</p>

Here the plural is inside the element, so the expansion is in the plural:

<p>Two <abbr title="Working Groups">WGs</abbr> worked on
this specification: the <abbr>WHATWG</abbr> and the
<abbr>HTMLWG</abbr>.</p>

Abbreviations do not have to be marked up using this element. It is expected to be useful in the following cases:

Abbreviations for which the author wants to give expansions, where using the abbr element with a title attribute is an alternative to including the expansion inline (e.g. in parentheses).
Abbreviations that are likely to be unfamiliar to the document's readers, for which authors are encouraged to either mark up the abbreviation using an abbr element with a title attribute or include the expansion inline in the text the first time the abbreviation is used.
Abbreviations whose presence needs to be semantically annotated, e.g. so that they can be identified from a style sheet and given specific styles, for which the abbr element can be used without a title attribute.

Providing an expansion in a title attribute once will not necessarily cause other abbr elements in the same document with the same contents but without a title attribute to behave as if they had the same expansion. Every abbr element is independent.

4.5.10 The `data` element

Where phrasing content is expected.

value - Machine-readable value

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLDataElement : HTMLElement {
           attribute DOMString value;
};

The data element represents its contents, along with a machine-readable form of those contents in the value attribute.

The value attribute must be present. Its value must be a representation of the element's contents in a machine-readable format.

When the value is date- or time-related, the more specific time element can be used instead.

The element can be used for several purposes.

When combined with microformats or microdata, the element serves to provide both a machine-readable value for the purposes of data processors, and a human-readable value for the purposes of rendering in a Web browser. In this case, the format to be used in the value attribute is determined by the microformats or microdata vocabulary in use.

The element can also, however, be used in conjunction with scripts in the page, for when a script has a literal value to store alongside a human-readable value. In such cases, the format to be used depends only on the needs of the script. (The data-* attributes can also be useful in such situations.)

The value IDL attribute must reflect the content attribute of the same name.

4.5.11 The `time` element

Where phrasing content is expected.

datetime - Machine-readable value

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

A valid yearless date string

interface HTMLTimeElement : HTMLElement {
           attribute DOMString dateTime;
};

The time element represents its contents, along with a machine-readable form of those contents in the datetime attribute. The kind of content is limited to various kinds of dates, times, time-zone offsets, and durations, as described below.

The datetime attribute may be present. If present, its value must be a representation of the element's contents in a machine-readable format.

A time element that does not have a datetime content attribute must not have any element descendants.

The datetime value of a time element is the value of the element's datetime content attribute, if it has one, or the element's textContent, if it does not.

The datetime value of a time element must match one of the following syntaxes.

A valid month string

<time>2011-11</time>

A valid date string

<time>2011-11-12</time>

<time>11-12</time>

A valid time string

<time>14:54</time>

<time>14:54:39</time>

<time>14:54:39.929</time>

A valid floating date and time string

<time>2011-11-12T14:54</time>

<time>2011-11-12T14:54:39</time>

<time>2011-11-12T14:54:39.929</time>

<time>2011-11-12 14:54</time>

<time>2011-11-12 14:54:39</time>

<time>2011-11-12 14:54:39.929</time>

Times with dates but without a time zone offset are useful for specifying events that are observed at the same specific time in each time zone, throughout a day. For example, the 2020 new year is celebrated at 2020-01-01 00:00 in each time zone, not at the same precise moment across all time zones. For events that occur at the same time across all time zones, for example a videoconference meeting, a valid global date and time string is likely more useful.

A valid time-zone offset string

<time>Z</time>

<time>+0000</time>

<time>+00:00</time>

<time>-0800</time>

<time>-08:00</time>

For times without dates (or times referring to events that recur on multiple dates), specifying the geographic location that controls the time is usually more useful than specifying a time zone offset, because geographic locations change time zone offsets with daylight savings time. In some cases, geographic locations even change time zone, e.g. when the boundaries of those time zones are redrawn, as happened with Samoa at the end of 2011. There exists a time zone database that describes the boundaries of time zones and what rules apply within each such zone, known as the time zone database. [TZDATABASE]

A valid global date and time string

<time>2011-11-12T14:54Z</time>

<time>2011-11-12T14:54:39Z</time>

<time>2011-11-12T14:54:39.929Z</time>

<time>2011-11-12T14:54+0000</time>

<time>2011-11-12T14:54:39+0000</time>

<time>2011-11-12T14:54:39.929+0000</time>

<time>2011-11-12T14:54+00:00</time>

<time>2011-11-12T14:54:39+00:00</time>

<time>2011-11-12T14:54:39.929+00:00</time>

<time>2011-11-12T06:54-0800</time>

<time>2011-11-12T06:54:39-0800</time>

<time>2011-11-12T06:54:39.929-0800</time>

<time>2011-11-12T06:54-08:00</time>

<time>2011-11-12T06:54:39-08:00</time>

<time>2011-11-12T06:54:39.929-08:00</time>

<time>2011-11-12 14:54Z</time>

<time>2011-11-12 14:54:39Z</time>

<time>2011-11-12 14:54:39.929Z</time>

<time>2011-11-12 14:54+0000</time>

<time>2011-11-12 14:54:39+0000</time>

<time>2011-11-12 14:54:39.929+0000</time>

<time>2011-11-12 14:54+00:00</time>

<time>2011-11-12 14:54:39+00:00</time>

<time>2011-11-12 14:54:39.929+00:00</time>

<time>2011-11-12 06:54-0800</time>

<time>2011-11-12 06:54:39-0800</time>

<time>2011-11-12 06:54:39.929-0800</time>

<time>2011-11-12 06:54-08:00</time>

<time>2011-11-12 06:54:39-08:00</time>

<time>2011-11-12 06:54:39.929-08:00</time>

Times with dates and a time zone offset are useful for specifying specific events, or recurring virtual events where the time is not anchored to a specific geographic location. For example, the precise time of an asteroid impact, or a particular meeting in a series of meetings held at 1400 UTC every day, regardless of whether any particular part of the world is observing daylight savings time or not. For events where the precise time varies by the local time zone offset of a specific geographic location, a valid floating date and time string combined with that geographic location is likely more useful.

A valid week string

<time>2011-W46</time>

Four or more ASCII digits, at least one of which is not "0" (U+0030)

<time>2011</time>

<time>0001</time>

A valid duration string

<time>PT4H18M3S</time>

<time>4h 18m 3s</time>

Many of the preceding valid syntaxes describe "floating" date and/or time values (they do not include a time-zone offset). Care is needed when converting floating time values to or from global ("incremental") time values (e.g., JavaScript's Date object). In many cases, an implicit time-of-day and time zone are used in the conversion and may result in unexpected changes to the value of the date itself. [TIMEZONES]

The machine-readable equivalent of the element's contents must be obtained from the element's datetime value by using the following algorithm:

If parsing a month string from the element's datetime value returns a month, that is the machine-readable equivalent; abort these steps.
If parsing a date string from the element's datetime value returns a date, that is the machine-readable equivalent; abort these steps.
If parsing a yearless date string from the element's datetime value returns a yearless date, that is the machine-readable equivalent; abort these steps.
If parsing a time string from the element's datetime value returns a time, that is the machine-readable equivalent; abort these steps.
If parsing a floating date and time string from the element's datetime value returns a floating date and time, that is the machine-readable equivalent; abort these steps.
If parsing a time-zone offset string from the element's datetime value returns a time-zone offset, that is the machine-readable equivalent; abort these steps.
If parsing a global date and time string from the element's datetime value returns a global date and time, that is the machine-readable equivalent; abort these steps.
If parsing a week string from the element's datetime value returns a week, that is the machine-readable equivalent; abort these steps.
If the element's datetime value consists of only ASCII digits, at least one of which is not "0" (U+0030), then the machine-readable equivalent is the base-ten interpretation of those digits, representing a year; abort these steps.
If parsing a duration string from the element's datetime value returns a duration, that is the machine-readable equivalent; abort these steps.
There is no machine-readable equivalent.

The algorithms referenced above are intended to be designed such that for any arbitrary string s, only one of the algorithms returns a value. A more efficient approach might be to create a single algorithm that parses all these data types in one pass; developing such an algorithm is left as an exercise to the reader.

The dateTime IDL attribute must reflect the element's datetime content attribute.

The time element can be used to encode dates, for example in microformats. The following shows a hypothetical way of encoding an event using a variant on hCalendar that uses the time element:

<div class="vevent">
 <a class="url" href="http://www.web2con.com/">http://www.web2con.com/</a>
  <span class="summary">Web 2.0 Conference</span>:
  <time class="dtstart" datetime="2005-10-05">October 5</time> -
  <time class="dtend" datetime="2005-10-07">7</time>,
  at the <span class="location">Argent Hotel, San Francisco, CA</span>
 </div>

Here, a fictional microdata vocabulary based on the Atom vocabulary is used with the time element to mark up a blog post's publication date.

<article itemscope itemtype="http://n.example.org/rfc4287">
 <h1 itemprop="title">Big tasks</h1>
 <footer>Published <time itemprop="published" datetime="2009-08-29">two days ago</time>.</footer>
 <p itemprop="content">Today, I went out and bought a bike for my kid.</p>
</article>

In this example, another article's publication date is marked up using time, this time using the schema.org microdata vocabulary:

<article itemscope itemtype="http://schema.org/BlogPosting">
 <h1 itemprop="headline">Small tasks</h1>
 <footer>Published <time itemprop="datePublished" datetime="2009-08-30">yesterday</time>.</footer>
 <p itemprop="articleBody">I put a bike bell on his bike.</p>
</article>

In the following snippet, the time element is used to encode a date in the ISO8601 format, for later processing by a script:

<p>Our first date was <time datetime="2006-09-23">a Saturday</time>.</p>

In this second snippet, the value includes a time:

<p>We stopped talking at <time datetime="2006-09-24T05:00-07:00">5am the next morning</time>.</p>

A script loaded by the page (and thus privy to the page's internal convention of marking up dates and times using the time element) could scan through the page and look at all the time elements therein to create an index of dates and times.

For example, this element conveys the string "Tuesday" with the additional semantic that the 12th of November 2011 is the meaning that corresponds to "Tuesday":

Today is <time datetime="2011-11-12">Tuesday</time>.

In this example, a specific time in the Pacific Standard Time timezone is specified:

Your next meeting is at <time datetime="2011-11-12T15:00-08:00">3pm</time>.

4.5.12 The `code` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The code element represents a fragment of computer code. This could be an XML element name, a file name, a computer program, or any other string that a computer would recognize.

There is no formal way to indicate the language of computer code being marked up. Authors who wish to mark code elements with the language used, e.g. so that syntax highlighting scripts can use the right rules, can use the class attribute, e.g. by adding a class prefixed with "language-" to the element.

The following example shows how the element can be used in a paragraph to mark up element names and computer code, including punctuation.

<p>The <code>code</code> element represents a fragment of computer
code.</p>

<p>When you call the <code>activate()</code> method on the
<code>robotSnowman</code> object, the eyes glow.</p>

<p>The example below uses the <code>begin</code> keyword to indicate
the start of a statement block. It is paired with an <code>end</code>
keyword, which is followed by the <code>.</code> punctuation character
(full stop) to indicate the end of the program.</p>

The following example shows how a block of code could be marked up using the pre and code elements.

<pre><code class="language-pascal">var i: Integer;
begin
   i := 1;
end.</code></pre>

A class is used in that example to indicate the language used.

See the pre element for more details.

4.5.13 The `var` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The var element represents a variable. This could be an actual variable in a mathematical expression or programming context, an identifier representing a constant, a symbol identifying a physical quantity, a function parameter, or just be a term used as a placeholder in prose.

In the paragraph below, the letter "n" is being used as a variable in prose:

<p>If there are <var>n</var> pipes leading to the ice
cream factory then I expect at <em>least</em> <var>n</var>
flavors of ice cream to be available for purchase!</p>

For mathematics, in particular for anything beyond the simplest of expressions, MathML is more appropriate. However, the var element can still be used to refer to specific variables that are then mentioned in MathML expressions.

In this example, an equation is shown, with a legend that references the variables in the equation. The expression itself is marked up with MathML, but the variables are mentioned in the figure's legend using var.

<figure>
 <math>
  <mi>a</mi>
  <mo>=</mo>
  <msqrt>
   <msup><mi>b</mi><mn>2</mn></msup>
   <mi>+</mi>
   <msup><mi>c</mi><mn>2</mn></msup>
  </msqrt>
 </math>
 <figcaption>
  Using Pythagoras' theorem to solve for the hypotenuse <var>a</var> of
  a triangle with sides <var>b</var> and <var>c</var>
 </figcaption>
</figure>

Here, the equation describing mass-energy equivalence is used in a sentence, and the var element is used to mark the variables and constants in that equation:

<p>Then he turned to the blackboard and picked up the chalk. After a few moment's
thought, he wrote <var>E</var> = <var>m</var> <var>c</var><sup>2</sup>. The teacher
looked pleased.</p>

4.5.14 The `samp` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The samp element represents (sample) output from a program or computing system.

See the pre and kbd elements for more details.

This example shows the samp element being used inline:

<p>The computer said <samp>Too much cheese in tray
two</samp> but I didn't know what that meant.</p>

This second example shows a block of sample output. Nested samp and kbd elements allow for the styling of specific elements of the sample output using a style sheet. There's also a few parts of the samp that are annotated with even more detailed markup, to enable very precise styling. To achieve this, span elements are used.

<pre><samp><span class="prompt">jdoe@mowmow:~$</span> <kbd>ssh demo.example.com</kbd>
Last login: Tue Apr 12 09:10:17 2005 from mowmow.example.com on pts/1
Linux demo 2.6.10-grsec+gg3+e+fhs6b+nfs+gr0501+++p3+c4a+gr2b-reslog-v6.189 #1 SMP Tue Feb 1 11:22:36 PST 2005 i686 unknown

<span class="prompt">jdoe@demo:~$</span> <span class="cursor">_</span></samp></pre>

4.5.15 The `kbd` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The kbd element represents user input (typically keyboard input, although it may also be used to represent other input, such as voice commands).

When the kbd element is nested inside a samp element, it represents the input as it was echoed by the system.

When the kbd element contains a samp element, it represents input based on system output, for example invoking a menu item.

When the kbd element is nested inside another kbd element, it represents an actual key or other single unit of input as appropriate for the input mechanism.

Here the kbd element is used to indicate keys to press:

<p>To make George eat an apple, press <kbd><kbd>Shift</kbd>+<kbd>F3</kbd></kbd></p>

In this second example, the user is told to pick a particular menu item. The outer kbd element marks up a block of input, with the inner kbd elements representing each individual step of the input, and the samp elements inside them indicating that the steps are input based on something being displayed by the system, in this case menu labels:

<p>To make George eat an apple, select
    <kbd><kbd><samp>File</samp></kbd>|<kbd><samp>Eat Apple...</samp></kbd></kbd>
</p>

Such precision isn't necessary; the following is equally fine:

<p>To make George eat an apple, select <kbd>File | Eat Apple...</kbd></p>

4.5.16 The `sub` and `sup` elements

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Use HTMLElement.

The sup element represents a superscript and the sub element represents a subscript.

These elements must be used only to mark up typographical conventions with specific meanings, not for typographical presentation for presentation's sake. For example, it would be inappropriate for the sub and sup elements to be used in the name of the LaTeX document preparation system. In general, authors should use these elements only if the absence of those elements would change the meaning of the content.

In certain languages, superscripts are part of the typographical conventions for some abbreviations.

<p>The most beautiful women are
<span lang="fr"><abbr>M<sup>lle</sup></abbr> Gwendoline</span> and
<span lang="fr"><abbr>M<sup>me</sup></abbr> Denise</span>.</p>

The sub element can be used inside a var element, for variables that have subscripts.

Here, the sub element is used to represent the subscript that identifies the variable in a family of variables:

<p>The coordinate of the <var>i</var>th point is
(<var>x<sub><var>i</var></sub></var>, <var>y<sub><var>i</var></sub></var>).
For example, the 10th point has coordinate
(<var>x<sub>10</sub></var>, <var>y<sub>10</sub></var>).</p>

Mathematical expressions often use subscripts and superscripts. Authors are encouraged to use MathML for marking up mathematics, but authors may opt to use sub and sup if detailed mathematical markup is not desired. [MATHML]

<var>E</var>=<var>m</var><var>c</var><sup>2</sup>

f(<var>x</var>, <var>n</var>) = log<sub>4</sub><var>x</var><sup><var>n</var></sup>

4.5.17 The `i` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The i element represents a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, transliteration, a thought, or a ship name in Western texts.

Terms in languages different from the main text should be annotated with lang attributes (or, in XML, lang attributes in the XML namespace).

The examples below show uses of the i element:

<p>The <i class="taxonomy">Felis silvestris catus</i> is cute.</p>
<p>The term <i>prose content</i> is defined above.</p>
<p>There is a certain <i lang="fr">je ne sais quoi</i> in the air.</p>

In the following example, a dream sequence is marked up using i elements.

<p>Raymond tried to sleep.</p>
<p><i>The ship sailed away on Thursday</i>, he
dreamt. <i>The ship had many people aboard, including a beautiful
princess called Carey. He watched her, day-in, day-out, hoping she
would notice him, but she never did.</i></p>
<p><i>Finally one night he picked up the courage to speak with
her—</i></p>
<p>Raymond woke with a start as the fire alarm rang out.</p>

Authors can use the class attribute on the i element to identify why the element is being used, so that if the style of a particular use (e.g. dream sequences as opposed to taxonomic terms) is to be changed at a later date, the author doesn't have to go through the entire document (or series of related documents) annotating each use.

Authors are encouraged to consider whether other elements might be more applicable than the i element, for instance the em element for marking up stress emphasis, or the dfn element to mark up the defining instance of a term.

Style sheets can be used to format i elements, just like any other element can be restyled. Thus, it is not the case that content in i elements will necessarily be italicized.

4.5.18 The `b` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The b element represents a span of text to which attention is being drawn for utilitarian purposes without conveying any extra importance and with no implication of an alternate voice or mood, such as key words in a document abstract, product names in a review, actionable words in interactive text-driven software, or an article lede.

The following example shows a use of the b element to highlight key words without marking them up as important:

<p>The <b>frobonitor</b> and <b>barbinator</b> components are fried.</p>

In the following example, objects in a text adventure are highlighted as being special by use of the b element.

<p>You enter a small room. Your <b>sword</b> glows
brighter. A <b>rat</b> scurries past the corner wall.</p>

Another case where the b element is appropriate is in marking up the lede (or lead) sentence or paragraph. The following example shows how a BBC article about kittens adopting a rabbit as their own could be marked up:

<article>
 <h2>Kittens 'adopted' by pet rabbit</h2>
 <p><b class="lede">Six abandoned kittens have found an
 unexpected new mother figure — a pet rabbit.</b></p>
 <p>Veterinary nurse Melanie Humble took the three-week-old
 kittens to her Aberdeen home.</p>
[...]

As with the i element, authors can use the class attribute on the b element to identify why the element is being used, so that if the style of a particular use is to be changed at a later date, the author doesn't have to go through annotating each use.

The b element should be used as a last resort when no other element is more appropriate. In particular, headings should use the h1 to h6 elements, stress emphasis should use the em element, importance should be denoted with the strong element, and text marked or highlighted should use the mark element.

The following would be incorrect usage:

<p><b>WARNING!</b> Do not frob the barbinator!</p>

In the previous example, the correct element to use would have been strong, not b.

Style sheets can be used to format b elements, just like any other element can be restyled. Thus, it is not the case that content in b elements will necessarily be boldened.

4.5.19 The `u` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The u element represents a span of text with an unarticulated, though explicitly rendered, non-textual annotation, such as labeling the text as being a proper name in Chinese text (a Chinese proper name mark), or labeling the text as being misspelt.

In most cases, another element is likely to be more appropriate: for marking stress emphasis, the em element should be used; for marking key words or phrases either the b element or the mark element should be used, depending on the context; for marking book titles, the cite element should be used; for labeling text with explicit textual annotations, the ruby element should be used; for labeling ship names in Western texts, the i element should be used.

The default rendering of the u element in visual presentations clashes with the conventional rendering of hyperlinks (underlining). Authors are encouraged to avoid using the u element where it could be confused for a hyperlink.

4.5.20 The `mark` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The mark element represents a run of text in one document marked or highlighted for reference purposes, due to its relevance in another context. When used in a quotation or other block of text referred to from the prose, it indicates a highlight that was not originally present but which has been added to bring the reader's attention to a part of the text that might not have been considered important by the original author when the block was originally written, but which is now under previously unexpected scrutiny. When used in the main prose of a document, it indicates a part of the document that has been highlighted due to its likely relevance to the user's current activity.

This example shows how the mark element can be used to bring attention to a particular part of a quotation:

<p lang="en-US">Consider the following quote:</p>
<blockquote lang="en-GB">
 <p>Look around and you will find, no-one's really
 <mark>colour</mark> blind.</p>
</blockquote>
<p lang="en-US">As we can tell from the <em>spelling</em> of the word,
the person writing this quote is clearly not American.</p>

(If the goal was to mark the element as misspelt, however, the u element, possibly with a class, would be more appropriate.)

Another example of the mark element is highlighting parts of a document that are matching some search string. If someone looked at a document, and the server knew that the user was searching for the word "kitten", then the server might return the document with one paragraph modified as follows:

<p>I also have some <mark>kitten</mark>s who are visiting me
these days. They're really cute. I think they like my garden! Maybe I
should adopt a <mark>kitten</mark>.</p>

In the following snippet, a paragraph of text refers to a specific part of a code fragment.

<p>The highlighted part below is where the error lies:</p>
<pre><code>var i: Integer;
begin
   i := <mark>1.1</mark>;
end.</code></pre>

This is separate from syntax highlighting, for which span is more appropriate. Combining both, one would get:

<p>The highlighted part below is where the error lies:</p>
<pre><code><span class=keyword>var</span> <span class=ident>i</span>: <span class=type>Integer</span>;
<span class=keyword>begin</span>
   <span class=ident>i</span> := <span class=literal><mark>1.1</mark></span>;
<span class=keyword>end</span>.</code></pre>

This is another example showing the use of mark to highlight a part of quoted text that was originally not emphasized. In this example, common typographic conventions have led the author to explicitly style mark elements in quotes to render in italics.

<article>
 <style>
  blockquote mark, q mark {
    font: inherit; font-style: italic;
    text-decoration: none;
    background: transparent; color: inherit;
  }
  .bubble em {
    font: inherit; font-size: larger;
    text-decoration: underline;
  }
 </style>
 <h1>She knew</h1>
 <p>Did you notice the subtle joke in the joke on panel 4?</p>
 <blockquote>
  <p class="bubble">I didn't <em>want</em> to believe. <mark>Of course
  on some level I realized it was a known-plaintext attack.</mark> But I
  couldn't admit it until I saw for myself.</p>
 </blockquote>
 <p>(Emphasis mine.) I thought that was great. It's so pedantic, yet it
 explains everything neatly.</p>
</article>

Note, incidentally, the distinction between the em element in this example, which is part of the original text being quoted, and the mark element, which is highlighting a part for comment.

The following example shows the difference between denoting the importance of a span of text (strong) as opposed to denoting the relevance of a span of text (mark). It is an extract from a textbook, where the extract has had the parts relevant to the exam highlighted. The safety warnings, important though they may be, are apparently not relevant to the exam.

<h3>Wormhole Physics Introduction</h3>

<p><mark>A wormhole in normal conditions can be held open for a
maximum of just under 39 minutes.</mark> Conditions that can increase
the time include a powerful energy source coupled to one or both of
the gates connecting the wormhole, and a large gravity well (such as a
black hole).</p>

<p><mark>Momentum is preserved across the wormhole. Electromagnetic
radiation can travel in both directions through a wormhole,
but matter cannot.</mark></p>

<p>When a wormhole is created, a vortex normally forms.
<strong>Warning: The vortex caused by the wormhole opening will
annihilate anything in its path.</strong> Vortexes can be avoided when
using sufficiently advanced dialing technology.</p>

<p><mark>An obstruction in a gate will prevent it from accepting a
wormhole connection.</mark></p>

4.5.21 The `ruby` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: See prose.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The ruby element allows one or more spans of phrasing content to be marked with ruby annotations. Ruby annotations are short runs of text presented alongside base text, primarily used in East Asian typography as a guide for pronunciation or to include other annotations. In Japanese, this form of typography is also known as furigana. Ruby text can appear on either side, and sometimes both sides, of the base text, and it is possible to control its position using CSS. A more complete introduction to ruby can be found in the Use Cases & Exploratory Approaches for Ruby Markup document as well as in CSS Ruby Module Level 1. [RUBY-UC] [CSSRUBY]

The content model of ruby elements consists of one or more of the following sequences:

One or more phrasing content nodes or rb elements.
One or more rt or rtc elements, each of which either immediately preceded or followed by an rp elements.

The ruby, rb, rtc, and rt elements can be used for a variety of kinds of annotations, including in particular (though by no means limited to) those described below. For more details on Japanese Ruby in particular, and how to render Ruby for Japanese, see Requirements for Japanese Text Layout. [JLREQ] The rp element can be used as fallback content when ruby rendering is not supported.

Mono-ruby for individual base characters

Annotations (the ruby text) are associated individually with each ideographic character (the base text). In Japanese this is typically hiragana or katakana characters used to provide readings of kanji characters.

<ruby>base<rt>annotation</ruby>

When no rb element is used, the base is implied, as above. But you can also make it explicit. This can be useful notably for styling, or when consecutive bases are to be treated as a group, as in the jukugo ruby example further down.

<ruby><rb>base<rt>annotation</ruby>

In the following example, notice how each annotation corresponds to a single base character.

<ruby>日<rt>に</rt></ruby><ruby>本<rt>ほん</rt></ruby>
<ruby>語<rt>ご</rt></ruby>で<ruby>書<rt>か</rt></ruby>
いた<ruby>作<rt>さく</rt></ruby><ruby>文<rt>ぶん</rt></ruby>です。

Ruby text interspersed in regular text provides structure akin to the following image:

An example of ruby text mixed up with regular text.

This example can also be written as follows, using one ruby element with two segments of base text and two annotations (one for each) rather than two back-to-back ruby elements each with one base text segment and annotation (as in the markup above):

<ruby>日<rt>に</rt>本<rt>ほん</rt>語<rt>ご</rt></ruby>
で<ruby>書<rt>か</rt></ruby>
いた<ruby>作<rt>さく</rt>文<rt>ぶん</rt></ruby>です。

Group ruby

Group ruby is often used where phonetic annotations don't map to discreet base characters, or for semantic glosses that span the whole base text. For example, the word "today" is written with the characters 今日, literally "this day". But it's pronounced きょう (kyou), which can't be broken down into a "this" part and a "day" part. In typical rendering, you can't split text that is annotated with group ruby; it has to wrap as a single unit onto the next line. When a ruby text annotation maps to a base that is comprised of more than one character, then that base is grouped.

The following group ruby:

Group ruby example with きょう annotating 今日

Can be marked up as follows:

<ruby>今日<rt>きょう</ruby>

Jukugo ruby

Jukugo refers to a Japanese compound noun, i.e. a word made up of more than one kanji character. Jukugo ruby is a term that is used not to describe ruby annotations over jukugo text, but rather to describe ruby with a behaviour slightly different from mono or group ruby. Jukugo ruby is similar to mono ruby, in that there is a strong association between ruby text and individual base characters, but the ruby text is typically rendered as grouped together over multiple ideographs when they are on the same line.

The distinction is captured in this example:

Example of jukugo ruby

Which can be marked up as follows:

<ruby>法<rb>華<rb>経<rt>ほ<rt>け<rt>きょう</ruby>

In this example, each rt element is paired with its respective rb element, the difference with an interleaved rb/rt approach being that the sequences of both base text and ruby annotations are implicitly placed in common containers so that the grouping information is captured.

For more details on Jukugo Ruby rendering, see Appendix F in the Requirements for Japanese Text Layout and Use Case C: Jukugo ruby in the Use Cases & Exploratory Approaches for Ruby Markup. [JLREQ] [RUBY-UC]

Inline ruby

In some contexts, for instance when the font size or line height are too small for ruby to be readable, it is desirable to inline the ruby annotation such that it appears in parentheses after the text it annotates. This also provides a convenient fallback strategy for user agents that do not support rendering ruby annotations.

Inlining takes grouping into account. For example, Tokyo is written with two kanji characters, 東, which is pronounced とう, and 京, which is pronounced きょう. Each base character should be annotated individually, but the fallback should be 東京(とうきょう) not 東(とう)京(きょう). This can be marked up as follows:

<ruby>東<rb>京<rt>とう<rt>きょう</ruby>

Note that the above markup will enable the usage of parentheses when inlining for browsers that support ruby layout, but for those that don't it will fail to provide parenthetical fallback. This is where the rp element is useful. It can be inserted into the above example to provide the appropriate fallback when ruby layout is not supported:

<ruby>東<rb>京<rp>(<rt>とう<rt>きょう<rp>)</ruby>

Text with both phonetic and semantic annotations (double-sided ruby)

Sometimes, ruby can be used to annotate a base twice.

In the following example, the Chinese word for San Francisco (旧金山, i.e. “old gold mountain”) is annotated both using pinyin to give the pronunciation, and with the original English.

San Francisco in Chinese, with both pinyin and the original English as annotations.

Which is marked up as follows:

<ruby><rb>旧<rb>金<rb>山<rt>jiù<rt>jīn<rt>shān<rtc>San Francisco</ruby>

In this example, a single base run of three base characters is annotated with three pinyin ruby text segments in a first (implicit) container, and an rtc element is introduced in order to provide a second single ruby text annotation being the city's English name.

We can also revisit our jukugo example above with 上手 ("skill") to show how it can be annotation in both kana and romaji phonetics while at the same time maintaining the pairing to bases and annotation grouping information.

上手 ("skill") annotated in both kana and romaji, shown in both jukugo and mono styles.

Which is marked up as follows:

<ruby><rb>上<rb>手<rt>じよう<rt>ず<rtc><rt>jou<rt>zu</ruby>

Text that is a direct child of the rtc element implicitly produces a ruby text segment as if it were contained in an rt element. In this contrived example, this is shown with some symbols that are given names in English and French with annotations intended to appear on either side of the base symbol.

<ruby>
  ♥<rt>Heart<rtc lang=fr>Cœur</rtc>
  ☘<rt>Shamrock<rtc lang=fr>Trèfle</rtc>
  ✶<rt>Star<rtc lang=fr>Étoile
</ruby>

Similarly, text directly inside a ruby element implicitly produces a ruby base as if it were contained in an rb element, and rt children of ruby are implicitly contained in an rtc container. In effect, the above example is equivalent (in meaning, though not in the DOM it produces) to the following:

<ruby>
  <rb>♥</rb><rtc><rt>Heart</rt></rtc><rtc lang=fr><rt>Cœur</rt></rtc>
  <rb>☘</rb><rtc><rt>Shamrock</rt></rtc><rtc lang=fr><rt>Trèfle</rt></rtc>
  <rb>✶</rb><rtc><rt>Star</rt></rtc><rtc lang=fr><rt>Étoile</rt></rtc>
</ruby>

Within a ruby element, content is parcelled into a series of ruby segments. Each ruby segment is described by:

Zero or more ruby bases, each of which is a DOM range that may contain phrasing content or an rb element.
A base range, that is a DOM range including all the bases. This is the ruby base container.
Zero or more ruby text containers which may correspond to explicit rtc elements, or to sequences of rt elements implicitly recognised as contained in an anonymous ruby text container.

Each ruby text container is described by zero or more ruby text annotations each of which is a DOM range that may contain phrasing content or an rt element, and an annotations range that is a range including all the annotations for that container. A ruby text container is also known (primarily in a CSS context) as a ruby annotation container.

Furthermore, a ruby element contains ignored ruby content. Ignored ruby content does not form part of the document's semantics. It consists of some inter-element whitespace and rp elements, the latter of which are used for legacy user agents that do not support ruby at all.

The process of annotation pairing associates ruby annotations with ruby bases. Within each ruby segment, each ruby base in the ruby base container is paired with one ruby text annotation from the ruby text container, in order. If there are not enough ruby text annotations in a ruby annotation container, the last one is associated with any excess ruby bases. (If there are not any in the ruby annotation container, an anonymous empty one is assumed to exist.) If there are not enough ruby bases, any remaining ruby text annotations are assumed to be associated with empty, anonymous bases inserted at the end of the ruby base container.

Note that the terms ruby segment, ruby base, ruby text annotation, ruby text container, ruby base container, and ruby annotation container have their equivalents in CSS Ruby Module Level 1. [CSSRUBY]

Informally, the segmentation and categorisation algorithm below performs a simple set of tasks. First it processes adjacent rb elements, text nodes, and non-ruby elements into a list of bases. Then it processes any number of rtc elements or sequences of rt elements that are considered to automatically map to an anonymous ruby text container. Put together these data items form a ruby segment as detailed in the data model above. It will continue to produce such segments until it reaches the end of the content of a given ruby element. The complexity of the algorithm below compared to this informal description stems from the need to support an author-friendly syntax and being mindful of inter-element white space.

At any particular time, the segmentation and categorisation of content of a ruby element is the result that would be obtained from running the following algorithm:

Let root be the ruby element for which the algorithm is being run.
Let index be 0.
Let ruby segments be an empty list.
Let current bases be an empty list of DOM ranges.
Let current bases range be null.
Let current bases range start be null.
Let current annotations be an empty list of DOM ranges.
Let current annotations range be null.
Let current annotations range start be null.
Let current annotation containers be an empty list.
Let current automatic base nodes be an empty list of DOM Nodes.
Let current automatic base range start be null.
Process a ruby child: If index is equal to or greater than the number of child nodes in root, then run the steps to commit a ruby segment, return ruby segments, and abort these steps.
Let current child be the indexth node in root.
If current child is not a Text node and is not an Element node, then increment index by one and jump to the step labelled process a ruby child.
If current child is an rp element, then increment index by one and jump to the step labelled process a ruby child. (Note that this has the effect of including this element in any range that we are currently processing. This is done intentionally so that misplaced rp can be processed correctly; semantically they are ignored all the same.)
If current child is an rt element, then run these substeps:
1. Run the steps to commit an automatic base.
2. Run the steps to commit the base range.
3. If current annotations is empty, set current annotations range start to the value of index.
4. Create a new DOM range whose start is the boundary point (root, index) and whose end is the boundary point (root, index plus one), and append it at the end of current annotations.
5. Increment index by one and jump to the step labelled process a ruby child.
If current child is an rtc element, then run these substeps:
1. Run the steps to commit an automatic base.
2. Run the steps to commit the base range.
3. Run the steps to commit current annotations.
4. Create a new ruby annotation container. It is described by the list of annotations returned by running the steps to process an rtc element and a DOM range whose start is the boundary point (root, index) and whose end is the boundary point (root, index plus one). Append this new ruby annotation container at the end of current annotation containers.
5. Increment index by one and jump to the step labelled process a ruby child.
If current child is a Text node and is inter-element whitespace, then run these substeps:
1. If current annotations is not empty, increment index by one and jump to the step labelled process a ruby child.
2. Run the following substeps:
  1. Let lookahead index be set to the value of index.
  2. Peek ahead: Increment lookahead index by one.
  3. If lookahead index is equal to or greater than the number of child nodes in root, then abort these substeps.
  4. Let peek child be the lookahead indexth node in root.
  5. If peek child is a Text node and is inter-element whitespace, then jump to the step labelled peek ahead.
  6. If peek child is an rt element, an rtc element, or an rp element, then set index to the value of lookahead index and jump to the step labelled process a ruby child.
If current annotations is not empty or if current annotation containers is not empty, then run the steps to commit a ruby segment.
If current child is an rb element, then run these substeps:
1. Run the steps to commit an automatic base.
2. If current bases is empty, then set current bases range start to the value of index.
3. Create a new DOM range whose start is the boundary point (root, index) and whose end is the boundary point (root, index plus one), and append it at the end of current bases.
4. Increment index by one and jump to the step labelled process a ruby child.
If current automatic base nodes is empty, set current automatic base range start to the value of index.
Append current child at the end of current automatic base nodes.
Increment index by one and jump to the step labelled process a ruby child.

When the steps above say to commit a ruby segment, it means to run the following steps at that point in the algorithm:

Run the steps to commit an automatic base.
If current bases, current annotations, and current annotation containers are all empty, abort these steps.
Run the steps to commit the base range.
Run the steps to commit current annotations.
Create a new ruby segment. It is described by a list of bases set to current bases, a base DOM range set to current bases range, and a list of ruby annotation containers that are the current annotation containers list. Append this new ruby segment at the end of ruby segments.
Let current bases be an empty list.
Let current bases range be null.
Let current bases range start be null.
Let current annotation containers be an empty list.

When the steps above say to commit the base range, it means to run the following steps at that point in the algorithm:

If current bases is empty, abort these steps.
If current bases range is not null, abort these steps.
Let current bases range be a DOM range whose start is the boundary point (root, current bases range start) and whose end is the boundary point (root, index).

When the steps above say to commit current annotations, it means to run the following steps at that point in the algorithm:

If current annotations is not empty and current annotations range is null let current annotations range be a DOM range whose start is the boundary point (root, current annotations range start) and whose end is the boundary point (root, index).
If current annotations is not empty, create a new ruby annotation container. It is described by an annotations list set to current annotations and a range set to current annotations range. Append this new ruby annotation container at the end of current annotation containers.
Let current annotations be an empty list of DOM ranges.
Let current annotations range be null.
Let current annotations range start be null.

When the steps above say to commit an automatic base, it means to run the following steps at that point in the algorithm:

If current automatic base nodes is empty, abort these steps.
If current automatic base nodes contains nodes that are not Text nodes, or Text nodes that are not inter-element whitespace, then run these substeps:
1. It current bases is empty, set current bases range start to the value of current automatic base range start.
2. Create a new DOM range whose start is the boundary point (root, current automatic base range start) and whose end is the boundary point (root, index), and append it at the end of current bases.
Let current automatic base nodes be an empty list of DOM Nodes.
Let current automatic base range start be null.

4.5.22 The `rb` element

Categories:: None.
Contexts in which this element can be used:: As a child of a ruby element.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: An rb element's end tag may be omitted if the rb element is immediately followed by an rb, rt, rtc or rp element, or if there is no more content in the parent element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The rb element marks the base text component of a ruby annotation. When it is the child of a ruby element, it doesn't represent anything itself, but its parent ruby element uses it as part of determining what it represents.

An rb element that is not a child of a ruby element represents the same thing as its children.

4.5.23 The `rt` element

Categories:: None.
Contexts in which this element can be used:: As a child of a ruby or of an rtc element.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: An rt element's end tag may be omitted if the rt element is immediately followed by an rb, rt, rtc or rp element, or if there is no more content in the parent element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The rt element marks the ruby text component of a ruby annotation. When it is the child of a ruby element or of an rtc element that is itself the child of a ruby element, it doesn't represent anything itself, but its ancestor ruby element uses it as part of determining what it represents.

An rt element that is not a child of a ruby element or of an rtc element that is itself the child of a ruby element represents the same thing as its children.

4.5.24 The `rtc` element

Categories:: None.
Contexts in which this element can be used:: As a child of a ruby element.
Content model:: Phrasing content or rt elements.
Content attributes:: Global attributes
Tag omission in text/html:: An rtc element's end tag may be omitted if the rtc element is immediately followed by an rb, rtc or rp element, or if there is no more content in the parent element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The rtc element marks a ruby text container for ruby text components in a ruby annotation. When it is the child of a ruby element it doesn't represent anything itself, but its parent ruby element uses it as part of determining what it represents.

An rtc element that is not a child of a ruby element represents the same thing as its children.

When an rtc element is processed as part of the segmentation and categorisation of content for a ruby element, the following algorithm defines how to process an rtc element:

Let root be the rtc element for which the algorithm is being run.
Let index be 0.
Let annotations be an empty list of DOM ranges.
Let current automatic annotation nodes be an empty list of DOM nodes.
Let current automatic annotation range start be null.
Process an rtc child: If index is equal to or greater than the number of child nodes in root, then run the steps to commit an automatic annotation, return annotations, and abort these steps.
Let current child be the indexth node in root.
If current child is an rt element, then run these substeps:
1. Run the steps to commit an automatic annotation.
2. Create a new DOM range whose start is the boundary point (root, index) and whose end is the boundary point (root, index plus one), and append it at the end of annotations.
3. Increment index by one and jump to the step labelled process an rtc child.
If current automatic annotation nodes is empty, set current automatic annotation range start to the value of index.
Append current child at the end of current automatic annotation nodes.
Increment index by one and jump to the step labelled process an rtc child.

When the steps above say to commit an automatic annotation, it means to run the following steps at that point in the algorithm:

If current automatic annotation nodes is empty, abort these steps.
If current automatic annotation nodes contains nodes that are not Text nodes, or Text nodes that are not inter-element whitespace, then create a new DOM range whose start is the boundary point (root, current automatic annotation range start) and whose end is the boundary point (root, index), and append it at the end of annotations.
Let current automatic annotation nodes be an empty list of DOM nodes.
Let current automatic annotation range start be null.

4.5.25 The `rp` element

Categories:: None.
Contexts in which this element can be used:: As a child of a ruby element, either immediately before or immediately after an rt or rtc element, but not between rt elements.
Content model:: Phrasing content.
Content attributes:: Global attributes
Tag omission in text/html:: An rp element's end tag may be omitted if the rp element is immediately followed by an rb, rt, rtc or rp element, or if there is no more content in the parent element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The rp element is used to provide fallback text to be shown by user agents that don't support ruby annotations. One widespread convention is to provide parentheses around the ruby text component of a ruby annotation.

The contents of the rp elements are typically not displayed by user agents which do support ruby annotations

An rp element that is a child of a ruby element represents nothing. An rp element whose parent element is not a ruby element represents its children.

The example shown previously, in which each ideograph in the text 漢字 is annotated with its phonetic reading, could be expanded to use rp so that in legacy user agents the readings are in parentheses (please note that white space has been introduced into this example in order to make it more readable):

...
<ruby>
  漢
  <rb>字</rb>
  <rp> (</rp>
  <rt>かん</rt>
  <rt>じ</rt>
  <rp>) </rp>
</ruby>
...

In conforming user agents the rendering would be as above, but in user agents that do not support ruby, the rendering would be:

... 漢字 (かんじ) ...

When there are multiple annotations for a segment, rp elements can also be placed between the annotations. Here is another copy of an earlier contrived example showing some symbols with names given in English and French using double-sided annotations, but this time with rp elements as well:

<ruby>
  ♥<rp>: </rp><rt>Heart</rt><rp>, </rp><rtc><rt lang=fr>Cœur</rt></rtc><rp>.</rp>
  ☘<rp>: </rp><rt>Shamrock</rt><rp>, </rp><rtc><rt lang=fr>Trèfle</rt></rtc><rp>.</rp>
  ✶<rp>: </rp><rt>Star</rt><rp>, </rp><rtc><rt lang=fr>Étoile</rt></rtc><rp>.</rp>
</ruby>

This would make the example render as follows in non-ruby-capable user agents:

♥: Heart, Cœur.
☘: Shamrock, Trèfle.
✶: Star, Étoile.

4.5.26 The `bdi` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes; Also, the dir global attribute has special semantics on this element.
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The bdi element represents a span of text that is to be isolated from its surroundings for the purposes of bidirectional text formatting. [BIDI]

The dir global attribute defaults to auto on this element (it never inherits from the parent element like with other elements).

This element has rendering requirements involving the bidirectional algorithm.

This element is especially useful when embedding user-generated content with an unknown directionality.

In this example, usernames are shown along with the number of posts that the user has submitted. If the bdi element were not used, the username of the Arabic user would end up confusing the text (the bidirectional algorithm would put the colon and the number "3" next to the word "User" rather than next to the word "posts").

<ul>
 <li>User <bdi>jcranmer</bdi>: 12 posts.
 <li>User <bdi>hober</bdi>: 5 posts.
 <li>User <bdi>إيان</bdi>: 3 posts.
</ul>

4.5.27 The `bdo` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Phrasing content.
Content attributes:: Global attributes; Also, the dir global attribute has special semantics on this element.
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The bdo element represents explicit text directionality formatting control for its children. It allows authors to override the Unicode bidirectional algorithm by explicitly specifying a direction override. [BIDI]

Authors must specify the dir attribute on this element, with the value ltr to specify a left-to-right override and with the value rtl to specify a right-to-left override. The auto value must not be specified.

This element has rendering requirements involving the bidirectional algorithm.

4.5.28 The `span` element

Where phrasing content is expected.

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLSpanElement : HTMLElement {};

The span element doesn't mean anything on its own, but can be useful when used together with the global attributes, e.g. class, lang, or dir. It represents its children.

In this example, a code fragment is marked up using span elements and class attributes so that its keywords and identifiers can be color-coded from CSS:

<pre><code class="lang-c"><span class="keyword">for</span> (<span class="ident">j</span> = 0; <span class="ident">j</span> &lt; 256; <span class="ident">j</span>++) {
  <span class="ident">i_t3</span> = (<span class="ident">i_t3</span> & 0x1ffff) | (<span class="ident">j</span> &lt;&lt; 17);
  <span class="ident">i_t6</span> = (((((((<span class="ident">i_t3</span> >> 3) ^ <span class="ident">i_t3</span>) >> 1) ^ <span class="ident">i_t3</span>) >> 8) ^ <span class="ident">i_t3</span>) >> 5) & 0xff;
  <span class="keyword">if</span> (<span class="ident">i_t6</span> == <span class="ident">i_t1</span>)
    <span class="keyword">break</span>;
}</code></pre>

4.5.29 The `br` element

Where phrasing content is expected.

Empty.

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLBRElement : HTMLElement {};

The br element represents a line break.

While line breaks are usually represented in visual media by physically moving subsequent text to a new line, a style sheet or user agent would be equally justified in causing line breaks to be rendered in a different manner, for instance as green dots, or as extra spacing.

br elements must be used only for line breaks that are actually part of the content, as in poems or addresses.

The following example is correct usage of the br element:

<p>P. Sherman<br>
42 Wallaby Way<br>
Sydney</p>

br elements must not be used for separating thematic groups in a paragraph.

The following examples are non-conforming, as they abuse the br element:

<p><a ...>34 comments.</a><br>
<a ...>Add a comment.</a></p>

<p><label>Name: <input name="name"></label><br>
<label>Address: <input name="address"></label></p>

Here are alternatives to the above, which are correct:

<p><a ...>34 comments.</a></p>
<p><a ...>Add a comment.</a></p>

<p><label>Name: <input name="name"></label></p>
<p><label>Address: <input name="address"></label></p>

If a paragraph consists of nothing but a single br element, it represents a placeholder blank line (e.g. as in a template). Such blank lines must not be used for presentation purposes.

Any content inside br elements must not be considered part of the surrounding text.

This element has rendering requirements involving the bidirectional algorithm.

4.5.30 The `wbr` element

Categories:: Flow content.; Phrasing content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Empty.
Content attributes:: Global attributes
Tag omission in text/html:: No end tag
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses HTMLElement.

The wbr element represents a line break opportunity.

In the following example, someone is quoted as saying something which, for effect, is written as one long word. However, to ensure that the text can be wrapped in a readable fashion, the individual words in the quote are separated using a wbr element.

<p>So then he pointed at the tiger and screamed
"there<wbr>is<wbr>no<wbr>way<wbr>you<wbr>are<wbr>ever<wbr>going<wbr>to<wbr>catch<wbr>me"!</p>

Here, especially long lines of code in a program listing have suggested wrapping points given using wbr elements.

<pre>...
Heading heading = Helm.HeadingFactory(HeadingCoordinates[1], <wbr>HeadingCoordinates[2], <wbr>HeadingCoordinates[3], <wbr>HeadingCoordinates[4]);
Course course = Helm.CourseFactory(Heading, <wbr>Maps.MapFactoryFromHeading(heading), <wbr>Speeds.GetMaximumSpeed().ConvertToWarp());
...</pre>

Any content inside wbr elements must not be considered part of the surrounding text.

This element has rendering requirements involving the bidirectional algorithm.

4.5.31 Usage summary

This section is non-normative.

Element	Purpose	Example
`a`	Hyperlinks	Visit my <a href="drinks.html">drinks</a> page.
`em`	Stress emphasis	I must say I <em>adore</em> lemonade.
`strong`	Importance	This tea is <strong>very hot</strong>.
`small`	Side comments	These grapes are made into wine. <small>Alcohol is addictive.</small>
`s`	Inaccurate text	Price: <s>£4.50</s> £2.00!
`cite`	Titles of works	The case <cite>Hugo v. Danielle</cite> is relevant here.
`q`	Quotations	The judge said <q>You can drink water from the fish tank</q> but advised against it.
`dfn`	Defining instance	The term <dfn>organic food</dfn> refers to food produced without synthetic chemicals.
`abbr`	Abbreviations	Organic food in Ireland is certified by the <abbr title="Irish Organic Farmers and Growers Association">IOFGA</abbr>.
`data`	Machine-readable equivalent	Available starting today! <data value="UPC:022014640201">North Coast Organic Apple Cider</data>
`time`	Machine-readable equivalent of date- or time-related data	Available starting on <time datetime="2011-11-12">November 12th</time>!
`code`	Computer code	The <code>fruitdb</code> program can be used for tracking fruit production.
`var`	Variables	If there are <var>n</var> fruit in the bowl, at least <var>n</var>÷2 will be ripe.
`samp`	Computer output	The computer said <samp>Unknown error -3</samp>.
`kbd`	User input	Hit <kbd>F1</kbd> to continue.
`sub`	Subscripts	Water is H<sub>2</sub>O.
`sup`	Superscripts	The Hydrogen in heavy water is usually <sup>2</sup>H.
`i`	Alternative voice	Lemonade consists primarily of <i>Citrus limon</i>.
`b`	Keywords	Take a <b>lemon</b> and squeeze it with a <b>juicer</b>.
`u`	Annotations	The mixture of apple juice and <u class="spelling">eldeflower</u> juice is very pleasant.
`mark`	Highlight	Elderflower cordial, with one <mark>part</mark> cordial to ten <mark>part</mark>s water, stands a<mark>part</mark> from the rest.
`ruby`, `rb`, `rp`, `rt`, `rtc`	Ruby annotations	<ruby> <rb>OJ <rp>(<rtc><rt>Orange Juice</rtc><rp>)</ruby>
`bdi`	Text directionality isolation	The recommended restaurant is <bdi lang="">My Juice Café (At The Beach)</bdi>.
`bdo`	Text directionality formatting	The proposal is to write English, but in reverse order. "Juice" would become "<bdo dir=rtl>Juice</bdo>"
`span`	Other	In French we call it <span lang="fr">sirop de sureau</span>.
`br`	Line break	Simply Orange Juice Company<br>Apopka, FL 32703<br>U.S.A.
`wbr`	Line breaking opportunity	www.simply<wbr>orange<wbr>juice.com

4.6 Edits

The ins and del elements represent edits to the document.

4.6.1 The `ins` element

Categories:: Flow content.; Phrasing content.; Palpable content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Transparent.
Content attributes:: Global attributes; cite - Link to the source of the quotation or more information about the edit; datetime - Date and (optionally) time of the change
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses the HTMLModElement interface.

The ins element represents an addition to the document.

The following represents the addition of a single paragraph:

<aside>
 <ins>
  <p> I like fruit. </p>
 </ins>
</aside>

As does the following, because everything in the aside element here counts as phrasing content and therefore there is just one paragraph:

<aside>
 <ins>
  Apples are <em>tasty</em>.
 </ins>
 <ins>
  So are pears.
 </ins>
</aside>

ins elements should not cross implied paragraph boundaries.

The following example represents the addition of two paragraphs, the second of which was inserted in two parts. The first ins element in this example thus crosses a paragraph boundary, which is considered poor form.

<aside>
 <!-- don't do this -->
 <ins datetime="2005-03-16 00:00Z">
  <p> I like fruit. </p>
  Apples are <em>tasty</em>.
 </ins>
 <ins datetime="2007-12-19 00:00Z">
  So are pears.
 </ins>
</aside>

Here is a better way of marking this up. It uses more elements, but none of the elements cross implied paragraph boundaries.

<aside>
 <ins datetime="2005-03-16 00:00Z">
  <p> I like fruit. </p>
 </ins>
 <ins datetime="2005-03-16 00:00Z">
  Apples are <em>tasty</em>.
 </ins>
 <ins datetime="2007-12-19 00:00Z">
  So are pears.
 </ins>
</aside>

4.6.2 The `del` element

Categories:: Flow content.; Phrasing content.
Contexts in which this element can be used:: Where phrasing content is expected.
Content model:: Transparent.
Content attributes:: Global attributes; cite - Link to the source of the quotation or more information about the edit; datetime - Date and (optionally) time of the change
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: Uses the HTMLModElement interface.

The del element represents a removal from the document.

del elements should not cross implied paragraph boundaries.

The following shows a "to do" list where items that have been done are crossed-off with the date and time of their completion.

<h1>To Do</h1>
<ul>
 <li>Empty the dishwasher</li>
 <li><del datetime="2009-10-11T01:25-07:00">Watch Walter Lewin's lectures</del></li>
 <li><del datetime="2009-10-10T23:38-07:00">Download more tracks</del></li>
 <li>Buy a printer</li>
</ul>

4.6.3 Attributes common to `ins` and `del` elements

The cite attribute may be used to specify the address of a document that explains the change. When that document is long, for instance the minutes of a meeting, authors are encouraged to include a fragment identifier pointing to the specific part of that document that discusses the change.

If the cite attribute is present, it must be a valid URL potentially surrounded by spaces that explains the change. To obtain the corresponding citation link, the value of the attribute must be resolved relative to the element. User agents may allow users to follow such citation links, but they are primarily intended for private use (e.g. by server-side scripts collecting statistics about a site's edits), not for readers.

The datetime attribute may be used to specify the time and date of the change.

If present, the datetime attribute's value must be a valid date string with optional time.

User agents must parse the datetime attribute according to the parse a date or time string algorithm. If that doesn't return a date or a global date and time, then the modification has no associated timestamp (the value is non-conforming; it is not a valid date string with optional time). Otherwise, the modification is marked as having been made at the given date or global date and time. If the given value is a global date and time then user agents should use the associated time-zone offset information to determine which time zone to present the given datetime in.

This value may be shown to the user, but it is primarily intended for private use.

The ins and del elements must implement the HTMLModElement interface:

interface HTMLModElement : HTMLElement {
           attribute DOMString cite;
           attribute DOMString dateTime;
};

The cite IDL attribute must reflect the element's cite content attribute. The dateTime IDL attribute must reflect the element's datetime content attribute.

4.6.4 Edits and paragraphs

This section is non-normative.

Since the ins and del elements do not affect paragraphing, it is possible, in some cases where paragraphs are implied (without explicit p elements), for an ins or del element to span both an entire paragraph or other non-phrasing content elements and part of another paragraph. For example:

<section>
 <ins>
  <p>
   This is a paragraph that was inserted.
  </p>
  This is another paragraph whose first sentence was inserted
  at the same time as the paragraph above.
 </ins>
 This is a second sentence, which was there all along.
</section>

By only wrapping some paragraphs in p elements, one can even get the end of one paragraph, a whole second paragraph, and the start of a third paragraph to be covered by the same ins or del element (though this is very confusing, and not considered good practice):

<section>
 This is the first paragraph. <ins>This sentence was
 inserted.
 <p>This second paragraph was inserted.</p>
 This sentence was inserted too.</ins> This is the
 third paragraph in this example.
 <!-- (don't do this) -->
</section>

However, due to the way implied paragraphs are defined, it is not possible to mark up the end of one paragraph and the start of the very next one using the same ins or del element. You instead have to use one (or two) p element(s) and two ins or del elements, as for example:

<section>
 <p>This is the first paragraph. <del>This sentence was
 deleted.</del></p>
 <p><del>This sentence was deleted too.</del> That
 sentence needed a separate &lt;del&gt; element.</p>
</section>

Partly because of the confusion described above, authors are strongly encouraged to always mark up all paragraphs with the p element, instead of having ins or del elements that cross implied paragraphs boundaries.

4.6.5 Edits and lists

This section is non-normative.

The content models of the ol and ul elements do not allow ins and del elements as children. Lists always represent all their items, including items that would otherwise have been marked as deleted.

To indicate that an item is inserted or deleted, an ins or del element can be wrapped around the contents of the li element. To indicate that an item has been replaced by another, a single li element can have one or more del elements followed by one or more ins elements.

In the following example, a list that started empty had items added and removed from it over time. The bits in the example that have been emphasized show the parts that are the "current" state of the list. The list item numbers don't take into account the edits, though.

<h1>Stop-ship bugs</h1>
<ol>
 <li><ins datetime="2008-02-12T15:20Z">Bug 225:
 Rain detector doesn't work in snow</ins></li>
 <li><del datetime="2008-03-01T20:22Z"><ins datetime="2008-02-14T12:02Z">Bug 228:
 Water buffer overflows in April</ins></del></li>
 <li><ins datetime="2008-02-16T13:50Z">Bug 230:
 Water heater doesn't use renewable fuels</ins></li>
 <li><del datetime="2008-02-20T21:15Z"><ins datetime="2008-02-16T14:25Z">Bug 232:
 Carbon dioxide emissions detected after startup</ins></del></li>
</ol>

In the following example, a list that started with just fruit was replaced by a list with just colors.

<h1>List of <del>fruits</del><ins>colors</ins></h1>
<ul>
 <li><del>Lime</del><ins>Green</ins></li>
 <li><del>Apple</del></li>
 <li>Orange</li>
 <li><del>Pear</del></li>
 <li><ins>Teal</ins></li>
 <li><del>Lemon</del><ins>Yellow</ins></li>
 <li>Olive</li>
 <li><ins>Purple</ins></li>
</ul>

4.6.6 Edits and tables

This section is non-normative.

The elements that form part of the table model have complicated content model requirements that do not allow for the ins and del elements, so indicating edits to a table can be difficult.

To indicate that an entire row or an entire column has been added or removed, the entire contents of each cell in that row or column can be wrapped in ins or del elements (respectively).

Here, a table's row has been added:

<table>
 <thead>
  <tr> <th> Game name           <th> Game publisher   <th> Verdict
 <tbody>
  <tr> <td> Diablo 2            <td> Blizzard         <td> 8/10
  <tr> <td> Portal              <td> Valve            <td> 10/10
  <tr> <td> <ins>Portal 2</ins> <td> <ins>Valve</ins> <td> <ins>10/10</ins>
</table>

Here, a column has been removed (the time at which it was removed is given also, as is a link to the page explaining why):

<table>
 <thead>
  <tr> <th> Game name           <th> Game publisher   <th> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">Verdict</del>
 <tbody>
  <tr> <td> Diablo 2            <td> Blizzard         <td> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">8/10</del>
  <tr> <td> Portal              <td> Valve            <td> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">10/10</del>
  <tr> <td> Portal 2            <td> Valve            <td> <del cite="/edits/r192" datetime="2011-05-02 14:23Z">10/10</del>
</table>

Generally speaking, there is no good way to indicate more complicated edits (e.g. that a cell was removed, moving all subsequent cells up or to the left).

4.7 Embedded content

4.7.1 The `img` element

Form-associated element.

If the element has a usemap attribute: Interactive content.

Where embedded content is expected.

Empty.

alt - Replacement text for use when images are not available

src - Address of the resource

crossorigin - How the element handles crossorigin requests

usemap - Name of image map to use

ismap - Whether the image is a server-side image map

width - Horizontal dimension

height - Vertical dimension

No end tag.

Allowed ARIA role attribute values:

presentation role only, for an img element whose alt attribute's value is empty (alt=""), otherwise any role value.

Any aria-* attributes applicable to the allowed roles.

[NamedConstructor=Image(optional unsigned long width, optional unsigned long height)]
interface HTMLImageElement : HTMLElement {
           attribute DOMString alt;
           attribute DOMString src;

           attribute DOMString crossOrigin;
           attribute DOMString useMap;
           attribute boolean isMap;
           attribute unsigned long width;
           attribute unsigned long height;
  readonly attribute unsigned long naturalWidth;
  readonly attribute unsigned long naturalHeight;
  readonly attribute boolean complete;
};

An img element represents an image.

The image given by the src attributes is the embedded content; the value of the alt attribute provides equivalent content for those who cannot process images or who have image loading disabled.

The requirements on the alt attribute's value are described in the next section.

The src attribute must be present, and must contain a valid non-empty URL potentially surrounded by spaces referencing a non-interactive, optionally animated, image resource that is neither paged nor scripted.

The requirements above imply that images can be static bitmaps (e.g. PNGs, GIFs, JPEGs), single-page vector documents (single-page PDFs, XML files with an SVG root element), animated bitmaps (APNGs, animated GIFs), animated vector graphics (XML files with an SVG root element that use declarative SMIL animation), and so forth. However, these definitions preclude SVG files with script, multipage PDF files, interactive MNG files, HTML documents, plain text documents, and so forth. [PNG] [GIF] [JPEG] [PDF] [XML] [APNG] [SVG] [MNG]

The img element must not be used as a layout tool. In particular, img elements should not be used to display transparent images, as such images rarely convey meaning and rarely add anything useful to the document.

The crossorigin attribute is a CORS settings attribute. Its purpose is to allow images from third-party sites that allow cross-origin access to be used with canvas.

An img is always in one of the following states:

Unavailable: The user agent hasn't obtained any image data.
Partially available: The user agent has obtained some of the image data.
Completely available: The user agent has obtained all of the image data and at least the image dimensions are available.
Broken: The user agent has obtained all of the image data that it can, but it cannot even decode the image enough to get the image dimensions (e.g. the image is corrupted, or the format is not supported, or no data could be obtained).

When an img element is either in the partially available state or in the completely available state, it is said to be available.

An img element is initially unavailable.

When an img element is available, it provides a paint source whose width is the image's intrinsic width, whose height is the image's intrinsic height, and whose appearance is the intrinsic appearance of the image.

In a browsing context where scripting is disabled, user agents may obtain images immediately or on demand. In a browsing context where scripting is enabled, user agents must obtain images immediately.

A user agent that obtains images immediately must synchronously update the image data of an img element whenever that element is created with a src attribute. A user agent that obtains images immediately must also synchronously update the image data of an img element whenever that element has its src or crossorigin attribute set, changed, or removed.

A user agent that obtains images on demand must update the image data of an img element whenever it needs the image data (i.e. on demand), but only if the img element has a src attribute, and only if the img element is in the unavailable state. When an img element's src or crossorigin attribute set, changed, or removed, if the user agent only obtains images on demand, the img element must return to the unavailable state.

Each img element has a last selected source, which must initially be null, and a current pixel density, which must initially be undefined.

When an img element has a current pixel density that is not 1.0, the element's image data must be treated as if its resolution, in device pixels per CSS pixels, was the current pixel density.

For example, if the current pixel density is 3.125, that means that there are 300 device pixels per CSS inch, and thus if the image data is 300x600, it has an intrinsic dimension of 96 CSS pixels by 192 CSS pixels.

Each Document object must have a list of available images. Each image in this list is identified by a tuple consisting of an absolute URL, a CORS settings attribute mode, and, if the mode is not No CORS, an origin. User agents may copy entries from one Document object's list of available images to another at any time (e.g. when the Document is created, user agents can add to it all the images that are loaded in other Documents), but must not change the keys of entries copied in this way when doing so. User agents may also remove images from such lists at any time (e.g. to save memory).

When the user agent is to update the image data of an img element, it must run the following steps:

Return the img element to the unavailable state.
If an instance of the fetching algorithm is still running for this element, then abort that algorithm, discarding any pending tasks generated by that algorithm.
Forget the img element's current image data, if any.
If the user agent cannot support images, or its support for images has been disabled, then abort these steps.
Otherwise, if the element has a src attribute specified and its value is not the empty string, let selected source be the value of the element's src attribute, and selected pixel density be 1.0. Otherwise, let selected source be null and selected pixel density be undefined.
Let the img element's last selected source be selected source and the img element's current pixel density be selected pixel density.
If selected source is not null, run these substeps:
1. Resolve selected source, relative to the element. If that is not successful, abort these steps.
2. Let key be a tuple consisting of the resulting absolute URL, the img element's crossorigin attribute's mode, and, if that mode is not No CORS, the Document object's origin.
3. If the list of available images contains an entry for key, then set the img element to the completely available state, update the presentation of the image appropriately, queue a task to fire a simple event named load at the img element, and abort these steps.
Asynchronously await a stable state, allowing the task that invoked this algorithm to continue. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ If another instance of this algorithm for this img element was started after this instance (even if it aborted and is no longer running), then abort these steps.

Only the last instance takes effect, to avoid multiple requests when, for example, the src and crossorigin attributes are all set in succession.
⌛ If selected source is null, then set the element to the broken state, queue a task to fire a simple event named error at the img element, and abort these steps.
⌛ Queue a task to fire a progress event named loadstart at the img element.
⌛ Do a potentially CORS-enabled fetch of the absolute URL that resulted from the earlier step, with the mode being the current state of the element's crossorigin content attribute, the origin being the origin of the img element's Document, and the default origin behaviour set to taint.

The resource obtained in this fashion, if any, is the img element's image data. It can be either CORS-same-origin or CORS-cross-origin; this affects the origin of the image itself (e.g. when used on a canvas).

Fetching the image must delay the load event of the element's document until the task that is queued by the networking task source once the resource has been fetched (defined below) has been run.

This, unfortunately, can be used to perform a rudimentary port scan of the user's local network (especially in conjunction with scripting, though scripting isn't actually necessary to carry out such an attack). User agents may implement cross-origin access control policies that are stricter than those described above to mitigate this attack, but unfortunately such policies are typically not compatible with existing Web content.

If the resource is CORS-same-origin, each task that is queued by the networking task source while the image is being fetched must fire a progress event named progress at the img element.
End the synchronous section, continuing the remaining steps asynchronously, but without missing any data from the fetch algorithm.
As soon as possible, jump to the first applicable entry from the following list:
If the resource type is multipart/x-mixed-replace

The next task that is queued by the networking task source while the image is being fetched must set the img element's state to partially available.

Each task that is queued by the networking task source while the image is being fetched must update the presentation of the image, but as each new body part comes in, it must replace the previous image. Once one body part has been completely decoded, the user agent must set the img element to the completely available state and queue a task to fire a simple event named load at the img element.

The progress and loadend events are not fired for multipart/x-mixed-replace image streams.

If the resource type and data corresponds to a supported image format, as described below
The next task that is queued by the networking task source while the image is being fetched must set the img element's state to partially available.

That task, and each subsequent task, that is queued by the networking task source while the image is being fetched must update the presentation of the image appropriately (e.g. if the image is a progressive JPEG, each packet can improve the resolution of the image).

Furthermore, the last task that is queued by the networking task source once the resource has been fetched must additionally run the steps for the matching entry in the following list:
If the download was successful and the user agent was able to determine the image's width and height
Set the img element to the completely available state.

Add the image to the list of available images using the key key.

If the resource is CORS-same-origin: fire a progress event named load at the img element.

If the resource is CORS-cross-origin: fire a simple event named load at the img element.

If the resource is CORS-same-origin: fire a progress event named loadend at the img element.

If the resource is CORS-cross-origin: fire a simple event named loadend at the img element.
Otherwise
Set the img element to the broken state.

If the resource is CORS-same-origin: fire a progress event named load at the img element.

If the resource is CORS-cross-origin: fire a simple event named load at the img element.

If the resource is CORS-same-origin: fire a progress event named loadend at the img element.

If the resource is CORS-cross-origin: fire a simple event named loadend at the img element.
Otherwise

Either the image data is corrupted in some fatal way such that the image dimensions cannot be obtained, or the image data is not in a supported file format; the user agent must set the img element to the broken state, abort the fetching algorithm, discarding any pending tasks generated by that algorithm, and then queue a task to first fire a simple event named error at the img element and then fire a simple event named loadend at the img element.

While a user agent is running the above algorithm for an element x, there must be a strong reference from the element's Document to the element x, even if that element is not in its Document.

When an img element is in the completely available state and the user agent can decode the media data without errors, then the img element is said to be fully decodable.

Whether the image is fetched successfully or not (e.g. whether the response code was a 2xx code or equivalent) must be ignored when determining the image's type and whether it is a valid image.

This allows servers to return images with error responses, and have them displayed.

The user agent should apply the image sniffing rules to determine the type of the image, with the image's associated Content-Type headers giving the official type. If these rules are not applied, then the type of the image must be the type given by the image's associated Content-Type headers.

User agents must not support non-image resources with the img element (e.g. XML files whose root element is an HTML element). User agents must not run executable code (e.g. scripts) embedded in the image resource. User agents must only display the first page of a multipage resource (e.g. a PDF file). User agents must not allow the resource to act in an interactive fashion, but should honor any animation in the resource.

This specification does not specify which image types are to be supported.

What an img element represents depends on the src attribute and the alt attribute.

If the src attribute is set and the alt attribute is set to the empty string

The image is either decorative or supplemental to the rest of the content, redundant with some other information in the document.

If the image is available and the user agent is configured to display that image, then the element represents the element's image data.

Otherwise, the element represents nothing, and may be omitted completely from the rendering. User agents may provide the user with a notification that an image is present but has been omitted from the rendering.

If the src attribute is set and the alt attribute is set to a value that isn't empty

The image is a key part of the content; the alt attribute gives a textual equivalent or replacement for the image.

If the image is available and the user agent is configured to display that image, then the element represents the element's image data.

Otherwise, the element represents the text given by the alt attribute. User agents may provide the user with a notification that an image is present but has been omitted from the rendering.

If the src attribute is set and the alt attribute is not

There is no textual equivalent of the image available.

If the image is available and the user agent is configured to display that image, then the element represents the element's image data.

Otherwise, the user agent should display some sort of indicator that there is an image that is not being rendered, and may, if requested by the user, or if so configured, or when required to provide contextual information in response to navigation, provide caption information for the image, derived as follows:

If the image is a descendant of a figure element that has a child figcaption element, and, ignoring the figcaption element and its descendants, the figure element has no Text node descendants other than inter-element whitespace, and no embedded content descendant other than the img element, then the contents of the first such figcaption element are the caption information; abort these steps.
There is no caption information.

If the src attribute is not set and either the alt attribute is set to the empty string or the alt attribute is not set at all

The element represents nothing.

Otherwise

The element represents the text given by the alt attribute.

The alt attribute does not represent advisory information. User agents must not present the contents of the alt attribute in the same way as content of the title attribute.

While user agents are encouraged to repair cases of missing alt attributes, authors must not rely on such behavior. Requirements for providing text to act as an alternative for images are described in detail below.

The contents of img elements, if any, are ignored for the purposes of rendering.

The usemap attribute, if present, can indicate that the image has an associated image map.

The ismap attribute, when used on an element that is a descendant of an a element with an href attribute, indicates by its presence that the element provides access to a server-side image map. This affects how events are handled on the corresponding a element.

The ismap attribute is a boolean attribute. The attribute must not be specified on an element that does not have an ancestor a element with an href attribute.

The img element supports dimension attributes.

The alt, src IDL attributes must reflect the respective content attributes of the same name.

The crossOrigin IDL attribute must reflect the crossorigin content attribute, limited to only known values.

The useMap IDL attribute must reflect the usemap content attribute.

The isMap IDL attribute must reflect the ismap content attribute.

image . width [ = value ]

image . height [ = value ]

These attributes return the actual rendered dimensions of the image, or zero if the dimensions are not known.

They can be set, to change the corresponding content attributes.

image . naturalWidth

image . naturalHeight

These attributes return the intrinsic dimensions of the image, or zero if the dimensions are not known.

image . complete

Returns true if the image has been completely downloaded or if no image is specified; otherwise, returns false.

image = new Image( [ width [, height ] ] )

Returns a new img element, with the width and height attributes set to the values passed in the relevant arguments, if applicable.

The IDL attributes width and height must return the rendered width and height of the image, in CSS pixels, if the image is being rendered, and is being rendered to a visual medium; or else the intrinsic width and height of the image, in CSS pixels, if the image is available but not being rendered to a visual medium; or else 0, if the image is not available. [CSS]

On setting, they must act as if they reflected the respective content attributes of the same name.

The IDL attributes naturalWidth and naturalHeight must return the intrinsic width and height of the image, in CSS pixels, if the image is available, or else 0. [CSS]

The IDL attribute complete must return true if any of the following conditions is true:

The src attribute is omitted.
The final task that is queued by the networking task source once the resource has been fetched has been queued.
The img element is completely available.
The img element is broken.

Otherwise, the attribute must return false.

The value of complete can thus change while a script is executing.

A constructor is provided for creating HTMLImageElement objects (in addition to the factory methods from DOM such as createElement()): Image(width, height). When invoked as a constructor, this must return a new HTMLImageElement object (a new img element). If the width argument is present, the new object's width content attribute must be set to width. If the height argument is also present, the new object's height content attribute must be set to height. The element's document must be the active document of the browsing context of the Window object on which the interface object of the invoked constructor is found.

4.7.1.1 Requirements for providing text to act as an alternative for images

Text alternatives, [WCAG] are a primary way of making visual information accessible, because they can be rendered through any sensory modality (for example, visual, auditory or tactile) to match the needs of the user. Providing text alternatives allows the information to be rendered in a variety of ways by a variety of user agents. For example, a person who cannot see a picture can have the text alternative read aloud using synthesized speech.

The alt attribute on images is a very important accessibility attribute. Authoring useful alt attribute content requires the author to carefully consider the context in which the image appears and the function that image may have in that context. The guidance included here addresses the most common ways authors use images. Additional guidance and techniques are available in Resources on Alternative Text for Images.

4.7.1.1.1 Examples of scenarios where users benefit from text alternatives for images

They have a very slow connection and are browsing with images disabled.
They have a vision impairment and use text to speech software.
They have a cognitive impairment and use text to speech software.
They are using a text-only browser.
They are listening to the page being read out by a voice Web browser.
They have images disabled to save on download costs.
They have problems loading images or the source of an image is wrong.

4.7.1.1.2 General guidelines

Except where otherwise specified, the alt attribute must be specified and its value must not be empty; the value must be an appropriate functional replacement for the image. The specific requirements for the alt attribute content depend on the image's function in the page, as described in the following sections.

To determine an appropriate text alternative it is important to think about why an image is being included in a page. What is its purpose? Thinking like this will help you to understand what is important about the image for the intended audience. Every image has a reason for being on a page, because it provides useful information, performs a function, labels an interactive element, enhances aesthetics or is purely decorative. Therefore, knowing what the image is for, makes writing an appropriate text alternative easier.

4.7.1.1.3 A link or button containing nothing but an image

When an a element that is a hyperlink, or a button element, has no text content but contains one or more images, include text in the alt attribute(s) that together convey the purpose of the link or button.

In this example, a user is asked to pick her preferred color from a list of three. Each color is given by an image, but for users who cannot view the images, the color names are included within the alt attributes of the images:

  <ul>
  <li><a href="red.html"><img src="red.jpeg" alt="Red"></a></li>
  <li><a href="green.html"><img src="green.jpeg" alt="Green"></a></li>
  <li><a href="blue.html"><img src="blue.jpeg" alt="Blue"></a></li>
  </ul>

In this example, a link contains a logo. The link points to the W3C web site from an external site. The text alternative is a brief description of the link target.

  <a href="http://w3.org">
  <img src="images/w3c_home.png" width="72" height="48" alt="W3C web site">
  </a>

This example is the same as the previous example, except that the link is on the W3C web site. The text alternative is a brief description of the link target.

  <a href="http://w3.org">
  <img src="images/w3c_home.png" width="72" height="48" alt="W3C home">
  </a>

Depending on the context in which an image of a logo is used it could be appropriate to provide an indication that the image is a logo as part of the text alternative. Refer to section 4.7.1.1.19 Logos, insignia, flags, or emblems.

In this example, a link contains a print preview icon. The link points to a version of the page with a print stylesheet applied. The text alternative is a brief description of the link target.

  <a href="preview.html">
  <img src="images/preview.png" width="32" height="30" alt="Print preview.">
  </a>

In this example, a button contains a search icon. The button submits a search form. The text alternative is a brief description of what the button does.

  <button>
  <img src="images/search.png" width="74" height="29" alt="Search">
  </button>

In this example, a company logo for the PIP Corporation has been split into the following two images, the first containing the word PIP and the second with the abbreviated word CO. The images are the sole content of a link to the PIPCO home page. In this case a brief description of the link target is provided. As the images are presented to the user as a single entity the text alternative PIP CO home is in the alt attribute of the first image.

Image containing the text 'PIP'. Image containing the text 'CO'.

  <a href="pipco-home.html">
  <img src="pip.gif" alt="PIP CO home"><img src="co.gif" alt="">
  </a>

4.7.1.1.4 Graphical Representations: Charts, diagrams, graphs, maps, illustrations

Users can benefit when content is presented in graphical form, for example as a flowchart, a diagram, a graph, or a map showing directions. Users also benefit when content presented in a graphical form is also provided in a textual format, these users include those who are unable to view the image (e.g. because they have a very slow connection, or because they are using a text-only browser, or because they are listening to the page being read out by a hands-free automobile voice Web browser, or because they have a visual impairment and use an assistive technology to render the text to speech).

In the following example we have an image of a pie chart, with text in the alt attribute representing the data shown in the pie chart:

Browser Share: Internet Explorer 25%, Firefox 40%, Chrome 25%, Safari 6% and Opera 4%.

  <img src="piechart.gif" alt="Pie chart: Browser Share - Internet Explorer 25%, Firefox 40%, Chrome 25%, Safari 6% and Opera 4%.">

In the case where an image repeats the previous paragraph in graphical form. The alt attribute content labels the image.

 <p>According to a recent study Firefox has a 40% browser share, Internet Explorer has 25%, Chrome has 25%, Safari has 6% and Opera has 4%.</p>
 <p><img src="piechart.gif" alt="Pie chart representing the data in the previous paragraph."></p>

It can be seen that when the image is not available, for example because the src attribute value is incorrect, the text alternative provides the user with a brief description of the image content:

Representation of the code snippet above.

In cases where the text alternative is lengthy, more than a sentence or two, or would benefit from the use of structured markup, provide a brief description or label using the alt attribute, and an associated text alternative.

Here's an example of a flowchart image, with a short text alternative included in the alt attribute, in this case the text alternative is a description of the link target as the image is the sole content of a link. The link points to a description, within the same document, of the process represented in the flowchart.

Flowchart: Dealing with a broken lamp.

  
  <a href="#desc"><img src="flowchart.gif" alt="Flowchart: Dealing with a broken lamp."></a>

  ...

  ...

  <div id="desc">
  <h2>Dealing with a broken lamp</h2>
  <ol>
  <li>Check if it's plugged in, if not, plug it in.</li>
  <li>If it still doesn't work; check if the bulb is burned out. If it is, replace the bulb.</li>
  <li>If it still doesn't work; buy a new lamp.</li>
  </ol>
  </div>

In this example, there is an image of a chart. It would be inappropriate to provide the information depicted in the chart as a plain text alternative in an alt attribute as the information is a data set. Instead a structured text alternative is provided below the image in the form of a data table using the data that is represented in the chart image.

Bar Chart showing average rainfall in millimetres by Country and Season.

Indications of the highest and lowest rainfall for each season have been included in the table, so trends easily identified in the chart are also available in the data table.

Average rainfall in millimetres by country and season.
	United Kingdom	Japan	Australia
Spring	5.3 (highest)	2.4	2 (lowest)
Summer	4.5 (highest)	3.4	2 (lowest)
Autumn	3.5 (highest)	1.8	1.5 (lowest)
Winter	1.5 (highest)	1.2	1 (lowest)

  <figure>
  <figcaption>Rainfall Data</figcaption>
  <img src="rainchart.gif" alt="Bar chart: Average rainfall in millimetres by Country and Season.">
  <table>
  <caption>Rainfall in millimetres by Country and Season.</caption>
  <tr><td><th scope="col">UK <th scope="col">Japan<th scope="col">Australia</tr>
  <tr><th scope="row">Spring <td>5.5 (highest)<td>2.4 <td>2 (lowest)</tr>
  <tr><th scope="row">Summer <td>4.5 (highest)<td>3.4<td>2 (lowest)</tr>
  <tr><th scope="row">Autumn <td>3.5 (highest) <td>1.8 <td>1.5 (lowest)</tr>
  <tr><th scope="row">Winter <td>1.5 (highest) <td>1.2 <td>1 lowest</tr>
  </table>
  </figure>

The figure element is used to group the Bar Chart image and data table. The figcaption element provides a caption for the grouped content.

4.7.1.1.5 Images of text

Sometimes, an image only contains text, and the purpose of the image is to display text using visual effects and /or fonts. It is strongly recommended that text styled using CSS be used, but if this is not possible, provide the same text in the alt attribute as is in the image.

This example shows an image of the text "Get Happy!" written in a fancy multi colored freehand style. The image makes up the content of a heading. In this case the text alternative for the image is "Get Happy!".

Get Happy!

<h1><img src="gethappy.gif" alt="Get Happy!"></h1>

In this example we have an advertising image consisting of text, the phrase "The BIG sale" is repeated 3 times, each time the text gets smaller and fainter, the last line reads "...ends Friday" In the context of use, as an advertisement, it is recommended that the image's text alternative only include the text "The BIG sale" once as the repetition is for visual effect and the repetition of the text for users who cannot view the image is unnecessary and could be confusing.

The big sale ...ends Friday.

  <p><img src="sale.gif" alt="The BIG sale ...ends Friday."></p>

In situations where there is also a photo or other graphic along with the image of text, ensure that the words in the image text are included in the text alternative, along with any other description of the image that conveys meaning to users who can view the image, so the information is also available to users who cannot view the image.

When an image is used to represent a character that cannot otherwise be represented in Unicode, for example gaiji, itaiji, or new characters such as novel currency symbols, the text alternative should be a more conventional way of writing the same thing, e.g. using the phonetic hiragana or katakana to give the character's pronunciation.

In this example from 1997, a new-fangled currency symbol that looks like a curly E with two bars in the middle instead of one is represented using an image. The alternative text gives the character's pronunciation.

Only 5.99!

<p>Only <img src="euro.png" alt="euro ">5.99!

An image should not be used if Unicode characters would serve an identical purpose. Only when the text cannot be directly represented using Unicode, e.g. because of decorations or because the character is not in the Unicode character set (as in the case of gaiji), would an image be appropriate.

If an author is tempted to use an image because their default system font does not support a given character, then Web Fonts are a better solution than images.

An illuminated manuscript might use graphics for some of its letters. The text alternative in such a situation is just the character that the image represents.

nce upon a time and a long long time ago...

<p><img src="initials/fancyO.png" alt="O">nce upon a time and a long long time ago...

4.7.1.1.6 Images that include text

Sometimes, an image consists of a graphics such as a chart and associated text. In this case it is recommended that the text in the image is included in the text alternative.

Consider an image containing a pie chart and associated text. It is recommended wherever possible to provide any associated text as text, not an image of text. If this is not possible include the text in the text alternative along with the pertinent information conveyed in the image.

Figure 1. Distribution of Articles by Journal
Category. Pie chart: Language=68%, Education=14% and Science=18%.

  <p><img src="figure1.gif" alt="Figure 1. Distribution of Articles by Journal Category. 
  Pie chart: Language=68%, Education=14% and Science=18%."></p>

Here's another example of the same pie chart image, showing a short text alternative included in the alt attribute and a longer text alternative in text. The figure and figcaption elements are used to associate the longer text alternative with the image. The alt attribute is used to label the image.

  <figure>
  <img src="figure1.gif" alt="Figure 1">
  <figcaption><strong>Figure 1.</strong> Distribution of Articles by Journal Category. 
  Pie chart: Language=68%, Education=14% and Science=18%.</figcaption>
  </figure>

The advantage of this method over the previous example is that the text alternative is available to all users at all times. It also allows structured mark up to be used in the text alternative, where as a text alternative provided using the alt attribute does not.

4.7.1.1.7 Images that enhance the themes or subject matter of the page content

An image that isn't discussed directly by the surrounding text but still has some relevance can be included in a page using the img element. Such images are more than mere decoration, they may augment the themes or subject matter of the page content and so still form part of the content. In these cases, it is recommended that a text alternative be provided.

Here is an example of an image closely related to the subject matter of the page content but not directly discussed. An image of a painting inspired by a poem, on a page reciting that poem. The following snippet shows an example. The image is a painting titled the "Lady of Shallot", it is inspired by the poem and its subject matter is derived from the poem. Therefore it is strongly recommended that a text alternative is provided. There is a short description of the content of the image in the alt attribute and a link below the image to a longer description located at the bottom of the document. At the end of the longer description there is also a link to further information about the painting.

A painting inspired by Alfred Tennyson's poem The Lady of Shalott

  <header>
  <h1>The Lady of Shalott</h1>
  <p>A poem by Alfred Lord Tennyson</p>
  </header>

  <img src="shalott.jpeg" alt="Painting of a  young woman with long hair, sitting in a wooden boat. ">
  <p><a href="#des">Description of the painting</a>.</p>


  <!-- Full Recitation of Alfred, Lord Tennyson's Poem.  -->

  ...
  ...
  ...
  <p id="des">The woman in the painting is wearing a flowing white dress. A large piece of intricately 
  patterned fabric is draped over the side. In her right hand she holds the chain mooring the boat. Her expression 
  is mournful. She stares at a crucifix lying in front of her. Beside it are three candles. Two have blown out. 
  <a href="http://bit.ly/5HJvVZ">Further information about the painting</a>.</p>

This example illustrates the provision of a text alternative identifying an image as a photo of the main subject of a page.

Portrait photo(black and white) of Robin, accompanied by a heading 'Robin Berjon' and a question
'what more needs to be said?'

    <img src="orateur_robin_berjon.png" alt="Portrait photo(black and white) of Robin.">
    <h1>Robin Berjon</h1>
    <p>What more needs to be said?</p>

4.7.1.1.8 A graphical representation of some of the surrounding text

In many cases, the image is actually just supplementary, and its presence merely reinforces the surrounding text. In these cases, the alt attribute must be present but its value must be the empty string.

In general, an image falls into this category if removing the image doesn't make the page any less useful, but including the image makes it a lot easier for users of visual browsers to understand the concept.

It is not always easy to write a useful text alternative for an image, another option is to provide a link to a description or further information about the image when one is available.

In this example of the same image, there is a short text alternative included in the alt attribute, and there is a link after the image. The link points to a page containing information about the painting.

The Lady of Shalott
A poem by Alfred Lord Tennyson.

Painting of a woman in a white flowing dress, sitting in a small boat.

About this painting.

Full recitation of Alfred, Lord Tennyson's poem.

  <header><h1>The Lady of Shalott</h1>
  <p>A poem by Alfred Lord Tennyson</p></header>
  <figure>
  <img src="shalott.jpeg" alt="Painting of a woman in a white flowing dress, sitting in a small boat.">
  <p><a href="http://bit.ly/5HJvVZ">About this painting.</a></p>
  </figure>
  <!-- Full Recitation of Alfred, Lord Tennyson's Poem.  -->

4.7.1.1.9 A purely decorative image that doesn't add any information

Purely decorative images are visual enhancements, decorations or embellishments that provide no function or information beyond aesthetics to users who can view the images.

Mark up purely decorative images so they can be ignored by assistive technology by using an empty alt attribute (alt=""). While it is not unacceptable to include decorative images inline, it is recommended if they are purely decorative to include the image using CSS.

Here's an example of an image being used as a decorative banner for a person's blog, the image offers no information and so an empty alt attribute is used.

Clara's Blog

Welcome to my blog...

  <header>
  <div><img src="border.gif" alt="" width="400" height="30"></div>
  <h1>Clara's Blog</h1>
  </header>
  <p>Welcome to my blog...</p>

4.7.1.1.10 Inline images

When images are used inline as part of the flow of text in a sentence, provide a word or phrase as a text alternative which makes sense in the context of the sentence it is apart of.

I you.

I <img src="heart.png" alt="love"> you.

My breaks.

My <img src="heart.png" alt="heart"> breaks.

4.7.1.1.11 A group of images that form a single larger picture with no links

When a picture has been sliced into smaller image files that are then displayed together to form the complete picture again, include a text alternative for one of the images using the alt attribute as per the relevant relevant guidance for the picture as a whole, and then include an empty alt attribute on the other images.

In this example, a picture representing a company logo for the PIP Corporation has been split into two pieces, the first containing the letters "PIP" and the second with the word "CO". The text alternatve PIP CO is in the alt attribute of the first image.

Image containing the text 'PIP'. Image containing the text 'CO'.

  <img src="pip.gif" alt="PIP CO"><img src="co.gif" alt="">

In the following example, a rating is shown as three filled stars and two empty stars. While the text alternative could have been "★★★☆☆", the author has instead decided to more helpfully give the rating in the form "3 out of 5". That is the text alternative of the first image, and the rest have empty alt attributes.

  <p>Rating: <meter max=5 value=3>
  <img src="1" alt="3 out of 5">
  <img src="1" alt=""><img src="1" alt="">
  <img src="0" alt=""><img src="0" alt="">
  </meter></p>

4.7.1.1.12 Image maps

If an img element has a usemap attribute which references a map element containing area elements that have href attributes, the img is considered to be interactive content. In such cases, always provide a text alternative for the image using the alt attribute.

Consider the following image which is a map of Katoomba, it has 2 interactive regions corresponding to the areas of North and South Katoomba:

Map of Katoomba.

The text alternative is a brief description of the image. The alt attribute on each of the area elements provides text describing the content of the target page of each linked region:

<p>View houses for sale in North Katoomba or South Katoomba:</p>
 <p><img src="imagemap.png" width="209" alt="Map of Katoomba" height="249" usemap="#Map">

 <map name="Map"> 
 <area shape="poly" coords="78,124,124,10,189,29,173,93,168,132,136,151,110,130" 
 href="north.html" alt="Houses in North Katoomba">
 <area shape="poly" coords="66,63,80,135,106,138,137,154,167,137,175,133,144,240,49,223,17,137,17,61" 
 alt="Houses in South Katoomba" href="south.html">
 </map>

4.7.1.1.13 A group of images that form a single larger picture with links

Generally, image maps should be used instead of slicing an image for links.

Sometimes, when you create a composite picture from multiple images, you may wish to link one or more of the images. Provide an alt attribute for each linked image to describe the purpose of the link.

In the following example, a composite picture is used to represent a "crocoduck"; a fictional creature which defies evolutionary principles by being part crocodile and part duck. You are asked to interact with the crocoduck, but you need to exercise caution...

crocodile's angry, chomping head duck's soft, feathery body

  <h1>The crocoduck</h1>
  <p>You encounter a strange creature called a "crocoduck". 
  The creature seems angry! Perhaps some friendly stroking will help to calm 
  it, but be careful not to stroke any crocodile parts. This would just enrage 
  the beast further.</p>
  <a href="?stroke=head"><img src="crocoduck1.png" alt="Stroke crocodile's angry, chomping head"></a> 
  <a href="?stroke=body"><img src="crocoduck2.png" alt="Stroke duck's soft, feathery body"></a>

4.7.1.1.14 Images of Pictures

Images of pictures or graphics include visual representations of objects, people, scenes, abstractions, etc. This non-text content, [WCAG] can convey a significant amount of information visually or provide a specific sensory experience, [WCAG] to a sighted person. Examples include photographs, paintings, drawings and artwork.

An appropriate text alternative for a picture is a brief description, or name [WCAG]. As in all text alternative authoring decisions, writing suitable text alternatives for pictures requires human judgment. The text value is subjective to the context where the image is used and the page author's writing style. Therefore, there is no single 'right' or 'correct' piece of alt text for any particular image. In addition to providing a short text alternative that gives a brief description of the non-text content, also providing supplemental content through another means when appropriate may be useful.

This first example shows an image uploaded to a photo-sharing site. The photo is of a cat, sitting in the bath. The image has a text alternative provided using the img element's alt attribute. It also has a caption provided by including the img element in a figure element and using a figcaption element to identify the caption text.

Lola the cat sitting under an umbrella in the bath tub.

Lola prefers a bath to a shower.

  <figure>
  <img src="664aef.jpg" alt="Lola the cat sitting under an umbrella in the bath tub.">
  <figcaption>Lola prefers a bath to a shower.</figcaption>
  </figure>

This example is of an image that defies a complete description, as the subject of the image is open to interpretation. The image has a text alternative in the alt attribute which gives users who cannot view the image a sense of what the image is. It also has a caption provided by including the img element in a figure element and using a figcaption element to identify the caption text.

An abstract, freeform, vertically symmetrical, black inkblot on a light background.

The first of the ten cards in the Rorschach test.

<figure>
  <img src="Rorschach1.jpg" alt="An abstract, freeform, vertically symmetrical, black inkblot on a light background.">
  <figcaption>The first of the ten cards in the Rorschach test.</figcaption>
  </figure>

4.7.1.1.15 Webcam images

Webcam images are static images that are automatically updated periodically. Typically the images are from a fixed viewpoint, the images may update on the page automatically as each new image is uploaded from the camera or the user may be required to refresh the page to view an updated image. Examples include traffic and weather cameras.

This example is fairly typical; the title and a time stamp are included in the image, automatically generated by the webcam software. It would be better if the text information was not included in the image, but as it is part of the image, include it part of the text alternative. A caption is also provided using the figure and figcaption elements. As the image is provided to give a visual indication of the current weather near a building, a link to a local weather forecast is provided, as with automatically generated and uploaded webcam images it may be impractical to provide such information as a text alternative.

The text of the alt attribute includes a prose version of the timestamp, designed to make the text more understandable when announced by text to speech software. The text alternative also includes a description of some aspects of what can be seen in the image which are unchanging, although weather conditions and time of day change.

Sopwith house weather cam. Taken on the 21/04/10 at 11:51 and 34 seconds. In the foreground are the safety
rails on the flat part of the roof. Nearby ther are low rise industrial buildings, beyond those are block of flats. In the distance there's a
church steeple.

View from the top of Sopwith house, looking towards North Kingston. This image is updated every hour.

View the latest weather details for Kingston upon Thames.

<figure>
  <img src="webcam1.jpg" alt="Sopwith house weather cam. Taken on the 21/04/10 at 11:51 and 34 seconds.
  In the foreground are the safety rails on the flat part of the roof. Nearby there are low rise industrial buildings,
  beyond are blocks of flats. In the distance there's a church steeple.">
  <figcaption>View from Sopwith house, looking towards north Kingston. This image is updated every hour.</figcaption>
  </figure>
  <p>View the <a href="http://news.bbc.co.uk/weather/forecast/4296?area=Kingston">latest weather details</a> for Kingston upon Thames.</p>

4.7.1.1.16 When a text alternative is not available at the time of publication

In some cases an image is included in a published document, but the author is unable to provide an appropriate text alternative. In such cases the minimum requirement is to provide a caption for the image using the figure and figcaption elements under the following conditions:

The img element is in a figure element
The figure element contains a figcaption element
The figcaption element contains content other than inter-element whitespace
Ignoring the figcaption element and its descendants, the figure element has no Text node descendants other than inter-element whitespace, and no embedded content descendant other than the img element.

In other words, the only content of the figure is an img element and a figcaption element, and the figcaption element must include (caption) content.

Such cases are to be kept to an absolute minimum. If there is even the slightest possibility of the author having the ability to provide real alternative text, then it would not be acceptable to omit the alt attribute.

In this example, a person uploads a photo, as part of a bulk upoad of many images, to a photo sharing site. The user has not provided a text alternative or a caption for the image. The site's authoring tool inserts a caption automatically using whatever useful information it has for the image. In this case it's the file name and date the photo was taken.

no text alternative provided

clara.jpg, taken on 12/11/2010.

  <figure>
  <img src="clara.jpg">
  <figcaption>clara.jpg, taken on 12/11/2010.</figcaption>
  </figure>

Notice that even in this example, as much useful information as possible is still included in the figcaption element.

In this second example, a person uploads a photo to a photo sharing site. She has provided a caption for the image but not a text alternative. This may be because the site does not provide users with the ability to add a text alternative in the alt attribute.

no text alternative provided

Eloisa with Princess Belle

  <figure>
  <img src="elo.jpg">
  <figcaption>Eloisa with Princess Belle</figcaption>
  </figure>

Sometimes the entire point of the image is that a textual description is not available, and the user is to provide the description. For example, software that displays images and asks for alternative text precisely for the purpose of then writing a page with correct alternative text. Such a page could have a table of images, like this:

<table>
  <tr><tr> <th> Image <th> Description<tr>
  <td>
  <figure>
  <img src="2421.png">
  <figcaption>Image 640 by 100, filename 'banner.gif'</figcaption>
  </figure>
  <td> <input name="alt2421">
  <tr>
  <td> <figure>
  <img src="2422.png">
  <figcaption>Image 200 by 480, filename 'ad3.gif'</figcaption>
  </figure>
  <td> <input name="alt2422">
  </table>

Since some users cannot use images at all (e.g. because they are blind) the alt attribute is only allowed to be omitted when no text alternative is available and none can be made available, as in the above examples.

4.7.1.1.17 An image not intended for the user

Generally authors should avoid using img elements for purposes other than showing images.

If an img element is being used for purposes other than showing an image, e.g. as part of a service to count page views, use an empty alt attribute.

An example of an img element used to collect web page statistics. The alt attribute is empty as the image has no meaning.

  <img src="http://server3.stats.com/count.pl?NeonMeatDream.com" width="0" height="0" alt="">

It is recommended for the example use above the width and height attributes be set to zero.

Another example use is when an image such as a spacer.gif is used to aid positioning of content. The alt attribute is empty as the image has no meaning.

  <img src="spacer.gif" width="10" height="10" alt="">

It is recommended that that CSS be used to position content instead of img elements.

4.7.1.1.18 Icon Images

An icon is usually a simple picture representing a program, action, data file or a concept. Icons are intended to help users of visual browsers to recognize features at a glance.

Use an empty alt attribute when an icon is supplemental to text conveying the same meaning.

In this example, we have a link pointing to a site's home page, the link contains a house icon image and the text "home". The image has an empty alt text. Where images are used in this way, it would also be appropriate to add the image using CSS

  <a href="home.html"><img src="home.gif" width="15" height="15" alt="">Home</a>

  #home:before
  { 
  content: url(home.png);
  }

  <a href="home.html" id="home">Home</a>

In this example, there is a warning message, with a warning icon. The word "Warning!" is in emphasized text next to the icon. As the information conveyed by the icon is redundant the img element is given an an empty alt attribute.

Warning! Your session is about to expire.

  <p><img src="warning.png" width="15" height="15" alt="">
  <strong>Warning!</strong> 
  Your session is about to expire</p>

When an icon conveys additional information not available in text, provide a text alternative.

In this example, there is a warning message, with a warning icon. The icon emphasizes the importance of the message and identifies it as a particular type of content.

Your session is about to expire.

  <p><img src="warning.png" width="15" height="15" alt="Warning!">
  Your session is about to expire</p>

4.7.1.1.19 Logos, insignia, flags, or emblems

Many pages include logos, insignia, flags, or emblems, which stand for a company, organization, project, band, software package, country, or other entity. What can be considered as an appropriate text alternative depends upon, like all images, the context in which the image is being used and what function it serves in the given context.

If a logo is the sole content of a link, provide a brief description of the link target in the alt attribute.

This example illustrates the use of the HTML5 logo as the sole content of a link to the HTML specification.

  <a href="http://dev.w3.org/html5/spec/spec.html">
  <img src="HTML5_Logo.png" alt="HTML 5.1 specification"></a>

If a logo is being used to represent the entity, e.g. as a page heading, provide the name of the entity being represented by the logo as the text alternative.

This example illustrates the use of the WebPlatform.org logo being used to represent itself.

and other developer resources

  <h2><img src="images/webplatform.png" alt="WebPlatform.org"> and other developer resources<h2>

The text alternative in the example above could also include the word "logo" to describe the type of image content. If so, it is suggested that square brackets be used to delineate this information: alt="[logo] WebPlatform.org".

If a logo is being used next to the name of the what that it represents, then the logo is supplemental. Include an empty alt attribute as the text alternative is already provided.

This example illustrates the use of a logo next to the name of the organization it represents.

WebPlatform.org

  <img src="images/webplatform1.png" alt=""> WebPlatform.org

If the logo is used alongside text discussing the subject or entity the logo represents, then provide a text alternative which describes the logo.

This example illustrates the use of a logo next to text discussing the subject the logo represents.

HTML5 logo: Shaped like a shield with the
text 'HTML' above and the numeral '5' prominent on the face of the shield.

HTML5 is a language for structuring and presenting content for the World Wide Web, a core technology of the Internet. It is the latest revision of the HTML standard (originally created in 1990 and most recently standardized as HTML4 in 1997) and currently remains under development. Its core aims have been to improve the language with support for the latest multimedia while keeping it easily readable by humans and consistently understood by computers and devices (web browsers, parsers etc.).

  <p><img src="HTML5_Logo.png" alt="HTML5 logo: Shaped like a shield with the 
  text 'HTML' above and the numeral '5' prominent on the face of the shield."></p>
  
  Information about HTML5

4.7.1.1.20 CAPTCHA Images

CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart". CAPTCHA images are used for security purposes to confirm that content is being accessed by a person rather than a computer. This authentication is done through visual verification of an image. CAPTCHA typically presents an image with characters or words in it that the user is to re-type. The image is usually distorted and has some noise applied to it to make the characters difficult to read.

To improve the accessibility of CAPTCHA provide text alternatives that identify and describe the purpose of the image, and provide alternative forms of the CAPTCHA using output modes for different types of sensory perception. For instance provide an audio alternative along with the visual image. Place the audio option right next to the visual one. This helps but is still problematic for people without sound cards, the deaf-blind, and some people with limited hearing. Another method is to include a form that asks a question along with the visual image. This helps but can be problematic for people with cognitive impairments.

It is strongly recommended that alternatives to CAPTCHA be used, as all forms of CAPTCHA introduce unacceptable barriers to entry for users with disabilities. Further information is available in Inaccessibility of CAPTCHA.

This example shows a CAPTCHA test which uses a distorted image of text. The text alternative in the alt attribute provides instructions for a user in the case where she cannot access the image content.

captcha containing the words 'aides' and 'sprucest'. The letters are distorted and the color of the letters and background is partially inverted,

Example code:

<img src="captcha.png" alt="If you cannot view this image an audio challenge is provided."> 
  <!-- audio CAPTCHA option that allows the user to listen and type the word -->  
  <!-- form that asks a question -->

4.7.1.1.21 Guidance for markup generators

Markup generators (such as WYSIWYG authoring tools) should, wherever possible, obtain a text alternative from their users. However, it is recognized that in many cases, this will not be possible.

For images that are the sole contents of links, markup generators should examine the link target to determine the title of the target, or the URL of the target, and use information obtained in this manner as the text alternative.

For images that have captions, markup generators should use the figure and figcaption elements to provide the image's caption.

As a last resort, implementors should either set the alt attribute to the empty string, under the assumption that the image is a purely decorative image that doesn't add any information but is still specific to the surrounding content, or omit the alt attribute altogether, under the assumption that the image is a key part of the content.

Markup generators may specify a generator-unable-to-provide-required-alt attribute on img elements for which they have been unable to obtain a text alternative and for which they have therefore omitted the alt attribute. The value of this attribute must be the empty string. Documents containing such attributes are not conforming, but conformance checkers will silently ignore this error.

This is intended to avoid markup generators from being pressured into replacing the error of omitting the alt attribute with the even more egregious error of providing phony text alternatives, because state-of-the-art automated conformance checkers cannot distinguish phony text alternatives from correct text alternatives.

Markup generators should generally avoid using the image's own file name as the text alternative. Similarly, markup generators should avoid generating text alternatives from any content that will be equally available to presentation user agents (e.g. Web browsers).

This is because once a page is generated, it will typically not be updated, whereas the browsers that later read the page can be updated by the user, therefore the browser is likely to have more up-to-date and finely-tuned heuristics than the markup generator did when generating the page.

4.7.1.1.22 Guidance for conformance checkers

A conformance checker must report the lack of an alt attribute as an error unless one of the conditions listed below applies:

The img element is in a figure element that satisfies the conditions described above.
The img element has a (non-conforming) generator-unable-to-provide-required-alt attribute whose value is the empty string. A conformance checker that is not reporting the lack of an alt attribute as an error must also not report the presence of the empty generator-unable-to-provide-required-alt attribute as an error. (This case does not represent a case where the document is conforming, only that the generator could not determine appropriate alternative text — validators are not required to show an error in this case, because such an error might encourage markup generators to include bogus alternative text purely in an attempt to silence validators. Naturally, conformance checkers may report the lack of an alt attribute as an error even in the presence of the generator-unable-to-provide-required-alt attribute; for example, there could be a user option to report all conformance errors even those that might be the more or less inevitable result of using a markup generator.)

4.7.2 The `iframe` element

Where embedded content is expected.

Text that conforms to the requirements given in the prose.

src - Address of the resource

srcdoc - A document to render in the iframe

name - Name of nested browsing context

sandbox - Security rules for nested content

width - Horizontal dimension

height - Vertical dimension

Neither tag is omissible

Allowed ARIA role attribute values:

application, document, img or presentation.

Any aria-* attributes applicable to the allowed roles.

interface HTMLIFrameElement : HTMLElement {
           attribute DOMString src;
           attribute DOMString srcdoc;
           attribute DOMString name;
  [PutForwards=value] readonly attribute DOMSettableTokenList sandbox;
           attribute DOMString width;
           attribute DOMString height;
  readonly attribute Document? contentDocument;
  readonly attribute WindowProxy? contentWindow;
};

The iframe element represents a nested browsing context.

The src attribute gives the address of a page that the nested browsing context is to contain. The attribute, if present, must be a valid non-empty URL potentially surrounded by spaces.

The srcdoc attribute gives the content of the page that the nested browsing context is to contain. The value of the attribute is the source of an iframe srcdoc document.

For iframe elements in HTML documents, the srcdoc attribute, if present, must have a value using the HTML syntax that consists of the following syntactic components, in the given order:

Any number of comments and space characters.
Optionally, a DOCTYPE.
Any number of comments and space characters.
The root element, in the form of an html element.
Any number of comments and space characters.

For iframe elements in XML documents, the srcdoc attribute, if present, must have a value that matches the production labeled document in the XML specification. [XML]

In the HTML syntax, authors need only remember to use """ (U+0022) characters to wrap the attribute contents and then to escape all """ (U+0022) and U+0026 AMPERSAND (&) characters, and to specify the sandbox attribute, to ensure safe embedding of content.

Due to restrictions of the XHTML syntax, in XML the "<" (U+003C) character needs to be escaped as well. In order to prevent attribute-value normalization, some of XML's whitespace characters — specifically "tab" (U+0009), "LF" (U+000A), and "CR" (U+000D) — also need to be escaped. [XML]

If the src attribute and the srcdoc attribute are both specified together, the srcdoc attribute takes priority. This allows authors to provide a fallback URL for legacy user agents that do not support the srcdoc attribute.

When an iframe element is inserted into a document, the user agent must create a nested browsing context, and then process the iframe attributes for the "first time".

When an iframe element is removed from a document, the user agent must discard the nested browsing context.

This happens without any unload events firing (the nested browsing context and its Document are discarded, not unloaded).

Whenever an iframe element with a nested browsing context has its srcdoc attribute set, changed, or removed, the user agent must process the iframe attributes.

Similarly, whenever an iframe element with a nested browsing context but with no srcdoc attribute specified has its src attribute set, changed, or removed, the user agent must process the iframe attributes.

When the user agent is to process the iframe attributes, it must run the first appropriate steps from the following list:

If the srcdoc attribute is specified

Navigate the element's child browsing context to a resource whose Content-Type is text/html, whose URL is about:srcdoc, and whose data consists of the value of the attribute. The resulting Document must be considered an iframe srcdoc document.

Otherwise, if the element has no src attribute specified, and the user agent is processing the iframe's attributes for the "first time"

Queue a task to run the iframe load event steps.

The task source for this task is the DOM manipulation task source.

Otherwise

If the value of the src attribute is missing, or its value is the empty string, let url be the string "about:blank".

Otherwise, resolve the value of the src attribute, relative to the iframe element.

If that is not successful, then let url be the string "about:blank". Otherwise, let url be the resulting absolute URL.
If there exists an ancestor browsing context whose active document's address, ignoring fragment identifiers, is equal to url, then abort these steps.
Navigate the element's child browsing context to url.

Any navigation required of the user agent in the process the iframe attributes algorithm must be completed as an explicit self-navigation override and with the iframe element's document's browsing context as the source browsing context.

Furthermore, if the active document of the element's child browsing context before such a navigation was not completely loaded at the time of the new navigation, then the navigation must be completed with replacement enabled.

Similarly, if the child browsing context's session history contained only one Document when the process the iframe attributes algorithm was invoked, and that was the about:blank Document created when the child browsing context was created, then any navigation required of the user agent in that algorithm must be completed with replacement enabled.

When a Document in an iframe is marked as completely loaded, the user agent must synchronously run the iframe load event steps.

A load event is also fired at the iframe element when it is created if no other data is loaded in it.

Each Document has an iframe load in progress flag and a mute iframe load flag. When a Document is created, these flags must be unset for that Document.

The iframe load event steps are as follows:

Let child document be the active document of the iframe element's nested browsing context.
If child document has its mute iframe load flag set, abort these steps.
Set child document's iframe load in progress flag.
Fire a simple event named load at the iframe element.
Unset child document's iframe load in progress flag.

This, in conjunction with scripting, can be used to probe the URL space of the local network's HTTP servers. User agents may implement cross-origin access control policies that are stricter than those described above to mitigate this attack, but unfortunately such policies are typically not compatible with existing Web content.

When the iframe's browsing context's active document is not ready for post-load tasks, and when anything in the iframe is delaying the load event of the iframe's browsing context's active document, and when the iframe's browsing context is in the delaying load events mode, the iframe must delay the load event of its document.

If, during the handling of the load event, the browsing context in the iframe is again navigated, that will further delay the load event.

If, when the element is created, the srcdoc attribute is not set, and the src attribute is either also not set or set but its value cannot be resolved, the browsing context will remain at the initial about:blank page.

If the user navigates away from this page, the iframe's corresponding WindowProxy object will proxy new Window objects for new Document objects, but the src attribute will not change.

The name attribute, if present, must be a valid browsing context name. The given value is used to name the nested browsing context. When the browsing context is created, if the attribute is present, the browsing context name must be set to the value of this attribute; otherwise, the browsing context name must be set to the empty string.

Whenever the name attribute is set, the nested browsing context's name must be changed to the new value. If the attribute is removed, the browsing context name must be set to the empty string.

The sandbox attribute, when specified, enables a set of extra restrictions on any content hosted by the iframe. Its value must be an unordered set of unique space-separated tokens that are ASCII case-insensitive. The allowed values are allow-forms, allow-pointer-lock, allow-popups, allow-same-origin, allow-scripts, and allow-top-navigation.

When the attribute is set, the content is treated as being from a unique origin, forms, scripts, and various potentially annoying APIs are disabled, links are prevented from targeting other browsing contexts, and plugins are secured. The allow-same-origin keyword causes the content to be treated as being from its real origin instead of forcing it into a unique origin; the allow-top-navigation keyword allows the content to navigate its top-level browsing context; and the allow-forms, allow-pointer-lock, allow-popups and allow-scripts keywords re-enable forms, the pointer lock API, popups, and scripts respectively. [POINTERLOCK]

Setting both the allow-scripts and allow-same-origin keywords together when the embedded page has the same origin as the page containing the iframe allows the embedded page to simply remove the sandbox attribute and then reload itself, effectively breaking out of the sandbox altogether.

These flags only take effect when the nested browsing context of the iframe is navigated. Removing them, or removing the entire sandbox attribute, has no effect on an already-loaded page.

Potentially hostile files should not be served from the same server as the file containing the iframe element. Sandboxing hostile content is of minimal help if an attacker can convince the user to just visit the hostile content directly, rather than in the iframe. To limit the damage that can be caused by hostile HTML content, it should be served from a separate dedicated domain. Using a different domain ensures that scripts in the files are unable to attack the site, even if the user is tricked into visiting those pages directly, without the protection of the sandbox attribute.

When an iframe element with a sandbox attribute has its nested browsing context created (before the initial about:blank Document is created), and when an iframe element's sandbox attribute is set or changed while it has a nested browsing context, the user agent must parse the sandboxing directive using the attribute's value as the input, and the iframe element's nested browsing context's iframe sandboxing flag set as the output.

When an iframe element's sandbox attribute is removed while it has a nested browsing context, the user agent must empty the iframe element's nested browsing context's iframe sandboxing flag set as the output.

In this example, some completely-unknown, potentially hostile, user-provided HTML content is embedded in a page. Because it is served from a separate domain, it is affected by all the normal cross-site restrictions. In addition, the embedded page has scripting disabled, plugins disabled, forms disabled, and it cannot navigate any frames or windows other than itself (or any frames or windows it itself embeds).

<p>We're not scared of you! Here is your content, unedited:</p>
<iframe sandbox src="http://usercontent.example.net/getusercontent.cgi?id=12193"></iframe>

It is important to use a separate domain so that if the attacker convinces the user to visit that page directly, the page doesn't run in the context of the site's origin, which would make the user vulnerable to any attack found in the page.

In this example, a gadget from another site is embedded. The gadget has scripting and forms enabled, and the origin sandbox restrictions are lifted, allowing the gadget to communicate with its originating server. The sandbox is still useful, however, as it disables plugins and popups, thus reducing the risk of the user being exposed to malware and other annoyances.

<iframe sandbox="allow-same-origin allow-forms allow-scripts"
        src="http://maps.example.com/embedded.html"></iframe>

Suppose a file A contained the following fragment:

<iframe sandbox="allow-same-origin allow-forms" src=B></iframe>

Suppose that file B contained an iframe also:

<iframe sandbox="allow-scripts" src=C></iframe>

Further, suppose that file C contained a link:

<a href=D>Link</a>

For this example, suppose all the files were served as text/html.

Page C in this scenario has all the sandboxing flags set. Scripts are disabled, because the iframe in A has scripts disabled, and this overrides the allow-scripts keyword set on the iframe in B. Forms are also disabled, because the inner iframe (in B) does not have the allow-forms keyword set.

Suppose now that a script in A removes all the sandbox attributes in A and B. This would change nothing immediately. If the user clicked the link in C, loading page D into the iframe in B, page D would now act as if the iframe in B had the allow-same-origin and allow-forms keywords set, because that was the state of the nested browsing context in the iframe in A when page B was loaded.

Generally speaking, dynamically removing or changing the sandbox attribute is ill-advised, because it can make it quite hard to reason about what will be allowed and what will not.

The iframe element supports dimension attributes for cases where the embedded content has specific dimensions (e.g. ad units have well-defined dimensions).

An iframe element never has fallback content, as it will always create a nested browsing context, regardless of whether the specified initial contents are successfully used.

Descendants of iframe elements represent nothing. (In legacy user agents that do not support iframe elements, the contents would be parsed as markup that could act as fallback content.)

When used in HTML documents, the allowed content model of iframe elements is text, except that invoking the HTML fragment parsing algorithm with the iframe element as the context element and the text contents as the input must result in a list of nodes that are all phrasing content, with no parse errors having occurred, with no script elements being anywhere in the list or as descendants of elements in the list, and with all the elements in the list (including their descendants) being themselves conforming.

The iframe element must be empty in XML documents.

The HTML parser treats markup inside iframe elements as text.

The IDL attributes src, srcdoc, name, and sandbox must reflect the respective content attributes of the same name.

The contentDocument IDL attribute must return the Document object of the active document of the iframe element's nested browsing context, if any and if its effective script origin is the same origin as the effective script origin specified by the incumbent settings object, or null otherwise.

The contentWindow IDL attribute must return the WindowProxy object of the iframe element's nested browsing context, if any, or null otherwise.

Here is an example of a page using an iframe to include advertising from an advertising broker:

<iframe src="http://ads.example.com/?customerid=923513721&amp;format=banner"
        width="468" height="60"></iframe>

4.7.3 The `embed` element

Where embedded content is expected.

Empty.

src - Address of the resource

type - Type of embedded resource

width - Horizontal dimension

height- Vertical dimension

Any other attribute that has no namespace (see prose).

Tag omission in text/html:

Allowed ARIA role attribute values:

application, document or img or presentation.

interface HTMLEmbedElement : HTMLElement {
           attribute DOMString src;
           attribute DOMString type;
           attribute DOMString width;
           attribute DOMString height;
  legacycaller any (any... arguments);
};

Depending on the type of content instantiated by the embed element, the node may also support other interfaces.

The embed element provides an integration point for an external (typically non-HTML) application or interactive content.

The src attribute gives the address of the resource being embedded. The attribute, if present, must contain a valid non-empty URL potentially surrounded by spaces.

The type attribute, if present, gives the MIME type by which the plugin to instantiate is selected. The value must be a valid MIME type. If both the type attribute and the src attribute are present, then the type attribute must specify the same type as the explicit Content-Type metadata of the resource given by the src attribute.

When the element is created with neither a src attribute nor a type attribute, and when attributes are removed such that neither attribute is present on the element anymore, and when the element has a media element ancestor, and when the element has an ancestor object element that is not showing its fallback content, any plugin instantiated for the element must be removed, and the embed element then represents nothing.

An embed element is said to be potentially active when the following conditions are all met simultaneously:

The element is in a Document or was in a Document the last time the event loop reached step 1.
The element's Document is fully active.
The element has either a src attribute set or a type attribute set (or both).
The element's src attribute is either absent or its value is not the empty string.
The element is not a descendant of a media element.
The element is not a descendant of an object element that is not showing its fallback content.
The element is being rendered, or was being rendered the last time the event loop reached step 1.

Whenever an embed element that was not potentially active becomes potentially active, and whenever a potentially active embed element that is remaining potentially active and has its src attribute set, changed, or removed or its type attribute set, changed, or removed, the user agent must queue a task using the embed task source to run the embed element setup steps.

The embed element setup steps are as follows:

If another task has since been queued to run the embed element setup steps for this element, then abort these steps.
If the element has a src attribute set
The user agent must resolve the value of the element's src attribute, relative to the element. If that is successful, the user agent should fetch the resulting absolute URL, from the element's browsing context scope origin if it has one. The task that is queued by the networking task source once the resource has been fetched must run the following steps:
1. If another task has since been queued to run the embed element setup steps for this element, then abort these steps.
2. Determine the type of the content being embedded, as follows (stopping at the first substep that determines the type):
  1. If the element has a type attribute, and that attribute's value is a type that a plugin supports, then the value of the type attribute is the content's type.
  2. Otherwise, if applying the URL parser algorithm to the URL of the specified resource (after any redirects) results in a parsed URL whose path component matches a pattern that a plugin supports, then the content's type is the type that that plugin can handle.
    
    For example, a plugin might say that it can handle resources with path components that end with the four character string ".swf".
  3. Otherwise, if the specified resource has explicit Content-Type metadata, then that is the content's type.
  4. Otherwise, the content has no type and there can be no appropriate plugin for it.
3. If the previous step determined that the content's type is image/svg+xml, then run the following substeps:
  1. If the embed element is not associated with a nested browsing context, associate the element with a newly created nested browsing context, and, if the element has a name attribute, set the browsing context name of the element's nested browsing context to the value of this attribute.
  2. Navigate the nested browsing context to the fetched resource, with replacement enabled, and with the embed element's document's browsing context as the source browsing context. (The src attribute of the embed element doesn't get updated if the browsing context gets further navigated to other locations.)
  3. The embed element now represents its associated nested browsing context.
4. Otherwise, find and instantiate an appropriate plugin based on the content's type, and hand that plugin the content of the resource, replacing any previously instantiated plugin for the element. The embed element now represents this plugin instance.
Whether the resource is fetched successfully or not (e.g. whether the response code was a 2xx code or equivalent) must be ignored when determining the content's type and when handing the resource to the plugin.

This allows servers to return data for plugins even with error responses (e.g. HTTP 500 Internal Server Error codes can still contain plugin data).

Fetching the resource must delay the load event of the element's document.
If the element has no src attribute set

The user agent should find and instantiate an appropriate plugin based on the value of the type attribute. The embed element now represents this plugin instance.

The embed element has no fallback content. If the user agent can't find a suitable plugin when attempting to find and instantiate one for the algorithm above, then the user agent must use a default plugin. This default could be as simple as saying "Unsupported Format".

Whenever an embed element that was potentially active stops being potentially active, any plugin that had been instantiated for that element must be unloaded.

When a plugin is to be instantiated but it cannot be secured and the sandboxed plugins browsing context flag is set on the embed element's Document's active sandboxing flag set, then the user agent must not instantiate the plugin, and must instead render the embed element in a manner that conveys that the plugin was disabled. The user agent may offer the user the option to override the sandbox and instantiate the plugin anyway; if the user invokes such an option, the user agent must act as if the conditions above did not apply for the purposes of this element.

Plugins that cannot be secured are disabled in sandboxed browsing contexts because they might not honor the restrictions imposed by the sandbox (e.g. they might allow scripting even when scripting in the sandbox is disabled). User agents should convey the danger of overriding the sandbox to the user if an option to do so is provided.

Any namespace-less attribute other than name, align, hspace, and vspace may be specified on the embed element, so long as its name is XML-compatible and contains no uppercase ASCII letters. These attributes are then passed as parameters to the plugin.

All attributes in HTML documents get lowercased automatically, so the restriction on uppercase letters doesn't affect such documents.

The four exceptions are to exclude legacy attributes that have side-effects beyond just sending parameters to the plugin.

The user agent should pass the names and values of all the attributes of the embed element that have no namespace to the plugin used, when one is instantiated.

The HTMLEmbedElement object representing the element must expose the scriptable interface of the plugin instantiated for the embed element, if any. At a minimum, this interface must implement the legacy caller operation. (It is suggested that the default behavior of this legacy caller operation, e.g. the behavior of the default plugin's legacy caller operation, be to throw a NotSupportedError exception.)

The embed element supports dimension attributes.

The IDL attributes src and type each must reflect the respective content attributes of the same name.

Here's a way to embed a resource that requires a proprietary plugin, like Flash:

<embed src="catgame.swf">

If the user does not have the plugin (for example if the plugin vendor doesn't support the user's platform), then the user will be unable to use the resource.

To pass the plugin a parameter "quality" with the value "high", an attribute can be specified:

<embed src="catgame.swf" quality="high">

This would be equivalent to the following, when using an object element instead:

<object data="catgame.swf">
 <param name="quality" value="high">
</object>

4.7.4 The `object` element

If the element has a usemap attribute: Interactive content.

Listed, submittable, and reassociateable form-associated element.

Where embedded content is expected.

Zero or more param elements, then, transparent.

data - Address of the resource

type - Type of embedded resource

typemustmatch - Whether the type attribute and the Content-Type value need to match for the resource to be used

name - Name of nested browsing context

usemap - Name of image map to use

form - Associates the control with a form element

width - Horizontal dimension

height - Vertical dimension

Tag omission in text/html:

Neither tag is omissible.

Allowed ARIA role attribute values:

application, document or img or presentation.

Any aria-* attributes applicable to the allowed roles.

interface HTMLObjectElement : HTMLElement {
           attribute DOMString data;
           attribute DOMString type;
           attribute boolean typeMustMatch;
           attribute DOMString name;
           attribute DOMString useMap;
  readonly attribute HTMLFormElement? form;
           attribute DOMString width;
           attribute DOMString height;
  readonly attribute Document? contentDocument;
  readonly attribute WindowProxy? contentWindow;

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);

  legacycaller any (any... arguments);
};

Depending on the type of content instantiated by the object element, the node also supports other interfaces.

The object element can represent an external resource, which, depending on the type of the resource, will either be treated as an image, as a nested browsing context, or as an external resource to be processed by a plugin.

The data attribute, if present, specifies the address of the resource. If present, the attribute must be a valid non-empty URL potentially surrounded by spaces.

Authors who reference resources from other origins that they do not trust are urged to use the typemustmatch attribute defined below. Without that attribute, it is possible in certain cases for an attacker on the remote host to use the plugin mechanism to run arbitrary scripts, even if the author has used features such as the Flash "allowScriptAccess" parameter.

The type attribute, if present, specifies the type of the resource. If present, the attribute must be a valid MIME type.

At least one of either the data attribute or the type attribute must be present.

The typemustmatch attribute is a boolean attribute whose presence indicates that the resource specified by the data attribute is only to be used if the value of the type attribute and the Content-Type of the aforementioned resource match.

The typemustmatch attribute must not be specified unless both the data attribute and the type attribute are present.

The name attribute, if present, must be a valid browsing context name. The given value is used to name the nested browsing context, if applicable.

Whenever one of the following conditions occur:

the element is created,
the element is popped off the stack of open elements of an HTML parser or XML parser,
the element is not on the stack of open elements of an HTML parser or XML parser, and it is either inserted into a document or removed from a document,
the element's Document changes whether it is fully active,
one of the element's ancestor object elements changes to or from showing its fallback content,
the element's classid attribute is set, changed, or removed,
the element's classid attribute is not present, and its data attribute is set, changed, or removed,
neither the element's classid attribute nor its data attribute are present, and its type attribute is set, changed, or removed,
the element changes from being rendered to not being rendered, or vice versa,

...the user agent must queue a task to run the following steps to (re)determine what the object element represents. The task source for this task is the DOM manipulation task source. This task being queued or actively running must delay the load event of the element's document.

If the user has indicated a preference that this object element's fallback content be shown instead of the element's usual behavior, then jump to the step below labeled fallback.

For example, a user could ask for the element's fallback content to be shown because that content uses a format that the user finds more accessible.
If the element has an ancestor media element, or has an ancestor object element that is not showing its fallback content, or if the element is not in a Document with a browsing context, or if the element's Document is not fully active, or if the element is still in the stack of open elements of an HTML parser or XML parser, or if the element is not being rendered, then jump to the step below labeled fallback.
If the classid attribute is present, and has a value that isn't the empty string, then: if the user agent can find a plugin suitable according to the value of the classid attribute, and either plugins aren't being sandboxed or that plugin can be secured, then that plugin should be used, and the value of the data attribute, if any, should be passed to the plugin. If no suitable plugin can be found, or if the plugin reports an error, jump to the step below labeled fallback.
If the data attribute is present and its value is not the empty string, then:
1. If the type attribute is present and its value is not a type that the user agent supports, and is not a type that the user agent can find a plugin for, then the user agent may jump to the step below labeled fallback without fetching the content to examine its real type.
2. Resolve the URL specified by the data attribute, relative to the element.
3. If that failed, fire a simple event named error at the element, then jump to the step below labeled fallback.
4. Fetch the resulting absolute URL, from the element's browsing context scope origin if it has one.
  
  Fetching the resource must delay the load event of the element's document until the task that is queued by the networking task source once the resource has been fetched (defined next) has been run.
  
  For the purposes of the application cache networking model, this fetch operation is not for a child browsing context (though it might end up being used for one after all, as defined below).
5. If the resource is not yet available (e.g. because the resource was not available in the cache, so that loading the resource required making a request over the network), then jump to the step below labeled fallback. The task that is queued by the networking task source once the resource is available must restart this algorithm from this step. Resources can load incrementally; user agents may opt to consider a resource "available" whenever enough data has been obtained to begin processing the resource.
6. If the load failed (e.g. there was an HTTP 404 error, there was a DNS error), fire a simple event named error at the element, then jump to the step below labeled fallback.
7. Determine the resource type, as follows:
  1. Let the resource type be unknown.
  2. If the object element has a type attribute and a typemustmatch attribute, and the resource has associated Content-Type metadata, and the type specified in the resource's Content-Type metadata is an ASCII case-insensitive match for the value of the element's type attribute, then let resource type be that type and jump to the step below labeled handler.
  3. If the object element has a typemustmatch attribute, jump to the step below labeled handler.
  4. If the user agent is configured to strictly obey Content-Type headers for this resource, and the resource has associated Content-Type metadata, then let the resource type be the type specified in the resource's Content-Type metadata, and jump to the step below labeled handler.
    
    This can introduce a vulnerability, wherein a site is trying to embed a resource that uses a particular plugin, but the remote site overrides that and instead furnishes the user agent with a resource that triggers a different plugin with different security characteristics.
  5. If there is a type attribute present on the object element, and that attribute's value is not a type that the user agent supports, but it is a type that a plugin supports, then let the resource type be the type specified in that type attribute, and jump to the step below labeled handler.
  6. Run the appropriate set of steps from the following list:
    If the resource has associated Content-Type metadata
    Let binary be false.
    
    If the type specified in the resource's Content-Type metadata is "text/plain", and the result of applying the rules for distinguishing if a resource is text or binary to the resource is that the resource is not text/plain, then set binary to true.
    
    If the type specified in the resource's Content-Type metadata is "application/octet-stream", then set binary to true.
    
    If binary is false, then let the resource type be the type specified in the resource's Content-Type metadata, and jump to the step below labeled handler.
    
    If there is a type attribute present on the object element, and its value is not application/octet-stream, then run the following steps:
    
    If the attribute's value is a type that a plugin supports, or the attribute's value is a type that starts with "image/" that is not also an XML MIME type, then let the resource type be the type specified in that type attribute.
    
    Jump to the step below labeled handler.
    Otherwise, if the resource does not have associated Content-Type metadata
    If there is a type attribute present on the object element, then let the tentative type be the type specified in that type attribute.
    
    Otherwise, let tentative type be the sniffed type of the resource.
    
    If tentative type is not application/octet-stream, then let resource type be tentative type and jump to the step below labeled handler.
  7. If applying the URL parser algorithm to the URL of the specified resource (after any redirects) results in a parsed URL whose path component matches a pattern that a plugin supports, then let resource type be the type that that plugin can handle.
    
    For example, a plugin might say that it can handle resources with path components that end with the four character string ".swf".
  It is possible for this step to finish, or for one of the substeps above to jump straight to the next step, with resource type still being unknown. In both cases, the next step will trigger fallback.
8. Handler: Handle the content as given by the first of the following cases that matches:
  
  If the resource type is not a type that the user agent supports, but it is a type that a plugin supports
  
  If plugins are being sandboxed and the plugin that supports resource type cannot be secured, jump to the step below labeled fallback.
  
  Otherwise, the user agent should use the plugin that supports resource type and pass the content of the resource to that plugin. If the plugin reports an error, then jump to the step below labeled fallback.
  
  If the resource type is an XML MIME type, or if the resource type does not start with "image/"
  
  The object element must be associated with a newly created nested browsing context, if it does not already have one.
  
  If the URL of the given resource is not about:blank, the element's nested browsing context must then be navigated to that resource, with replacement enabled, and with the object element's document's browsing context as the source browsing context. (The data attribute of the object element doesn't get updated if the browsing context gets further navigated to other locations.)
  
  If the URL of the given resource is about:blank, then, instead, the user agent must queue a task to fire a simple event named load at the object element. No load event is fired at the about:blank document itself.
  
  The object element represents the nested browsing context.
  
  If the name attribute is present, the browsing context name must be set to the value of this attribute; otherwise, the browsing context name must be set to the empty string.
  
  In certain situations, e.g. if the resource was fetched from an application cache but it is an HTML file with a manifest attribute that points to a different application cache manifest, the navigation of the browsing context will be restarted so as to load the resource afresh from the network or a different application cache. Even if the resource is then found to have a different type, it is still used as part of a nested browsing context: only the navigate algorithm is restarted, not this object algorithm.
  
  If the resource type starts with "image/", and support for images has not been disabled
  
  Apply the image sniffing rules to determine the type of the image.
  
  The object element represents the specified image. The image is not a nested browsing context.
  
  If the image cannot be rendered, e.g. because it is malformed or in an unsupported format, jump to the step below labeled fallback.
  
  Otherwise
  
  The given resource type is not supported. Jump to the step below labeled fallback.
  
  If the previous step ended with the resource type being unknown, this is the case that is triggered.
9. The element's contents are not part of what the object element represents.
10. Once the resource is completely loaded, queue a task to fire a simple event named load at the element.
  
  The task source for this task is the DOM manipulation task source.
If the data attribute is absent but the type attribute is present, and the user agent can find a plugin suitable according to the value of the type attribute, and either plugins aren't being sandboxed or the plugin can be secured, then that plugin should be used. If these conditions cannot be met, or if the plugin reports an error, jump to the step below labeled fallback.
Fallback: The object element represents the element's children, ignoring any leading param element children. This is the element's fallback content. If the element has an instantiated plugin, then unload it.

When the algorithm above instantiates a plugin, the user agent should pass to the plugin used the names and values of all the attributes on the element, in the order they were added to the element, with the attributes added by the parser being ordered in source order, followed by a parameter named "PARAM" whose value is null, followed by all the names and values of parameters given by param elements that are children of the object element, in tree order. If the plugin supports a scriptable interface, the HTMLObjectElement object representing the element should expose that interface. The object element represents the plugin. The plugin is not a nested browsing context.

Plugins are considered sandboxed for the purpose of an object element if the sandboxed plugins browsing context flag is set on the object element's Document's active sandboxing flag set.

Due to the algorithm above, the contents of object elements act as fallback content, used only when referenced resources can't be shown (e.g. because it returned a 404 error). This allows multiple object elements to be nested inside each other, targeting multiple user agents with different capabilities, with the user agent picking the first one it supports.

Whenever the name attribute is set, if the object element has a nested browsing context, its name must be changed to the new value. If the attribute is removed, if the object element has a browsing context, the browsing context name must be set to the empty string.

The usemap attribute, if present while the object element represents an image, can indicate that the object has an associated image map. The attribute must be ignored if the object element doesn't represent an image.

The form attribute is used to explicitly associate the object element with its form owner.

Constraint validation: object elements are always barred from constraint validation.

The object element supports dimension attributes.

The IDL attributes data, type and name each must reflect the respective content attributes of the same name. The typeMustMatch IDL attribute must reflect the typemustmatch content attribute. The useMap IDL attribute must reflect the usemap content attribute.

The contentDocument IDL attribute must return the Document object of the active document of the object element's nested browsing context, if any and if its effective script origin is the same origin as the effective script origin specified by the incumbent settings object, or null otherwise.

The contentWindow IDL attribute must return the WindowProxy object of the object element's nested browsing context, if it has one; otherwise, it must return null.

The willValidate, validity, and validationMessage attributes, and the checkValidity(), and setCustomValidity() methods, are part of the constraint validation API. The form IDL attribute is part of the element's forms API.

All object elements have a legacy caller operation. If the object element has an instantiated plugin that supports a scriptable interface that defines a legacy caller operation, then that must be the behavior of the object's legacy caller operation. Otherwise, the object's legacy caller operation must be to throw a NotSupportedError exception.

In the following example, a Java applet is embedded in a page using the object element. (Generally speaking, it is better to avoid using applets like these and instead use native JavaScript and HTML to provide the functionality, since that way the application will work on all Web browsers without requiring a third-party plugin. Many devices, especially embedded devices, do not support third-party technologies like Java.)

<figure>
 <object type="application/x-java-applet">
  <param name="code" value="MyJavaClass">
  <p>You do not have Java available, or it is disabled.</p>
 </object>
 <figcaption>My Java Clock</figcaption>
</figure>

In this example, an HTML page is embedded in another using the object element.

<figure>
 <object data="clock.html"></object>
 <figcaption>My HTML Clock</figcaption>
</figure>

The following example shows how a plugin can be used in HTML (in this case the Flash plugin, to show a video file). Fallback is provided for users who do not have Flash enabled, in this case using the video element to show the video for those using user agents that support video, and finally providing a link to the video for those who have neither Flash nor a video-capable browser.

<p>Look at my video:
 <object type="application/x-shockwave-flash">
  <param name=movie value="http://video.example.com/library/watch.swf">
  <param name=allowfullscreen value=true>
  <param name=flashvars value="http://video.example.com/vids/315981">
  <video controls src="http://video.example.com/vids/315981">
   <a href="http://video.example.com/vids/315981">View video</a>.
  </video>
 </object>
</p>

4.7.5 The `param` element

None.

As a child of an object element, before any flow content.

Empty.

name - Name of parameter

value - Value of parameter

Tag omission in text/html:

Allowed ARIA role attribute values:

None

interface HTMLParamElement : HTMLElement {
           attribute DOMString name;
           attribute DOMString value;
};

The param element defines parameters for plugins invoked by object elements. It does not represent anything on its own.

The name attribute gives the name of the parameter.

The value attribute gives the value of the parameter.

Both attributes must be present. They may have any value.

If both attributes are present, and if the parent element of the param is an object element, then the element defines a parameter with the given name-value pair.

If either the name or value of a parameter defined by a param element that is the child of an object element that represents an instantiated plugin changes, and if that plugin is communicating with the user agent using an API that features the ability to update the plugin when the name or value of a parameter so changes, then the user agent must appropriately exercise that ability to notify the plugin of the change.

The IDL attributes name and value must both reflect the respective content attributes of the same name.

The following example shows how the param element can be used to pass a parameter to a plugin, in this case the O3D plugin.

<!DOCTYPE HTML>
<html lang="en">
  <head>
   <title>O3D Utah Teapot</title>
  </head>
  <body>
   <p>
    <object type="application/vnd.o3d.auto">
     <param name="o3d_features" value="FloatingPointTextures">
     <img src="o3d-teapot.png"
          title="3D Utah Teapot illustration rendered using O3D."
          alt="When O3D renders the Utah Teapot, it appears as a squat
          teapot with a shiny metallic finish on which the
          surroundings are reflected, with a faint shadow caused by
          the lighting.">
     <p>To see the teapot actually rendered by O3D on your
     computer, please download and install the <a
     href="http://code.google.com/apis/o3d/docs/gettingstarted.html#install">O3D plugin</a>.</p>
    </object>
    <script src="o3d-teapot.js"></script>
   </p>
  </body>
</html>

4.7.6 The `video` element

If the element has a controls attribute: Interactive content.

Where embedded content is expected.

If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.

If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.

src - Address of the resource

crossorigin - How the element handles crossorigin requests

poster - Poster frame to show prior to video playback

preload - Hints how much buffering the media resource will likely need

autoplay - Hint that the media resource can be started automatically when the page is loaded

mediagroup - Groups media elements together with an implicit MediaController

loop - Whether to loop the media resource

muted - Whether to mute the media resource by default

controls - Show user agent controls

width - Horizontal dimension

height - Vertical dimension

Tag omission in text/html

Neither tag is omissible

Allowed ARIA role attribute values:

application.

Any aria-* attributes applicable to the allowed roles.

interface HTMLVideoElement : HTMLMediaElement {
           attribute unsigned long width;
           attribute unsigned long height;
  readonly attribute unsigned long videoWidth;
  readonly attribute unsigned long videoHeight;
           attribute DOMString poster;
};

A video element is used for playing videos or movies, and audio files with captions.

Content may be provided inside the video element. User agents should not show this content to the user; it is intended for older Web browsers which do not support video, so that legacy video plugins can be tried, or to show text to the users of these older browsers informing them of how to access the video contents.

In particular, this content is not intended to address accessibility concerns. To make video content accessible to the partially sighted, the blind, the hard-of-hearing, the deaf, and those with other physical or cognitive disabilities, a variety of features are available. Captions can be provided, either embedded in the video stream or as external files using the track element. Sign-language tracks can be provided, again either embedded in the video stream or by synchronizing multiple video elements using the mediagroup attribute or a MediaController object. Audio descriptions can be provided, either as a separate track embedded in the video stream, or a separate audio track in an audio element slaved to the same controller as the video element(s), or in text form using a WebVTT file referenced using the track element and synthesized into speech by the user agent. WebVTT can also be used to provide chapter titles. For users who would rather not use a media element at all, transcripts or other textual alternatives can be provided by simply linking to them in the prose near the video element. [WEBVTT]

The video element is a media element whose media data is ostensibly video data, possibly with associated audio data.

The src, preload, autoplay, mediagroup, loop, muted, and controls attributes are the attributes common to all media elements.

The poster attribute gives the address of an image file that the user agent can show while no video data is available. The attribute, if present, must contain a valid non-empty URL potentially surrounded by spaces.

If the specified resource is to be used, then, when the element is created or when the poster attribute is set, changed, or removed, the user agent must run the following steps to determine the element's poster frame (regardless of the value of the element's show poster flag):

If there is an existing instance of this algorithm running for this video element, abort that instance of this algorithm without changing the poster frame.
If the poster attribute's value is the empty string or if the attribute is absent, then there is no poster frame; abort these steps.
Resolve the poster attribute's value relative to the element. If this fails, then there is no poster frame; abort these steps.
Fetch the resulting absolute URL, from the element's Document's origin. This must delay the load event of the element's document.
If an image is thus obtained, the poster frame is that image. Otherwise, there is no poster frame.

The image given by the poster attribute, the poster frame, is intended to be a representative frame of the video (typically one of the first non-blank frames) that gives the user an idea of what the video is like.

A video element represents what is given for the first matching condition in the list below:

When no video data is available (the element's readyState attribute is either HAVE_NOTHING, or HAVE_METADATA but no video data has yet been obtained at all, or the element's readyState attribute is any subsequent value but the media resource does not have a video channel): The video element represents its poster frame, if any, or else transparent black with no intrinsic dimensions.
When the video element is paused, the current playback position is the first frame of video, and the element's show poster flag is set: The video element represents its poster frame, if any, or else the first frame of the video.
When the video element is paused, and the frame of video corresponding to the current playback position is not available (e.g. because the video is seeking or buffering)
When the video element is neither potentially playing nor paused (e.g. when seeking or stalled): The video element represents the last frame of the video to have been rendered.
When the video element is paused: The video element represents the frame of video corresponding to the current playback position.
Otherwise (the video element has a video channel and is potentially playing): The video element represents the frame of video at the continuously increasing "current" position. When the current playback position changes such that the last frame rendered is no longer the frame corresponding to the current playback position in the video, the new frame must be rendered.

Which frame in a video stream corresponds to a particular playback position is defined by the video stream's format.

The video element also represents any text track cues whose text track cue active flag is set and whose text track is in the showing mode, and any audio from the media resource, at the current playback position.

Any audio associated with the media resource must, if played, be played synchronized with the current playback position, at the element's effective media volume.

In addition to the above, the user agent may provide messages to the user (such as "buffering", "no video loaded", "error", or more detailed information) by overlaying text or icons on the video or other areas of the element's playback area, or in another appropriate manner.

User agents that cannot render the video may instead make the element represent a link to an external video playback utility or to the video data itself.

When a video element's media resource has a video channel, the element provides a paint source whose width is the media resource's intrinsic width, whose height is the media resource's intrinsic height, and whose appearance is the frame of video corresponding to the current playback position, if that is available, or else (e.g. when the video is seeking or buffering) its previous appearance, if any, or else (e.g. because the video is still loading the first frame) blackness.

video . videoWidth
video . videoHeight: These attributes return the intrinsic dimensions of the video, or zero if the dimensions are not known.

The intrinsic width and intrinsic height of the media resource are the dimensions of the resource in CSS pixels after taking into account the resource's dimensions, aspect ratio, clean aperture, resolution, and so forth, as defined for the format used by the resource. If an anamorphic format does not define how to apply the aspect ratio to the video data's dimensions to obtain the "correct" dimensions, then the user agent must apply the ratio by increasing one dimension and leaving the other unchanged.

The videoWidth IDL attribute must return the intrinsic width of the video in CSS pixels. The videoHeight IDL attribute must return the intrinsic height of the video in CSS pixels. If the element's readyState attribute is HAVE_NOTHING, then the attributes must return 0.

Whenever the intrinsic width or intrinsic height of the video changes (including, for example, because the selected video track was changed), if the element's readyState attribute is not HAVE_NOTHING, the user agent must queue a task to fire a simple event named resize at the media element.

The video element supports dimension attributes.

In the absence of style rules to the contrary, video content should be rendered inside the element's playback area such that the video content is shown centered in the playback area at the largest possible size that fits completely within it, with the video content's aspect ratio being preserved. Thus, if the aspect ratio of the playback area does not match the aspect ratio of the video, the video will be shown letterboxed or pillarboxed. Areas of the element's playback area that do not contain the video represent nothing.

In user agents that implement CSS, the above requirement can be implemented by using the style rule suggested in the rendering section.

The intrinsic width of a video element's playback area is the intrinsic width of the poster frame, if that is available and the element currently represents its poster frame; otherwise, it is the intrinsic width of the video resource, if that is available; otherwise the intrinsic width is missing.

The intrinsic height of a video element's playback area is the intrinsic height of the poster frame, if that is available and the element currently represents its poster frame; otherwise it is the intrinsic height of the video resource, if that is available; otherwise the intrinsic height is missing.

The default object size is a width of 300 CSS pixels and a height of 150 CSS pixels. [CSSIMAGES]

User agents should provide controls to enable or disable the display of closed captions, audio description tracks, and other additional data associated with the video stream, though such features should, again, not interfere with the page's normal rendering.

User agents may allow users to view the video content in manners more suitable to the user (e.g. full-screen or in an independent resizable window). As for the other user interface features, controls to enable this should not interfere with the page's normal rendering unless the user agent is exposing a user interface. In such an independent context, however, user agents may make full user interfaces visible, with, e.g., play, pause, seeking, and volume controls, even if the controls attribute is absent.

User agents may allow video playback to affect system features that could interfere with the user's experience; for example, user agents could disable screensavers while video playback is in progress.

The poster IDL attribute must reflect the poster content attribute.

This example shows how to detect when a video has failed to play correctly:

<script>
 function failed(e) {
   // video playback failed - show a message saying why
   switch (e.target.error.code) {
     case e.target.error.MEDIA_ERR_ABORTED:
       alert('You aborted the video playback.');
       break;
     case e.target.error.MEDIA_ERR_NETWORK:
       alert('A network error caused the video download to fail part-way.');
       break;
     case e.target.error.MEDIA_ERR_DECODE:
       alert('The video playback was aborted due to a corruption problem or because the video used features your browser did not support.');
       break;
     case e.target.error.MEDIA_ERR_SRC_NOT_SUPPORTED:
       alert('The video could not be loaded, either because the server or network failed or because the format is not supported.');
       break;
     default:
       alert('An unknown error occurred.');
       break;
   }
 }
</script>
<p><video src="tgif.vid" autoplay controls onerror="failed(event)"></video></p>
<p><a href="tgif.vid">Download the video file</a>.</p>

4.7.7 The `audio` element

If the element has a controls attribute: Interactive content.

If the element has a controls attribute: Palpable content.

Where embedded content is expected.

If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.

If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.

src - Address of the resource

crossorigin - How the element handles crossorigin requests

preload - Hints how much buffering the media resource will likely need

autoplay - Hint that the media resource can be started automatically when the page is loaded

mediagroup - Groups media elements together with an implicit MediaController

loop - Whether to loop the media resource

muted - Whether to mute the media resource by default

controls - Show user agent controls

Tag omission in text/html

Neither tag is omissible

Allowed ARIA role attribute values:

application.

Any aria-* attributes applicable to the allowed roles.

[NamedConstructor=Audio(optional DOMString src)]
interface HTMLAudioElement : HTMLMediaElement {};

An audio element represents a sound or audio stream.

Content may be provided inside the audio element. User agents should not show this content to the user; it is intended for older Web browsers which do not support audio, so that legacy audio plugins can be tried, or to show text to the users of these older browsers informing them of how to access the audio contents.

In particular, this content is not intended to address accessibility concerns. To make audio content accessible to the deaf or to those with other physical or cognitive disabilities, a variety of features are available. If captions or a sign language video are available, the video element can be used instead of the audio element to play the audio, allowing users to enable the visual alternatives. Chapter titles can be provided to aid navigation, using the track element and a WebVTT file. And, naturally, transcripts or other textual alternatives can be provided by simply linking to them in the prose near the audio element. [WEBVTT]

The audio element is a media element whose media data is ostensibly audio data.

The src, preload, autoplay, mediagroup, loop, muted, and controls attributes are the attributes common to all media elements.

When an audio element is potentially playing, it must have its audio data played synchronized with the current playback position, at the element's effective media volume.

When an audio element is not potentially playing, audio must not play for the element.

audio = new Audio( [ url ] ): Returns a new audio element, with the src attribute set to the value passed in the argument, if applicable.

A constructor is provided for creating HTMLAudioElement objects (in addition to the factory methods from DOM such as createElement()): Audio(src). When invoked as a constructor, it must return a new HTMLAudioElement object (a new audio element). The element must have its preload attribute set to the literal value "auto". If the src argument is present, the object created must have its src content attribute set to the provided value, and the user agent must invoke the object's resource selection algorithm before returning. The element's document must be the active document of the browsing context of the Window object on which the interface object of the invoked constructor is found.

4.7.8 The `source` element

None.

As a child of a media element, before any flow content or track elements.

Empty.

src - Address of the resource

type - Type of embedded resource

Tag omission in text/html:

Allowed ARIA role attribute values:

None

interface HTMLSourceElement : HTMLElement {
           attribute DOMString src;
           attribute DOMString type;
           attribute DOMString media;
};

The source element allows authors to specify multiple alternative media resources for media elements. It does not represent anything on its own.

The src attribute gives the address of the media resource. The value must be a valid non-empty URL potentially surrounded by spaces. This attribute must be present.

Dynamically modifying a source element and its attribute when the element is already inserted in a video or audio element will have no effect. To change what is playing, just use the src attribute on the media element directly, possibly making use of the canPlayType() method to pick from amongst available resources. Generally, manipulating source elements manually after the document has been parsed is an unnecessarily complicated approach.

The type attribute gives the type of the media resource, to help the user agent determine if it can play this media resource before fetching it. If specified, its value must be a valid MIME type. The codecs parameter, which certain MIME types define, might be necessary to specify exactly how the resource is encoded. [RFC6381]

The following list shows some examples of how to use the codecs= MIME parameter in the type attribute.

H.264 Constrained baseline profile video (main and extended video compatible) level 3 and Low-Complexity AAC audio in MP4 container

<source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'>

H.264 Extended profile video (baseline-compatible) level 3 and Low-Complexity AAC audio in MP4 container

<source src='video.mp4' type='video/mp4; codecs="avc1.58A01E, mp4a.40.2"'>

H.264 Main profile video level 3 and Low-Complexity AAC audio in MP4 container

<source src='video.mp4' type='video/mp4; codecs="avc1.4D401E, mp4a.40.2"'>

H.264 'High' profile video (incompatible with main, baseline, or extended profiles) level 3 and Low-Complexity AAC audio in MP4 container

<source src='video.mp4' type='video/mp4; codecs="avc1.64001E, mp4a.40.2"'>

MPEG-4 Visual Simple Profile Level 0 video and Low-Complexity AAC audio in MP4 container

<source src='video.mp4' type='video/mp4; codecs="mp4v.20.8, mp4a.40.2"'>

MPEG-4 Advanced Simple Profile Level 0 video and Low-Complexity AAC audio in MP4 container

<source src='video.mp4' type='video/mp4; codecs="mp4v.20.240, mp4a.40.2"'>

MPEG-4 Visual Simple Profile Level 0 video and AMR audio in 3GPP container

<source src='video.3gp' type='video/3gpp; codecs="mp4v.20.8, samr"'>

Theora video and Vorbis audio in Ogg container

<source src='video.ogv' type='video/ogg; codecs="theora, vorbis"'>

Theora video and Speex audio in Ogg container

<source src='video.ogv' type='video/ogg; codecs="theora, speex"'>

Vorbis audio alone in Ogg container

<source src='audio.ogg' type='audio/ogg; codecs=vorbis'>

Speex audio alone in Ogg container

<source src='audio.spx' type='audio/ogg; codecs=speex'>

FLAC audio alone in Ogg container

<source src='audio.oga' type='audio/ogg; codecs=flac'>

Dirac video and Vorbis audio in Ogg container

<source src='video.ogv' type='video/ogg; codecs="dirac, vorbis"'>

The media attribute gives the intended media type of the media resource, to help the user agent determine if this media resource is useful to the user before fetching it. Its value must be a valid media query.

The resource selection algorithm is defined in such a way that when the media attribute is omitted the user agent acts the same as if the value was "all", i.e. by default the media resource is suitable for all media.

If a source element is inserted as a child of a media element that has no src attribute and whose networkState has the value NETWORK_EMPTY, the user agent must invoke the media element's resource selection algorithm.

The IDL attributes src, type, and media must reflect the respective content attributes of the same name.

If the author isn't sure if user agents will all be able to render the media resources provided, the author can listen to the error event on the last source element and trigger fallback behavior:

<script>
 function fallback(video) {
   // replace <video> with its contents
   while (video.hasChildNodes()) {
     if (video.firstChild instanceof HTMLSourceElement)
       video.removeChild(video.firstChild);
     else
       video.parentNode.insertBefore(video.firstChild, video);
   }
   video.parentNode.removeChild(video);
 }
</script>
<video controls autoplay>
 <source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'>
 <source src='video.ogv' type='video/ogg; codecs="theora, vorbis"'
         onerror="fallback(parentNode)">
 ...
</video>

4.7.9 The `track` element

None.

As a child of a media element, before any flow content.

Empty.

kind - The type of text track

src - Address of the resource

srclang - Language of the text track

label - User-visible label

default - Enable the track if no other text track is more suitable

Tag omission in text/html:

Allowed ARIA role attribute values:

None

interface HTMLTrackElement : HTMLElement {
           attribute DOMString kind;
           attribute DOMString src;
           attribute DOMString srclang;
           attribute DOMString label;
           attribute boolean default;

  const unsigned short NONE = 0;
  const unsigned short LOADING = 1;
  const unsigned short LOADED = 2;
  const unsigned short ERROR = 3;
  readonly attribute unsigned short readyState;

  readonly attribute TextTrack track;
};

The track element allows authors to specify explicit external timed text tracks for media elements. It does not represent anything on its own.

The kind attribute is an enumerated attribute. The following table lists the keywords defined for this attribute. The keyword given in the first cell of each row maps to the state given in the second cell.

Keyword	State	Brief description
`subtitles`	Subtitles	Transcription or translation of the dialogue, suitable for when the sound is available but not understood (e.g. because the user does not understand the language of the media resource's audio track). Overlaid on the video.
`captions`	Captions	Transcription or translation of the dialogue, sound effects, relevant musical cues, and other relevant audio information, suitable for when sound is unavailable or not clearly audible (e.g. because it is muted, drowned-out by ambient noise, or because the user is deaf). Overlaid on the video; labeled as appropriate for the hard-of-hearing.
`descriptions`	Descriptions	Textual descriptions of the video component of the media resource, intended for audio synthesis when the visual component is obscured, unavailable, or not usable (e.g. because the user is interacting with the application without a screen while driving, or because the user is blind). Synthesized as audio.
`chapters`	Chapters	Chapter titles, intended to be used for navigating the media resource. Displayed as an interactive (potentially nested) list in the user agent's interface.
`metadata`	Metadata	Tracks intended for use from script. Not displayed by the user agent.

The attribute may be omitted. The missing value default is the subtitles state.

The src attribute gives the address of the text track data. The value must be a valid non-empty URL potentially surrounded by spaces. This attribute must be present.

If the element has a src attribute whose value is not the empty string and whose value, when the attribute was set, could be successfully resolved relative to the element, then the element's track URL is the resulting absolute URL. Otherwise, the element's track URL is the empty string.

If the element's track URL identifies a WebVTT resource, and the element's kind attribute is not in the metadata state, then the WebVTT file must be a WebVTT file using cue text. [WEBVTT]

Furthermore, if the element's track URL identifies a WebVTT resource, and the element's kind attribute is in the chapters state, then the WebVTT file must be both a WebVTT file using chapter title text and a WebVTT file using only nested cues. [WEBVTT]

The srclang attribute gives the language of the text track data. The value must be a valid BCP 47 language tag. This attribute must be present if the element's kind attribute is in the subtitles state. [BCP47]

If the element has a srclang attribute whose value is not the empty string, then the element's track language is the value of the attribute. Otherwise, the element has no track language.

The label attribute gives a user-readable title for the track. This title is used by user agents when listing subtitle, caption, and audio description tracks in their user interface.

The value of the label attribute, if the attribute is present, must not be the empty string. Furthermore, there must not be two track element children of the same media element whose kind attributes are in the same state, whose srclang attributes are both missing or have values that represent the same language, and whose label attributes are again both missing or both have the same value.

If the element has a label attribute whose value is not the empty string, then the element's track label is the value of the attribute. Otherwise, the element's track label is an empty string.

The default attribute is a boolean attribute, which, if specified, indicates that the track is to be enabled if the user's preferences do not indicate that another track would be more appropriate.

Each media element must have no more than one track element child whose kind attribute is in the subtitles or captions state and whose default attribute is specified.

Each media element must have no more than one track element child whose kind attribute is in the description state and whose default attribute is specified.

Each media element must have no more than one track element child whose kind attribute is in the chapters state and whose default attribute is specified.

There is no limit on the number of track elements whose kind attribute is in the metadata state and whose default attribute is specified.

track . readyState

Returns the text track readiness state, represented by a number from the following list:

track . NONE (0): The text track not loaded state.
track . LOADING (1): The text track loading state.
track . LOADED (2): The text track loaded state.
track . ERROR (3): The text track failed to load state.

track . track

Returns the TextTrack object corresponding to the text track of the track element.

The readyState attribute must return the numeric value corresponding to the text track readiness state of the track element's text track, as defined by the following list:

NONE (numeric value 0): The text track not loaded state.
LOADING (numeric value 1): The text track loading state.
LOADED (numeric value 2): The text track loaded state.
ERROR (numeric value 3): The text track failed to load state.

The track IDL attribute must, on getting, return the track element's text track's corresponding TextTrack object.

The src, srclang, label, and default IDL attributes must reflect the respective content attributes of the same name. The kind IDL attribute must reflect the content attribute of the same name, limited to only known values.

This video has subtitles in several languages:

<video src="brave.webm">
 <track kind=subtitles src=brave.en.vtt srclang=en label="English">
 <track kind=captions src=brave.en.hoh.vtt srclang=en label="English for the Hard of Hearing">
 <track kind=subtitles src=brave.fr.vtt srclang=fr lang=fr label="Français">
 <track kind=subtitles src=brave.de.vtt srclang=de lang=de label="Deutsch">
</video>

(The lang attributes on the last two describe the language of the label attribute, not the language of the subtitles themselves. The language of the subtitles is given by the srclang attribute.)

4.7.10 Media elements

Media elements (audio and video, in this specification) implement the following interface:

enum CanPlayTypeEnum { "" /* empty string */, "maybe", "probably" };
interface HTMLMediaElement : HTMLElement {

  // error state
  readonly attribute MediaError? error;

  // network state
           attribute DOMString src;
  readonly attribute DOMString currentSrc;
           attribute DOMString crossOrigin;
  const unsigned short NETWORK_EMPTY = 0;
  const unsigned short NETWORK_IDLE = 1;
  const unsigned short NETWORK_LOADING = 2;
  const unsigned short NETWORK_NO_SOURCE = 3;
  readonly attribute unsigned short networkState;
           attribute DOMString preload;
  readonly attribute TimeRanges buffered;
  void load();
  CanPlayTypeEnum canPlayType(DOMString type);

  // ready state
  const unsigned short HAVE_NOTHING = 0;
  const unsigned short HAVE_METADATA = 1;
  const unsigned short HAVE_CURRENT_DATA = 2;
  const unsigned short HAVE_FUTURE_DATA = 3;
  const unsigned short HAVE_ENOUGH_DATA = 4;
  readonly attribute unsigned short readyState;
  readonly attribute boolean seeking;

  // playback state
           attribute double currentTime;
  readonly attribute unrestricted double duration;
  Date getStartDate();
  readonly attribute boolean paused;
           attribute double defaultPlaybackRate;
           attribute double playbackRate;
  readonly attribute TimeRanges played;
  readonly attribute TimeRanges seekable;
  readonly attribute boolean ended;
           attribute boolean autoplay;
           attribute boolean loop;
  void play();
  void pause();

  // media controller
           attribute DOMString mediaGroup;
           attribute MediaController? controller;

  // controls
           attribute boolean controls;
           attribute double volume;
           attribute boolean muted;
           attribute boolean defaultMuted;

  // tracks
  readonly attribute AudioTrackList audioTracks;
  readonly attribute VideoTrackList videoTracks;
  readonly attribute TextTrackList textTracks;
  TextTrack addTextTrack(TextTrackKind kind, optional DOMString label = "", optional DOMString language = "");
};

The media element attributes, src, crossorigin, preload, autoplay, mediagroup, loop, muted, and controls, apply to all media elements. They are defined in this section.

Media elements are used to present audio data, or video and audio data, to the user. This is referred to as media data in this section, since this section applies equally to media elements for audio or for video. The term media resource is used to refer to the complete set of media data, e.g. the complete video file, or complete audio file.

A media resource can have multiple audio and video tracks. For the purposes of a media element, the video data of the media resource is only that of the currently selected track (if any) given by the element's videoTracks attribute, and the audio data of the media resource is the result of mixing all the currently enabled tracks (if any) given by the element's audioTracks attribute.

Both audio and video elements can be used for both audio and video. The main difference between the two is simply that the audio element has no playback area for visual content (such as video or captions), whereas the video element does.

Except where otherwise explicitly specified, the task source for all the tasks queued in this section and its subsections is the media element event task source of the media element in question.

4.7.10.1 Error codes

media . error

Returns a MediaError object representing the current error state of the element.

Returns null if there is no error.

All media elements have an associated error status, which records the last error the element encountered since its resource selection algorithm was last invoked. The error attribute, on getting, must return the MediaError object created for this last error, or null if there has not been an error.

interface MediaError {
  const unsigned short MEDIA_ERR_ABORTED = 1;
  const unsigned short MEDIA_ERR_NETWORK = 2;
  const unsigned short MEDIA_ERR_DECODE = 3;
  const unsigned short MEDIA_ERR_SRC_NOT_SUPPORTED = 4;
  readonly attribute unsigned short code;
};

media . error . code: Returns the current error's error code, from the list below.

The code attribute of a MediaError object must return the code for the error, which must be one of the following:

MEDIA_ERR_ABORTED (numeric value 1): The fetching process for the media resource was aborted by the user agent at the user's request.
MEDIA_ERR_NETWORK (numeric value 2): A network error of some description caused the user agent to stop fetching the media resource, after the resource was established to be usable.
MEDIA_ERR_DECODE (numeric value 3): An error of some description occurred while decoding the media resource, after the resource was established to be usable.
MEDIA_ERR_SRC_NOT_SUPPORTED (numeric value 4): The media resource indicated by the src attribute was not suitable.

4.7.10.2 Location of the media resource

The src content attribute on media elements gives the address of the media resource (video, audio) to show. The attribute, if present, must contain a valid non-empty URL potentially surrounded by spaces.

The crossorigin content attribute on media elements is a CORS settings attribute.

If a src attribute of a media element is set or changed, the user agent must invoke the media element's media element load algorithm. (Removing the src attribute does not do this, even if there are source elements present.)

The src IDL attribute on media elements must reflect the content attribute of the same name.

The crossOrigin IDL attribute must reflect the crossorigin content attribute, limited to only known values.

media . currentSrc

Returns the address of the current media resource.

Returns the empty string when there is no media resource.

The currentSrc IDL attribute is initially the empty string. Its value is changed by the resource selection algorithm defined below.

There are two ways to specify a media resource, the src attribute, or source elements. The attribute overrides the elements.

4.7.10.3 MIME types

A media resource can be described in terms of its type, specifically a MIME type, in some cases with a codecs parameter. (Whether the codecs parameter is allowed or not depends on the MIME type.) [RFC6381]

Types are usually somewhat incomplete descriptions; for example "video/mpeg" doesn't say anything except what the container type is, and even a type like "video/mp4; codecs="avc1.42E01E, mp4a.40.2"" doesn't include information like the actual bitrate (only the maximum bitrate). Thus, given a type, a user agent can often only know whether it might be able to play media of that type (with varying levels of confidence), or whether it definitely cannot play media of that type.

A type that the user agent knows it cannot render is one that describes a resource that the user agent definitely does not support, for example because it doesn't recognize the container type, or it doesn't support the listed codecs.

The MIME type "application/octet-stream" with no parameters is never a type that the user agent knows it cannot render. User agents must treat that type as equivalent to the lack of any explicit Content-Type metadata when it is used to label a potential media resource.

Only the MIME type "application/octet-stream" with no parameters is special-cased here; if any parameter appears with it, it will be treated just like any other MIME type. This is a deviation from the rule that unknown MIME type parameters should be ignored.

media . canPlayType(type): Returns the empty string (a negative response), "maybe", or "probably" based on how confident the user agent is that it can play media resources of the given type.

The canPlayType(type) method must return the empty string if type is a type that the user agent knows it cannot render or is the type "application/octet-stream"; it must return "probably" if the user agent is confident that the type represents a media resource that it can render if used in with this audio or video element; and it must return "maybe" otherwise. Implementors are encouraged to return "maybe" unless the type can be confidently established as being supported or not. Generally, a user agent should never return "probably" for a type that allows the codecs parameter if that parameter is not present.

This script tests to see if the user agent supports a (fictional) new format to dynamically decide whether to use a video element or a plugin:

<section id="video">
 <p><a href="playing-cats.nfv">Download video</a></p>
</section>
<script>
 var videoSection = document.getElementById('video');
 var videoElement = document.createElement('video');
 var support = videoElement.canPlayType('video/x-new-fictional-format;codecs="kittens,bunnies"');
 if (support != "probably" && "New Fictional Video Plugin" in navigator.plugins) {
   // not confident of browser support
   // but we have a plugin
   // so use plugin instead
   videoElement = document.createElement("embed");
 } else if (support == "") {
   // no support from browser and no plugin
   // do nothing
   videoElement = null;
 }
 if (videoElement) {
   while (videoSection.hasChildNodes())
     videoSection.removeChild(videoSection.firstChild);
   videoElement.setAttribute("src", "playing-cats.nfv");
   videoSection.appendChild(videoElement);
 }
</script>

The type attribute of the source element allows the user agent to avoid downloading resources that use formats it cannot render.

4.7.10.4 Network states

media . networkState: Returns the current state of network activity for the element, from the codes in the list below.

As media elements interact with the network, their current network activity is represented by the networkState attribute. On getting, it must return the current network state of the element, which must be one of the following values:

NETWORK_EMPTY (numeric value 0): The element has not yet been initialized. All attributes are in their initial states.
NETWORK_IDLE (numeric value 1): The element's resource selection algorithm is active and has selected a resource, but it is not actually using the network at this time.
NETWORK_LOADING (numeric value 2): The user agent is actively trying to download data.
NETWORK_NO_SOURCE (numeric value 3): The element's resource selection algorithm is active, but it has not yet found a resource to use.

The resource selection algorithm defined below describes exactly when the networkState attribute changes value and what events fire to indicate changes in this state.

4.7.10.5 Loading the media resource

media . load(): Causes the element to reset and start selecting and loading a new media resource from scratch.

All media elements have an autoplaying flag, which must begin in the true state, and a delaying-the-load-event flag, which must begin in the false state. While the delaying-the-load-event flag is true, the element must delay the load event of its document.

When the load() method on a media element is invoked, the user agent must run the media element load algorithm.

The media element load algorithm consists of the following steps.

Abort any already-running instance of the resource selection algorithm for this element.
If there are any tasks from the media element's media element event task source in one of the task queues, then remove those tasks.

If there are any tasks that were queued by the resource selection algorithm (including the algorithms that it itself invokes) for this same media element from the DOM manipulation task source in one of the task queues, then remove those tasks.

Basically, pending events and callbacks for the media element are discarded when the media element starts loading a new resource.
If the media element's networkState is set to NETWORK_LOADING or NETWORK_IDLE, queue a task to fire a simple event named abort at the media element.
If the media element's networkState is not set to NETWORK_EMPTY, then run these substeps:
1. Queue a task to fire a simple event named emptied at the media element.
2. If a fetching process is in progress for the media element, the user agent should stop it.
3. Forget the media element's media-resource-specific tracks.
4. If readyState is not set to HAVE_NOTHING, then set it to that state.
5. If the paused attribute is false, then set it to true.
6. If seeking is true, set it to false.
7. Set the current playback position to 0.
  
  Set the official playback position to 0.
  
  If this changed the official playback position, then queue a task to fire a simple event named timeupdate at the media element.
8. Set the initial playback position to 0.
9. Set the timeline offset to Not-a-Number (NaN).
10. Update the duration attribute to Not-a-Number (NaN).
  
  The user agent will not fire a durationchange event for this particular change of the duration.
Set the playbackRate attribute to the value of the defaultPlaybackRate attribute.
Set the error attribute to null and the autoplaying flag to true.
Invoke the media element's resource selection algorithm.
Playback of any previously playing media resource for this element stops.

The resource selection algorithm for a media element is as follows. This algorithm is always invoked synchronously, but one of the first steps in the algorithm is to return and continue running the remaining steps asynchronously, meaning that it runs in the background with scripts and other tasks running in parallel. In addition, this algorithm interacts closely with the event loop mechanism; in particular, it has synchronous sections (which are triggered as part of the event loop algorithm). Steps in such sections are marked with ⌛.

Set the element's networkState attribute to the NETWORK_NO_SOURCE value.
Set the element's show poster flag to true.
Set the media element's delaying-the-load-event flag to true (this delays the load event).
Asynchronously await a stable state, allowing the task that invoked this algorithm to continue. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ If the media element's blocked-on-parser flag is false, then populate the list of pending text tracks.
⌛ If the media element has a src attribute, then let mode be attribute.

⌛ Otherwise, if the media element does not have a src attribute but has a source element child, then let mode be children and let candidate be the first such source element child in tree order.

⌛ Otherwise the media element has neither a src attribute nor a source element child: set the networkState to NETWORK_EMPTY, and abort these steps; the synchronous section ends.
⌛ Set the media element's networkState to NETWORK_LOADING.
⌛ Queue a task to fire a simple event named loadstart at the media element.
If mode is attribute, then run these substeps:
1. ⌛ If the src attribute's value is the empty string, then end the synchronous section, and jump down to the failed with attribute step below.
2. ⌛ Let absolute URL be the absolute URL that would have resulted from resolving the URL specified by the src attribute's value relative to the media element when the src attribute was last changed.
3. ⌛ If absolute URL was obtained successfully, set the currentSrc attribute to absolute URL.
4. End the synchronous section, continuing the remaining steps asynchronously.
5. If absolute URL was obtained successfully, run the resource fetch algorithm with absolute URL. If that algorithm returns without aborting this one, then the load failed.
6. Failed with attribute: Reaching this step indicates that the media resource failed to load or that the given URL could not be resolved. Queue a task to run the following steps, using the DOM manipulation task source:
  1. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_SRC_NOT_SUPPORTED.
  2. Forget the media element's media-resource-specific tracks.
  3. Set the element's networkState attribute to the NETWORK_NO_SOURCE value.
  4. Set the element's show poster flag to true.
  5. Fire a simple event named error at the media element.
  6. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
7. Wait for the task queued by the previous step to have executed.
8. Abort these steps. Until the load() method is invoked or the src attribute is changed, the element won't attempt to load another resource.
Otherwise, the source elements will be used; run these substeps:
1. ⌛ Let pointer be a position defined by two adjacent nodes in the media element's child list, treating the start of the list (before the first child in the list, if any) and end of the list (after the last child in the list, if any) as nodes in their own right. One node is the node before pointer, and the other node is the node after pointer. Initially, let pointer be the position between the candidate node and the next node, if there are any, or the end of the list, if it is the last node.
  
  As nodes are inserted and removed into the media element, pointer must be updated as follows:
  
  If a new node is inserted between the two nodes that define pointer
  
  Let pointer be the point between the node before pointer and the new node. In other words, insertions at pointer go after pointer.
  
  If the node before pointer is removed
  
  Let pointer be the point between the node after pointer and the node before the node after pointer. In other words, pointer doesn't move relative to the remaining nodes.
  
  If the node after pointer is removed
  
  Let pointer be the point between the node before pointer and the node after the node before pointer. Just as with the previous case, pointer doesn't move relative to the remaining nodes.
  
  Other changes don't affect pointer.
2. ⌛ Process candidate: If candidate does not have a src attribute, or if its src attribute's value is the empty string, then end the synchronous section, and jump down to the failed with elements step below.
3. ⌛ Let absolute URL be the absolute URL that would have resulted from resolving the URL specified by candidate's src attribute's value relative to the candidate when the src attribute was last changed.
4. ⌛ If absolute URL was not obtained successfully, then end the synchronous section, and jump down to the failed with elements step below.
5. ⌛ If candidate has a type attribute whose value, when parsed as a MIME type (including any codecs described by the codecs parameter, for types that define that parameter), represents a type that the user agent knows it cannot render, then end the synchronous section, and jump down to the failed with elements step below.
6. ⌛ If candidate has a media attribute whose value does not match the environment, then end the synchronous section, and jump down to the failed with elements step below.
7. ⌛ Set the currentSrc attribute to absolute URL.
8. End the synchronous section, continuing the remaining steps asynchronously.
9. Run the resource fetch algorithm with absolute URL. If that algorithm returns without aborting this one, then the load failed.
10. Failed with elements: Queue a task, using the DOM manipulation task source, to fire a simple event named error at the candidate element.
11. Asynchronously await a stable state. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
12. ⌛ Forget the media element's media-resource-specific tracks.
13. ⌛ Find next candidate: Let candidate be null.
14. ⌛ Search loop: If the node after pointer is the end of the list, then jump to the waiting step below.
15. ⌛ If the node after pointer is a source element, let candidate be that element.
16. ⌛ Advance pointer so that the node before pointer is now the node that was after pointer, and the node after pointer is the node after the node that used to be after pointer, if any.
17. ⌛ If candidate is null, jump back to the search loop step. Otherwise, jump back to the process candidate step.
18. ⌛ Waiting: Set the element's networkState attribute to the NETWORK_NO_SOURCE value.
19. ⌛ Set the element's show poster flag to true.
20. ⌛ Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
21. End the synchronous section, continuing the remaining steps asynchronously.
22. Wait until the node after pointer is a node other than the end of the list. (This step might wait forever.)
23. Asynchronously await a stable state. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
24. ⌛ Set the element's delaying-the-load-event flag back to true (this delays the load event again, in case it hasn't been fired yet).
25. ⌛ Set the networkState back to NETWORK_LOADING.
26. ⌛ Jump back to the find next candidate step above.

The resource fetch algorithm for a media element and a given absolute URL is as follows:

Let the current media resource be the resource given by the absolute URL passed to this algorithm. This is now the element's media resource.
Remove all media-resource-specific text tracks from the media element's list of pending text tracks, if any.
Optionally, run the following substeps. This is the expected behavior if the user agent intends to not attempt to fetch the resource until the user requests it explicitly (e.g. as a way to implement the preload attribute's none keyword).
1. Set the networkState to NETWORK_IDLE.
2. Queue a task to fire a simple event named suspend at the element, using the DOM manipulation task source.
3. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
4. Wait for the task to be run.
5. Wait for an implementation-defined event (e.g. the user requesting that the media element begin playback).
6. Set the element's delaying-the-load-event flag back to true (this delays the load event again, in case it hasn't been fired yet).
7. Set the networkState to NETWORK_LOADING.
Perform a potentially CORS-enabled fetch of the current media resource's absolute URL, with the mode being the state of the media element's crossorigin content attribute, the origin being the origin of the media element's Document, and the default origin behaviour set to taint.

The resource obtained in this fashion, if any, contains the media data. It can be CORS-same-origin or CORS-cross-origin; this affects whether subtitles referenced in the media data are exposed in the API and, for video elements, whether a canvas gets tainted when the video is drawn on it.

While the load is not suspended (see below), every 350ms (±200ms) or for every byte received, whichever is least frequent, queue a task to fire a simple event named progress at the element.

The stall timeout is a user-agent defined length of time, which should be about three seconds. When a media element that is actively attempting to obtain media data has failed to receive any data for a duration equal to the stall timeout, the user agent must queue a task to fire a simple event named stalled at the element.

User agents may allow users to selectively block or slow media data downloads. When a media element's download has been blocked altogether, the user agent must act as if it was stalled (as opposed to acting as if the connection was closed). The rate of the download may also be throttled automatically by the user agent, e.g. to balance the download with other connections sharing the same bandwidth.

User agents may decide to not download more content at any time, e.g. after buffering five minutes of a one hour media resource, while waiting for the user to decide whether to play the resource or not, while waiting for user input in an interactive resource, or when the user navigates away from the page. When a media element's download has been suspended, the user agent must queue a task, using the DOM manipulation task source, to set the networkState to NETWORK_IDLE and fire a simple event named suspend at the element. If and when downloading of the resource resumes, the user agent must queue a task to set the networkState to NETWORK_LOADING. Between the queuing of these tasks, the load is suspended (so progress events don't fire, as described above).

The preload attribute provides a hint regarding how much buffering the author thinks is advisable, even in the absence of the autoplay attribute.

When a user agent decides to completely stall a download, e.g. if it is waiting until the user starts playback before downloading any further content, the element's delaying-the-load-event flag must be set to false. This stops delaying the load event.

The user agent may use whatever means necessary to fetch the resource (within the constraints put forward by this and other specifications); for example, reconnecting to the server in the face of network errors, using HTTP range retrieval requests, or switching to a streaming protocol. The user agent must consider a resource erroneous only if it has given up trying to fetch it.

This specification does not currently say whether or how to check the MIME types of the media resources, or whether or how to perform file type sniffing using the actual file data. Implementors differ in their intentions on this matter and it is therefore unclear what the right solution is. In the absence of any requirement here, the HTTP specification's strict requirement to follow the Content-Type header prevails ("Content-Type specifies the media type of the underlying data." ... "If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource.").

The networking task source tasks to process the data as it is being fetched must, when appropriate, include the relevant substeps from the following list:
If the media data cannot be fetched at all, due to network errors, causing the user agent to give up trying to fetch the resource

If the media data can be fetched but is found by inspection to be in an unsupported format, or can otherwise not be rendered at all
DNS errors, HTTP 4xx and 5xx errors (and equivalents in other protocols), and other fatal network errors that occur before the user agent has established whether the current media resource is usable, as well as the file using an unsupported container format, or using unsupported codecs for all the data, must cause the user agent to execute the following steps:
1. The user agent should cancel the fetching process.
2. Abort this subalgorithm, returning to the resource selection algorithm.
If the media resource is found to have an audio track
1. Create an AudioTrack object to represent the audio track.
2. Update the media element's audioTracks attribute's AudioTrackList object with the new AudioTrack object.
3. Fire a trusted event with the name addtrack, that does not bubble and is not cancelable, and that uses the TrackEvent interface, with the track attribute initialized to the new AudioTrack object, at this AudioTrackList object.
If the media resource is found to have a video track
1. Create a VideoTrack object to represent the video track.
2. Update the media element's videoTracks attribute's VideoTrackList object with the new VideoTrack object.
3. Fire a trusted event with the name addtrack, that does not bubble and is not cancelable, and that uses the TrackEvent interface, with the track attribute initialized to the new VideoTrack object, at this VideoTrackList object.
Once enough of the media data has been fetched to determine the duration of the media resource, its dimensions, and other metadata
This indicates that the resource is usable. The user agent must follow these substeps:
1. Establish the media timeline for the purposes of the current playback position, the earliest possible position, and the initial playback position, based on the media data.
2. Update the timeline offset to the date and time that corresponds to the zero time in the media timeline established in the previous step, if any. If no explicit time and date is given by the media resource, the timeline offset must be set to Not-a-Number (NaN).
3. Set the current playback position and the official playback position to the earliest possible position.
4. Update the duration attribute with the time of the last frame of the resource, if known, on the media timeline established above. If it is not known (e.g. a stream that is in principle infinite), update the duration attribute to the value positive Infinity.
  
  The user agent will queue a task to fire a simple event named durationchange at the element at this point.
5. For video elements, set the videoWidth and videoHeight attributes, and queue a task to fire a simple event named resize at the media element.
  
  Further resize events will be fired if the dimensions subsequently change.
6. Set the readyState attribute to HAVE_METADATA.
  
  A loadedmetadata DOM event will be fired as part of setting the readyState attribute to a new value.
7. Let jumped be false.
8. If the media element's default playback start position is greater than zero, then seek to that time, and let jumped be true.
9. Let the media element's default playback start position be zero.
10. If either the media resource or the address of the current media resource indicate a particular start time, then set the initial playback position to that time and, if jumped is still false, seek to that time and let jumped be true.
  
  For example, with media formats that support the Media Fragments URI fragment identifier syntax, the fragment identifier can be used to indicate a start position. [MEDIAFRAG]
11. If either the media resource or the address of the current media resource indicate a particular set of audio or video tracks to enable, or if the user agent has information that would enable it to select specific tracks to improve the user's experience, then the relevant audio tracks must be enabled in the element's audioTracks object, and, of the relevant video tracks, the one that is listed first in the element's videoTracks object must be selected. All other tracks must be disabled.
  
  This could again be triggered by Media Fragments URI fragment identifier syntax, but it could also be triggered e.g. by the user agent selecting a 5.1 surround sound audio track over a stereo audio track. [MEDIAFRAG]
12. If the media element has a current media controller, then: if jumped is true and the initial playback position, relative to the current media controller's timeline, is greater than the current media controller's media controller position, then seek the media controller to the media element's initial playback position, relative to the current media controller's timeline; otherwise, seek the media element to the media controller position, relative to the media element's timeline.
Once the readyState attribute reaches HAVE_CURRENT_DATA, after the loadeddata event has been fired, set the element's delaying-the-load-event flag to false. This stops delaying the load event.

A user agent that is attempting to reduce network usage while still fetching the metadata for each media resource would also stop buffering at this point, following the rules described previously, which involve the networkState attribute switching to the NETWORK_IDLE value and a suspend event firing.

The user agent is required to determine the duration of the media resource and go through this step before playing.
Once the entire media resource has been fetched (but potentially before any of it has been decoded)

Fire a simple event named progress at the media element.

Set the networkState to NETWORK_IDLE and fire a simple event named suspend at the media element.

If the user agent ever discards any media data and then needs to resume the network activity to obtain it again, then it must queue a task to set the networkState to NETWORK_LOADING.

If the user agent can keep the media resource loaded, then the algorithm will continue to its final step below, which aborts the algorithm.

If the connection is interrupted after some media data has been received, causing the user agent to give up trying to fetch the resource
Fatal network errors that occur after the user agent has established whether the current media resource is usable (i.e. once the media element's readyState attribute is no longer HAVE_NOTHING) must cause the user agent to execute the following steps:
1. The user agent should cancel the fetching process.
2. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_NETWORK.
3. Fire a simple event named error at the media element.
4. Set the element's networkState attribute to the NETWORK_IDLE value.
5. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
6. Abort the overall resource selection algorithm.
If the media data is corrupted
Fatal errors in decoding the media data that occur after the user agent has established whether the current media resource is usable must cause the user agent to execute the following steps:
1. The user agent should cancel the fetching process.
2. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_DECODE.
3. Fire a simple event named error at the media element.
4. If the media element's readyState attribute has a value equal to HAVE_NOTHING, set the element's networkState attribute to the NETWORK_EMPTY value, set the element's show poster flag to true, and fire a simple event named emptied at the element.
  
  Otherwise, set the element's networkState attribute to the NETWORK_IDLE value.
5. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
6. Abort the overall resource selection algorithm.
If the media data fetching process is aborted by the user
The fetching process is aborted by the user, e.g. because the user pressed a "stop" button, the user agent must execute the following steps. These steps are not followed if the load() method itself is invoked while these steps are running, as the steps above handle that particular kind of abort.
1. The user agent should cancel the fetching process.
2. Set the error attribute to a new MediaError object whose code attribute is set to MEDIA_ERR_ABORTED.
3. Fire a simple event named abort at the media element.
4. If the media element's readyState attribute has a value equal to HAVE_NOTHING, set the element's networkState attribute to the NETWORK_EMPTY value, set the element's show poster flag to true, and fire a simple event named emptied at the element.
  
  Otherwise, set the element's networkState attribute to the NETWORK_IDLE value.
5. Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
6. Abort the overall resource selection algorithm.
If the media data can be fetched but has non-fatal errors or uses, in part, codecs that are unsupported, preventing the user agent from rendering the content completely correctly but not preventing playback altogether

The server returning data that is partially usable but cannot be optimally rendered must cause the user agent to render just the bits it can handle, and ignore the rest.

If the media resource is found to declare a media-resource-specific text track that the user agent supports

If the media data is CORS-same-origin, run the steps to expose a media-resource-specific text track with the relevant data.

Cross-origin videos do not expose their subtitles, since that would allow attacks such as hostile sites reading subtitles from confidential videos on a user's intranet.
When the networking task source has queued the last task as part of fetching the media resource (i.e. once the download has completed), if the fetching process completes without errors, including decoding the media data, and if all of the data is available to the user agent without network access, then, the user agent must move on to the next step. This might never happen, e.g. when streaming an infinite resource such as Web radio, or if the resource is longer than the user agent's ability to cache data.

While the user agent might still need network access to obtain parts of the media resource, the user agent must remain on this step.

For example, if the user agent has discarded the first half of a video, the user agent will remain at this step even once the playback has ended, because there is always the chance the user will seek back to the start. In fact, in this situation, once playback has ended, the user agent will end up firing a suspend event, as described earlier.
If the user agent ever reaches this step (which can only happen if the entire resource gets loaded and kept available): abort the overall resource selection algorithm.

When a media element is to forget the media element's media-resource-specific tracks, the user agent must remove from the media element's list of text tracks all the media-resource-specific text tracks, then empty the media element's audioTracks attribute's AudioTrackList object, then empty the media element's videoTracks attribute's VideoTrackList object. No events (in particular, no removetrack events) are fired as part of this; the error and emptied events, fired by the algorithms that invoke this one, can be used instead.

The preload attribute is an enumerated attribute. The following table lists the keywords and states for the attribute — the keywords in the left column map to the states in the cell in the second column on the same row as the keyword. The attribute can be changed even once the media resource is being buffered or played; the descriptions in the table below are to be interpreted with that in mind.

Keyword	State	Brief description
`none`	None	Hints to the user agent that either the author does not expect the user to need the media resource, or that the server wants to minimize unnecessary traffic. This state does not provide a hint regarding how aggressively to actually download the media resource if buffering starts anyway (e.g. once the user hits "play").
`metadata`	Metadata	Hints to the user agent that the author does not expect the user to need the media resource, but that fetching the resource metadata (dimensions, track list, duration, etc), and maybe even the first few frames, is reasonable. If the user agent precisely fetches no more than the metadata, then the media element will end up with its `readyState` attribute set to `HAVE_METADATA`; typically though, some frames will be obtained as well and it will probably be `HAVE_CURRENT_DATA` or `HAVE_FUTURE_DATA`. When the media resource is playing, hints to the user agent that bandwidth is to be considered scarce, e.g. suggesting throttling the download so that the media data is obtained at the slowest possible rate that still maintains consistent playback.
`auto`	Automatic	Hints to the user agent that the user agent can put the user's needs first without risk to the server, up to and including optimistically downloading the entire resource.

The empty string is also a valid keyword, and maps to the Automatic state. The attribute's missing value default is user-agent defined, though the Metadata state is suggested as a compromise between reducing server load and providing an optimal user experience.

Authors might switch the attribute from "none" or "metadata" to "auto" dynamically once the user begins playback. For example, on a page with many videos this might be used to indicate that the many videos are not to be downloaded unless requested, but that once one is requested it is to be downloaded aggressively.

The preload attribute is intended to provide a hint to the user agent about what the author thinks will lead to the best user experience. The attribute may be ignored altogether, for example based on explicit user preferences or based on the available connectivity.

The preload IDL attribute must reflect the content attribute of the same name, limited to only known values.

The autoplay attribute can override the preload attribute (since if the media plays, it naturally has to buffer first, regardless of the hint given by the preload attribute). Including both is not an error, however.

media . buffered: Returns a TimeRanges object that represents the ranges of the media resource that the user agent has buffered.

The buffered attribute must return a new static normalized TimeRanges object that represents the ranges of the media resource, if any, that the user agent has buffered, at the time the attribute is evaluated. Users agents must accurately determine the ranges available, even for media streams where this can only be determined by tedious inspection.

Typically this will be a single range anchored at the zero point, but if, e.g. the user agent uses HTTP range requests in response to seeking, then there could be multiple ranges.

User agents may discard previously buffered data.

Thus, a time position included within a range of the objects return by the buffered attribute at one time can end up being not included in the range(s) of objects returned by the same attribute at later times.

4.7.10.6 Offsets into the media resource

media . duration

Returns the length of the media resource, in seconds, assuming that the start of the media resource is at time zero.

Returns NaN if the duration isn't available.

Returns Infinity for unbounded streams.

media . currentTime [ = value ]

Returns the official playback position, in seconds.

Can be set, to seek to the given time.

Will throw an InvalidStateError exception if there is no selected media resource or if there is a current media controller.

A media resource has a media timeline that maps times (in seconds) to positions in the media resource. The origin of a timeline is its earliest defined position. The duration of a timeline is its last defined position.

Establishing the media timeline: If the media resource somehow specifies an explicit timeline whose origin is not negative (i.e. gives each frame a specific time offset and gives the first frame a zero or positive offset), then the media timeline should be that timeline. (Whether the media resource can specify a timeline or not depends on the media resource's format.) If the media resource specifies an explicit start time and date, then that time and date should be considered the zero point in the media timeline; the timeline offset will be the time and date, exposed using the getStartDate() method.

If the media resource has a discontinuous timeline, the user agent must extend the timeline used at the start of the resource across the entire resource, so that the media timeline of the media resource increases linearly starting from the earliest possible position (as defined below), even if the underlying media data has out-of-order or even overlapping time codes.

For example, if two clips have been concatenated into one video file, but the video format exposes the original times for the two clips, the video data might expose a timeline that goes, say, 00:15..00:29 and then 00:05..00:38. However, the user agent would not expose those times; it would instead expose the times as 00:15..00:29 and 00:29..01:02, as a single video.

In the rare case of a media resource that does not have an explicit timeline, the zero time on the media timeline should correspond to the first frame of the media resource. In the even rarer case of a media resource with no explicit timings of any kind, not even frame durations, the user agent must itself determine the time for each frame in a user-agent-defined manner.

An example of a file format with no explicit timeline but with explicit frame durations is the Animated GIF format. An example of a file format with no explicit timings at all is the JPEG-push format (multipart/x-mixed-replace with JPEG frames, often used as the format for MJPEG streams).

If, in the case of a resource with no timing information, the user agent will nonetheless be able to seek to an earlier point than the first frame originally provided by the server, then the zero time should correspond to the earliest seekable time of the media resource; otherwise, it should correspond to the first frame received from the server (the point in the media resource at which the user agent began receiving the stream).

At the time of writing, there is no known format that lacks explicit frame time offsets yet still supports seeking to a frame before the first frame sent by the server.

Consider a stream from a TV broadcaster, which begins streaming on a sunny Friday afternoon in October, and always sends connecting user agents the media data on the same media timeline, with its zero time set to the start of this stream. Months later, user agents connecting to this stream will find that the first frame they receive has a time with millions of seconds. The getStartDate() method would always return the date that the broadcast started; this would allow controllers to display real times in their scrubber (e.g. "2:30pm") rather than a time relative to when the broadcast began ("8 months, 4 hours, 12 minutes, and 23 seconds").

Consider a stream that carries a video with several concatenated fragments, broadcast by a server that does not allow user agents to request specific times but instead just streams the video data in a predetermined order, with the first frame delivered always being identified as the frame with time zero. If a user agent connects to this stream and receives fragments defined as covering timestamps 2010-03-20 23:15:00 UTC to 2010-03-21 00:05:00 UTC and 2010-02-12 14:25:00 UTC to 2010-02-12 14:35:00 UTC, it would expose this with a media timeline starting at 0s and extending to 3,600s (one hour). Assuming the streaming server disconnected at the end of the second clip, the duration attribute would then return 3,600. The getStartDate() method would return a Date object with a time corresponding to 2010-03-20 23:15:00 UTC. However, if a different user agent connected five minutes later, it would (presumably) receive fragments covering timestamps 2010-03-20 23:20:00 UTC to 2010-03-21 00:05:00 UTC and 2010-02-12 14:25:00 UTC to 2010-02-12 14:35:00 UTC, and would expose this with a media timeline starting at 0s and extending to 3,300s (fifty five minutes). In this case, the getStartDate() method would return a Date object with a time corresponding to 2010-03-20 23:20:00 UTC.

In both of these examples, the seekable attribute would give the ranges that the controller would want to actually display in its UI; typically, if the servers don't support seeking to arbitrary times, this would be the range of time from the moment the user agent connected to the stream up to the latest frame that the user agent has obtained; however, if the user agent starts discarding earlier information, the actual range might be shorter.

In any case, the user agent must ensure that the earliest possible position (as defined below) using the established media timeline, is greater than or equal to zero.

The media timeline also has an associated clock. Which clock is used is user-agent defined, and may be media resource-dependent, but it should approximate the user's wall clock.

All the media elements that share current media controller use the same clock for their media timeline.

Media elements have a current playback position, which must initially (i.e. in the absence of media data) be zero seconds. The current playback position is a time on the media timeline.

Media elements also have an official playback position, which must initially be set to zero seconds. The official playback position is an approximation of the current playback position that is kept stable while scripts are running.

Media elements also have a default playback start position, which must initially be set to zero seconds. This time is used to allow the element to be seeked even before the media is loaded.

Each media element has a show poster flag. When a media element is created, this flag must be set to true. This flag is used to control when the user agent is to show a poster frame for a video element instead of showing the video contents.

The currentTime attribute must, on getting, return the media element's default playback start position, unless that is zero, in which case it must return the element's official playback position. The returned value must be expressed in seconds. On setting, if the media element has a current media controller, then the user agent must throw an InvalidStateError exception; otherwise, if the media element's readyState is HAVE_NOTHING, then it must set the media element's default playback start position to the new value; otherwise, it must set the official playback position to the new value and then seek to the new value. The new value must be interpreted as being in seconds.

Media elements have an initial playback position, which must initially (i.e. in the absence of media data) be zero seconds. The initial playback position is updated when a media resource is loaded. The initial playback position is a time on the media timeline.

If the media resource is a streaming resource, then the user agent might be unable to obtain certain parts of the resource after it has expired from its buffer. Similarly, some media resources might have a media timeline that doesn't start at zero. The earliest possible position is the earliest position in the stream or resource that the user agent can ever obtain again. It is also a time on the media timeline.

The earliest possible position is not explicitly exposed in the API; it corresponds to the start time of the first range in the seekable attribute's TimeRanges object, if any, or the current playback position otherwise.

When the earliest possible position changes, then: if the current playback position is before the earliest possible position, the user agent must seek to the earliest possible position; otherwise, if the user agent has not fired a timeupdate event at the element in the past 15 to 250ms and is not still running event handlers for such an event, then the user agent must queue a task to fire a simple event named timeupdate at the element.

Because of the above requirement and the requirement in the resource fetch algorithm that kicks in when the metadata of the clip becomes known, the current playback position can never be less than the earliest possible position.

If at any time the user agent learns that an audio or video track has ended and all media data relating to that track corresponds to parts of the media timeline that are before the earliest possible position, the user agent may queue a task to first remove the track from the audioTracks attribute's AudioTrackList object or the videoTracks attribute's VideoTrackList object as appropriate and then fire a trusted event with the name removetrack, that does not bubble and is not cancelable, and that uses the TrackEvent interface, with the track attribute initialized to the AudioTrack or VideoTrack object representing the track, at the media element's aforementioned AudioTrackList or VideoTrackList object.

The duration attribute must return the time of the end of the media resource, in seconds, on the media timeline. If no media data is available, then the attributes must return the Not-a-Number (NaN) value. If the media resource is not known to be bounded (e.g. streaming radio, or a live event with no announced end time), then the attribute must return the positive Infinity value.

The user agent must determine the duration of the media resource before playing any part of the media data and before setting readyState to a value equal to or greater than HAVE_METADATA, even if doing so requires fetching multiple parts of the resource.

When the length of the media resource changes to a known value (e.g. from being unknown to known, or from a previously established length to a new length) the user agent must queue a task to fire a simple event named durationchange at the media element. (The event is not fired when the duration is reset as part of loading a new media resource.) If the duration is changed such that the current playback position ends up being greater than the time of the end of the media resource, then the user agent must also seek to the time of the end of the media resource.

If an "infinite" stream ends for some reason, then the duration would change from positive Infinity to the time of the last frame or sample in the stream, and the durationchange event would be fired. Similarly, if the user agent initially estimated the media resource's duration instead of determining it precisely, and later revises the estimate based on new information, then the duration would change and the durationchange event would be fired.

Some video files also have an explicit date and time corresponding to the zero time in the media timeline, known as the timeline offset. Initially, the timeline offset must be set to Not-a-Number (NaN).

The getStartDate() method must return a new Date object representing the current timeline offset.

The loop attribute is a boolean attribute that, if specified, indicates that the media element is to seek back to the start of the media resource upon reaching the end.

The loop attribute has no effect while the element has a current media controller.

The loop IDL attribute must reflect the content attribute of the same name.

4.7.10.7 Ready states

media . readyState: Returns a value that expresses the current state of the element with respect to rendering the current playback position, from the codes in the list below.

Media elements have a ready state, which describes to what degree they are ready to be rendered at the current playback position. The possible values are as follows; the ready state of a media element at any particular time is the greatest value describing the state of the element:

HAVE_NOTHING (numeric value 0)

No information regarding the media resource is available. No data for the current playback position is available. Media elements whose networkState attribute are set to NETWORK_EMPTY are always in the HAVE_NOTHING state.

HAVE_METADATA (numeric value 1)

Enough of the resource has been obtained that the duration of the resource is available. In the case of a video element, the dimensions of the video are also available. The API will no longer throw an exception when seeking. No media data is available for the immediate current playback position.

HAVE_CURRENT_DATA (numeric value 2)

Data for the immediate current playback position is available, but either not enough data is available that the user agent could successfully advance the current playback position in the direction of playback at all without immediately reverting to the HAVE_METADATA state, or there is no more data to obtain in the direction of playback. For example, in video this corresponds to the user agent having data from the current frame, but not the next frame, when the current playback position is at the end of the current frame; and to when playback has ended.

HAVE_FUTURE_DATA (numeric value 3)

Data for the immediate current playback position is available, as well as enough data for the user agent to advance the current playback position in the direction of playback at least a little without immediately reverting to the HAVE_METADATA state, and the text tracks are ready. For example, in video this corresponds to the user agent having data for at least the current frame and the next frame when the current playback position is at the instant in time between the two frames, or to the user agent having the video data for the current frame and audio data to keep playing at least a little when the current playback position is in the middle of a frame. The user agent cannot be in this state if playback has ended, as the current playback position can never advance in this case.

HAVE_ENOUGH_DATA (numeric value 4)

All the conditions described for the HAVE_FUTURE_DATA state are met, and, in addition, either of the following conditions is also true:

The user agent estimates that data is being fetched at a rate where the current playback position, if it were to advance at the effective playback rate, would not overtake the available data before playback reaches the end of the media resource.
The user agent has entered a state where waiting longer will not result in further data being obtained, and therefore nothing would be gained by delaying playback any further. (For example, the buffer might be full.)

In practice, the difference between HAVE_METADATA and HAVE_CURRENT_DATA is negligible. Really the only time the difference is relevant is when painting a video element onto a canvas, where it distinguishes the case where something will be drawn (HAVE_CURRENT_DATA or greater) from the case where nothing is drawn (HAVE_METADATA or less). Similarly, the difference between HAVE_CURRENT_DATA (only the current frame) and HAVE_FUTURE_DATA (at least this frame and the next) can be negligible (in the extreme, only one frame). The only time that distinction really matters is when a page provides an interface for "frame-by-frame" navigation.

When the ready state of a media element whose networkState is not NETWORK_EMPTY changes, the user agent must follow the steps given below:

Apply the first applicable set of substeps from the following list:
If the previous ready state was HAVE_NOTHING, and the new ready state is HAVE_METADATA

Queue a task to fire a simple event named loadedmetadata at the element.

Before this task is run, as part of the event loop mechanism, the rendering will have been updated to resize the video element if appropriate.

If the previous ready state was HAVE_METADATA and the new ready state is HAVE_CURRENT_DATA or greater

If this is the first time this occurs for this media element since the load() algorithm was last invoked, the user agent must queue a task to fire a simple event named loadeddata at the element.

If the new ready state is HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA, then the relevant steps below must then be run also.

If the previous ready state was HAVE_FUTURE_DATA or more, and the new ready state is HAVE_CURRENT_DATA or less

If the media element was potentially playing before its readyState attribute changed to a value lower than HAVE_FUTURE_DATA, and the element has not ended playback, and playback has not stopped due to errors, paused for user interaction, or paused for in-band content, the user agent must queue a task to fire a simple event named timeupdate at the element, and queue a task to fire a simple event named waiting at the element.

If the previous ready state was HAVE_CURRENT_DATA or less, and the new ready state is HAVE_FUTURE_DATA

The user agent must queue a task to fire a simple event named canplay at the element.

If the element's paused attribute is false, the user agent must queue a task to fire a simple event named playing at the element.

If the new ready state is HAVE_ENOUGH_DATA
If the previous ready state was HAVE_CURRENT_DATA or less, the user agent must queue a task to fire a simple event named canplay at the element, and, if the element's paused attribute is false, queue a task to fire a simple event named playing at the element.

If the autoplaying flag is true, and the paused attribute is true, and the media element has an autoplay attribute specified, and the media element's Document's active sandboxing flag set does not have the sandboxed automatic features browsing context flag set, then the user agent may also run the following substeps:
1. Set the paused attribute to false.
2. If the element's show poster flag is true, set it to false and run the time marches on steps.
3. Queue a task to fire a simple event named play at the element.
4. Queue a task to fire a simple event named playing at the element.
User agents do not need to support autoplay, and it is suggested that user agents honor user preferences on the matter. Authors are urged to use the autoplay attribute rather than using script to force the video to play, so as to allow the user to override the behavior if so desired.

In any case, the user agent must finally queue a task to fire a simple event named canplaythrough at the element.
If the media element has a current media controller, then report the controller state for the media element's current media controller.

It is possible for the ready state of a media element to jump between these states discontinuously. For example, the state of a media element can jump straight from HAVE_METADATA to HAVE_ENOUGH_DATA without passing through the HAVE_CURRENT_DATA and HAVE_FUTURE_DATA states.

The readyState IDL attribute must, on getting, return the value described above that describes the current ready state of the media element.

The autoplay attribute is a boolean attribute. When present, the user agent (as described in the algorithm described herein) will automatically begin playback of the media resource as soon as it can do so without stopping.

Authors are urged to use the autoplay attribute rather than using script to trigger automatic playback, as this allows the user to override the automatic playback when it is not desired, e.g. when using a screen reader. Authors are also encouraged to consider not using the automatic playback behavior at all, and instead to let the user agent wait for the user to start playback explicitly.

The autoplay IDL attribute must reflect the content attribute of the same name.

4.7.10.8 Playing the media resource

media . paused

Returns true if playback is paused; false otherwise.

media . ended

Returns true if playback has reached the end of the media resource.

media . defaultPlaybackRate [ = value ]

Returns the default rate of playback, for when the user is not fast-forwarding or reversing through the media resource.

Can be set, to change the default rate of playback.

The default rate has no direct effect on playback, but if the user switches to a fast-forward mode, when they return to the normal playback mode, it is expected that the rate of playback will be returned to the default rate of playback.

When the element has a current media controller, the defaultPlaybackRate attribute is ignored and the current media controller's defaultPlaybackRate is used instead.

media . playbackRate [ = value ]

Returns the current rate playback, where 1.0 is normal speed.

Can be set, to change the rate of playback.

When the element has a current media controller, the playbackRate attribute is ignored and the current media controller's playbackRate is used instead.

media . played

Returns a TimeRanges object that represents the ranges of the media resource that the user agent has played.

media . play()

Sets the paused attribute to false, loading the media resource and beginning playback if necessary. If the playback had ended, will restart it from the start.

media . pause()

Sets the paused attribute to true, loading the media resource if necessary.

The paused attribute represents whether the media element is paused or not. The attribute must initially be true.

A media element is a blocked media element if its readyState attribute is in the HAVE_NOTHING state, the HAVE_METADATA state, or the HAVE_CURRENT_DATA state, or if the element has paused for user interaction or paused for in-band content.

A media element is said to be potentially playing when its paused attribute is false, the element has not ended playback, playback has not stopped due to errors, the element either has no current media controller or has a current media controller but is not blocked on its media controller, and the element is not a blocked media element.

A waiting DOM event can be fired as a result of an element that is potentially playing stopping playback due to its readyState attribute changing to a value lower than HAVE_FUTURE_DATA.

A media element is said to have ended playback when:

The element's readyState attribute is HAVE_METADATA or greater, and
Either:
- The current playback position is the end of the media resource, and
- The direction of playback is forwards, and
- Either the media element does not have a loop attribute specified, or the media element has a current media controller.
Or:
- The current playback position is the earliest possible position, and
- The direction of playback is backwards.

The ended attribute must return true if, the last time the event loop reached step 1, the media element had ended playback and the direction of playback was forwards, and false otherwise.

A media element is said to have stopped due to errors when the element's readyState attribute is HAVE_METADATA or greater, and the user agent encounters a non-fatal error during the processing of the media data, and due to that error, is not able to play the content at the current playback position.

A media element is said to have paused for user interaction when its paused attribute is false, the readyState attribute is either HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA and the user agent has reached a point in the media resource where the user has to make a selection for the resource to continue. If the media element has a current media controller when this happens, then the user agent must report the controller state for the media element's current media controller. If the media element has a current media controller when the user makes a selection, allowing playback to resume, the user agent must similarly report the controller state for the media element's current media controller.

It is possible for a media element to have both ended playback and paused for user interaction at the same time.

When a media element that is potentially playing stops playing because it has paused for user interaction, the user agent must queue a task to fire a simple event named timeupdate at the element.

A media element is said to have paused for in-band content when its paused attribute is false, the readyState attribute is either HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA and the user agent has suspended playback of the media resource in order to play content that is temporally anchored to the media resource and has a non-zero length, or to play content that is temporally anchored to a segment of the media resource but has a length longer than that segment. If the media element has a current media controller when this happens, then the user agent must report the controller state for the media element's current media controller. If the media element has a current media controller when the user agent unsuspends playback, the user agent must similarly report the controller state for the media element's current media controller.

One example of when a media element would be paused for in-band content is when the user agent is playing audio descriptions from an external WebVTT file, and the synthesized speech generated for a cue is longer than the time between the text track cue start time and the text track cue end time.

When the current playback position reaches the end of the media resource when the direction of playback is forwards, then the user agent must follow these steps:

If the media element has a loop attribute specified and does not have a current media controller, then seek to the earliest possible position of the media resource and abort these steps.
As defined above, the ended IDL attribute starts returning true once the event loop's current task ends.
Queue a task to fire a simple event named timeupdate at the media element.
Queue a task that, if the media element does not have a current media controller, and the media element has still ended playback, and the direction of playback is still forwards, and paused is false, changes paused to true and fires a simple event named pause at the media element.
Queue a task to fire a simple event named ended at the media element.
If the media element has a current media controller, then report the controller state for the media element's current media controller.

When the current playback position reaches the earliest possible position of the media resource when the direction of playback is backwards, then the user agent must only queue a task to fire a simple event named timeupdate at the element.

The defaultPlaybackRate attribute gives the desired speed at which the media resource is to play, as a multiple of its intrinsic speed. The attribute is mutable: on getting it must return the last value it was set to, or 1.0 if it hasn't yet been set; on setting the attribute must be set to the new value.

The defaultPlaybackRate is used by the user agent when it exposes a user interface to the user.

The playbackRate attribute gives the effective playback rate (assuming there is no current media controller overriding it), which is the speed at which the media resource plays, as a multiple of its intrinsic speed. If it is not equal to the defaultPlaybackRate, then the implication is that the user is using a feature such as fast forward or slow motion playback. The attribute is mutable: on getting it must return the last value it was set to, or 1.0 if it hasn't yet been set; on setting the attribute must be set to the new value, and the playback will change speed (if the element is potentially playing and there is no current media controller).

When the defaultPlaybackRate or playbackRate attributes change value (either by being set by script or by being changed directly by the user agent, e.g. in response to user control) the user agent must queue a task to fire a simple event named ratechange at the media element.

The defaultPlaybackRate and playbackRate attributes have no effect when the media element has a current media controller; the namesake attributes on the MediaController object are used instead in that situation.

The played attribute must return a new static normalized TimeRanges object that represents the ranges of points on the media timeline of the media resource reached through the usual monotonic increase of the current playback position during normal playback, if any, at the time the attribute is evaluated.

When the play() method on a media element is invoked, the user agent must run the following steps.

If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's resource selection algorithm.
If the playback has ended and the direction of playback is forwards, and the media element does not have a current media controller, seek to the earliest possible position of the media resource.

This will cause the user agent to queue a task to fire a simple event named timeupdate at the media element.
If the media element has a current media controller, then bring the media element up to speed with its new media controller.
If the media element's paused attribute is true, run the following substeps:
1. Change the value of paused to false.
2. If the show poster flag is true, set the element's show poster flag to false and run the time marches on steps.
3. Queue a task to fire a simple event named play at the element.
4. If the media element's readyState attribute has the value HAVE_NOTHING, HAVE_METADATA, or HAVE_CURRENT_DATA, queue a task to fire a simple event named waiting at the element.
  
  Otherwise, the media element's readyState attribute has the value HAVE_FUTURE_DATA or HAVE_ENOUGH_DATA: queue a task to fire a simple event named playing at the element.
Set the media element's autoplaying flag to false.
If the media element has a current media controller, then report the controller state for the media element's current media controller.

When the pause() method is invoked, and when the user agent is required to pause the media element, the user agent must run the following steps:

If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's resource selection algorithm.
Set the media element's autoplaying flag to false.
If the media element's paused attribute is false, run the following steps:
1. Change the value of paused to true.
2. Queue a task to fire a simple event named timeupdate at the element.
3. Queue a task to fire a simple event named pause at the element.
4. Set the official playback position to the current playback position.
If the media element has a current media controller, then report the controller state for the media element's current media controller.

The effective playback rate is not necessarily the element's playbackRate. When a media element has a current media controller, its effective playback rate is the MediaController's media controller playback rate. Otherwise, the effective playback rate is just the element's playbackRate. Thus, the current media controller overrides the media element.

If the effective playback rate is positive or zero, then the direction of playback is forwards. Otherwise, it is backwards.

When a media element is potentially playing and its Document is a fully active Document, its current playback position must increase monotonically at effective playback rate units of media time per unit time of the media timeline's clock. (This specification always refers to this as an increase, but that increase could actually be a decrease if the effective playback rate is negative.)

The effective playback rate can be 0.0, in which case the current playback position doesn't move, despite playback not being paused (paused doesn't become true, and the pause event doesn't fire).

This specification doesn't define how the user agent achieves the appropriate playback rate — depending on the protocol and media available, it is plausible that the user agent could negotiate with the server to have the server provide the media data at the appropriate rate, so that (except for the period between when the rate is changed and when the server updates the stream's playback rate) the client doesn't actually have to drop or interpolate any frames.

Any time the user agent provides a stable state, the official playback position must be set to the current playback position.

When the direction of playback is backwards, any corresponding audio must be muted. When the effective playback rate is so low or so high that the user agent cannot play audio usefully, the corresponding audio must also be muted. If the effective playback rate is not 1.0, the user agent may apply pitch adjustments to the audio as necessary to render it faithfully.

Media elements that are potentially playing while not in a Document must not play any video, but should play any audio component. Media elements must not stop playing just because all references to them have been removed; only once a media element is in a state where no further audio could ever be played by that element may the element be garbage collected.

It is possible for an element to which no explicit references exist to play audio, even if such an element is not still actively playing: for instance, it could have a current media controller that still has references and can still be unpaused, or it could be unpaused but stalled waiting for content to buffer.

Each media element has a list of newly introduced cues, which must be initially empty. Whenever a text track cue is added to the list of cues of a text track that is in the list of text tracks for a media element, that cue must be added to the media element's list of newly introduced cues. Whenever a text track is added to the list of text tracks for a media element, all of the cues in that text track's list of cues must be added to the media element's list of newly introduced cues. When a media element's list of newly introduced cues has new cues added while the media element's show poster flag is not set, then the user agent must run the time marches on steps.

When a text track cue is removed from the list of cues of a text track that is in the list of text tracks for a media element, and whenever a text track is removed from the list of text tracks of a media element, if the media element's show poster flag is not set, then the user agent must run the time marches on steps.

When the current playback position of a media element changes (e.g. due to playback or seeking), the user agent must run the time marches on steps. If the current playback position changes while the steps are running, then the user agent must wait for the steps to complete, and then must immediately rerun the steps. (These steps are thus run as often as possible or needed — if one iteration takes a long time, this can cause certain cues to be skipped over as the user agent rushes ahead to "catch up".)

The time marches on steps are as follows:

Let current cues be a list of cues, initialized to contain all the cues of all the hidden or showing text tracks of the media element (not the disabled ones) whose start times are less than or equal to the current playback position and whose end times are greater than the current playback position.
Let other cues be a list of cues, initialized to contain all the cues of hidden and showing text tracks of the media element that are not present in current cues.
Let last time be the current playback position at the time this algorithm was last run for this media element, if this is not the first time it has run.
If the current playback position has, since the last time this algorithm was run, only changed through its usual monotonic increase during normal playback, then let missed cues be the list of cues in other cues whose start times are greater than or equal to last time and whose end times are less than or equal to the current playback position. Otherwise, let missed cues be an empty list.
Remove all the cues in missed cues that are also in the media element's list of newly introduced cues, and then empty the element's list of newly introduced cues.
If the time was reached through the usual monotonic increase of the current playback position during normal playback, and if the user agent has not fired a timeupdate event at the element in the past 15 to 250ms and is not still running event handlers for such an event, then the user agent must queue a task to fire a simple event named timeupdate at the element. (In the other cases, such as explicit seeks, relevant events get fired as part of the overall process of changing the current playback position.)

The event thus is not to be fired faster than about 66Hz or slower than 4Hz (assuming the event handlers don't take longer than 250ms to run). User agents are encouraged to vary the frequency of the event based on the system load and the average cost of processing the event each time, so that the UI updates are not any more frequent than the user agent can comfortably handle while decoding the video.
If all of the cues in current cues have their text track cue active flag set, none of the cues in other cues have their text track cue active flag set, and missed cues is empty, then abort these steps.
If the time was reached through the usual monotonic increase of the current playback position during normal playback, and there are cues in other cues that have their text track cue pause-on-exit flag set and that either have their text track cue active flag set or are also in missed cues, then immediately pause the media element.

In the other cases, such as explicit seeks, playback is not paused by going past the end time of a cue, even if that cue has its text track cue pause-on-exit flag set.
Let events be a list of tasks, initially empty. Each task in this list will be associated with a text track, a text track cue, and a time, which are used to sort the list before the tasks are queued.

Let affected tracks be a list of text tracks, initially empty.

When the steps below say to prepare an event named event for a text track cue target with a time time, the user agent must run these substeps:
1. Let track be the text track with which the text track cue target is associated.
2. Create a task to fire a simple event named event at target.
3. Add the newly created task to events, associated with the time time, the text track track, and the text track cue target.
4. Add track to affected tracks.
For each text track cue in missed cues, prepare an event named enter for the TextTrackCue object with the text track cue start time.
For each text track cue in other cues that either has its text track cue active flag set or is in missed cues, prepare an event named exit for the TextTrackCue object with the later of the text track cue end time and the text track cue start time.
For each text track cue in current cues that does not have its text track cue active flag set, prepare an event named enter for the TextTrackCue object with the text track cue start time.
Sort the tasks in events in ascending time order (tasks with earlier times first).

Further sort tasks in events that have the same time by the relative text track cue order of the text track cues associated with these tasks.

Finally, sort tasks in events that have the same time and same text track cue order by placing tasks that fire enter events before those that fire exit events.
Queue each task in events, in list order.
Sort affected tracks in the same order as the text tracks appear in the media element's list of text tracks, and remove duplicates.
For each text track in affected tracks, in the list order, queue a task to fire a simple event named cuechange at the TextTrack object, and, if the text track has a corresponding track element, to then fire a simple event named cuechange at the track element as well.
Set the text track cue active flag of all the cues in the current cues, and unset the text track cue active flag of all the cues in the other cues.
Run the rules for updating the text track rendering of each of the text tracks in affected tracks that are showing. For example, for text tracks based on WebVTT, the rules for updating the display of WebVTT text tracks. [WEBVTT]

For the purposes of the algorithm above, a text track cue is considered to be part of a text track only if it is listed in the text track list of cues, not merely if it is associated with the text track.

If the media element's Document stops being a fully active document, then the playback will stop until the document is active again.

When a media element is removed from a Document, the user agent must run the following steps:

Asynchronously await a stable state, allowing the task that removed the media element from the Document to continue. The synchronous section consists of all the remaining steps of this algorithm. (Steps in the synchronous section are marked with ⌛.)
⌛ If the media element is in a Document, abort these steps.
⌛ If the media element's networkState attribute has the value NETWORK_EMPTY, abort these steps.
⌛ Pause the media element.

4.7.10.9 Seeking

media . seeking: Returns true if the user agent is currently seeking.
media . seekable: Returns a TimeRanges object that represents the ranges of the media resource to which it is possible for the user agent to seek.

The seeking attribute must initially have the value false.

When the user agent is required to seek to a particular new playback position in the media resource, optionally with the approximate-for-speed flag set, it means that the user agent must run the following steps. This algorithm interacts closely with the event loop mechanism; in particular, it has a synchronous section (which is triggered as part of the event loop algorithm). Steps in that section are marked with ⌛.

Set the media element's show poster flag to false.
If the media element's readyState is HAVE_NOTHING, abort these steps.
If the element's seeking IDL attribute is true, then another instance of this algorithm is already running. Abort that other instance of the algorithm without waiting for the step that it is running to complete.
Set the seeking IDL attribute to true.
If the seek was in response to a DOM method call or setting of an IDL attribute, then continue the script. The remainder of these steps must be run asynchronously. With the exception of the steps marked with ⌛, they could be aborted at any time by another instance of this algorithm being invoked.
If the new playback position is later than the end of the media resource, then let it be the end of the media resource instead.
If the new playback position is less than the earliest possible position, let it be that position instead.
If the (possibly now changed) new playback position is not in one of the ranges given in the seekable attribute, then let it be the position in one of the ranges given in the seekable attribute that is the nearest to the new playback position. If two positions both satisfy that constraint (i.e. the new playback position is exactly in the middle between two ranges in the seekable attribute) then use the position that is closest to the current playback position. If there are no ranges given in the seekable attribute then set the seeking IDL attribute to false and abort these steps.
If the approximate-for-speed flag is set, adjust the new playback position to a value that will allow for playback to resume promptly. If new playback position before this step is before current playback position, then the adjusted new playback position must also be before the current playback position. Similarly, if the new playback position before this step is after current playback position, then the adjusted new playback position must also be after the current playback position.

For example, the user agent could snap to the nearest key frame, so that it doesn't have to spend time decoding then discarding intermediate frames before resuming playback.
Queue a task to fire a simple event named seeking at the element.
Set the current playback position to the given new playback position.

If the media element was potentially playing immediately before it started seeking, but seeking caused its readyState attribute to change to a value lower than HAVE_FUTURE_DATA, then a waiting will be fired at the element.

The currentTime attribute does not get updated asynchronously, as it returns the official playback position, not the current playback position.
Wait until the user agent has established whether or not the media data for the new playback position is available, and, if it is, until it has decoded enough data to play back that position.
Await a stable state. The synchronous section consists of all the remaining steps of this algorithm. (Steps in the synchronous section are marked with ⌛.)
⌛ Set the seeking IDL attribute to false.
⌛ Run the time marches on steps.
⌛ Queue a task to fire a simple event named timeupdate at the element.
⌛ Queue a task to fire a simple event named seeked at the element.

The seekable attribute must return a new static normalized TimeRanges object that represents the ranges of the media resource, if any, that the user agent is able to seek to, at the time the attribute is evaluated.

If the user agent can seek to anywhere in the media resource, e.g. because it is a simple movie file and the user agent and the server support HTTP Range requests, then the attribute would return an object with one range, whose start is the time of the first frame (the earliest possible position, typically zero), and whose end is the same as the time of the first frame plus the duration attribute's value (which would equal the time of the last frame, and might be positive Infinity).

The range might be continuously changing, e.g. if the user agent is buffering a sliding window on an infinite stream. This is the behavior seen with DVRs viewing live TV, for instance.

Media resources might be internally scripted or interactive. Thus, a media element could play in a non-linear fashion. If this happens, the user agent must act as if the algorithm for seeking was used whenever the current playback position changes in a discontinuous fashion (so that the relevant events fire). If the media element has a current media controller, then the user agent must seek the media controller appropriately instead.

4.7.10.10 Media resources with multiple media tracks

A media resource can have multiple embedded audio and video tracks. For example, in addition to the primary video and audio tracks, a media resource could have foreign-language dubbed dialogues, director's commentaries, audio descriptions, alternative angles, or sign-language overlays.

media . audioTracks: Returns an AudioTrackList object representing the audio tracks available in the media resource.
media . videoTracks: Returns a VideoTrackList object representing the video tracks available in the media resource.

The audioTracks attribute of a media element must return a live AudioTrackList object representing the audio tracks available in the media element's media resource. The same object must be returned each time.

The videoTracks attribute of a media element must return a live VideoTrackList object representing the video tracks available in the media element's media resource. The same object must be returned each time.

There are only ever one AudioTrackList object and one VideoTrackList object per media element, even if another media resource is loaded into the element: the objects are reused. (The AudioTrack and VideoTrack objects are not, though.)

In this example, a script defines a function that takes a URL to a video and a reference to an element where the video is to be placed. That function then tries to load the video, and, once it is loaded, checks to see if there is a sign-language track available. If there is, it also displays that track. Both tracks are just placed in the given container; it's assumed that styles have been applied to make this work in a pretty way!

<script>
 function loadVideo(url, container) {
   var controller = new MediaController();
   var video = document.createElement('video');
   video.src = url;
   video.autoplay = true;
   video.controls = true;
   video.controller = controller;
   container.appendChild(video);
   video.onloadedmetadata = function (event) {
     for (var i = 0; i < video.videoTracks.length; i += 1) {
       if (video.videoTracks[i].kind == 'sign') {
         var sign = document.createElement('video');
         sign.src = url + '#track=' + video.videoTracks[i].id; 
         sign.autoplay = true;
         sign.controller = controller;
         container.appendChild(sign);
         return;
       }
     }
   };
 }
</script>

4.7.10.10.1 `AudioTrackList` and `VideoTrackList` objects

The AudioTrackList and VideoTrackList interfaces are used by attributes defined in the previous section.

interface AudioTrackList : EventTarget {
  readonly attribute unsigned long length;
  getter AudioTrack (unsigned long index);
  AudioTrack? getTrackById(DOMString id);

           attribute EventHandler onchange;
           attribute EventHandler onaddtrack;
           attribute EventHandler onremovetrack;
};

interface AudioTrack {
  readonly attribute DOMString id;
  readonly attribute DOMString kind;
  readonly attribute DOMString label;
  readonly attribute DOMString language;
           attribute boolean enabled;
};

interface VideoTrackList : EventTarget {
  readonly attribute unsigned long length;
  getter VideoTrack (unsigned long index);
  VideoTrack? getTrackById(DOMString id);
  readonly attribute long selectedIndex;

           attribute EventHandler onchange;
           attribute EventHandler onaddtrack;
           attribute EventHandler onremovetrack;
};

interface VideoTrack {
  readonly attribute DOMString id;
  readonly attribute DOMString kind;
  readonly attribute DOMString label;
  readonly attribute DOMString language;
           attribute boolean selected;
};

media . audioTracks . length

media . videoTracks . length

Returns the number of tracks in the list.

audioTrack = media . audioTracks[index]

videoTrack = media . videoTracks[index]

Returns the specified AudioTrack or VideoTrack object.

audioTrack = media . audioTracks . getTrackById( id )

videoTrack = media . videoTracks . getTrackById( id )

Returns the AudioTrack or VideoTrack object with the given identifier, or null if no track has that identifier.

audioTrack . id

videoTrack . id

Returns the ID of the given track. This is the ID that can be used with a fragment identifier if the format supports the Media Fragments URI syntax, and that can be used with the getTrackById() method. [MEDIAFRAG]

audioTrack . kind

videoTrack . kind

Returns the category the given track falls into. The possible track categories are given below.

audioTrack . label

videoTrack . label

Returns the label of the given track, if known, or the empty string otherwise.

audioTrack . language

videoTrack . language

Returns the language of the given track, if known, or the empty string otherwise.

audioTrack . enabled [ = value ]

Returns true if the given track is active, and false otherwise.

Can be set, to change whether the track is enabled or not. If multiple audio tracks are enabled simultaneously, they are mixed.

media . videoTracks . selectedIndex

Returns the index of the currently selected track, if any, or −1 otherwise.

videoTrack . selected [ = value ]

Returns true if the given track is active, and false otherwise.

Can be set, to change whether the track is selected or not. Either zero or one video track is selected; selecting a new track while a previous one is selected will unselect the previous one.

An AudioTrackList object represents a dynamic list of zero or more audio tracks, of which zero or more can be enabled at a time. Each audio track is represented by an AudioTrack object.

A VideoTrackList object represents a dynamic list of zero or more video tracks, of which zero or one can be selected at a time. Each video track is represented by a VideoTrack object.

Tracks in AudioTrackList and VideoTrackList objects must be consistently ordered. If the media resource is in a format that defines an order, then that order must be used; otherwise, the order must be the relative order in which the tracks are declared in the media resource. The order used is called the natural order of the list.

Each track in a TrackList thus has an index; the first has the index 0, and each subsequent track is numbered one higher than the previous one. If a media resource dynamically adds or removes audio or video tracks, then the indices of the tracks will change dynamically. If the media resource changes entirely, then all the previous tracks will be removed and replaced with new tracks.

The AudioTrackList.length and VideoTrackList.length attributes must return the number of tracks represented by their objects at the time of getting.

The supported property indices of AudioTrackList and VideoTrackList objects at any instant are the numbers from zero to the number of tracks represented by the respective object minus one, if any tracks are represented. If an AudioTrackList or VideoTrackList object represents no tracks, it has no supported property indices.

To determine the value of an indexed property for a given index index in an AudioTrackList or VideoTrackList object list, the user agent must return the AudioTrack or VideoTrack object that represents the indexth track in list.

The AudioTrackList.getTrackById(id) and VideoTrackList.getTrackById(id) methods must return the first AudioTrack or VideoTrack object (respectively) in the AudioTrackList or VideoTrackList object (respectively) whose identifier is equal to the value of the id argument (in the natural order of the list, as defined above). When no tracks match the given argument, the methods must return null.

The AudioTrack and VideoTrack objects represent specific tracks of a media resource. Each track can have an identifier, category, label, and language. These aspects of a track are permanent for the lifetime of the track; even if a track is removed from a media resource's AudioTrackList or VideoTrackList objects, those aspects do not change.

In addition, AudioTrack objects can each be enabled or disabled; this is the audio track's enabled state. When an AudioTrack is created, its enabled state must be set to false (disabled). The resource fetch algorithm can override this.

Similarly, a single VideoTrack object per VideoTrackList object can be selected, this is the video track's selection state. When a VideoTrack is created, its selection state must be set to false (not selected). The resource fetch algorithm can override this.

The AudioTrack.id and VideoTrack.id attributes must return the identifier of the track, if it has one, or the empty string otherwise. If the media resource is in a format that supports the Media Fragments URI fragment identifier syntax, the identifier returned for a particular track must be the same identifier that would enable the track if used as the name of a track in the track dimension of such a fragment identifier. [MEDIAFRAG]

For example, in Ogg files, this would be the Name header field of the track. [OGGSKELETONHEADERS]

The AudioTrack.kind and VideoTrack.kind attributes must return the category of the track, if it has one, or the empty string otherwise.

The category of a track is the string given in the first column of the table below that is the most appropriate for the track based on the definitions in the table's second and third columns, as determined by the metadata included in the track in the media resource. The cell in the third column of a row says what the category given in the cell in the first column of that row applies to; a category is only appropriate for an audio track if it applies to audio tracks, and a category is only appropriate for video tracks if it applies to video tracks. Categories must only be returned for AudioTrack objects if they are appropriate for audio, and must only be returned for VideoTrack objects if they are appropriate for video.

Return values for `AudioTrack.kind()` and `VideoTrack.kind()`
Category	Definition	Applies to...
"`alternative`"	A possible alternative to the main track, e.g. a different take of a song (audio), or a different angle (video).	Audio and video.
"`captions`"	A version of the main video track with captions burnt in. (For legacy content; new content would use text tracks.)	Video only.
"`descriptions`"	An audio description of a video track.	Audio only.
"`main`"	The primary audio or video track.	Audio and video.
"`main-desc`"	The primary audio track, mixed with audio descriptions.	Audio only.
"`sign`"	A sign-language interpretation of an audio track.	Video only.
"`subtitles`"	A version of the main video track with subtitles burnt in. (For legacy content; new content would use text tracks.)	Video only.
"`translation`"	A translated version of the main audio track.	Audio only.
"`commentary`"	Commentary on the primary audio or video track, e.g. a director's commentary.	Audio and video.
"" (empty string)	No explicit kind, or the kind given by the track's metadata is not recognised by the user agent.	Audio and video.

The AudioTrack.label and VideoTrack.label attributes must return the label of the track, if it has one, or the empty string otherwise.

The AudioTrack.language and VideoTrack.language attributes must return the BCP 47 language tag of the language of the track, if it has one, or the empty string otherwise. If the user agent is not able to express that language as a BCP 47 language tag (for example because the language information in the media resource's format is a free-form string without a defined interpretation), then the method must return the empty string, as if the track had no language.

Source attribute values for id, kind, label and language of multitrack audio and video tracks as described for the relevant media resource format. [INBANDTRACKS]

The AudioTrack.enabled attribute, on getting, must return true if the track is currently enabled, and false otherwise. On setting, it must enable the track if the new value is true, and disable it otherwise. (If the track is no longer in an AudioTrackList object, then the track being enabled or disabled has no effect beyond changing the value of the attribute on the AudioTrack object.)

Whenever an audio track in an AudioTrackList is enabled or disabled, the user agent must queue a task to fire a simple event named change at the AudioTrackList object.

The VideoTrackList.selectedIndex attribute must return the index of the currently selected track, if any. If the VideoTrackList object does not currently represent any tracks, or if none of the tracks are selected, it must instead return −1.

The VideoTrack.selected attribute, on getting, must return true if the track is currently selected, and false otherwise. On setting, it must select the track if the new value is true, and unselect it otherwise. If the track is in a VideoTrackList, then all the other VideoTrack objects in that list must be unselected. (If the track is no longer in a VideoTrackList object, then the track being selected or unselected has no effect beyond changing the value of the attribute on the VideoTrack object.)

Whenever a track in a VideoTrackList that was previously not selected is selected, the user agent must queue a task to fire a simple event named change at the VideoTrackList object. This task must be queued before the task that fires the resize event, if any.

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by all objects implementing the AudioTrackList and VideoTrackList interfaces:

Event handler	Event handler event type
`onchange`	`change`
`onaddtrack`	`addtrack`
`onremovetrack`	`removetrack`

The task source for the tasks listed in this section is the DOM manipulation task source.

4.7.10.10.2 Selecting specific audio and video tracks declaratively

The audioTracks and videoTracks attributes allow scripts to select which track should play, but it is also possible to select specific tracks declaratively, by specifying particular tracks in the fragment identifier of the URL of the media resource. The format of the fragment identifier depends on the MIME type of the media resource. [RFC2046] [URL]

In this example, a video that uses a format that supports the Media Fragments URI fragment identifier syntax is embedded in such a way that the alternative angles labeled "Alternative" are enabled instead of the default video track. [MEDIAFRAG]

<video src="myvideo#track=Alternative"></video>

4.7.10.11 Synchronising multiple media elements

4.7.10.11.1 Introduction

Each media element can have a MediaController. A MediaController is an object that coordinates the playback of multiple media elements, for instance so that a sign-language interpreter track can be overlaid on a video track, with the two being kept in sync.

By default, a media element has no MediaController. An implicit MediaController can be assigned using the mediagroup content attribute. An explicit MediaController can be assigned directly using the controller IDL attribute.

Media elements with a MediaController are said to be slaved to their controller. The MediaController modifies the playback rate and the playback volume of each of the media elements slaved to it, and ensures that when any of its slaved media elements unexpectedly stall, the others are stopped at the same time.

When a media element is slaved to a MediaController, its playback rate is fixed to that of the other tracks in the same MediaController, and any looping is disabled.

4.7.10.11.2 Media controllers

enum MediaControllerPlaybackState { "waiting", "playing", "ended" };
[Constructor]
interface MediaController : EventTarget {
  readonly attribute unsigned short readyState; // uses HTMLMediaElement.readyState's values

  readonly attribute TimeRanges buffered;
  readonly attribute TimeRanges seekable;
  readonly attribute unrestricted double duration;
           attribute double currentTime;

  readonly attribute boolean paused;
  readonly attribute MediaControllerPlaybackState playbackState;
  readonly attribute TimeRanges played;
  void pause();
  void unpause();
  void play(); // calls play() on all media elements as well

           attribute double defaultPlaybackRate;
           attribute double playbackRate;

           attribute double volume;
           attribute boolean muted;

           attribute EventHandler onemptied;
           attribute EventHandler onloadedmetadata;
           attribute EventHandler onloadeddata;
           attribute EventHandler oncanplay;
           attribute EventHandler oncanplaythrough;
           attribute EventHandler onplaying;
           attribute EventHandler onended;
           attribute EventHandler onwaiting;

           attribute EventHandler ondurationchange;
           attribute EventHandler ontimeupdate;
           attribute EventHandler onplay;
           attribute EventHandler onpause;
           attribute EventHandler onratechange;
           attribute EventHandler onvolumechange;
};

controller = new MediaController()

Returns a new MediaController object.

media . controller [ = controller ]

Returns the current MediaController for the media element, if any; returns null otherwise.

Can be set, to set an explicit MediaController. Doing so removes the mediagroup attribute, if any.

controller . readyState

Returns the state that the MediaController was in the last time it fired events as a result of reporting the controller state. The values of this attribute are the same as for the readyState attribute of media elements.

controller . buffered

Returns a TimeRanges object that represents the intersection of the time ranges for which the user agent has all relevant media data for all the slaved media elements.

controller . seekable

Returns a TimeRanges object that represents the intersection of the time ranges into which the user agent can seek for all the slaved media elements.

controller . duration

Returns the difference between the earliest playable moment and the latest playable moment (not considering whether the data in question is actually buffered or directly seekable, but not including time in the future for infinite streams). Will return zero if there is no media.

controller . currentTime [ = value ]

Returns the current playback position, in seconds, as a position between zero time and the current duration.

Can be set, to seek to the given time.

controller . paused

Returns true if playback is paused; false otherwise. When this attribute is true, any media element slaved to this controller will be stopped.

controller . playbackState

Returns the state that the MediaController was in the last time it fired events as a result of reporting the controller state. The value of this attribute is either "playing", indicating that the media is actively playing, "ended", indicating that the media is not playing because playback has reached the end of all the slaved media elements, or "waiting", indicating that the media is not playing for some other reason (e.g. the MediaController is paused).

controller . pause()

Sets the paused attribute to true.

controller . unpause()

Sets the paused attribute to false.

controller . play()

Sets the paused attribute to false and invokes the play() method of each slaved media element.

controller . played

Returns a TimeRanges object that represents the union of the time ranges in all the slaved media elements that have been played.

controller . defaultPlaybackRate [ = value ]

Returns the default rate of playback.

Can be set, to change the default rate of playback.

This default rate has no direct effect on playback, but if the user switches to a fast-forward mode, when they return to the normal playback mode, it is expected that rate of playback (playbackRate) will be returned to this default rate.

controller . playbackRate [ = value ]

Returns the current rate of playback.

Can be set, to change the rate of playback.

controller . volume [ = value ]

Returns the current playback volume multiplier, as a number in the range 0.0 to 1.0, where 0.0 is the quietest and 1.0 the loudest.

Can be set, to change the volume multiplier.

Throws an IndexSizeError exception if the new value is not in the range 0.0 .. 1.0.

controller . muted [ = value ]

Returns true if all audio is muted (regardless of other attributes either on the controller or on any media elements slaved to this controller), and false otherwise.

Can be set, to change whether the audio is muted or not.

A media element can have a current media controller, which is a MediaController object. When a media element is created without a mediagroup attribute, it does not have a current media controller. (If it is created with such an attribute, then that attribute initializes the current media controller, as defined below.)

The slaved media elements of a MediaController are the media elements whose current media controller is that MediaController. All the slaved media elements of a MediaController must use the same clock for their definition of their media timeline's unit time. When the user agent is required to act on each slaved media element in turn, they must be processed in the order that they were last associated with the MediaController.

The controller attribute on a media element, on getting, must return the element's current media controller, if any, or null otherwise. On setting, the user agent must run the following steps:

Let m be the media element in question.
Let old controller be m's current media controller, if it currently has one, and null otherwise.
Let new controller be null.
Let m have no current media controller, if it currently has one.
Remove the element's mediagroup content attribute, if any.
If the new value is null, then jump to the update controllers step below.
Let m's current media controller be the new value.
Let new controller be m's current media controller.
Bring the media element up to speed with its new media controller.
Update controllers: If old controller and new controller are the same (whether both null or both the same controller) then abort these steps.
If old controller is not null and still has one or more slaved media elements, then report the controller state for old controller.
If new controller is not null, then report the controller state for new controller.

The MediaController() constructor, when invoked, must return a newly created MediaController object.

The readyState attribute must return the value to which it was most recently set. When the MediaController object is created, the attribute must be set to the value 0 (HAVE_NOTHING). The value is updated by the report the controller state algorithm below.

The seekable attribute must return a new static normalized TimeRanges object that represents the intersection of the ranges of the media resources of the slaved media elements that the user agent is able to seek to, at the time the attribute is evaluated.

The buffered attribute must return a new static normalized TimeRanges object that represents the intersection of the ranges of the media resources of the slaved media elements that the user agent has buffered, at the time the attribute is evaluated. Users agents must accurately determine the ranges available, even for media streams where this can only be determined by tedious inspection.

The duration attribute must return the media controller duration.

Every 15 to 250ms, or whenever the MediaController's media controller duration changes, whichever happens least often, the user agent must queue a task to fire a simple event named durationchange at the MediaController. If the MediaController's media controller duration decreases such that the media controller position is greater than the media controller duration, the user agent must immediately seek the media controller to media controller duration.

The currentTime attribute must return the media controller position on getting, and on setting must seek the media controller to the new value.

Every 15 to 250ms, or whenever the MediaController's media controller position changes, whichever happens least often, the user agent must queue a task to fire a simple event named timeupdate at the MediaController.

When a MediaController is created it is a playing media controller. It can be changed into a paused media controller and back either via the user agent's user interface (when the element is exposing a user interface to the user) or by script using the APIs defined in this section (see below).

The paused attribute must return true if the MediaController object is a paused media controller, and false otherwise.

When the pause() method is invoked, if the MediaController is a playing media controller then the user agent must change the MediaController into a paused media controller, queue a task to fire a simple event named pause at the MediaController, and then report the controller state of the MediaController.

When the unpause() method is invoked, if the MediaController is a paused media controller, the user agent must change the MediaController into a playing media controller, queue a task to fire a simple event named play at the MediaController, and then report the controller state of the MediaController.

When the play() method is invoked, the user agent must invoke the play() method of each slaved media element in turn, and then invoke the unpause method of the MediaController.

The playbackState attribute must return the value to which it was most recently set. When the MediaController object is created, the attribute must be set to the value "waiting". The value is updated by the report the controller state algorithm below.

The played attribute must return a new static normalized TimeRanges object that represents the union of the ranges of points on the media timelines of the media resources of the slaved media elements that the user agent has so far reached through the usual monotonic increase of their current playback positions during normal playback, at the time the attribute is evaluated.

A MediaController has a media controller default playback rate and a media controller playback rate, which must both be set to 1.0 when the MediaController object is created.

The defaultPlaybackRate attribute, on getting, must return the MediaController's media controller default playback rate, and on setting, must set the MediaController's media controller default playback rate to the new value, then queue a task to fire a simple event named ratechange at the MediaController.

The playbackRate attribute, on getting, must return the MediaController's media controller playback rate, and on setting, must set the MediaController's media controller playback rate to the new value, then queue a task to fire a simple event named ratechange at the MediaController.

A MediaController has a media controller volume multiplier, which must be set to 1.0 when the MediaController object is created, and a media controller mute override, much must initially be false.

The volume attribute, on getting, must return the MediaController's media controller volume multiplier, and on setting, if the new value is in the range 0.0 to 1.0 inclusive, must set the MediaController's media controller volume multiplier to the new value and queue a task to fire a simple event named volumechange at the MediaController. If the new value is outside the range 0.0 to 1.0 inclusive, then, on setting, an IndexSizeError exception must be thrown instead.

The muted attribute, on getting, must return the MediaController's media controller mute override, and on setting, must set the MediaController's media controller mute override to the new value and queue a task to fire a simple event named volumechange at the MediaController.

The media resources of all the slaved media elements of a MediaController have a defined temporal relationship which provides relative offsets between the zero time of each such media resource: for media resources with a timeline offset, their relative offsets are the difference between their timeline offset; the zero times of all the media resources without a timeline offset are not offset from each other (i.e. the origins of their timelines are cotemporal); and finally, the zero time of the media resource with the earliest timeline offset (if any) is not offset from the zero times of the media resources without a timeline offset (i.e. the origins of media resources without a timeline offset are further cotemporal with the earliest defined point on the timeline of the media resource with the earliest timeline offset).

The media resource end position of a media resource in a media element is defined as follows: if the media resource has a finite and known duration, the media resource end position is the duration of the media resource's timeline (the last defined position on that timeline); otherwise, the media resource's duration is infinite or unknown, and the media resource end position is the time of the last frame of media data currently available for that media resource.

Each MediaController also has its own defined timeline. On this timeline, all the media resources of all the slaved media elements of the MediaController are temporally aligned according to their defined offsets. The media controller duration of that MediaController is the time from the earliest earliest possible position, relative to this MediaController timeline, of any of the media resources of the slaved media elements of the MediaController, to the time of the latest media resource end position of the media resources of the slaved media elements of the MediaController, again relative to this MediaController timeline.

Each MediaController has a media controller position. This is the time on the MediaController's timeline at which the user agent is trying to play the slaved media elements. When a MediaController is created, its media controller position is initially zero.

When the user agent is to bring a media element up to speed with its new media controller, it must seek that media element to the MediaController's media controller position relative to the media element's timeline.

When the user agent is to seek the media controller to a particular new playback position, it must follow these steps:

If the new playback position is less than zero, then set it to zero.
If the new playback position is greater than the media controller duration, then set it to the media controller duration.
Set the media controller position to the new playback position.
Seek each slaved media element to the new playback position relative to the media element timeline.

A MediaController is a restrained media controller if the MediaController is a playing media controller, and none of its slaved media elements are blocked media elements, but either at least one of its slaved media elements whose autoplaying flag is true still has its paused attribute set to true, or, all of its slaved media elements have their paused attribute set to true.

A MediaController is a blocked media controller if the MediaController is a paused media controller, or if any of its slaved media elements are blocked media elements, or if any of its slaved media elements whose autoplaying flag is true still have their paused attribute set to true, or if all of its slaved media elements have their paused attribute set to true.

A media element is blocked on its media controller if the MediaController is a blocked media controller, or if its media controller position is either before the media resource's earliest possible position relative to the MediaController's timeline or after the end of the media resource relative to the MediaController's timeline.

When a MediaController is not a blocked media controller and it has at least one slaved media element whose Document is a fully active Document, the MediaController's media controller position must increase monotonically at media controller playback rate units of time on the MediaController's timeline per unit time of the clock used by its slaved media elements.

When the zero point on the timeline of a MediaController moves relative to the timelines of the slaved media elements by a time difference ΔT, the MediaController's media controller position must be decremented by ΔT.

In some situations, e.g. when playing back a live stream without buffering anything, the media controller position would increase monotonically as described above at the same rate as the ΔT described in the previous paragraph decreases it, with the end result that for all intents and purposes, the media controller position would appear to remain constant (probably with the value 0).

A MediaController has a most recently reported readiness state, which is a number from 0 to 4 derived from the numbers used for the media element readyState attribute, and a most recently reported playback state, which is either playing, waiting, or ended.

When a MediaController is created, its most recently reported readiness state must be set to 0, and its most recently reported playback state must be set to waiting.

When a user agent is required to report the controller state for a MediaController, the user agent must run the following steps:

If the MediaController has no slaved media elements, let new readiness state be 0.

Otherwise, let it have the lowest value of the readyState IDL attributes of all of its slaved media elements.

If the MediaController's most recently reported readiness state is less than the new readiness state, then run these substeps:

Let next state be the MediaController's most recently reported readiness state.
Loop: Increment next state by one.
Queue a task to run the following steps:
1. Set the MediaController's readyState attribute to the value next state.
2. Fire a simple event at the MediaController object, whose name is the event name corresponding to the value of next state given in the table below.
If next state is less than new readiness state, then return to the step labeled loop.

Otherwise, if the MediaController's most recently reported readiness state is greater than new readiness state then queue a task to fire a simple event at the MediaController object, whose name is the event name corresponding to the value of new readiness state given in the table below.

Value of `new readiness state`	Event name
0	`emptied`
1	`loadedmetadata`
2	`loadeddata`
3	`canplay`
4	`canplaythrough`

Let the MediaController's most recently reported readiness state be new readiness state.
Initialize new playback state by setting it to the state given for the first matching condition from the following list:

If the MediaController has no slaved media elements

Let new playback state be waiting.

If all of the MediaController's slaved media elements have ended playback and the media controller playback rate is positive or zero

Let new playback state be ended.

If the MediaController is a blocked media controller

Let new playback state be waiting.

Otherwise

Let new playback state be playing.
If the MediaController's most recently reported playback state is not equal to new playback state and the new playback state is ended, then queue a task that, if the MediaController object is a playing media controller, and all of the MediaController's slaved media elements have still ended playback, and the media controller playback rate is still positive or zero, changes the MediaController object to a paused media controller and then fires a simple event named pause at the MediaController object.

If the MediaController's most recently reported playback state is not equal to new playback state then queue a task to run the following steps:

Set the MediaController's playbackState attribute to the value given in the second column of the row of the following table whose first column contains the new playback state.
Fire a simple event at the MediaController object, whose name is the value given in the third column of the row of the following table whose first column contains the new playback state.

New playback state	New value for `playbackState`	Event name
playing	"`playing`"	`playing`
waiting	"`waiting`"	`waiting`
ended	"`ended`"	`ended`

Let the MediaController's most recently reported playback state be new playback state.

Event handler	Event handler event type
`onemptied`	`emptied`
`onloadedmetadata`	`loadedmetadata`
`onloadeddata`	`loadeddata`
`oncanplay`	`canplay`
`oncanplaythrough`	`canplaythrough`
`onplaying`	`playing`
`onended`	`ended`
`onwaiting`	`waiting`
`ondurationchange`	`durationchange`
`ontimeupdate`	`timeupdate`
`onplay`	`play`
`onpause`	`pause`
`onratechange`	`ratechange`
`onvolumechange`	`volumechange`

The task source for the tasks listed in this section is the DOM manipulation task source.

4.7.10.11.3 Assigning a media controller declaratively

The mediagroup content attribute on media elements can be used to link multiple media elements together by implicitly creating a MediaController. The value is text; media elements with the same value are automatically linked by the user agent.

When a media element is created with a mediagroup attribute, and when a media element's mediagroup attribute is set, changed, or removed, the user agent must run the following steps:

Let m be the media element in question.
Let old controller be m's current media controller, if it currently has one, and null otherwise.
Let new controller be null.
Let m have no current media controller, if it currently has one.
If m's mediagroup attribute is being removed, then jump to the update controllers step below.
If there is another media element whose Document is the same as m's Document (even if one or both of these elements are not actually in the Document), and which also has a mediagroup attribute, and whose mediagroup attribute has the same value as the new value of m's mediagroup attribute, then let controller be that media element's current media controller.

Otherwise, let controller be a newly created MediaController.
Let m's current media controller be controller.
Let new controller be m's current media controller.
Bring the media element up to speed with its new media controller.
Update controllers: If old controller and new controller are the same (whether both null or both the same controller) then abort these steps.
If old controller is not null and still has one or more slaved media elements, then report the controller state for old controller.
If new controller is not null, then report the controller state for new controller.

The mediaGroup IDL attribute on media elements must reflect the mediagroup content attribute.

Multiple media elements referencing the same media resource will share a single network request. This can be used to efficiently play two (video) tracks from the same media resource in two different places on the screen. Used with the mediagroup attribute, these elements can also be kept synchronised.

In this example, a sign-languge interpreter track from a movie file is overlaid on the primary video track of that same video file using two video elements, some CSS, and an implicit MediaController:

<article>
 <style>
  div { margin: 1em auto; position: relative; width: 400px; height: 300px; }
  video { position; absolute; bottom: 0; right: 0; }
  video:first-child { width: 100%; height: 100%; }
  video:last-child { width: 30%; }
 </style>
 <div>
  <video src="movie.vid#track=Video&amp;track=English" autoplay controls mediagroup=movie></video>
  <video src="movie.vid#track=sign" autoplay mediagroup=movie></video>
 </div>
</article>

4.7.10.12 Timed text tracks

4.7.10.12.1 Text track model

A media element can have a group of associated text tracks, known as the media element's list of text tracks. The text tracks are sorted as follows:

The text tracks corresponding to track element children of the media element, in tree order.
Any text tracks added using the addTextTrack() method, in the order they were added, oldest first.
Any media-resource-specific text tracks (text tracks corresponding to data in the media resource), in the order defined by the media resource's format specification.

A text track consists of:

The kind of text track

This decides how the track is handled by the user agent. The kind is represented by a string. The possible strings are:

subtitles
captions
descriptions
chapters
metadata

The kind of track can change dynamically, in the case of a text track corresponding to a track element.

A label

This is a human-readable string intended to identify the track for the user.

The label of a track can change dynamically, in the case of a text track corresponding to a track element.

When a text track label is the empty string, the user agent should automatically generate an appropriate label from the text track's other properties (e.g. the kind of text track and the text track's language) for use in its user interface. This automatically-generated label is not exposed in the API.

An in-band metadata track dispatch type

This is a string extracted from the media resource specifically for in-band metadata tracks to enable such tracks to be dispatched to different scripts in the document.

For example, a traditional TV station broadcast streamed on the Web and augmented with Web-specific interactive features could include text tracks with metadata for ad targeting, trivia game data during game shows, player states during sports games, recipe information during food programs, and so forth. As each program starts and ends, new tracks might be added or removed from the stream, and as each one is added, the user agent could bind them to dedicated script modules using the value of this attribute.

Other than for in-band metadata text tracks, the in-band metadata track dispatch type is the empty string. How this value is populated for different media formats is described in steps to expose a media-resource-specific text track.

A language

This is a string (a BCP 47 language tag) representing the language of the text track's cues. [BCP47]

The language of a text track can change dynamically, in the case of a text track corresponding to a track element.

A readiness state

One of the following:

Not loaded: Indicates that the text track's cues have not been obtained.
Loading: Indicates that the text track is loading and there have been no fatal errors encountered so far. Further cues might still be added to the track by the parser.
Loaded: Indicates that the text track has been loaded with no fatal errors.
Failed to load: Indicates that the text track was enabled, but when the user agent attempted to obtain it, this failed in some way (e.g. URL could not be resolved, network error, unknown text track format). Some or all of the cues are likely missing and will not be obtained.

The readiness state of a text track changes dynamically as the track is obtained.

A mode

One of the following:

Disabled: Indicates that the text track is not active. Other than for the purposes of exposing the track in the DOM, the user agent is ignoring the text track. No cues are active, no events are fired, and the user agent will not attempt to obtain the track's cues.
Hidden: Indicates that the text track is active, but that the user agent is not actively displaying the cues. If no attempt has yet been made to obtain the track's cues, the user agent will perform such an attempt momentarily. The user agent is maintaining a list of which cues are active, and events are being fired accordingly.
Showing: Indicates that the text track is active. If no attempt has yet been made to obtain the track's cues, the user agent will perform such an attempt momentarily. The user agent is maintaining a list of which cues are active, and events are being fired accordingly. In addition, for text tracks whose kind is subtitles or captions, the cues are being overlaid on the video as appropriate; for text tracks whose kind is descriptions, the user agent is making the cues available to the user in a non-visual fashion; and for text tracks whose kind is chapters, the user agent is making available to the user a mechanism by which the user can navigate to any point in the media resource by selecting a cue.

A list of zero or more cues

A list of text track cues, along with rules for updating the text track rendering. For example, for WebVTT, the rules for updating the display of WebVTT text tracks. [WEBVTT]

The list of cues of a text track can change dynamically, either because the text track has not yet been loaded or is still loading, or due to DOM manipulation.

Each text track has a corresponding TextTrack object.

Each media element has a list of pending text tracks, which must initially be empty, a blocked-on-parser flag, which must initially be false, and a did-perform-automatic-track-selection flag, which must also initially be false.

When the user agent is required to populate the list of pending text tracks of a media element, the user agent must add to the element's list of pending text tracks each text track in the element's list of text tracks whose text track mode is not disabled and whose text track readiness state is loading.

Whenever a track element's parent node changes, the user agent must remove the corresponding text track from any list of pending text tracks that it is in.

Whenever a text track's text track readiness state changes to either loaded or failed to load, the user agent must remove it from any list of pending text tracks that it is in.

When a media element is created by an HTML parser or XML parser, the user agent must set the element's blocked-on-parser flag to true. When a media element is popped off the stack of open elements of an HTML parser or XML parser, the user agent must honor user preferences for automatic text track selection, populate the list of pending text tracks, and set the element's blocked-on-parser flag to false.

The text tracks of a media element are ready when both the element's list of pending text tracks is empty and the element's blocked-on-parser flag is false.

Each media element has a pending text track change notification flag, which must initially be unset.

Whenever a text track that is in a media element's list of text tracks has its text track mode change value, the user agent must run the following steps for the media element:

If the media element's pending text track change notification flag is set, abort these steps.
Set the media element's pending text track change notification flag.
Queue a task that runs the following substeps:
1. Unset the media element's pending text track change notification flag.
2. Fire a simple event named change at the media element's textTracks attribute's TextTrackList object.
If the media element's show poster flag is not set, run the time marches on steps.

The task source for the tasks listed in this section is the DOM manipulation task source.

A text track cue is the unit of time-sensitive data in a text track, corresponding for instance for subtitles and captions to the text that appears at a particular time and disappears at another time.

Each text track cue consists of:

An identifier

An arbitrary string.

A start time

The time, in seconds and fractions of a second, that describes the beginning of the range of the media data to which the cue applies.

An end time

The time, in seconds and fractions of a second, that describes the end of the range of the media data to which the cue applies.

A pause-on-exit flag

A boolean indicating whether playback of the media resource is to pause when the end of the range to which the cue applies is reached.

Some additional format-specific data

Additional fields, as needed for the format. For example, WebVTT has a text track cue writing direction and so forth. [WEBVTT]

The data of the cue

The raw data of the cue, and rules for rendering the cue in isolation.

The precise nature of this data is defined by the format. For example, WebVTT uses text.

The text track cue start time and text track cue end time can be negative. (The current playback position can never be negative, though, so cues entirely before time zero cannot be active.)

Each text track cue has a corresponding TextTrackCue object (or more specifically, an object that inherits from TextTrackCue — for example, WebVTT cues use the VTTCue interface). A text track cue's in-memory representation can be dynamically changed through this TextTrackCue API. [WEBVTT]

A text track cue is associated with rules for updating the text track rendering, as defined by the specification for the specific kind of text track cue. These rules are used specifically when the object representing the cue is added to a TextTrack object using the addCue() method.

In addition, each text track cue has two pieces of dynamic information:

The active flag

This flag must be initially unset. The flag is used to ensure events are fired appropriately when the cue becomes active or inactive, and to make sure the right cues are rendered.

The user agent must synchronously unset this flag whenever the text track cue is removed from its text track's text track list of cues; whenever the text track itself is removed from its media element's list of text tracks or has its text track mode changed to disabled; and whenever the media element's readyState is changed back to HAVE_NOTHING. When the flag is unset in this way for one or more cues in text tracks that were showing prior to the relevant incident, the user agent must, after having unset the flag for all the affected cues, apply the rules for updating the text track rendering of those text tracks. For example, for text tracks based on WebVTT, the rules for updating the display of WebVTT text tracks. [WEBVTT]

The display state

This is used as part of the rendering model, to keep cues in a consistent position. It must initially be empty. Whenever the text track cue active flag is unset, the user agent must empty the text track cue display state.

The text track cues of a media element's text tracks are ordered relative to each other in the text track cue order, which is determined as follows: first group the cues by their text track, with the groups being sorted in the same order as their text tracks appear in the media element's list of text tracks; then, within each group, cues must be sorted by their start time, earliest first; then, any cues with the same start time must be sorted by their end time, latest first; and finally, any cues with identical end times must be sorted in the order they were last added to their respective text track list of cues, oldest first (so e.g. for cues from a WebVTT file, that would initially be the order in which the cues were listed in the file). [WEBVTT]

4.7.10.12.2 Sourcing in-band text tracks

A media-resource-specific text track is a text track that corresponds to data found in the media resource.

Rules for processing and rendering such data are defined by the relevant specifications, e.g. the specification of the video format if the media resource is a video.

When a media resource contains data that the user agent recognises and supports as being equivalent to a text track, the user agent runs the steps to expose a media-resource-specific text track with the relevant data, as follows.

Associate the relevant data with a new text track and its corresponding new TextTrack object. The text track is a media-resource-specific text track.
Set the new text track's kind, label, and language based on the semantics of the relevant data, as defined for the relevant format [INBANDTRACKS]. If there is no label in that data, then the label must be set to the empty string.
Associate the text track list of cues with the rules for updating the text track rendering appropriate for the format in question.
If the new text track's kind is metadata, then set the text track in-band metadata track dispatch type as follows, based on the type of the media resource:

If the media resource is an Ogg file

The text track in-band metadata track dispatch type must be set to the value of the Role header field. [OGGSKELETONHEADERS]

If the media resource is a WebM file

The text track in-band metadata track dispatch type must be set to the value of the CodecID element. [WEBMCG]

If the media resource is an MPEG-2 file

Let stream type be the value of the "stream_type" field describing the text track's type in the file's program map section, interpreted as an 8-bit unsigned integer. Let length be the value of the "ES_info_length" field for the track in the same part of the program map section, interpreted as an integer as defined by the MPEG-2 specification. Let descriptor bytes be the length bytes following the "ES_info_length" field. The text track in-band metadata track dispatch type must be set to the concatenation of the stream type byte and the zero or more descriptor bytes bytes, expressed in hexadecimal using uppercase ASCII hex digits. [MPEG2]

If the media resource is an MPEG-4 file

Let the first stsd box of the first stbl box of the first minf box of the first mdia box of the text track's trak box in the first moov box of the file be the stsd box, if any. If the file has no stsd box, or if the stsd box has neither a mett box nor a metx box, then the text track in-band metadata track dispatch type must be set to the empty string. Otherwise, if the stsd box has a mett box then the text track in-band metadata track dispatch type must be set to the concatenation of the string "mett", a U+0020 SPACE character, and the value of the first mime_format field of the first mett box of the stsd box, or the empty string if that field is absent in that box. Otherwise, if the stsd box has no mett box but has a metx box then the text track in-band metadata track dispatch type must be set to the concatenation of the string "metx", a U+0020 SPACE character, and the value of the first namespace field of the first metx box of the stsd box, or the empty string if that field is absent in that box. [MPEG4]

If the media resource is a DASH media resource

The text track in-band metadata track dispatch type must be set to the concatenation of the "AdaptationSet" element attributes and all child Role descriptors. [DASH]
Populate the new text track's list of cues with the cues parsed so far, following the guidelines for exposing cues, and begin updating it dynamically as necessary.
Set the new text track's readiness state to loaded.
Set the new text track's mode to the mode consistent with the user's preferences and the requirements of the relevant specification for the data.
Add the new text track to the media element's list of text tracks.
Fire a trusted event with the name addtrack, that does not bubble and is not cancelable, and that uses the TrackEvent interface, with the track attribute initialized to the text track's TextTrack object, at the media element's textTracks attribute's TextTrackList object.

4.7.10.12.3 Sourcing out-of-band text tracks

When a track element is created, it must be associated with a new text track (with its value set as defined below) and its corresponding new TextTrack object.

The text track kind is determined from the state of the element's kind attribute according to the following table; for a state given in a cell of the first column, the kind is the string given in the second column:

State	String
Subtitles	`subtitles`
Captions	`captions`
Descriptions	`descriptions`
Chapters	`chapters`
Metadata	`metadata`

The text track label is the element's track label.

The text track language is the element's track language, if any, or the empty string otherwise.

As the kind, label, and srclang attributes are set, changed, or removed, the text track must update accordingly, as per the definitions above.

Changes to the track URL are handled in the algorithm below.

The text track readiness state is initially not loaded, and the text track mode is initially disabled.

The text track list of cues is initially empty. It is dynamically modified when the referenced file is parsed. Associated with the list are the rules for updating the text track rendering appropriate for the format in question; for WebVTT, this is the rules for updating the display of WebVTT text tracks. [WEBVTT]

When a track element's parent element changes and the new parent is a media element, then the user agent must add the track element's corresponding text track to the media element's list of text tracks, and then queue a task to fire a trusted event with the name addtrack, that does not bubble and is not cancelable, and that uses the TrackEvent interface, with the track attribute initialized to the text track's TextTrack object, at the media element's textTracks attribute's TextTrackList object.

When a track element's parent element changes and the old parent was a media element, then the user agent must remove the track element's corresponding text track from the media element's list of text tracks, and then queue a task to fire a trusted event with the name removetrack, that does not bubble and is not cancelable, and that uses the TrackEvent interface, with the track attribute initialized to the text track's TextTrack object, at the media element's textTracks attribute's TextTrackList object.

When a text track corresponding to a track element is added to a media element's list of text tracks, the user agent must queue a task to run the following steps for the media element:

If the element's blocked-on-parser flag is true, abort these steps.
If the element's did-perform-automatic-track-selection flag is true, abort these steps.
Honor user preferences for automatic text track selection for this element.

When the user agent is required to honor user preferences for automatic text track selection for a media element, the user agent must run the following steps:

Perform automatic text track selection for subtitles and captions.
Perform automatic text track selection for descriptions.
Perform automatic text track selection for chapters.
If there are any text tracks in the media element's list of text tracks whose text track kind is metadata that correspond to track elements with a default attribute set whose text track mode is set to disabled, then set the text track mode of all such tracks to hidden
Set the element's did-perform-automatic-track-selection flag to true.

When the steps above say to perform automatic text track selection for one or more text track kinds, it means to run the following steps:

Let candidates be a list consisting of the text tracks in the media element's list of text tracks whose text track kind is one of the kinds that were passed to the algorithm, if any, in the order given in the list of text tracks.
If candidates is empty, then abort these steps.
If any of the text tracks in candidates have a text track mode set to showing, abort these steps.
If the user has expressed an interest in having a track from candidates enabled based on its text track kind, text track language, and text track label, then set its text track mode to showing.

For example, the user could have set a browser preference to the effect of "I want French captions whenever possible", or "If there is a subtitle track with 'Commentary' in the title, enable it", or "If there are audio description tracks available, enable one, ideally in Swiss German, but failing that in Standard Swiss German or Standard German".

Otherwise, if there are any text tracks in candidates that correspond to track elements with a default attribute set whose text track mode is set to disabled, then set the text track mode of the first such track to showing.

When a text track corresponding to a track element experiences any of the following circumstances, the user agent must start the track processing model for that text track and its track element:

The track element is created.
The text track has its text track mode changed.
The track element's parent element changes and the new parent is a media element.

When a user agent is to start the track processing model for a text track and its track element, it must run the following algorithm. This algorithm interacts closely with the event loop mechanism; in particular, it has a synchronous section (which is triggered as part of the event loop algorithm). The steps in that section are marked with ⌛.

If another occurrence of this algorithm is already running for this text track and its track element, abort these steps, letting that other algorithm take care of this element.
If the text track's text track mode is not set to one of hidden or showing, abort these steps.
If the text track's track element does not have a media element as a parent, abort these steps.
Run the remainder of these steps asynchronously, allowing whatever caused these steps to run to continue.
Top: Await a stable state. The synchronous section consists of the following steps. (The steps in the synchronous section are marked with ⌛.)
⌛ Set the text track readiness state to loading.
⌛ Let URL be the track URL of the track element.
⌛ If the track element's parent is a media element then let CORS mode be the state of the parent media element's crossorigin content attribute. Otherwise, let CORS mode be No CORS.
End the synchronous section, continuing the remaining steps asynchronously.
If URL is not the empty string, perform a potentially CORS-enabled fetch of URL, with the mode being CORS mode, the origin being the origin of the track element's Document, and the default origin behaviour set to fail.

The resource obtained in this fashion, if any, contains the text track data. If any data is obtained, it is by definition CORS-same-origin (cross-origin resources that are not suitably CORS-enabled do not get this far).

The tasks queued by the fetching algorithm on the networking task source to process the data as it is being fetched must determine the type of the resource. If the type of the resource is not a supported text track format, the load will fail, as described below. Otherwise, the resource's data must be passed to the appropriate parser (e.g. the WebVTT parser) as it is received, with the text track list of cues being used for that parser's output. [WEBVTT]

The appropriate parser will synchronously (during these networking task source tasks) and incrementally (as each such task is run with whatever data has been received from the network) update the text track list of cues.

This specification does not currently say whether or how to check the MIME types of text tracks, or whether or how to perform file type sniffing using the actual file data. Implementors differ in their intentions on this matter and it is therefore unclear what the right solution is. In the absence of any requirement here, the HTTP specification's strict requirement to follow the Content-Type header prevails ("Content-Type specifies the media type of the underlying data." ... "If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource.").

If the fetching algorithm fails for any reason (network error, the server returns an error code, a cross-origin check fails, etc), if URL is the empty string, or if the type of the resource is not a supported text track format, then run these steps:
1. Queue a task to first change the text track readiness state to failed to load and then fire a simple event named error at the track element.
2. Wait until the text track readiness state is no longer set to loading.
3. Wait until the track URL is no longer equal to URL, at the same time as the text track mode is set to hidden or showing.
4. Jump to the step labeled top.
If the fetching algorithm does not fail, then the final task that is queued by the networking task source must run the following steps after it has tried to parse the data:
1. Change the text track readiness state to loaded.
2. If the file was successfully processed, fire a simple event named load at the track element.
  
  Otherwise, the file was not successfully processed (e.g. the format in question is an XML format and the file contained a well-formedness error that the XML specification requires be detected and reported to the application); fire a simple event named error at the track element.
3. Wait until the track URL is no longer equal to URL, at the same time as the text track mode is set to hidden or showing.
4. Jump back to the step labeled top.
If, while the fetching algorithm is active, either:
- the track URL changes so that it is no longer equal to URL, while the text track mode is set to hidden or showing; or
- the text track mode changes to hidden or showing, while the track URL is not equal to URL
...then the user agent must run the following steps:
1. Abort the fetching algorithm, discarding any pending tasks generated by that algorithm (and in particular, not adding any cues to the text track list of cues after the moment the URL changed).
2. Jump back to the step labeled top.
Until one of the above circumstances occurs, the user agent must remain on this step.

Whenever a track element has its src attribute set, changed, or removed, the user agent must synchronously empty the element's text track's text track list of cues. (This also causes the algorithm above to stop adding cues from the resource being obtained using the previously given URL, if any.)

4.7.10.12.4 Guidelines for exposing cues in various formats as text track cues

How a specific format's text track cues are to be interpreted for the purposes of processing by an HTML user agent is defined by that format [INBANDTRACKS]. In the absence of such a specification, this section provides some constraints within which implementations can attempt to consistently expose such formats.

To support the text track model of HTML, each unit of timed data is converted to a text track cue. Where the mapping of the format's features to the aspects of a text track cue as defined in this specification are not defined, implementations must ensure that the mapping is consistent with the definitions of the aspects of a text track cue as defined above, as well as with the following constraints:

The text track cue identifier: Should be set to the empty string if the format has no obvious analogue to a per-cue identifier.
The text track cue pause-on-exit flag: Should be set to false.

4.7.10.12.5 Text track API

interface TextTrackList : EventTarget {
  readonly attribute unsigned long length;
  getter TextTrack (unsigned long index);
  TextTrack? getTrackById(DOMString id);

           attribute EventHandler onchange;
           attribute EventHandler onaddtrack;
           attribute EventHandler onremovetrack;
};

media . textTracks . length: Returns the number of text tracks associated with the media element (e.g. from track elements). This is the number of text tracks in the media element's list of text tracks.
media . textTracks[ n ]: Returns the TextTrack object representing the nth text track in the media element's list of text tracks.
textTrack = media . textTracks . getTrackById( id ): Returns the TextTrack object with the given identifier, or null if no track has that identifier.
track . track: Returns the TextTrack object representing the track element's text track.

A TextTrackList object represents a dynamically updating list of text tracks in a given order.

The textTracks attribute of media elements must return a TextTrackList object representing the TextTrack objects of the text tracks in the media element's list of text tracks, in the same order as in the list of text tracks. The same object must be returned each time the attribute is accessed. [WEBIDL]

The length attribute of a TextTrackList object must return the number of text tracks in the list represented by the TextTrackList object.

The supported property indices of a TextTrackList object at any instant are the numbers from zero to the number of text tracks in the list represented by the TextTrackList object minus one, if any. If there are no text tracks in the list, there are no supported property indices.

To determine the value of an indexed property of a TextTrackList object for a given index index, the user agent must return the indexth text track in the list represented by the TextTrackList object.

The getTrackById(id) method must return the first TextTrack in the TextTrackList object whose id IDL attribute would return a value equal to the value of the id argument. When no tracks match the given argument, the method must return null.

enum TextTrackMode { "disabled",  "hidden",  "showing" };
enum TextTrackKind { "subtitles",  "captions",  "descriptions",  "chapters",  "metadata" };
interface TextTrack : EventTarget {
  readonly attribute TextTrackKind kind;
  readonly attribute DOMString label;
  readonly attribute DOMString language;

  readonly attribute DOMString id;
  readonly attribute DOMString inBandMetadataTrackDispatchType;

           attribute TextTrackMode mode;

  readonly attribute TextTrackCueList? cues;
  readonly attribute TextTrackCueList? activeCues;

  void addCue(TextTrackCue cue);
  void removeCue(TextTrackCue cue);

           attribute EventHandler oncuechange;
};

textTrack = media . addTextTrack( kind [, label [, language ] ] )

Creates and returns a new TextTrack object, which is also added to the media element's list of text tracks.

textTrack . kind

Returns the text track kind string.

textTrack . label

Returns the text track label, if there is one, or the empty string otherwise (indicating that a custom label probably needs to be generated from the other attributes of the object if the object is exposed to the user).

textTrack . language

Returns the text track language string.

textTrack . id

Returns the ID of the given track.

For in-band tracks, this is the ID that can be used with a fragment identifier if the format supports the Media Fragments URI syntax, and that can be used with the getTrackById() method. [MEDIAFRAG]

For TextTrack objects corresponding to track elements, this is the ID of the track element.

textTrack . inBandMetadataTrackDispatchType

Returns the text track in-band metadata track dispatch type string.

textTrack . mode [ = value ]

Returns the text track mode, represented by a string from the following list:

"disabled": The text track disabled mode.
"hidden": The text track hidden mode.
"showing": The text track showing mode.

Can be set, to change the mode.

textTrack . cues

Returns the text track list of cues, as a TextTrackCueList object.

textTrack . activeCues

Returns the text track cues from the text track list of cues that are currently active (i.e. that start before the current playback position and end after it), as a TextTrackCueList object.

textTrack . addCue( cue )

Adds the given cue to textTrack's text track list of cues.

textTrack . removeCue( cue )

Removes the given cue from textTrack's text track list of cues.

The addTextTrack(kind, label, language) method of media elements, when invoked, must run the following steps:

Create a new TextTrack object.
Create a new text track corresponding to the new object, and set its text track kind to kind, its text track label to label, its text track language to language, its text track readiness state to the text track loaded state, its text track mode to the text track hidden mode, and its text track list of cues to an empty list.

Initially, the text track list of cues is not associated with any rules for updating the text track rendering. When a text track cue is added to it, the text track list of cues has its rules permanently set accordingly.
Add the new text track to the media element's list of text tracks.
Queue a task to fire a trusted event with the name addtrack, that does not bubble and is not cancelable, and that uses the TrackEvent interface, with the track attribute initialized to the new text track's TextTrack object, at the media element's textTracks attribute's TextTrackList object.
Return the new TextTrack object.

The kind attribute must return the text track kind of the text track that the TextTrack object represents.

The label attribute must return the text track label of the text track that the TextTrack object represents.

The language attribute must return the text track language of the text track that the TextTrack object represents.

The id attribute returns the track's identifier, if it has one, or the empty string otherwise. For tracks that correspond to track elements, the track's identifier is the value of the element's id attribute, if any. For in-band tracks, the track's identifier is specified by the media resource. If the media resource is in a format that supports the Media Fragments URI fragment identifier syntax, the identifier returned for a particular track must be the same identifier that would enable the track if used as the name of a track in the track dimension of such a fragment identifier. [MEDIAFRAG]

The inBandMetadataTrackDispatchType attribute must return the text track in-band metadata track dispatch type of the text track that the TextTrack object represents.

The mode attribute, on getting, must return the string corresponding to the text track mode of the text track that the TextTrack object represents, as defined by the following list:

"disabled": The text track disabled mode.
"hidden": The text track hidden mode.
"showing": The text track showing mode.

On setting, if the new value isn't equal to what the attribute would currently return, the new value must be processed as follows:

If the new value is "disabled": Set the text track mode of the text track that the TextTrack object represents to the text track disabled mode.
If the new value is "hidden": Set the text track mode of the text track that the TextTrack object represents to the text track hidden mode.
If the new value is "showing": Set the text track mode of the text track that the TextTrack object represents to the text track showing mode.

If the text track mode of the text track that the TextTrack object represents is not the text track disabled mode, then the cues attribute must return a live TextTrackCueList object that represents the subset of the text track list of cues of the text track that the TextTrack object represents whose end times occur at or after the earliest possible position when the script started, in text track cue order. Otherwise, it must return null. When an object is returned, the same object must be returned each time.

The earliest possible position when the script started is whatever the earliest possible position was the last time the event loop reached step 1.

If the text track mode of the text track that the TextTrack object represents is not the text track disabled mode, then the activeCues attribute must return a live TextTrackCueList object that represents the subset of the text track list of cues of the text track that the TextTrack object represents whose active flag was set when the script started, in text track cue order. Otherwise, it must return null. When an object is returned, the same object must be returned each time.

A text track cue's active flag was set when the script started if its text track cue active flag was set the last time the event loop reached step 1.

The addCue(cue) method of TextTrack objects, when invoked, must run the following steps:

If the text track list of cues does not yet have any associated rules for updating the text track rendering, then associate the text track list of cues with the rules for updating the text track rendering appropriate to cue.
If text track list of cues' associated rules for updating the text track rendering are not the same rules for updating the text track rendering as appropriate for cue, then throw an InvalidStateError exception and abort these steps.
If the given cue is in a text track list of cues, then remove cue from that text track list of cues.
Add cue to the method's TextTrack object's text track's text track list of cues.

The removeCue(cue) method of TextTrack objects, when invoked, must run the following steps:

If the given cue is not currently listed in the method's TextTrack object's text track's text track list of cues, then throw a NotFoundError exception and abort these steps.
Remove cue from the method's TextTrack object's text track's text track list of cues.

In this example, an audio element is used to play a specific sound-effect from a sound file containing many sound effects. A cue is used to pause the audio, so that it ends exactly at the end of the clip, even if the browser is busy running some script. If the page had relied on script to pause the audio, then the start of the next clip might be heard if the browser was not able to run the script at the exact time specified.

var sfx = new Audio('sfx.wav');
var sounds = sfx.addTextTrack('metadata');

// add sounds we care about
function addFX(start, end, name) {
  var cue = new VTTCue(start, end, '');
  cue.id = name;
  cue.pauseOnExit = true;
  sounds.addCue(cue);
}
addFX(12.783, 13.612, 'dog bark');
addFX(13.612, 15.091, 'kitten mew'))

function playSound(id) {
  sfx.currentTime = sounds.getCueById(id).startTime;
  sfx.play();
}

// play a bark as soon as we can
sfx.oncanplaythrough = function () {
  playSound('dog bark');
}
// meow when the user tries to leave
window.onbeforeunload = function () {
  playSound('kitten mew');
  return 'Are you sure you want to leave this awesome page?';
}

interface TextTrackCueList {
  readonly attribute unsigned long length;
  getter TextTrackCue (unsigned long index);
  TextTrackCue? getCueById(DOMString id);
};

cuelist . length

Returns the number of cues in the list.

cuelist[index]

Returns the text track cue with index index in the list. The cues are sorted in text track cue order.

cuelist . getCueById( id )

Returns the first text track cue (in text track cue order) with text track cue identifier id.

Returns null if none of the cues have the given identifier or if the argument is the empty string.

A TextTrackCueList object represents a dynamically updating list of text track cues in a given order.

The length attribute must return the number of cues in the list represented by the TextTrackCueList object.

The supported property indices of a TextTrackCueList object at any instant are the numbers from zero to the number of cues in the list represented by the TextTrackCueList object minus one, if any. If there are no cues in the list, there are no supported property indices.

To determine the value of an indexed property for a given index index, the user agent must return the indexth text track cue in the list represented by the TextTrackCueList object.

The getCueById(id) method, when called with an argument other than the empty string, must return the first text track cue in the list represented by the TextTrackCueList object whose text track cue identifier is id, if any, or null otherwise. If the argument is the empty string, then the method must return null.

interface TextTrackCue : EventTarget {
  readonly attribute TextTrack? track;

           attribute DOMString id;
           attribute double startTime;
           attribute double endTime;
           attribute boolean pauseOnExit;

           attribute EventHandler onenter;
           attribute EventHandler onexit;
};

cue . track

Returns the TextTrack object to which this text track cue belongs, if any, or null otherwise.

cue . id [ = value ]

Returns the text track cue identifier.

Can be set.

cue . startTime [ = value ]

Returns the text track cue start time, in seconds.

Can be set.

cue . endTime [ = value ]

Returns the text track cue end time, in seconds.

Can be set.

cue . pauseOnExit [ = value ]

Returns true if the text track cue pause-on-exit flag is set, false otherwise.

Can be set.

The track attribute, on getting, must return the TextTrack object of the text track in whose list of cues the text track cue that the TextTrackCue object represents finds itself, if any; or null otherwise.

The id attribute, on getting, must return the text track cue identifier of the text track cue that the TextTrackCue object represents. On setting, the text track cue identifier must be set to the new value.

The startTime attribute, on getting, must return the text track cue start time of the text track cue that the TextTrackCue object represents, in seconds. On setting, the text track cue start time must be set to the new value, interpreted in seconds; then, if the TextTrackCue object's text track cue is in a text track's list of cues, and that text track is in a media element's list of text tracks, and the media element's show poster flag is not set, then run the time marches on steps for that media element.

The endTime attribute, on getting, must return the text track cue end time of the text track cue that the TextTrackCue object represents, in seconds. On setting, the text track cue end time must be set to the new value, interpreted in seconds; then, if the TextTrackCue object's text track cue is in a text track's list of cues, and that text track is in a media element's list of text tracks, and the media element's show poster flag is not set, then run the time marches on steps for that media element.

The pauseOnExit attribute, on getting, must return true if the text track cue pause-on-exit flag of the text track cue that the TextTrackCue object represents is set; or false otherwise. On setting, the text track cue pause-on-exit flag must be set if the new value is true, and must be unset otherwise.

4.7.10.12.6 Text tracks describing chapters

Chapters are segments of a media resource with a given title. Chapters can be nested, in the same way that sections in a document outline can have subsections.

Each text track cue in a text track being used for describing chapters has three key features: the text track cue start time, giving the start time of the chapter, the text track cue end time, giving the end time of the chapter, and the text track cue data giving the chapter title.

The rules for constructing the chapter tree from a text track are as follows. They produce a potentially nested list of chapters, each of which have a start time, end time, title, and a list of nested chapters. This algorithm discards cues that do not correctly nest within each other, or that are out of order.

Let list be a copy of the list of cues of the text track being processed.
Remove from list any text track cue whose text track cue end time is before its text track cue start time.
Let output be an empty list of chapters, where a chapter is a record consisting of a start time, an end time, a title, and a (potentially empty) list of nested chapters. For the purpose of this algorithm, each chapter also has a parent chapter.
Let current chapter be a stand-in chapter whose start time is negative infinity, whose end time is positive infinity, and whose list of nested chapters is output. (This is just used to make the algorithm easier to describe.)
Loop: If list is empty, jump to the step labeled end.
Let current cue be the first cue in list, and then remove it from list.
If current cue's text track cue start time is less than the start time of current chapter, then return to the step labeled loop.
While current cue's text track cue start time is greater than or equal to current chapter's end time, let current chapter be current chapter's parent chapter.
If current cue's text track cue end time is greater than the end time of current chapter, then return to the step labeled loop.
Create a new chapter new chapter, whose start time is current cue's text track cue start time, whose end time is current cue's text track cue end time, whose title is current cue's text track cue data interpreted according to its rules for rendering the cue in isolation, and whose list of nested chapters is empty.

For WebVTT, the rules for rendering the cue in isolation are the rules for interpreting WebVTT cue text. [WEBVTT]
Append new chapter to current chapter's list of nested chapters, and let current chapter be new chapter's parent.
Let current chapter be new chapter.
Return to the step labeled loop.
End: Return output.

The following snippet of a WebVTT file shows how nested chapters can be marked up. The file describes three 50-minute chapters, "Astrophysics", "Computational Physics", and "General Relativity". The first has three subchapters, the second has four, and the third has two. [WEBVTT]

WEBVTT

00:00:00.000 --> 00:50:00.000
Astrophysics

00:00:00.000 --> 00:10:00.000
Introduction to Astrophysics

00:10:00.000 --> 00:45:00.000
The Solar System

00:00:00.000 --> 00:10:00.000
Coursework Description

00:50:00.000 --> 01:40:00.000
Computational Physics

00:50:00.000 --> 00:55:00.000
Introduction to Programming

00:55:00.000 --> 01:30:00.000
Data Structures

01:30:00.000 --> 01:35:00.000
Answers to Last Exam

01:35:00.000 --> 01:40:00.000
Coursework Description

01:40:00.000 --> 02:30:00.000
General Relativity

01:40:00.000 --> 02:00:00.000
Tensor Algebra

02:00:00.000 --> 02:30:00.000
The General Relativistic Field Equations

4.7.10.12.7 Event definitions

The following are the event handlers that (and their corresponding event handler event types) must be supported, as event handler IDL attributes, by all objects implementing the TextTrackList interface:

Event handler	Event handler event type
`onchange`	`change`
`onaddtrack`	`addtrack`
`onremovetrack`	`removetrack`

Event handler	Event handler event type
`oncuechange`	`cuechange`

Event handler	Event handler event type
`onenter`	`enter`
`onexit`	`exit`

4.7.10.13 User interface

The controls attribute is a boolean attribute. If present, it indicates that the author has not provided a scripted controller and would like the user agent to provide its own set of controls.

If the attribute is present, or if scripting is disabled for the media element, then the user agent should expose a user interface to the user. This user interface should include features to begin playback, pause playback, seek to an arbitrary position in the content (if the content supports arbitrary seeking), change the volume, change the display of closed captions or embedded sign-language tracks, select different audio tracks or turn on audio descriptions, and show the media content in manners more suitable to the user (e.g. full-screen video or in an independent resizable window). Other controls may also be made available.

If the media element has a current media controller, then the user agent should expose audio tracks from all the slaved media elements (although avoiding duplicates if the same media resource is being used several times). If a media resource's audio track exposed in this way has no known name, and it is the only audio track for a particular media element, the user agent should use the element's title attribute, if any, as the name (or as part of the name) of that track.

Even when the attribute is absent, however, user agents may provide controls to affect playback of the media resource (e.g. play, pause, seeking, and volume controls), but such features should not interfere with the page's normal rendering. For example, such features could be exposed in the media element's context menu. The user agent may implement this simply by exposing a user interface to the user as described above (as if the controls attribute was present).

If the user agent exposes a user interface to the user by displaying controls over the media element, then the user agent should suppress any user interaction events while the user agent is interacting with this interface. (For example, if the user clicks on a video's playback control, mousedown events and so forth would not simultaneously be fired at elements on the page.)

Where possible (specifically, for starting, stopping, pausing, and unpausing playback, for seeking, for changing the rate of playback, for fast-forwarding or rewinding, for listing, enabling, and disabling text tracks, and for muting or changing the volume of the audio), user interface features exposed by the user agent must be implemented in terms of the DOM API described above, so that, e.g., all the same events fire.

When a media element has a current media controller, the user agent's user interface for pausing and unpausing playback, for seeking, for changing the rate of playback, for fast-forwarding or rewinding, and for muting or changing the volume of audio of the entire group must be implemented in terms of the MediaController API exposed on that current media controller. When a media element has a current media controller, and all the slaved media elements of that MediaController are paused, the user agent should also unpause all the slaved media elements when the user invokes a user agent interface control for beginning playback.

The "play" function in the user agent's interface must set the playbackRate attribute to the value of the defaultPlaybackRate attribute before invoking the play() method. When a media element has a current media controller, the attributes and method with those names on that MediaController object must be used. Otherwise, the attributes and method with those names on the media element itself must be used.

Features such as fast-forward or rewind must be implemented by only changing the playbackRate attribute (and not the defaultPlaybackRate attribute). Again, when a media element has a current media controller, the attributes with those names on that MediaController object must be used; otherwise, the attributes with those names on the media element itself must be used.

When a media element has a current media controller, seeking must be implemented in terms of the currentTime attribute on that MediaController object. Otherwise, the user agent must directly seek to the requested position in the media element's media timeline. For media resources where seeking to an arbitrary position would be slow, user agents are encouraged to use the approximate-for-speed flag when seeking in response to the user manipulating an approximate position interface such as a seek bar.

When a media element has a current media controller, user agents may additionally provide the user with controls that directly manipulate an individual media element without affecting the MediaController, but such features are considered relatively advanced and unlikely to be useful to most users.

The activation behavior of a media element that is exposing a user interface to the user must be to run the following steps:

If the media element has a current media controller, and that current media controller is a restrained media controller, then invoke the play() method of the MediaController.
Otherwise, if the media element has a current media controller, and that current media controller is a paused media controller, then invoke the unpause() method of the MediaController.
Otherwise, if the media element has a current media controller, then that current media controller is a playing media controller; invoke the pause() method of the MediaController.
Otherwise, the media element has no current media controller; if the media element's paused attribute is true, then invoke the play() method on the media element.
Otherwise, the media element has no current media controller, and the media element's paused attribute is false; invoke the pause() method on the media element.

For the purposes of listing chapters in the media resource, only text tracks in the media element's list of text tracks that are showing and whose text track kind is chapters should be used. Such tracks must be interpreted according to the rules for constructing the chapter tree from a text track. When seeking in response to a user maniplating a chapter selection interface, user agents should not use the approximate-for-speed flag.

The controls IDL attribute must reflect the content attribute of the same name.

media . volume [ = value ]

Returns the current playback volume, as a number in the range 0.0 to 1.0, where 0.0 is the quietest and 1.0 the loudest.

Can be set, to change the volume.

Throws an IndexSizeError exception if the new value is not in the range 0.0 .. 1.0.

media . muted [ = value ]

Returns true if audio is muted, overriding the volume attribute, and false if the volume attribute is being honored.

Can be set, to change whether the audio is muted or not.

The volume attribute must return the playback volume of any audio portions of the media element, in the range 0.0 (silent) to 1.0 (loudest). Initially, the volume should be 1.0, but user agents may remember the last set value across sessions, on a per-site basis or otherwise, so the volume may start at other values. On setting, if the new value is in the range 0.0 to 1.0 inclusive, the playback volume of any audio portions of the media element must be set to the new value. If the new value is outside the range 0.0 to 1.0 inclusive, then, on setting, an IndexSizeError exception must be thrown instead.

The muted attribute must return true if the audio output is muted and false otherwise. Initially, the audio output should not be muted (false), but user agents may remember the last set value across sessions, on a per-site basis or otherwise, so the muted state may start as muted (true). On setting, if the new value is true then the audio output should be muted and if the new value is false it should be unmuted.

Whenever either of the values that would be returned by the volume and muted attributes change, the user agent must queue a task to fire a simple event named volumechange at the media element.

An element's effective media volume is determined as follows:

If the user has indicated that the user agent is to override the volume of the element, then the element's effective media volume is the volume desired by the user. Abort these steps.
If the element's audio output is muted, the element's effective media volume is zero. Abort these steps.
If the element has a current media controller and that MediaController object's media controller mute override is true, the element's effective media volume is zero. Abort these steps.
Let volume be the playback volume of the audio portions of the media element, in range 0.0 (silent) to 1.0 (loudest).
If the element has a current media controller, multiply volume by that MediaController object's media controller volume multiplier.
The element's effective media volume is volume, interpreted relative to the range 0.0 to 1.0, with 0.0 being silent, and 1.0 being the loudest setting, values in between increasing in loudness. The range need not be linear. The loudest setting may be lower than the system's loudest possible setting; for example the user could have set a maximum volume.

The muted attribute on media elements is a boolean attribute that controls the default state of the audio output of the media resource, potentially overriding user preferences.

When a media element is created, if it has a muted attribute specified, the user agent must mute the media element's audio output, overriding any user preference.

The defaultMuted IDL attribute must reflect the muted content attribute.

This attribute has no dynamic effect (it only controls the default state of the element).

This video (an advertisement) autoplays, but to avoid annoying users, it does so without sound, and allows the user to turn the sound on.

<video src="adverts.cgi?kind=video" controls autoplay loop muted></video>

4.7.10.14 Time ranges

Objects implementing the TimeRanges interface represent a list of ranges (periods) of time.

interface TimeRanges {
  readonly attribute unsigned long length;
  double start(unsigned long index);
  double end(unsigned long index);
};

media . length

Returns the number of ranges in the object.

time = media . start(index)

Returns the time for the start of the range with the given index.

Throws an IndexSizeError exception if the index is out of range.

time = media . end(index)

Returns the time for the end of the range with the given index.

Throws an IndexSizeError exception if the index is out of range.

The length IDL attribute must return the number of ranges represented by the object.

The start(index) method must return the position of the start of the indexth range represented by the object, in seconds measured from the start of the timeline that the object covers.

The end(index) method must return the position of the end of the indexth range represented by the object, in seconds measured from the start of the timeline that the object covers.

These methods must throw IndexSizeError exceptions if called with an index argument greater than or equal to the number of ranges represented by the object.

When a TimeRanges object is said to be a normalized TimeRanges object, the ranges it represents must obey the following criteria:

The start of a range must be greater than the end of all earlier ranges.
The start of a range must be less than the end of that same range.

In other words, the ranges in such an object are ordered, don't overlap, aren't empty, and don't touch (adjacent ranges are folded into one bigger range).

Ranges in a TimeRanges object must be inclusive.

Thus, the end of a range would be equal to the start of a following adjacent (touching but not overlapping) range. Similarly, a range covering a whole timeline anchored at zero would have a start equal to zero and an end equal to the duration of the timeline.

The timelines used by the objects returned by the buffered, seekable and played IDL attributes of media elements must be that element's media timeline.

4.7.10.15 Event definitions

[Constructor(DOMString type, optional TrackEventInit eventInitDict)]
interface TrackEvent : Event {
  readonly attribute (VideoTrack or AudioTrack or TextTrack) track;
};

dictionary TrackEventInit : EventInit {
  (VideoTrack or AudioTrack or TextTrack) track;
};

event . track: Returns the track object (TextTrack, AudioTrack, or VideoTrack) to which the event relates.

The track attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to null. It represents the context information for the event.

4.7.10.16 Event summary

This section is non-normative.

The following events fire on media elements as part of the processing model described above:

Event name	Interface	Fired when...	Preconditions
`loadstart`	`Event`	The user agent begins looking for media data, as part of the resource selection algorithm.	`networkState` equals `NETWORK_LOADING`
`progress`	`Event`	The user agent is fetching media data.	`networkState` equals `NETWORK_LOADING`
`suspend`	`Event`	The user agent is intentionally not currently fetching media data.	`networkState` equals `NETWORK_IDLE`
`abort`	`Event`	The user agent stops fetching the media data before it is completely downloaded, but not due to an error.	`error` is an object with the code `MEDIA_ERR_ABORTED`. `networkState` equals either `NETWORK_EMPTY` or `NETWORK_IDLE`, depending on when the download was aborted.
`error`	`Event`	An error occurs while fetching the media data.	`error` is an object with the code `MEDIA_ERR_NETWORK` or higher. `networkState` equals either `NETWORK_EMPTY` or `NETWORK_IDLE`, depending on when the download was aborted.
`emptied`	`Event`	A media element whose `networkState` was previously not in the `NETWORK_EMPTY` state has just switched to that state (either because of a fatal error during load that's about to be reported, or because the `load()` method was invoked while the resource selection algorithm was already running).	`networkState` is `NETWORK_EMPTY`; all the IDL attributes are in their initial states.
`stalled`	`Event`	The user agent is trying to fetch media data, but data is unexpectedly not forthcoming.	`networkState` is `NETWORK_LOADING`.
`loadedmetadata`	`Event`	The user agent has just determined the duration and dimensions of the media resource and the text tracks are ready.	`readyState` is newly equal to `HAVE_METADATA` or greater for the first time.
`loadeddata`	`Event`	The user agent can render the media data at the current playback position for the first time.	`readyState` newly increased to `HAVE_CURRENT_DATA` or greater for the first time.
`canplay`	`Event`	The user agent can resume playback of the media data, but estimates that if playback were to be started now, the media resource could not be rendered at the current playback rate up to its end without having to stop for further buffering of content.	`readyState` newly increased to `HAVE_FUTURE_DATA` or greater.
`canplaythrough`	`Event`	The user agent estimates that if playback were to be started now, the media resource could be rendered at the current playback rate all the way to its end without having to stop for further buffering.	`readyState` is newly equal to `HAVE_ENOUGH_DATA`.
`playing`	`Event`	Playback is ready to start after having been paused or delayed due to lack of media data.	`readyState` is newly equal to or greater than `HAVE_FUTURE_DATA` and `paused` is false, or `paused` is newly false and `readyState` is equal to or greater than `HAVE_FUTURE_DATA`. Even if this event fires, the element might still not be potentially playing, e.g. if the element is blocked on its media controller (e.g. because the current media controller is paused, or another slaved media element is stalled somehow, or because the media resource has no data corresponding to the media controller position), or the element is paused for user interaction or paused for in-band content.
`waiting`	`Event`	Playback has stopped because the next frame is not available, but the user agent expects that frame to become available in due course.	`readyState` is equal to or less than `HAVE_CURRENT_DATA`, and `paused` is false. Either `seeking` is true, or the current playback position is not contained in any of the ranges in `buffered`. It is possible for playback to stop for other reasons without `paused` being false, but those reasons do not fire this event (and when those situations resolve, a separate `playing` event is not fired either): e.g. the element is newly blocked on its media controller, or playback ended, or playback stopped due to errors, or the element has paused for user interaction or paused for in-band content.
`seeking`	`Event`	The `seeking` IDL attribute changed to true, and the user agent has started seeking to a new position.
`seeked`	`Event`	The `seeking` IDL attribute changed to false after the current playback position was changed.
`ended`	`Event`	Playback has stopped because the end of the media resource was reached.	`currentTime` equals the end of the media resource; `ended` is true.
`durationchange`	`Event`	The `duration` attribute has just been updated.
`timeupdate`	`Event`	The current playback position changed as part of normal playback or in an especially interesting way, for example discontinuously.
`play`	`Event`	The element is no longer paused. Fired after the `play()` method has returned, or when the `autoplay` attribute has caused playback to begin.	`paused` is newly false.
`pause`	`Event`	The element has been paused. Fired after the `pause()` method has returned.	`paused` is newly true.
`ratechange`	`Event`	Either the `defaultPlaybackRate` or the `playbackRate` attribute has just been updated.
`resize`	`Event`	One or both of the `videoWidth` and `videoHeight` attributes have just been updated.	Media element is a `video` element; `readyState` is not `HAVE_NOTHING`
`volumechange`	`Event`	Either the `volume` attribute or the `muted` attribute has changed. Fired after the relevant attribute's setter has returned.

The following events fire on MediaController objects:

Event name	Interface	Fired when...
`emptied`	`Event`	All the slaved media elements newly have `readyState` set to `HAVE_NOTHING` or greater, or there are no longer any slaved media elements.
`loadedmetadata`	`Event`	All the slaved media elements newly have `readyState` set to `HAVE_METADATA` or greater.
`loadeddata`	`Event`	All the slaved media elements newly have `readyState` set to `HAVE_CURRENT_DATA` or greater.
`canplay`	`Event`	All the slaved media elements newly have `readyState` set to `HAVE_FUTURE_DATA` or greater.
`canplaythrough`	`Event`	All the slaved media elements newly have `readyState` set to `HAVE_ENOUGH_DATA` or greater.
`playing`	`Event`	The `MediaController` is no longer a blocked media controller.
`ended`	`Event`	The `MediaController` has reached the end of all the slaved media elements.
`waiting`	`Event`	The `MediaController` is now a blocked media controller.
`ended`	`Event`	All the slaved media elements have newly ended playback.
`durationchange`	`Event`	The `duration` attribute has just been updated.
`timeupdate`	`Event`	The media controller position changed.
`play`	`Event`	The `paused` attribute is newly false.
`pause`	`Event`	The `paused` attribute is newly true.
`ratechange`	`Event`	Either the `defaultPlaybackRate` attribute or the `playbackRate` attribute has just been updated.
`volumechange`	`Event`	Either the `volume` attribute or the `muted` attribute has just been updated.

The following events fire on AudioTrackList, VideoTrackList, and TextTrackList objects:

Event name	Interface	Fired when...
`change`	`Event`	One or more tracks in the track list has been enabled or disabled.
`addtrack`	`TrackEvent`	A track has been added to the track list.
`removetrack`	`TrackEvent`	A track has been removed from the track list.

4.7.10.17 Security and privacy considerations

The main security and privacy implications of the video and audio elements come from the ability to embed media cross-origin. There are two directions that threats can flow: from hostile content to a victim page, and from a hostile page to victim content.

If a victim page embeds hostile content, the threat is that the content might contain scripted code that attempts to interact with the Document that embeds the content. To avoid this, user agents must ensure that there is no access from the content to the embedding page. In the case of media content that uses DOM concepts, the embedded content must be treated as if it was in its own unrelated top-level browsing context.

For instance, if an SVG animation was embedded in a video element, the user agent would not give it access to the DOM of the outer page. From the perspective of scripts in the SVG resource, the SVG file would appear to be in a lone top-level browsing context with no parent.

If a hostile page embeds victim content, the threat is that the embedding page could obtain information from the content that it would not otherwise have access to. The API does expose some information: the existence of the media, its type, its duration, its size, and the performance characteristics of its host. Such information is already potentially problematic, but in practice the same information can more or less be obtained using the img element, and so it has been deemed acceptable.

However, significantly more sensitive information could be obtained if the user agent further exposes metadata within the content such as subtitles or chapter titles. Such information is therefore only exposed if the video resource passes a CORS resource sharing check. The crossorigin attribute allows authors to control how this check is performed. [CORS]

Without this restriction, an attacker could trick a user running within a corporate network into visiting a site that attempts to load a video from a previously leaked location on the corporation's intranet. If such a video included confidential plans for a new product, then being able to read the subtitles would present a serious confidentiality breach.

4.7.10.18 Best practices for authors using media elements

This section is non-normative.

Playing audio and video resources on small devices such as set-top boxes or mobile phones is often constrained by limited hardware resources in the device. For example, a device might only support three simultaneous videos. For this reason, it is a good practice to release resources held by media elements when they are done playing, either by being very careful about removing all references to the element and allowing it to be garbage collected, or, even better, by removing the element's src attribute and any source element descendants, and invoking the element's load() method.

Similarly, when the playback rate is not exactly 1.0, hardware, software, or format limitations can cause video frames to be dropped and audio to be choppy or muted.

4.7.10.19 Best practices for implementors of media elements

This section is non-normative.

How accurately various aspects of the media element API are implemented is considered a quality-of-implementation issue.

For example, when implementing the buffered attribute, how precise an implementation reports the ranges that have been buffered depends on how carefully the user agent inspects the data. Since the API reports ranges as times, but the data is obtained in byte streams, a user agent receiving a variable-bit-rate stream might only be able to determine precise times by actually decoding all of the data. User agents aren't required to do this, however; they can instead return estimates (e.g. based on the average bit rate seen so far) which get revised as more information becomes available.

As a general rule, user agents are urged to be conservative rather than optimistic. For example, it would be bad to report that everything had been buffered when it had not.

Another quality-of-implementation issue would be playing a video backwards when the codec is designed only for forward playback (e.g. there aren't many key frames, and they are far apart, and the intervening frames only have deltas from the previous frame). User agents could do a poor job, e.g. only showing key frames; however, better implementations would do more work and thus do a better job, e.g. actually decoding parts of the video forwards, storing the complete frames, and then playing the frames backwards.

Similarly, while implementations are allowed to drop buffered data at any time (there is no requirement that a user agent keep all the media data obtained for the lifetime of the media element), it is again a quality of implementation issue: user agents with sufficient resources to keep all the data around are encouraged to do so, as this allows for a better user experience. For example, if the user is watching a live stream, a user agent could allow the user only to view the live video; however, a better user agent would buffer everything and allow the user to seek through the earlier material, pause it, play it forwards and backwards, etc.

When multiple tracks are synchronised with a MediaController, it is possible for scripts to add and remove media elements from the MediaController's list of slaved media elements, even while these tracks are playing. How smoothly the media plays back in such situations is another quality-of-implementation issue.

When a media element that is paused is removed from a document and not reinserted before the next time the event loop spins, implementations that are resource constrained are encouraged to take that opportunity to release all hardware resources (like video planes, networking resources, and data buffers) used by the media element. (User agents still have to keep track of the playback position and so forth, though, in case playback is later restarted.)

4.7.11 The `map` element

Where phrasing content is expected.

Transparent.

name - Name of image map to reference from the usemap attribute

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

None

interface HTMLMapElement : HTMLElement {
           attribute DOMString name;
  readonly attribute HTMLCollection areas;
  readonly attribute HTMLCollection images;
};

The map element, in conjunction with an img element and any area element descendants, defines an image map. The element represents its children.

The name attribute gives the map a name so that it can be referenced. The attribute must be present and must have a non-empty value with no space characters. The value of the name attribute must not be a compatibility-caseless match for the value of the name attribute of another map element in the same document. If the id attribute is also specified, both attributes must have the same value.

map . areas: Returns an HTMLCollection of the area elements in the map.
map . images: Returns an HTMLCollection of the img and object elements that use the map.

The areas attribute must return an HTMLCollection rooted at the map element, whose filter matches only area elements.

The images attribute must return an HTMLCollection rooted at the Document node, whose filter matches only img and object elements that are associated with this map element according to the image map processing model.

The IDL attribute name must reflect the content attribute of the same name.

Image maps can be defined in conjunction with other content on the page, to ease maintenance. This example is of a page with an image map at the top of the page and a corresponding set of text links at the bottom.

<!DOCTYPE HTML>
<TITLE>Babies™: Toys</TITLE>
<HEADER>
 <H1>Toys</H1>
 <IMG SRC="/images/menu.gif"
      ALT="Babies™ navigation menu. Select a department to go to its page."
      USEMAP="#NAV">
</HEADER>
 ...
<FOOTER>
 <MAP NAME="NAV">
  <P>
   <A HREF="/clothes/">Clothes</A>
   <AREA ALT="Clothes" COORDS="0,0,100,50" HREF="/clothes/"> |
   <A HREF="/toys/">Toys</A>
   <AREA ALT="Toys" COORDS="0,0,100,50" HREF="/toys/"> |
   <A HREF="/food/">Food</A>
   <AREA ALT="Food" COORDS="0,0,100,50" HREF="/food/"> |
   <A HREF="/books/">Books</A>
   <AREA ALT="Books" COORDS="0,0,100,50" HREF="/books/">
 </MAP>
</FOOTER>

4.7.12 The `area` element

Where phrasing content is expected, but only if there is a map element ancestor or a template element ancestor.

Empty.

alt - Replacement text for use when images are not available

coords - Coordinates for the shape to be created in an image map

download - Whether to download the resource instead of navigating to it, and its file name if so

href - Address of the hyperlink

hreflang - Language of the linked resource

rel Relationship between the document containing the hyperlink and the destination resource

shape - The kind of shape to be created in an image map

target - Browsing context for hyperlink navigation

type - Hint for the type of the referenced resource

Tag omission in text/html:

Allowed ARIA role attribute values:

link role (default - do not set).

Any aria-* attributes applicable to the allowed roles.

interface HTMLAreaElement : HTMLElement {
           attribute DOMString alt;
           attribute DOMString coords;
           attribute DOMString shape;
           attribute DOMString target;
           attribute DOMString download;

           attribute DOMString rel;
  readonly attribute DOMTokenList relList;
           attribute DOMString hreflang;
           attribute DOMString type;
};
HTMLAreaElement implements URLUtils;

The area element represents either a hyperlink with some text and a corresponding area on an image map, or a dead area on an image map.

An area element with a parent node must have a map element ancestor or a template element ancestor.

If the area element has an href attribute, then the area element represents a hyperlink. In this case, the alt attribute must be present. It specifies the text of the hyperlink. Its value must be text that, when presented with the texts specified for the other hyperlinks of the image map, and with the alternative text of the image, but without the image itself, provides the user with the same kind of choice as the hyperlink would when used without its text but with its shape applied to the image. The alt attribute may be left blank if there is another area element in the same image map that points to the same resource and has a non-blank alt attribute.

If the area element has no href attribute, then the area represented by the element cannot be selected, and the alt attribute must be omitted.

In both cases, the shape and coords attributes specify the area.

The shape attribute is an enumerated attribute. The following table lists the keywords defined for this attribute. The states given in the first cell of the rows with keywords give the states to which those keywords map. Some of the keywords are non-conforming, as noted in the last column.

State	Keywords	Notes
Circle state	`circle`
Circle state	`circ`	Non-conforming
Default state	`default`
Polygon state	`poly`
Polygon state	`polygon`	Non-conforming
Rectangle state	`rect`
Rectangle state	`rectangle`	Non-conforming

The attribute may be omitted. The missing value default is the rectangle state.

The coords attribute must, if specified, contain a valid list of integers. This attribute gives the coordinates for the shape described by the shape attribute. The processing for this attribute is described as part of the image map processing model.

In the circle state, area elements must have a coords attribute present, with three integers, the last of which must be non-negative. The first integer must be the distance in CSS pixels from the left edge of the image to the center of the circle, the second integer must be the distance in CSS pixels from the top edge of the image to the center of the circle, and the third integer must be the radius of the circle, again in CSS pixels.

In the default state state, area elements must not have a coords attribute. (The area is the whole image.)

In the polygon state, area elements must have a coords attribute with at least six integers, and the number of integers must be even. Each pair of integers must represent a coordinate given as the distances from the left and the top of the image in CSS pixels respectively, and all the coordinates together must represent the points of the polygon, in order.

In the rectangle state, area elements must have a coords attribute with exactly four integers, the first of which must be less than the third, and the second of which must be less than the fourth. The four points must represent, respectively, the distance from the left edge of the image to the left side of the rectangle, the distance from the top edge to the top side, the distance from the left edge to the right side, and the distance from the top edge to the bottom side, all in CSS pixels.

When user agents allow users to follow hyperlinks or download hyperlinks created using the area element, as described in the next section, the href, target, download, and attributes decide how the link is followed. The rel, hreflang, and type attributes may be used to indicate to the user the likely nature of the target resource before the user follows the link.

The target, download, rel, hreflang, and type attributes must be omitted if the href attribute is not present.

The activation behavior of area elements is to run the following steps:

If the area element's Document is not fully active, then abort these steps.
If the area element has a download attribute and the algorithm is not allowed to show a popup, or the element's target attribute is present and applying the rules for choosing a browsing context given a browsing context name, using the value of the target attribute as the browsing context name, would result in there not being a chosen browsing context, then run these substeps:
1. If there is an entry settings object, throw an InvalidAccessError exception.
2. Abort these steps without following the hyperlink.
Otherwise, the user agent must follow the hyperlink or download the hyperlink created by the area element, if any, and as determined by the download attribute and any expressed user preference.

The IDL attributes alt, coords, target, download, rel, hreflang, and type, each must reflect the respective content attributes of the same name.

The IDL attribute shape must reflect the shape content attribute.

The IDL attribute relList must reflect the rel content attribute.

The area element also supports the URLUtils interface. [URL]

The element's URLUtils interface's get the base algorithm must simply return the element's base URL.

The element's URLUtils interface's query encoding is the document's character encoding.

When the element's URLUtils interface invokes its update steps with a string value, the user agent must set the element's href content attribute to the string value.

4.7.13 Image maps

4.7.13.1 Authoring

An image map allows geometric areas on an image to be associated with hyperlinks.

An image, in the form of an img element or an object element representing an image, may be associated with an image map (in the form of a map element) by specifying a usemap attribute on the img or object element. The usemap attribute, if specified, must be a valid hash-name reference to a map element.

Consider an image that looks as follows:

A line with four shapes in it, equally spaced: a red hollow box, a green circle, a blue triangle, and a yellow four-pointed star.

If we wanted just the colored areas to be clickable, we could do it as follows:

<p>
 Please select a shape:
 <img src="shapes.png" usemap="#shapes"
      alt="Four shapes are available: a red hollow box, a green circle, a blue triangle, and a yellow four-pointed star.">
 <map name="shapes">
  <area shape=rect coords="50,50,100,100"> <!-- the hole in the red box -->
  <area shape=rect coords="25,25,125,125" href="red.html" alt="Red box.">
  <area shape=circle coords="200,75,50" href="green.html" alt="Green circle.">
  <area shape=poly coords="325,25,262,125,388,125" href="blue.html" alt="Blue triangle.">
  <area shape=poly coords="450,25,435,60,400,75,435,90,450,125,465,90,500,75,465,60"
        href="yellow.html" alt="Yellow star.">
 </map>
</p>

4.7.13.2 Processing model

If an img element or an object element representing an image has a usemap attribute specified, user agents must process it as follows:

First, rules for parsing a hash-name reference to a map element must be followed. This will return either an element (the map) or null.
If that returned null, then abort these steps. The image is not associated with an image map after all.
Otherwise, the user agent must collect all the area elements that are descendants of the map. Let those be the areas.

Having obtained the list of area elements that form the image map (the areas), interactive user agents must process the list in one of two ways.

If the user agent intends to show the text that the img element represents, then it must use the following steps.

In user agents that do not support images, or that have images disabled, object elements cannot represent images, and thus this section never applies (the fallback content is shown instead). The following steps therefore only apply to img elements.

Remove all the area elements in areas that have no href attribute.
Remove all the area elements in areas that have no alt attribute, or whose alt attribute's value is the empty string, if there is another area element in areas with the same value in the href attribute and with a non-empty alt attribute.
Each remaining area element in areas represents a hyperlink. Those hyperlinks should all be made available to the user in a manner associated with the text of the img.

In this context, user agents may represent area and img elements with no specified alt attributes, or whose alt attributes are the empty string or some other non-visible text, in a user-agent-defined fashion intended to indicate the lack of suitable author-provided text.

If the user agent intends to show the image and allow interaction with the image to select hyperlinks, then the image must be associated with a set of layered shapes, taken from the area elements in areas, in reverse tree order (so the last specified area element in the map is the bottom-most shape, and the first element in the map, in tree order, is the top-most shape).

Each area element in areas must be processed as follows to obtain a shape to layer onto the image:

Find the state that the element's shape attribute represents.
Use the rules for parsing a list of integers to parse the element's coords attribute, if it is present, and let the result be the coords list. If the attribute is absent, let the coords list be the empty list.

If the number of items in the coords list is less than the minimum number given for the area element's current state, as per the following table, then the shape is empty; abort these steps.

State	Minimum number of items
Circle state	3
Default state	0
Polygon state	6
Rectangle state	4

Check for excess items in the coords list as per the entry in the following list corresponding to the shape attribute's state:

Circle state

Drop any items in the list beyond the third.

Default state

Drop all items in the list.

Polygon state

Drop the last item if there's an odd number of items.

Rectangle state

Drop any items in the list beyond the fourth.
If the shape attribute represents the rectangle state, and the first number in the list is numerically less than the third number in the list, then swap those two numbers around.
If the shape attribute represents the rectangle state, and the second number in the list is numerically less than the fourth number in the list, then swap those two numbers around.
If the shape attribute represents the circle state, and the third number in the list is less than or equal to zero, then the shape is empty; abort these steps.
Now, the shape represented by the element is the one described for the entry in the list below corresponding to the state of the shape attribute:

Circle state

Let x be the first number in coords, y be the second number, and r be the third number.

The shape is a circle whose center is x CSS pixels from the left edge of the image and y CSS pixels from the top edge of the image, and whose radius is r pixels.

Default state

The shape is a rectangle that exactly covers the entire image.

Polygon state

Let x_i be the (2i)th entry in coords, and y_i be the (2i+1)th entry in coords (the first entry in coords being the one with index 0).

Let the coordinates be (x_i, y_i), interpreted in CSS pixels measured from the top left of the image, for all integer values of i from 0 to (N/2)-1, where N is the number of items in coords.

The shape is a polygon whose vertices are given by the coordinates, and whose interior is established using the even-odd rule. [GRAPHICS]

Rectangle state

Let x₁ be the first number in coords, y₁ be the second number, x₂ be the third number, and y₂ be the fourth number.

The shape is a rectangle whose top-left corner is given by the coordinate (x₁, y₁) and whose bottom right corner is given by the coordinate (x₂, y₂), those coordinates being interpreted as CSS pixels from the top left corner of the image.

For historical reasons, the coordinates must be interpreted relative to the displayed image after any stretching caused by the CSS 'width' and 'height' properties (or, for non-CSS browsers, the image element's width and height attributes — CSS browsers map those attributes to the aforementioned CSS properties).

Browser zoom features and transforms applied using CSS or SVG do not affect the coordinates.

Pointing device interaction with an image associated with a set of layered shapes per the above algorithm must result in the relevant user interaction events being first fired to the top-most shape covering the point that the pointing device indicated, if any, or to the image element itself, if there is no shape covering that point. User agents may also allow individual area elements representing hyperlinks to be selected and activated (e.g. using a keyboard).

Because a map element (and its area elements) can be associated with multiple img and object elements, it is possible for an area element to correspond to multiple focusable areas of the document.

Image maps are live; if the DOM is mutated, then the user agent must act as if it had rerun the algorithms for image maps.

4.7.14 MathML

The math element from the MathML namespace falls into the embedded content, phrasing content, and flow content categories for the purposes of the content models in this specification.

User agents must handle text other than inter-element whitespace found in MathML elements whose content models do not allow straight text by pretending for the purposes of MathML content models, layout, and rendering that that text is actually wrapped in an mtext element in the MathML namespace. (Such text is not, however, conforming.)

User agents must act as if any MathML element whose contents does not match the element's content model was replaced, for the purposes of MathML layout and rendering, by an merror element in the MathML namespace containing some appropriate error message.

To enable authors to use MathML tools that only accept MathML in its XML form, interactive HTML user agents are encouraged to provide a way to export any MathML fragment as an XML namespace-well-formed XML fragment.

The semantics of MathML elements are defined by the MathML specification and other applicable specifications. [MATHML]

Here is an example of the use of MathML in an HTML document:

<!DOCTYPE html>
<html>
 <head>
  <title>The quadratic formula</title>
 </head>
 <body>
  <h1>The quadratic formula</h1>
  <p>
   <math>
    <mi>x</mi>
    <mo>=</mo>
    <mfrac>
     <mrow>
      <mo form="prefix">−</mo> <mi>b</mi>
      <mo>±</mo>
      <msqrt>
       <msup> <mi>b</mi> <mn>2</mn> </msup>
       <mo>−</mo>
       <mn>4</mn> <mo>⁢</mo> <mi>a</mi> <mo>⁢</mo> <mi>c</mi>
      </msqrt>
     </mrow>
     <mrow>
      <mn>2</mn> <mo>⁢</mo> <mi>a</mi>
     </mrow>
    </mfrac>
   </math>
  </p>
 </body>
</html>

4.7.15 SVG

The svg element from the SVG namespace falls into the embedded content, phrasing content, and flow content categories for the purposes of the content models in this specification.

To enable authors to use SVG tools that only accept SVG in its XML form, interactive HTML user agents are encouraged to provide a way to export any SVG fragment as an XML namespace-well-formed XML fragment.

When the SVG foreignObject element contains elements from the HTML namespace, such elements must all be flow content. [SVG]

The content model for title elements in the SVG namespace inside HTML documents is phrasing content. (This further constrains the requirements given in the SVG specification.)

The semantics of SVG elements are defined by the SVG specification and other applicable specifications. [SVG]

The SVG specification includes requirements regarding the handling of elements in the DOM that are not in the SVG namespace, that are in SVG fragments, and that are not included in a foreignObject element. This specification does not define any processing for elements in SVG fragments that are not in the HTML namespace; they are considered neither conforming nor non-conforming from the perspective of this specification.

4.7.16 Dimension attributes

Author requirements: The width and height attributes on img, iframe, embed, object, video, and, when their type attribute is in the Image Button state, input elements may be specified to give the dimensions of the visual content of the element (the width and height respectively, relative to the nominal direction of the output medium), in CSS pixels. The attributes, if specified, must have values that are valid non-negative integers.

The specified dimensions given may differ from the dimensions specified in the resource itself, since the resource may have a resolution that differs from the CSS pixel resolution. (On screens, CSS pixels have a resolution of 96ppi, but in general the CSS pixel resolution depends on the reading distance.) If both attributes are specified, then one of the following statements must be true:

specified width - 0.5 ≤ specified height * target ratio ≤ specified width + 0.5
specified height - 0.5 ≤ specified width / target ratio ≤ specified height + 0.5
specified height = specified width = 0

The target ratio is the ratio of the intrinsic width to the intrinsic height in the resource. The specified width and specified height are the values of the width and height attributes respectively.

The two attributes must be omitted if the resource in question does not have both an intrinsic width and an intrinsic height.

If the two attributes are both zero, it indicates that the element is not intended for the user (e.g. it might be a part of a service to count page views).

The dimension attributes are not intended to be used to stretch the image.

User agent requirements: User agents are expected to use these attributes as hints for the rendering.

The width and height IDL attributes on the iframe, embed, object, and video elements must reflect the respective content attributes of the same name.

For iframe, embed, and object the IDL attributes are DOMString; for video the IDL attributes are unsigned long.

The corresponding IDL attributes for img and input elements are defined in those respective elements' sections, as they are slightly more specific to those elements' other behaviors.

4.8 Links

Links are a conceptual construct, created by a, area, and link elements, that represent a connection between two resources, one of which is the current Document. There are two kinds of links in HTML:

Links to external resources: These are links to resources that are to be used to augment the current document, generally automatically processed by the user agent.
Hyperlinks: These are links to other resources that are generally exposed to the user by the user agent so that the user can cause the user agent to navigate to those resources, e.g. to visit them in a browser or download them.

For link elements with an href attribute and a rel attribute, links must be created for the keywords of the rel attribute, as defined for those keywords in the link types section.

Similarly, for a and area elements with an href attribute and a rel attribute, links must be created for the keywords of the rel attribute as defined for those keywords in the link types section. Unlike link elements, however, a and area element with an href attribute that either do not have a rel attribute, or whose rel attribute has no keywords that are defined as specifying hyperlinks, must also create a hyperlink. This implied hyperlink has no special meaning (it has no link type) beyond linking the element's document to the resource given by the element's href attribute.

A hyperlink can have one or more hyperlink annotations that modify the processing semantics of that hyperlink.

4.8.1 Links created by `a` and `area` elements

The href attribute on a and area elements must have a value that is a valid URL potentially surrounded by spaces.

The href attribute on a and area elements is not required; when those elements do not have href attributes they do not create hyperlinks.

The target attribute, if present, must be a valid browsing context name or keyword. It gives the name of the browsing context that will be used. User agents use this name when following hyperlinks.

When an a or area element's activation behavior is invoked, the user agent may allow the user to indicate a preference regarding whether the hyperlink is to be used for navigation or whether the resource it specifies is to be downloaded.

In the absence of a user preference, the default should be navigation if the element has no download attribute, and should be to download the specified resource if it does.

Whether determined by the user's preferences or via the presence or absence of the attribute, if the decision is to use the hyperlink for navigation then the user agent must follow the hyperlink, and if the decision is to use the hyperlink to download a resource, the user agent must download the hyperlink. These terms are defined in subsequent sections below.

The download attribute, if present, indicates that the author intends the hyperlink to be used for downloading a resource. The attribute may have a value; the value, if any, specifies the default file name that the author recommends for use in labeling the resource in a local file system. There are no restrictions on allowed values, but authors are cautioned that most file systems have limitations with regard to what punctuation is supported in file names, and user agents are likely to adjust file names accordingly.

The rel attribute on a and area elements controls what kinds of links the elements create. The attribute's value must be a set of space-separated tokens. The allowed keywords and their meanings are defined below.

The rel attribute has no default value. If the attribute is omitted or if none of the values in the attribute are recognized by the user agent, then the document has no particular relationship with the destination resource other than there being a hyperlink between the two.

The hreflang attribute on a and area elements that create hyperlinks, if present, gives the language of the linked resource. It is purely advisory. The value must be a valid BCP 47 language tag. [BCP47] User agents must not consider this attribute authoritative — upon fetching the resource, user agents must use only language information associated with the resource to determine its language, not metadata included in the link to the resource.

The type attribute, if present, gives the MIME type of the linked resource. It is purely advisory. The value must be a valid MIME type. User agents must not consider the type attribute authoritative — upon fetching the resource, user agents must not use metadata included in the link to the resource to determine its type.

4.8.2 Following hyperlinks

When a user follows a hyperlink created by an element subject, the user agent must run the following steps:

Let replace be false.
Let source be the browsing context that contains the Document object with which subject in question is associated.
If the user indicated a specific browsing context when following the hyperlink, or if the user agent is configured to follow hyperlinks by navigating a particular browsing context, then let target be that browsing context.

Otherwise, if subject is an a or area element that has a target attribute, then let target be the browsing context that is chosen by applying the rules for choosing a browsing context given a browsing context name, using the value of the target attribute as the browsing context name. If these rules result in the creation of a new browsing context, set replace to true.

Otherwise, if target is an a or area element with no target attribute, but the Document contains a base element with a target attribute, then let target be the browsing context that is chosen by applying the rules for choosing a browsing context given a browsing context name, using the value of the target attribute of the first such base element as the browsing context name. If these rules result in the creation of a new browsing context, set replace to true.

Otherwise, let target be the browsing context that subject itself is in.
Resolve the URL given by the href attribute of that element, relative to that element.
If that is successful, let URL be the resulting absolute URL.

Otherwise, if resolving the URL failed, the user agent may report the error to the user in a user-agent-specific manner, may queue a task to navigate the target browsing context to an error page to report the error, or may ignore the error and do nothing. In any case, the user agent must then abort these steps.
In the case of server-side image maps, append the hyperlink suffix to URL.
Queue a task to navigate the target browsing context to URL. If replace is true, the navigation must be performed with replacement enabled. The source browsing context must be source.

The task source for the tasks mentioned above is the DOM manipulation task source.

4.8.3 Downloading resources

In some cases, resources are intended for later use rather than immediate viewing. To indicate that a resource is intended to be downloaded for use later, rather than immediately used, the download attribute can be specified on the a or area element that creates the hyperlink to that resource.

The attribute can furthermore be given a value, to specify the file name that user agents are to use when storing the resource in a file system. This value can be overridden by the Content-Disposition HTTP header's filename parameters. [RFC6266]

In cross-origin situations, the download attribute has to be combined with the Content-Disposition HTTP header, specifically with the attachment disposition type, to avoid the user being warned of possibly nefarious activity. (This is to protect users from being made to download sensitive personal or confidential information without their full understanding.)

When a user downloads a hyperlink created by an element, the user agent must run the following steps:

Resolve the URL given by the href attribute of that element, relative to that element.
If resolving the URL fails, the user agent may report the error to the user in a user-agent-specific manner, may navigate to an error page to report the error, or may ignore the error and do nothing. In either case, the user agent must abort these steps.
Otherwise, let URL be the resulting absolute URL.
In the case of server-side image maps, append the hyperlink suffix to URL.
Return to whatever algorithm invoked these steps and continue these steps asynchronously.
Fetch URL and handle the resulting resource as a download.

When a user agent is to handle a resource obtained from a fetch algorithm as a download, it should provide the user with a way to save the resource for later use, if a resource is successfully obtained; or otherwise should report any problems downloading the file to the user.

If the user agent needs a file name for a resource being handled as a download, it should select one using the following algorithm.

This algorithm is intended to mitigate security dangers involved in downloading files from untrusted sites, and user agents are strongly urged to follow it.

Let filename be the void value.
If the resource has a Content-Disposition header, that header specifies the attachment disposition type, and the header includes file name information, then let filename have the value specified by the header, and jump to the step labeled sanitize below. [RFC6266]
Let interface origin be the origin of the Document in which the download or navigate action resulting in the download was initiated, if any.
Let resource origin be the origin of the URL of the resource being downloaded, unless that URL's scheme component is data, in which case let resource origin be the same as the interface origin, if any.
If there is no interface origin, then let trusted operation be true. Otherwise, let trusted operation be true if resource origin is the same origin as interface origin, and false otherwise.
If trusted operation is true and the resource has a Content-Disposition header and that header includes file name information, then let filename have the value specified by the header, and jump to the step labeled sanitize below. [RFC6266]
If the download was not initiated from a hyperlink created by an a or area element, or if the element of the hyperlink from which it was initiated did not have a download attribute when the download was initiated, or if there was such an attribute but its value when the download was initiated was the empty string, then jump to the step labeled no proposed file name.
Let proposed filename have the value of the download attribute of the element of the hyperlink that initiated the download at the time the download was initiated.
If trusted operation is true, let filename have the value of proposed filename, and jump to the step labeled sanitize below.
If the resource has a Content-Disposition header and that header specifies the attachment disposition type, let filename have the value of proposed filename, and jump to the step labeled sanitize below. [RFC6266]
No proposed file name: If trusted operation is true, or if the user indicated a preference for having the resource in question downloaded, let filename have a value derived from the URL of the resource in a user-agent-defined manner, and jump to the step labeled sanitize below.
Act in a user-agent-defined manner to safeguard the user from a potentially hostile cross-origin download. If the download is not to be aborted, then let filename be set to the user's preferred file name or to a file name selected by the user agent, and jump to the step labeled sanitize below.

If the algorithm reaches this step, then a download was begun from a different origin than the resource being downloaded, and the origin did not mark the file as suitable for downloading, and the download was not initiated by the user. This could be because a download attribute was used to trigger the download, or because the resource in question is not of a type that the user agent supports.

This could be dangerous, because, for instance, a hostile server could be trying to get a user to unknowingly download private information and then re-upload it to the hostile server, by tricking the user into thinking the data is from the hostile server.

Thus, it is in the user's interests that the user be somehow notified that the resource in question comes from quite a different source, and to prevent confusion, any suggested file name from the potentially hostile interface origin should be ignored.
Sanitize: Optionally, allow the user to influence filename. For example, a user agent could prompt the user for a file name, potentially providing the value of filename as determined above as a default value.
Adjust filename to be suitable for the local file system.

For example, this could involve removing characters that are not legal in file names, or trimming leading and trailing whitespace.
If the platform conventions do not in any way use extensions to determine the types of file on the file system, then return filename as the file name and abort these steps.
Let claimed type be the type given by the resource's Content-Type metadata, if any is known. Let named type be the type given by filename's extension, if any is known. For the purposes of this step, a type is a mapping of a MIME type to an extension.
If named type is consistent with the user's preferences (e.g. because the value of filename was determined by prompting the user), then return filename as the file name and abort these steps.
If claimed type and named type are the same type (i.e. the type given by the resource's Content-Type metadata is consistent with the type given by filename's extension), then return filename as the file name and abort these steps.
If the claimed type is known, then alter filename to add an extension corresponding to claimed type.

Otherwise, if named type is known to be potentially dangerous (e.g. it will be treated by the platform conventions as a native executable, shell script, HTML application, or executable-macro-capable document) then optionally alter filename to add a known-safe extension (e.g. ".txt").

This last step would make it impossible to download executables, which might not be desirable. As always, implementors are forced to balance security and usability in this matter.
Return filename as the file name.

For the purposes of this algorithm, a file extension consists of any part of the file name that platform conventions dictate will be used for identifying the type of the file. For example, many operating systems use the part of the file name following the last dot (".") in the file name to determine the type of the file, and from that the manner in which the file is to be opened or executed.

User agents should ignore any directory or path information provided by the resource itself, its URL, and any download attribute, in deciding where to store the resulting file in the user's file system.

4.8.4 Link types

The following table summarizes the link types that are defined by this specification. This table is non-normative; the actual definitions for the link types are given in the next few sections.

In this section, the term referenced document refers to the resource identified by the element representing the link, and the term current document refers to the resource within which the element representing the link finds itself.

To determine which link types apply to a link, a, or area element, the element's rel attribute must be split on spaces. The resulting tokens are the link types that apply to that element.

Except where otherwise specified, a keyword must not be specified more than once per rel attribute.

Link types are always ASCII case-insensitive, and must be compared as such.

Thus, rel="next" is the same as rel="NEXT".

Link type	Effect on...		Brief description
Link type	`link`	`a` and `area`	Brief description
`alternate`	Hyperlink	Hyperlink	Gives alternate representations of the current document.
`author`	Hyperlink	Hyperlink	Gives a link to the author of the current document or article.
`bookmark`	not allowed	Hyperlink	Gives the permalink for the nearest ancestor section.
`help`	Hyperlink	Hyperlink	Provides a link to context-sensitive help.
`icon`	External Resource	not allowed	Imports an icon to represent the current document.
`license`	Hyperlink	Hyperlink	Indicates that the main content of the current document is covered by the copyright license described by the referenced document.
`next`	Hyperlink	Hyperlink	Indicates that the current document is a part of a series, and that the next document in the series is the referenced document.
`nofollow`	not allowed	Annotation	Indicates that the current document's original author or publisher does not endorse the referenced document.
`noreferrer`	not allowed	Annotation	Requires that the user agent not send an HTTP `Referer` (sic) header if the user follows the hyperlink.
`prefetch`	External Resource	External Resource	Specifies that the target resource should be preemptively cached.
`prev`	Hyperlink	Hyperlink	Indicates that the current document is a part of a series, and that the previous document in the series is the referenced document.
`search`	Hyperlink	Hyperlink	Gives a link to a resource that can be used to search through the current document and its related pages.
`stylesheet`	External Resource	not allowed	Imports a stylesheet.
`tag`	not allowed	Hyperlink	Gives a tag (identified by the given address) that applies to the current document.

Some of the types described below list synonyms for these values. These are to be handled as specified by user agents, but must not be used in documents.

4.8.4.1 Link type "`alternate`"

The alternate keyword may be used with link, a, and area elements.

The meaning of this keyword depends on the values of the other attributes.

If the element is a link element and the rel attribute also contains the keyword stylesheet

The alternate keyword modifies the meaning of the stylesheet keyword in the way described for that keyword. The alternate keyword does not create a link of its own.

If the alternate keyword is used with the type attribute set to the value application/rss+xml or the value application/atom+xml

The keyword creates a hyperlink referencing a syndication feed (though not necessarily syndicating exactly the same content as the current page).

The first link, a, or area element in the document (in tree order) with the alternate keyword used with the type attribute set to the value application/rss+xml or the value application/atom+xml must be treated as the default syndication feed for the purposes of feed autodiscovery.

The following link element gives the syndication feed for the current page:

<link rel="alternate" type="application/atom+xml" href="data.xml">

The following extract offers various different syndication feeds:

<p>You can access the planets database using Atom feeds:</p>
<ul>
 <li><a href="recently-visited-planets.xml" rel="alternate" type="application/atom+xml">Recently Visited Planets</a></li>
 <li><a href="known-bad-planets.xml" rel="alternate" type="application/atom+xml">Known Bad Planets</a></li>
 <li><a href="unexplored-planets.xml" rel="alternate" type="application/atom+xml">Unexplored Planets</a></li>
</ul>

Otherwise

The keyword creates a hyperlink referencing an alternate representation of the current document.

The nature of the referenced document is given by the hreflang, and type attributes.

If the alternate keyword is used with the hreflang attribute, and that attribute's value differs from the root element's language, it indicates that the referenced document is a translation.

If the alternate keyword is used with the type attribute, it indicates that the referenced document is a reformulation of the current document in the specified format.

The hreflang and type attributes can be combined when specified with the alternate keyword.

For example, the following link is a French translation that uses the PDF format:

<link rel=alternate type=application/pdf hreflang=fr href=manual-fr>

This relationship is transitive — that is, if a document links to two other documents with the link type "alternate", then, in addition to implying that those documents are alternative representations of the first document, it is also implying that those two documents are alternative representations of each other.

4.8.4.2 Link type "`author`"

The author keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

For a and area elements, the author keyword indicates that the referenced document provides further information about the author of the nearest article element ancestor of the element defining the hyperlink, if there is one, or of the page as a whole, otherwise.

For link elements, the author keyword indicates that the referenced document provides further information about the author for the page as a whole.

The "referenced document" can be, and often is, a mailto: URL giving the e-mail address of the author. [MAILTO]

Synonyms: For historical reasons, user agents must also treat link, a, and area elements that have a rev attribute with the value "made" as having the author keyword specified as a link relationship.

4.8.4.3 Link type "`bookmark`"

The bookmark keyword may be used with a and area elements. This keyword creates a hyperlink.

The bookmark keyword gives a permalink for the nearest ancestor article element of the linking element in question, or of the section the linking element is most closely associated with, if there are no ancestor article elements.

The following snippet has three permalinks. A user agent could determine which permalink applies to which part of the spec by looking at where the permalinks are given.

 ...
 <body>
  <h1>Example of permalinks</h1>
  <div id="a">
   <h2>First example</h2>
   <p><a href="a.html" rel="bookmark">This permalink applies to
   only the content from the first H2 to the second H2</a>. The DIV isn't
   exactly that section, but it roughly corresponds to it.</p>
  </div>
  <h2>Second example</h2>
  <article id="b">
   <p><a href="b.html" rel="bookmark">This permalink applies to
   the outer ARTICLE element</a> (which could be, e.g., a blog post).</p>
   <article id="c">
    <p><a href="c.html" rel="bookmark">This permalink applies to
    the inner ARTICLE element</a> (which could be, e.g., a blog comment).</p>
   </article>
  </article>
 </body>
 ...

4.8.4.4 Link type "`help`"

The help keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

For a and area elements, the help keyword indicates that the referenced document provides further help information for the parent of the element defining the hyperlink, and its children.

In the following example, the form control has associated context-sensitive help. The user agent could use this information, for example, displaying the referenced document if the user presses the "Help" or "F1" key.

 <p><label> Topic: <input name=topic> <a href="help/topic.html" rel="help">(Help)</a></label></p>

For link elements, the help keyword indicates that the referenced document provides help for the page as a whole.

For a and area elements, on some browsers, the help keyword causes the link to use a different cursor.

4.8.4.5 Link type "`icon`"

The icon keyword may be used with link elements. This keyword creates an external resource link.

The specified resource is an icon representing the page or site, and should be used by the user agent when representing the page in the user interface.

Icons could be auditory icons, visual icons, or other kinds of icons. If multiple icons are provided, the user agent must select the most appropriate icon according to the type, media, and sizes attributes. If there are multiple equally appropriate icons, user agents must use the last one declared in tree order at the time that the user agent collected the list of icons. If the user agent tries to use an icon but that icon is determined, upon closer examination, to in fact be inappropriate (e.g. because it uses an unsupported format), then the user agent must try the next-most-appropriate icon as determined by the attributes.

User agents are not required to update icons when the list of icons changes, but are encouraged to do so.

There is no default type for resources given by the icon keyword. However, for the purposes of determining the type of the resource, user agents must expect the resource to be an image.

The sizes attribute gives the sizes of icons for visual media. Its value, if present, is merely advisory. User agents may use the value to decide which icon(s) to use if multiple icons are available.

If specified, the attribute must have a value that is an unordered set of unique space-separated tokens which are ASCII case-insensitive. Each value must be either an ASCII case-insensitive match for the string "any", or a value that consists of two valid non-negative integers that do not have a leading "0" (U+0030) character and that are separated by a single U+0078 LATIN SMALL LETTER X or U+0058 LATIN CAPITAL LETTER X character.

The keywords represent icon sizes in raw pixels (as opposed to CSS pixels).

An icon that is 50 CSS pixels wide intended for displays with a device pixel density of two device pixels per CSS pixel (2x, 192dpi) would have a width of 100 raw pixels. This feature does not support indicating that a different resource is to be used for small high-resolution icons vs large low-resolution icons (e.g. 50×50 2x vs 100×100 1x).

To parse and process the attribute's value, the user agent must first split the attribute's value on spaces, and must then parse each resulting keyword to determine what it represents.

The any keyword represents that the resource contains a scalable icon, e.g. as provided by an SVG image.

Other keywords must be further parsed as follows to determine what they represent:

If the keyword doesn't contain exactly one U+0078 LATIN SMALL LETTER X or U+0058 LATIN CAPITAL LETTER X character, then this keyword doesn't represent anything. Abort these steps for that keyword.
Let width string be the string before the "x" or "X".
Let height string be the string after the "x" or "X".
If either width string or height string start with a "0" (U+0030) character or contain any characters other than ASCII digits, then this keyword doesn't represent anything. Abort these steps for that keyword.
Apply the rules for parsing non-negative integers to width string to obtain width.
Apply the rules for parsing non-negative integers to height string to obtain height.
The keyword represents that the resource contains a bitmap icon with a width of width device pixels and a height of height device pixels.

The keywords specified on the sizes attribute must not represent icon sizes that are not actually available in the linked resource.

In the absence of a link with the icon keyword, for Documents obtained over HTTP or HTTPS, user agents may instead attempt to fetch and use an icon with the absolute URL obtained by resolving the URL "/favicon.ico" against the document's address, as if the page had declared that icon using the icon keyword.

The following snippet shows the top part of an application with several icons.

<!DOCTYPE HTML>
<html>
 <head>
  <title>lsForums — Inbox</title>
  <link rel=icon href=favicon.png sizes="16x16" type="image/png">
  <link rel=icon href=windows.ico sizes="32x32 48x48" type="image/vnd.microsoft.icon">
  <link rel=icon href=mac.icns sizes="128x128 512x512 8192x8192 32768x32768">
  <link rel=icon href=iphone.png sizes="57x57" type="image/png">
  <link rel=icon href=gnome.svg sizes="any" type="image/svg+xml">
  <link rel=stylesheet href=lsforums.css>
  <script src=lsforums.js></script>
  <meta name=application-name content="lsForums">
 </head>
 <body>
  ...

For historical reasons, the icon keyword may be preceded by the keyword "shortcut". If the "shortcut" keyword is present, it must be come immediately before the icon keyword and the two keywords must be separated by only a single U+0020 SPACE character.

4.8.4.6 Link type "`license`"

The license keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

The license keyword indicates that the referenced document provides the copyright license terms under which the main content of the current document is provided.

This specification defines the main content of a document and content that is not deemed to be part of that main content via the main element. The distinction should be made clear to the user.

Consider a photo sharing site. A page on that site might describe and show a photograph, and the page might be marked up as follows:

<!DOCTYPE HTML>
<html>
 <head>
  <title>Exampl Pictures: Kissat</title>
  <link rel="stylesheet" href="/style/default">
 </head>
 <body>
  <h1>Kissat</h1>
  <nav>
   <a href="../">Return to photo index</a>
  </nav>
  
  <main>
  <figure>
   <img src="/pix/39627052_fd8dcd98b5.jpg">
   <figcaption>Kissat</figcaption>
  </figure>
  <p>One of them has six toes!</p>
  <p><small>This photograph is <a rel="license" href="http://www.opensource.org/licenses/mit-license.php">MIT Licensed</a></small></p>
  </main>
  <footer>
   <a href="/">Home</a> | <a href="../">Photo index</a>
   <p><small>© copyright 2009 Exampl Pictures. All Rights Reserved.</small></p>
  </footer>
 </body>
</html>

In this case the license applies to just the photo (the main content of the document), not the whole document. In particular not the design of the page itself, which is covered by the copyright given at the bottom of the document. This should be made clear in the text referencing the licensing link and could also be made clearer in the styling (e.g. making the license link prominently positioned near the photograph, while having the page copyright in small text at the foot of the page, or adding a border to the main element.)

Synonyms: For historical reasons, user agents must also treat the keyword "copyright" like the license keyword.

4.8.4.7 Link type "`nofollow`"

The nofollow keyword may be used with a and area elements. This keyword does not create a hyperlink, but annotates any other hyperlinks created by the element (the implied hyperlink, if no other keywords create one).

The nofollow keyword indicates that the link is not endorsed by the original author or publisher of the page, or that the link to the referenced document was included primarily because of a commercial relationship between people affiliated with the two pages.

4.8.4.8 Link type "`noreferrer`"

The noreferrer keyword may be used with a and area elements. This keyword does not create a hyperlink, but annotates any other hyperlinks created by the element (the implied hyperlink, if no other keywords create one).

It indicates that no referrer information is to be leaked when following the link.

If a user agent follows a link defined by an a or area element that has the noreferrer keyword, the user agent must not include a Referer (sic) HTTP header (or equivalent for other protocols) in the request.

This keyword also causes the opener attribute to remain null if the hyperlink creates a new browsing context.

4.8.4.9 Link type "`prefetch`"

The prefetch keyword may be used with link, a, and area elements. This keyword creates an external resource link.

The prefetch keyword indicates that preemptively fetching and caching the specified resource is likely to be beneficial, as it is highly likely that the user will require this resource.

There is no default type for resources given by the prefetch keyword.

4.8.4.10 Link type "`search`"

The search keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

The search keyword indicates that the referenced document provides an interface specifically for searching the document and its related resources.

OpenSearch description documents can be used with link elements and the search link type to enable user agents to autodiscover search interfaces. [OPENSEARCH]

4.8.4.11 Link type "`stylesheet`"

The stylesheet keyword may be used with link elements. This keyword creates an external resource link that contributes to the styling processing model.

The specified resource is a resource that describes how to present the document. Exactly how the resource is to be processed depends on the actual type of the resource.

If the alternate keyword is also specified on the link element, then the link is an alternative stylesheet; in this case, the title attribute must be specified on the link element, with a non-empty value.

The default type for resources given by the stylesheet keyword is text/css.

The appropriate times to obtain the resource are:

When the external resource link's link element is inserted into a document.
When the external resource link is created on a link element that is already in a Document.
When the href attribute of the link element of an external resource link that is already in a Document is changed.

Quirk: If the document has been set to quirks mode, has the same origin as the URL of the external resource, and the Content-Type metadata of the external resource is not a supported style sheet type, the user agent must instead assume it to be text/css.

4.8.4.12 Link type "`tag`"

The tag keyword may be used with a and area elements. This keyword creates a hyperlink.

The tag keyword indicates that the tag that the referenced document represents applies to the current document.

Since it indicates that the tag applies to the current document, it would be inappropriate to use this keyword in the markup of a tag cloud, which lists the popular tags across a set of pages.

This document is about some gems, and so it is tagged with "http://en.wikipedia.org/wiki/Gemstone" to unambiguously categorise it as applying to the "jewel" kind of gems, and not to, say, the towns in the US, the Ruby package format, or the Swiss locomotive class:

<!DOCTYPE HTML>
<html>
 <head>
  <title>My Precious</title>
 </head>
 <body>
  <header><h1>My precious</h1> <p>Summer 2012</p></header>
  <p>Recently I managed to dispose of a red gem that had been
  bothering me. I now have a much nicer blue sapphire.</p>
  <p>The red gem had been found in a bauxite stone while I was digging
  out the office level, but nobody was willing to haul it away. The
  same red gem stayed there for literally years.</p>
  <footer>
   Tags: <a rel=tag href="http://en.wikipedia.org/wiki/Gemstone">Gemstone</a>
  </footer>
 </body>
</html>

In this document, there are two articles. The "tag" link, however, applies to the whole page (and would do so wherever it was placed, including if it was within the article elements).

<!DOCTYPE HTML>
<html>
 <head>
  <title>Gem 4/4</title>
 </head>
 <body>
  <article>
   <h1>801: Steinbock</h1>
   <p>The number 801 Gem 4/4 electro-diesel has an ibex and was rebuilt in 2002.</p>
  </article>
  <article>
   <h1>802: Murmeltier</h1>
   <figure>
    <img src="http://upload.wikimedia.org/wikipedia/commons/b/b0/Trains_de_la_Bernina_en_hiver_2.jpg"
         alt="The 802 was red with pantographs and tall vents on the side.">
    <figcaption>The 802 in the 1980s, above Lago Bianco.</figcaption>
   </figure>
   <p>The number 802 Gem 4/4 electro-diesel has a marmot and was rebuilt in 2003.</p>
  </article>
  <p class="topic"><a rel=tag href="http://en.wikipedia.org/wiki/Rhaetian_Railway_Gem_4/4">Gem 4/4</a></p>
 </body>
</html>

4.8.4.13 Sequential link types

Some documents form part of a sequence of documents.

A sequence of documents is one where each document can have a previous sibling and a next sibling. A document with no previous sibling is the start of its sequence, a document with no next sibling is the end of its sequence.

A document may be part of multiple sequences.

4.8.4.13.1 Link type "`next`"

The next keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

The next keyword indicates that the document is part of a sequence, and that the link is leading to the document that is the next logical document in the sequence.

4.8.4.13.2 Link type "`prev`"

The prev keyword may be used with link, a, and area elements. This keyword creates a hyperlink.

The prev keyword indicates that the document is part of a sequence, and that the link is leading to the document that is the previous logical document in the sequence.

Synonyms: For historical reasons, user agents must also treat the keyword "previous" like the prev keyword.

4.8.4.14 Other link types

Extensions to the predefined set of link types may be registered in the microformats wiki existing-rel-values page. [MFREL]

Anyone is free to edit the microformats wiki existing-rel-values page at any time to add a type. Extension types must be specified with the following information:

Keyword

The actual value being defined. The value should not be confusingly similar to any other defined value (e.g. differing only in case).

If the value contains a ":" (U+003A) character, it must also be an absolute URL.

Effect on... link

One of the following:

Not allowed: The keyword must not be specified on link elements.
Hyperlink: The keyword may be specified on a link element; it creates a hyperlink.
External Resource: The keyword may be specified on a link element; it creates an external resource link.

Effect on... a and area

One of the following:

Not allowed: The keyword must not be specified on a and area elements.
Hyperlink: The keyword may be specified on a and area elements; it creates a hyperlink.
External Resource: The keyword may be specified on a and area elements; it creates an external resource link.
Hyperlink Annotation: The keyword may be specified on a and area elements; it annotates other hyperlinks created by the element.

Brief description

A short non-normative description of what the keyword's meaning is.

Specification

A link to a more detailed description of the keyword's semantics and requirements. It could be another page on the Wiki, or a link to an external page.

Synonyms

A list of other keyword values that have exactly the same processing requirements. Authors should not use the values defined to be synonyms, they are only intended to allow user agents to support legacy content. Anyone may remove synonyms that are not used in practice; only names that need to be processed as synonyms for compatibility with legacy content are to be registered in this way.

Status

One of the following:

Proposed: The keyword has not received wide peer review and approval. Someone has proposed it and is, or soon will be, using it.
Ratified: The keyword has received wide peer review and approval. It has a specification that unambiguously defines how to handle pages that use the keyword, including when they use it in incorrect ways.
Discontinued: The keyword has received wide peer review and it has been found wanting. Existing pages are using this keyword, but new pages should avoid it. The "brief description" and "specification" entries will give details of what authors should use instead, if anything.

If a keyword is found to be redundant with existing values, it should be removed and listed as a synonym for the existing value.

If a keyword is registered in the "proposed" state for a period of a month or more without being used or specified, then it may be removed from the registry.

If a keyword is added with the "proposed" status and found to be redundant with existing values, it should be removed and listed as a synonym for the existing value. If a keyword is added with the "proposed" status and found to be harmful, then it should be changed to "discontinued" status.

Anyone can change the status at any time, but should only do so in accordance with the definitions above.

Conformance checkers may use the information given on the microformats wiki existing-rel-values page to establish if a value is allowed or not: values defined in this specification or marked as "proposed" or "ratified" must be accepted when used on the elements for which they apply as described in the "Effect on..." field, whereas values marked as "discontinued" or values not containing a U+003A COLON character but not listed in either this specification or on the aforementioned page must be reported as invalid. The remaining values must be accepted as valid if they are absolute URLs containing US-ASCII characters only and rejected otherwise. Conformance checkers may cache this information (e.g. for performance reasons or to avoid the use of unreliable network connectivity).

Note: Even URL-valued link types are compared ASCII-case-insensitively. Validators might choose to warn about characters U+0041 (LATIN CAPITAL LETTER A) through U+005A (LATIN CAPITAL LETTER Z) (inclusive) in the pre-case-folded form of link types that contain a colon.

When an author uses a new type not defined by either this specification or the Wiki page, conformance checkers should offer to add the value to the Wiki, with the details described above, with the "proposed" status.

Types defined as extensions in the microformats wiki existing-rel-values page with the status "proposed" or "ratified" may be used with the rel attribute on link, a, and area elements in accordance to the "Effect on..." field. [MFREL]

4.9 Tabular data

4.9.1 The `table` element

Where flow content is expected.

In this order: optionally a caption element, followed by zero or more colgroup elements, followed optionally by a thead element, followed optionally by a tfoot element, followed by either zero or more tbody elements or one or more tr elements, followed optionally by a tfoot element (but there can only be one tfoot element child in total), optionally intermixed with one or more script-supporting elements.

border

sortable - Enables a sorting interface for the table

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLTableElement : HTMLElement {
           attribute HTMLTableCaptionElement? caption;
  HTMLElement createCaption();
  void deleteCaption();
           attribute HTMLTableSectionElement? tHead;
  HTMLElement createTHead();
  void deleteTHead();
           attribute HTMLTableSectionElement? tFoot;
  HTMLElement createTFoot();
  void deleteTFoot();
  readonly attribute HTMLCollection tBodies;
  HTMLElement createTBody();
  readonly attribute HTMLCollection rows;
  HTMLElement insertRow(optional long index = -1);
  void deleteRow(long index);
           attribute DOMString border;
};

The table element represents data with more than one dimension, in the form of a table.

The table element takes part in the table model. Tables have rows, columns, and cells given by their descendants. The rows and columns form a grid; a table's cells must completely cover that grid without overlap.

Precise rules for determining whether this conformance requirement is met are described in the description of the table model.

Authors are encouraged to provide information describing how to interpret complex tables. Guidance on how to provide such information is given below.

Tables should not be used as layout aids. Historically, many Web authors have tables in HTML as a way to control their page layout making it difficult to extract tabular data from such documents. In particular, users of accessibility tools, like screen readers, are likely to find it very difficult to navigate pages with tables used for layout. If a table is to be used for layout it must be marked with the attribute role="presentation" for a user agent to properly represent the table to an assistive technology and to properly convey the intent of the author to tools that wish to extract tabular data from the document.

There are a variety of alternatives to using HTML tables for layout, primarily using CSS positioning and the CSS table model. [CSS]

The border attribute may be specified on a table element to explicitly indicate that the table element is not being used for layout purposes. If specified, the attribute's value must either be the empty string or the value "1". The attribute is used by certain user agents as an indication that borders should be drawn around cells of the table.

Tables can be complicated to understand and navigate. To help users with this, user agents should clearly delineate cells in a table from each other, unless the user agent has classified the table as a layout table.

Authors and implementors are encouraged to consider using some of the table design techniques described below to make tables easier to navigate for users.

User agents, especially those that do table analysis on arbitrary content, are encouraged to find heuristics to determine which tables actually contain data and which are merely being used for layout. This specification does not define a precise heuristic, but the following are suggested as possible indicators:

Feature	Indication
The use of the `role` attribute with the value `presentation`	Probably a layout table
The use of the `border` attribute with the non-conforming value 0	Probably a layout table
The use of the non-conforming `cellspacing` and `cellpadding` attributes with the value 0	Probably a layout table
The use of `caption`, `thead`, or `th` elements	Probably a non-layout table
The use of the `headers` and `scope` attributes	Probably a non-layout table
The use of the `border` attribute with a value other than 0	Probably a non-layout table
Explicit visible borders set using CSS	Probably a non-layout table
The use of the `summary` attribute	Not a good indicator (both layout and non-layout tables have historically been given this attribute)

It is quite possible that the above suggestions are wrong. Implementors are urged to provide feedback elaborating on their experiences with trying to create a layout table detection heuristic.

table . caption [ = value ]

Returns the table's caption element.

Can be set, to replace the caption element. If the new value is not a caption element, throws a HierarchyRequestError exception.

caption = table . createCaption()

Ensures the table has a caption element, and returns it.

table . deleteCaption()

Ensures the table does not have a caption element.

table . tHead [ = value ]

Returns the table's thead element.

Can be set, to replace the thead element. If the new value is not a thead element, throws a HierarchyRequestError exception.

thead = table . createTHead()

Ensures the table has a thead element, and returns it.

table . deleteTHead()

Ensures the table does not have a thead element.

table . tFoot [ = value ]

Returns the table's tfoot element.

Can be set, to replace the tfoot element. If the new value is not a tfoot element, throws a HierarchyRequestError exception.

tfoot = table . createTFoot()

Ensures the table has a tfoot element, and returns it.

table . deleteTFoot()

Ensures the table does not have a tfoot element.

table . tBodies

Returns an HTMLCollection of the tbody elements of the table.

tbody = table . createTBody()

Creates a tbody element, inserts it into the table, and returns it.

table . rows

Returns an HTMLCollection of the tr elements of the table.

tr = table . insertRow( [ index ] )

Creates a tr element, along with a tbody if required, inserts them into the table at the position given by the argument, and returns the tr.

The position is relative to the rows in the table. The index −1, which is the default if the argument is omitted, is equivalent to inserting at the end of the table.

If the given position is less than −1 or greater than the number of rows, throws an IndexSizeError exception.

table . deleteRow(index)

Removes the tr element with the given position in the table.

The position is relative to the rows in the table. The index −1 is equivalent to deleting the last row of the table.

If the given position is less than −1 or greater than the index of the last row, or if there are no rows, throws an IndexSizeError exception.

The caption IDL attribute must return, on getting, the first caption element child of the table element, if any, or null otherwise. On setting, if the new value is a caption element, the first caption element child of the table element, if any, must be removed, and the new value must be inserted as the first node of the table element. If the new value is not a caption element, then a HierarchyRequestError DOM exception must be thrown instead.

The createCaption() method must return the first caption element child of the table element, if any; otherwise a new caption element must be created, inserted as the first node of the table element, and then returned.

The deleteCaption() method must remove the first caption element child of the table element, if any.

The tHead IDL attribute must return, on getting, the first thead element child of the table element, if any, or null otherwise. On setting, if the new value is a thead element, the first thead element child of the table element, if any, must be removed, and the new value must be inserted immediately before the first element in the table element that is neither a caption element nor a colgroup element, if any, or at the end of the table if there are no such elements. If the new value is not a thead element, then a HierarchyRequestError DOM exception must be thrown instead.

The createTHead() method must return the first thead element child of the table element, if any; otherwise a new thead element must be created and inserted immediately before the first element in the table element that is neither a caption element nor a colgroup element, if any, or at the end of the table if there are no such elements, and then that new element must be returned.

The deleteTHead() method must remove the first thead element child of the table element, if any.

The tFoot IDL attribute must return, on getting, the first tfoot element child of the table element, if any, or null otherwise. On setting, if the new value is a tfoot element, the first tfoot element child of the table element, if any, must be removed, and the new value must be inserted immediately before the first element in the table element that is neither a caption element, a colgroup element, nor a thead element, if any, or at the end of the table if there are no such elements. If the new value is not a tfoot element, then a HierarchyRequestError DOM exception must be thrown instead.

The createTFoot() method must return the first tfoot element child of the table element, if any; otherwise a new tfoot element must be created and inserted immediately before the first element in the table element that is neither a caption element, a colgroup element, nor a thead element, if any, or at the end of the table if there are no such elements, and then that new element must be returned.

The deleteTFoot() method must remove the first tfoot element child of the table element, if any.

The tBodies attribute must return an HTMLCollection rooted at the table node, whose filter matches only tbody elements that are children of the table element.

The createTBody() method must create a new tbody element, insert it immediately after the last tbody element child in the table element, if any, or at the end of the table element if the table element has no tbody element children, and then must return the new tbody element.

The rows attribute must return an HTMLCollection rooted at the table node, whose filter matches only tr elements that are either children of the table element, or children of thead, tbody, or tfoot elements that are themselves children of the table element. The elements in the collection must be ordered such that those elements whose parent is a thead are included first, in tree order, followed by those elements whose parent is either a table or tbody element, again in tree order, followed finally by those elements whose parent is a tfoot element, still in tree order.

The behavior of the insertRow(index) method depends on the state of the table. When it is called, the method must act as required by the first item in the following list of conditions that describes the state of the table and the index argument:

If index is less than −1 or greater than the number of elements in rows collection:: The method must throw an IndexSizeError exception.
If the rows collection has zero elements in it, and the table has no tbody elements in it:: The method must create a tbody element, then create a tr element, then append the tr element to the tbody element, then append the tbody element to the table element, and finally return the tr element.
If the rows collection has zero elements in it:: The method must create a tr element, append it to the last tbody element in the table, and return the tr element.
If index is −1 or equal to the number of items in rows collection:: The method must create a tr element, and append it to the parent of the last tr element in the rows collection. Then, the newly created tr element must be returned.
Otherwise:: The method must create a tr element, insert it immediately before the indexth tr element in the rows collection, in the same parent, and finally must return the newly created tr element.

When the deleteRow(index) method is called, the user agent must run the following steps:

If index is equal to −1, then index must be set to the number of items in the rows collection, minus one.
Now, if index is less than zero, or greater than or equal to the number of elements in the rows collection, the method must instead throw an IndexSizeError exception, and these steps must be aborted.
Otherwise, the method must remove the indexth element in the rows collection from its parent.

The border IDL attribute must reflect the content attribute of the same name.

Here is an example of a table being used to mark up a Sudoku puzzle. Observe the lack of headers, which are not necessary in such a table.

<section>
 <style>
  table { border-collapse: collapse; border: solid thick; }
  colgroup, tbody { border: solid medium; }
  td { border: solid thin; height: 1.4em; width: 1.4em; text-align: center; padding: 0; }
 </style>
 <h1>Today's Sudoku</h1>
 <table>
  <colgroup><col><col><col>
  <colgroup><col><col><col>
  <colgroup><col><col><col>
  <tbody>
   <tr> <td> 1 <td>   <td> 3 <td> 6 <td>   <td> 4 <td> 7 <td>   <td> 9
   <tr> <td>   <td> 2 <td>   <td>   <td> 9 <td>   <td>   <td> 1 <td>
   <tr> <td> 7 <td>   <td>   <td>   <td>   <td>   <td>   <td>   <td> 6
  <tbody>
   <tr> <td> 2 <td>   <td> 4 <td>   <td> 3 <td>   <td> 9 <td>   <td> 8
   <tr> <td>   <td>   <td>   <td>   <td>   <td>   <td>   <td>   <td>
   <tr> <td> 5 <td>   <td>   <td> 9 <td>   <td> 7 <td>   <td>   <td> 1
  <tbody>
   <tr> <td> 6 <td>   <td>   <td>   <td> 5 <td>   <td>   <td>   <td> 2
   <tr> <td>   <td>   <td>   <td>   <td> 7 <td>   <td>   <td>   <td>
   <tr> <td> 9 <td>   <td>   <td> 8 <td>   <td> 2 <td>   <td>   <td> 5
 </table>
</section>

4.9.1.1 Techniques for describing tables

For tables that consist of more than just a grid of cells with headers in the first row and headers in the first column, and for any table in general where the reader might have difficulty understanding the content, authors should include explanatory information introducing the table. This information is useful for all users, but is especially useful for users who cannot see the table, e.g. users of screen readers.

Such explanatory information should introduce the purpose of the table, outline its basic cell structure, highlight any trends or patterns, and generally teach the user how to use the table.

For instance, the following table:

Characteristics with positive and negative sides
Negative	Characteristic	Positive
Sad	Mood	Happy
Failing	Grade	Passing

...might benefit from a description explaining the way the table is laid out, something like "Characteristics are given in the second column, with the negative side in the left column and the positive side in the right column".

There are a variety of ways to include this information, such as:

In prose, surrounding the table

<p id="summary">In the following table, characteristics are 
given in the second column, with the negative side in the left column and the positive
side in the right column.</p>
<table aria-describedby="summary">
 <caption>Characteristics with positive and negative sides</caption>
 <thead>
  <tr>
   <th id="n"> Negative
   <th> Characteristic
   <th> Positive
 <tbody>
  <tr>
   <td headers="n r1"> Sad
   <th id="r1"> Mood
   <td> Happy
  <tr>
   <td headers="n r2"> Failing
   <th id="r2"> Grade
   <td> Passing
</table>

In the example above the aria-describedby attribute is used to explicitly associate the information with the table for assistive technology users.

In the table's caption

<table>
 <caption>
  <strong>Characteristics with positive and negative sides.</strong>
  <p>Characteristics are given in the second column, with the
  negative side in the left column and the positive side in the right
  column.</p>
 </caption>
 <thead>
  <tr>
   <th id="n"> Negative
   <th> Characteristic
   <th> Positive
 <tbody>
  <tr>
   <td headers="n r1"> Sad
   <th id="r1"> Mood
   <td> Happy
  <tr>
   <td headers="n r2"> Failing
   <th id="r2"> Grade
   <td> Passing
</table>

Next to the table, in the same figure

<figure>
 <figcaption>Characteristics with positive and negative sides</figcaption>
 <p>Characteristics are given in the second column, with the
 negative side in the left column and the positive side in the right
 column.</p>
 <table>
  <thead>
   <tr>
    <th id="n"> Negative
    <th> Characteristic
    <th> Positive
  <tbody>
   <tr>
    <td headers="n r1"> Sad
    <th id="r1"> Mood
    <td> Happy
   <tr>
    <td headers="n r2"> Failing
    <th id="r2"> Grade
    <td> Passing
 </table>
</figure>

Next to the table, in a figure's figcaption

<figure>
 <figcaption>
  <strong>Characteristics with positive and negative sides</strong>
  <p>Characteristics are given in the second column, with the
  negative side in the left column and the positive side in the right
  column.</p>
 </figcaption>
 <table>
  <thead>
   <tr>
    <th id="n"> Negative
    <th> Characteristic
    <th> Positive
  <tbody>
   <tr>
    <td headers="n r1"> Sad
    <th id="r1"> Mood
    <td> Happy
   <tr>
    <td headers="n r2"> Failing
    <th id="r2"> Grade
    <td> Passing
 </table>
</figure>

Authors may also use other techniques, or combinations of the above techniques, as appropriate.

The best option, of course, rather than writing a description explaining the way the table is laid out, is to adjust the table such that no explanation is needed.

In the case of the table used in the examples above, a simple rearrangement of the table so that the headers are on the top and left sides removes the need for an explanation as well as removing the need for the use of headers attributes:

<table>
 <caption>Characteristics with positive and negative sides</caption>
 <thead>
  <tr>
   <th> Characteristic
   <th> Negative
   <th> Positive
 <tbody>
  <tr>
   <th> Mood
   <td> Sad
   <td> Happy
  <tr>
   <th> Grade
   <td> Failing
   <td> Passing
</table>

4.9.1.2 Techniques for table design

Good table design is key to making tables more readable and usable.

In visual media, providing column and row borders and alternating row backgrounds can be very effective to make complicated tables more readable.

For tables with large volumes of numeric content, using monospaced fonts can help users see patterns, especially in situations where a user agent does not render the borders. (Unfortunately, for historical reasons, not rendering borders on tables is a common default.)

In speech media, table cells can be distinguished by reporting the corresponding headers before reading the cell's contents, and by allowing users to navigate the table in a grid fashion, rather than serializing the entire contents of the table in source order.

Authors are encouraged to use CSS to achieve these effects.

User agents are encouraged to render tables using these techniques whenever the page does not use CSS and the table is not classified as a layout table.

4.9.2 The `caption` element

None.

As the first element child of a table element.

Flow content, but with no descendant table elements.

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLTableCaptionElement : HTMLElement {};

The caption element represents the title of the table that is its parent, if it has a parent and that is a table element.

The caption element takes part in the table model.

When a table element is the only content in a figure element other than the figcaption, the caption element should be omitted in favor of the figcaption.

A caption can introduce context for a table, making it significantly easier to understand.

Consider, for instance, the following table:

	1	2	3	4	5	6
1	2	3	4	5	6	7
2	3	4	5	6	7	8
3	4	5	6	7	8	9
4	5	6	7	8	9	10
5	6	7	8	9	10	11
6	7	8	9	10	11	12

In the abstract, this table is not clear. However, with a caption giving the table's number (for reference in the main prose) and explaining its use, it makes more sense:

<caption>
<p>Table 1.
<p>This table shows the total score obtained from rolling two
six-sided dice. The first row represents the value of the first die,
the first column the value of the second die. The total is given in
the cell that corresponds to the values of the two dice.
</caption>

This provides the user with more context:

Table 1.
This table shows the total score obtained from rolling two six-sided dice. The first row represents the value of the first die, the first column the value of the second die. The total is given in the cell that corresponds to the values of the two dice.
	1	2	3	4	5	6
1	2	3	4	5	6	7
2	3	4	5	6	7	8
3	4	5	6	7	8	9
4	5	6	7	8	9	10
5	6	7	8	9	10	11
6	7	8	9	10	11	12

4.9.3 The `colgroup` element

None.

As a child of a table element, after any caption elements and before any thead, tbody, tfoot, and tr elements.

If the span attribute is present: Empty.

If the span attribute is absent: Zero or more col and template elements.

span - Number of columns spanned by the element

Tag omission in text/html:

A colgroup element's end tag may be omitted if the colgroup element is not immediately followed by a space character or a comment. A colgroup element's end tag may be omitted if the colgroup element is not immediately followed by a space character or a comment.

Allowed ARIA role attribute values:

None

interface HTMLTableColElement : HTMLElement {
           attribute unsigned long span;
};

The colgroup element represents a group of one or more columns in the table that is its parent, if it has a parent and that is a table element.

If the colgroup element contains no col elements, then the element may have a span content attribute specified, whose value must be a valid non-negative integer greater than zero.

The colgroup element and its span attribute take part in the table model.

The span IDL attribute must reflect the content attribute of the same name. The value must be limited to only non-negative numbers greater than zero.

4.9.4 The `col` element

Categories:: None.
Contexts in which this element can be used:: As a child of a colgroup element that doesn't have a span attribute.
Content model:: Empty.
Content attributes:: Global attributes; span - Number of columns spanned by the element
Tag omission in text/html:: No end tag.
Allowed ARIA role attribute values:: None
Allowed ARIA state and property attributes:: Global aria-* attributes
DOM interface:: HTMLTableColElement, same as for colgroup elements. This interface defines one member, span.

If a col element has a parent and that is a colgroup element that itself has a parent that is a table element, then the col element represents one or more columns in the column group represented by that colgroup.

The element may have a span content attribute specified, whose value must be a valid non-negative integer greater than zero.

The col element and its span attribute take part in the table model.

The span IDL attribute must reflect the content attribute of the same name. The value must be limited to only non-negative numbers greater than zero.

4.9.5 The `tbody` element

None.

As a child of a table element, after any caption, colgroup, and thead elements, but only if there are no tr elements that are children of the table element.

Zero or more tr and script-supporting elements

Tag omission in text/html:

A tbody element's start tag may be omitted if the first thing inside the tbody element is a tr element, and if the element is not immediately preceded by a tbody, thead, or tfoot element whose end tag has been omitted. (It can't be omitted if the element is empty.). A tbody element's end tag may be omitted if the tbody element is immediately followed by a tbody or tfoot element, or if there is no more content in the parent element.

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLTableSectionElement : HTMLElement {
  readonly attribute HTMLCollection rows;
  HTMLElement insertRow(optional long index = -1);
  void deleteRow(long index);
};

The HTMLTableSectionElement interface is also used for thead and tfoot elements.

The tbody element represents a block of rows that consist of a body of data for the parent table element, if the tbody element has a parent and it is a table.

The tbody element takes part in the table model.

tbody . rows

Returns an HTMLCollection of the tr elements of the table section.

tr = tbody . insertRow( [ index ] )

Creates a tr element, inserts it into the table section at the position given by the argument, and returns the tr.

The position is relative to the rows in the table section. The index −1, which is the default if the argument is omitted, is equivalent to inserting at the end of the table section.

If the given position is less than −1 or greater than the number of rows, throws an IndexSizeError exception.

tbody . deleteRow(index)

Removes the tr element with the given position in the table section.

The position is relative to the rows in the table section. The index −1 is equivalent to deleting the last row of the table section.

If the given position is less than −1 or greater than the index of the last row, or if there are no rows, throws an IndexSizeError exception.

The rows attribute must return an HTMLCollection rooted at the element, whose filter matches only tr elements that are children of the element.

The insertRow(index) method must, when invoked on an element table section, act as follows:

If index is less than −1 or greater than the number of elements in the rows collection, the method must throw an IndexSizeError exception.

If index is −1 or equal to the number of items in the rows collection, the method must create a tr element, append it to the element table section, and return the newly created tr element.

Otherwise, the method must create a tr element, insert it as a child of the table section element, immediately before the indexth tr element in the rows collection, and finally must return the newly created tr element.

The deleteRow(index) method must remove the indexth element in the rows collection from its parent. If index is less than zero or greater than or equal to the number of elements in the rows collection, the method must instead throw an IndexSizeError exception.

4.9.6 The `thead` element

Categories:: None.
Contexts in which this element can be used:: As a child of a table element, after any caption, and colgroup elements and before any tbody, tfoot, and tr elements, but only if there are no other thead elements that are children of the table element.
Content model:: Zero or more tr and script-supporting elements
Content attributes:: Global attributes
Tag omission in text/html:: A thead element's end tag may be omitted if the thead element is immediately followed by a tbody or tfoot element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: HTMLTableSectionElement, as defined for tbody elements.

The thead element represents the block of rows that consist of the column labels (headers) for the parent table element, if the thead element has a parent and it is a table.

The thead element takes part in the table model.

This example shows a thead element being used. Notice the use of both th and td elements in the thead element: the first row is the headers, and the second row is an explanation of how to fill in the table.

<table>
 <caption> School auction sign-up sheet </caption>
 <thead>
  <tr>
   <th><label for=e1>Name</label>
   <th><label for=e2>Product</label>
   <th><label for=e3>Picture</label>
   <th><label for=e4>Price</label>
  <tr>
   <td>Your name here
   <td>What are you selling?
   <td>Link to a picture
   <td>Your reserve price
 <tbody>
  <tr>
   <td>Ms Danus
   <td>Doughnuts
   <td><img src="http://example.com/mydoughnuts.png" title="Doughnuts from Ms Danus">
   <td>$45
  <tr>
   <td><input id=e1 type=text name=who required form=f>
   <td><input id=e2 type=text name=what required form=f>
   <td><input id=e3 type=url name=pic form=f>
   <td><input id=e4 type=number step=0.01 min=0 value=0 required form=f>
</table>
<form id=f action="/auction.cgi">
 <input type=button name=add value="Submit">
</form>

4.9.7 The `tfoot` element

Categories:: None.
Contexts in which this element can be used:: As a child of a table element, after any caption, colgroup, and thead elements and before any tbody and tr elements, but only if there are no other tfoot elements that are children of the table element.; As a child of a table element, after any caption, colgroup, thead, tbody, and tr elements, but only if there are no other tfoot elements that are children of the table element.
Content model:: Zero or more tr and script-supporting elements
Content attributes:: Global attributes
Tag omission in text/html:: A tfoot element's end tag may be omitted if the tfoot element is immediately followed by a tbody element, or if there is no more content in the parent element.
Allowed ARIA role attribute values:: Any role value.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.
DOM interface:: HTMLTableSectionElement, as defined for tbody elements.

The tfoot element represents the block of rows that consist of the column summaries (footers) for the parent table element, if the tfoot element has a parent and it is a table.

The tfoot element takes part in the table model.

4.9.8 The `tr` element

None.

As a child of a thead element.

As a child of a tbody element.

As a child of a tfoot element.

As a child of a table element, after any caption, colgroup, and thead elements, but only if there are no tbody elements that are children of the table element.

Zero or more td, th, and script-supporting elements

Tag omission in text/html:

A tr element's end tag may be omitted if the tr element is immediately followed by another tr element, or if there is no more content in the parent element.

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLTableRowElement : HTMLElement {
  readonly attribute long rowIndex;
  readonly attribute long sectionRowIndex;
  readonly attribute HTMLCollection cells;
  HTMLElement insertCell(optional long index = -1);
  void deleteCell(long index);
};

The tr element represents a row of cells in a table.

The tr element takes part in the table model.

tr . rowIndex

Returns the position of the row in the table's rows list.

Returns −1 if the element isn't in a table.

tr . sectionRowIndex

Returns the position of the row in the table section's rows list.

Returns −1 if the element isn't in a table section.

tr . cells

Returns an HTMLCollection of the td and th elements of the row.

cell = tr . insertCell( [ index ] )

Creates a td element, inserts it into the table row at the position given by the argument, and returns the td.

The position is relative to the cells in the row. The index −1, which is the default if the argument is omitted, is equivalent to inserting at the end of the row.

If the given position is less than −1 or greater than the number of cells, throws an IndexSizeError exception.

tr . deleteCell(index)

Removes the td or th element with the given position in the row.

The position is relative to the cells in the row. The index −1 is equivalent to deleting the last cell of the row.

If the given position is less than −1 or greater than the index of the last cell, or if there are no cells, throws an IndexSizeError exception.

The rowIndex attribute must, if the element has a parent table element, or a parent tbody, thead, or tfoot element and a grandparent table element, return the index of the tr element in that table element's rows collection. If there is no such table element, then the attribute must return −1.

The sectionRowIndex attribute must, if the element has a parent table, tbody, thead, or tfoot element, return the index of the tr element in the parent element's rows collection (for tables, that's the HTMLTableElement.rows collection; for table sections, that's the HTMLTableRowElement.rows collection). If there is no such parent element, then the attribute must return −1.

The cells attribute must return an HTMLCollection rooted at the tr element, whose filter matches only td and th elements that are children of the tr element.

The insertCell(index) method must act as follows:

If index is less than −1 or greater than the number of elements in the cells collection, the method must throw an IndexSizeError exception.

If index is equal to −1 or equal to the number of items in cells collection, the method must create a td element, append it to the tr element, and return the newly created td element.

Otherwise, the method must create a td element, insert it as a child of the tr element, immediately before the indexth td or th element in the cells collection, and finally must return the newly created td element.

The deleteCell(index) method must remove the indexth element in the cells collection from its parent. If index is less than zero or greater than or equal to the number of elements in the cells collection, the method must instead throw an IndexSizeError exception.

4.9.9 The `td` element

Sectioning root.

As a child of a tr element.

colspan - Number of columns that the cell is to span

rowspan - Number of rows that the cell is to span

headers - The header cells for this cell

Tag omission in text/html:

A td element's end tag may be omitted if the td element is immediately followed by a td or th element, or if there is no more content in the parent element.

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLTableDataCellElement : HTMLTableCellElement {};

The td element represents a data cell in a table.

The td element and its colspan, rowspan, and headers attributes take part in the table model.

User agents, especially in non-visual environments or where displaying the table as a 2D grid is impractical, may give the user context for the cell when rendering the contents of a cell; for instance, giving its position in the table model, or listing the cell's header cells (as determined by the algorithm for assigning header cells). When a cell's header cells are being listed, user agents may use the value of abbr attributes on those header cells, if any, instead of the contents of the header cells themselves.

4.9.10 The `th` element

None.

As a child of a tr element.

Flow content, but with no header, footer, sectioning content, or heading content descendants.

colspan - Number of columns that the cell is to span

rowspan - Number of rows that the cell is to span

headers - The headers for this cell

scope - Specifies which cells the header cell applies to

abbr - Alternative label to use for the header cell when referencing the cell in other contexts

sorted - Column sort direction and ordinality

Tag omission in text/html:

A th element's end tag may be omitted if the th element is immediately followed by a td or th element, or if there is no more content in the parent element.

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

interface HTMLTableHeaderCellElement : HTMLTableCellElement {
           attribute DOMString scope;
           attribute DOMString abbr;
};

The th element represents a header cell in a table.

The th element may have a scope content attribute specified. The scope attribute is an enumerated attribute with five states, four of which have explicit keywords:

The row keyword, which maps to the row state: The row state means the header cell applies to some of the subsequent cells in the same row(s).
The col keyword, which maps to the column state: The column state means the header cell applies to some of the subsequent cells in the same column(s).
The rowgroup keyword, which maps to the row group state: The row group state means the header cell applies to all the remaining cells in the row group. A th element's scope attribute must not be in the row group state if the element is not anchored in a row group.
The colgroup keyword, which maps to the column group state: The column group state means the header cell applies to all the remaining cells in the column group. A th element's scope attribute must not be in the column group state if the element is not anchored in a column group.
The auto state: The auto state makes the header cell apply to a set of cells selected based on context.

The scope attribute's missing value default is the auto state.

The th element may have an abbr content attribute specified. Its value must be an alternative label for the header cell, to be used when referencing the cell in other contexts (e.g. when describing the header cells that apply to a data cell). It is typically an abbreviated form of the full header cell, but can also be an expansion, or merely a different phrasing.

The th element and its colspan, rowspan, headers, and scope attributes take part in the table model.

The scope IDL attribute must reflect the content attribute of the same name, limited to only known values.

The following example shows how the scope attribute's rowgroup value affects which data cells a header cell applies to.

Here is a markup fragment showing a table:

The tbody elements in this example identify the range of the row groups.

<table>
   <caption>Measurement of legs and tails in Cats and English speakers</caption>
 <thead>
  <tr> <th> ID <th> Measurement <th> Average <th> Maximum
 <tbody>
  <tr> <td> <th scope=rowgroup> Cats <td> <td>
  <tr> <td> 93 <th scope=row> Legs <td> 3.5 <td> 4
  <tr> <td> 10 <th scope=row> Tails <td> 1 <td> 1
 </tbody>
 <tbody>
  <tr> <td> <th scope=rowgroup> English speakers <td> <td>
  <tr> <td> 32 <th scope=row> Legs <td> 2.67 <td> 4
  <tr> <td> 35 <th scope=row> Tails <td> 0.33 <td> 1
 </tbody>
  </table>

This would result in the following table:

Measurement of legs and tails in Cats and English speakers
ID	Measurement	Average	Maximum
	Cats
93	Legs	3.5	4
10	Tails	1	1
	English speakers
32	Legs	2.67	4
35	Tails	0.33	1

The header cells in row 1 ('ID', 'Measurement', 'Average' and 'Maximum') each apply only to the cells in their column.

The header cells with a scope=rowgroup ('Cats' and 'English speakers') apply to all the cells in their row group other than the cells (to their left) in column 1:

The header 'Cats' (row 2, column 2) applies to the headers 'Legs' (row 3, column 2) and 'Tails' (row 4, column 2) and to the data cells in rows 2, 3 and 4 of the 'Average' and 'Maximum' columns.

The header 'English speakers' (row 5, column 2) applies to the headers 'Legs' (row 6, column 2) and 'Tails' (row 7, column 2) and to the data cells in rows 5, 6 and 7 of the 'Average' and 'Maximum' columns.

Each of the 'Legs' and 'Tails' header cells has a scope=row and therefore apply to the data cells (to the right) in their row, from the 'Average' and 'Maximum' columns.

Representation of the example
table overlayed with arrows indicating which cells each header applies to.

4.9.11 Attributes common to `td` and `th` elements

The td and th elements may have a colspan content attribute specified, whose value must be a valid non-negative integer greater than zero.

The td and th elements may also have a rowspan content attribute specified, whose value must be a valid non-negative integer. For this attribute, the value zero means that the cell is to span all the remaining rows in the row group.

These attributes give the number of columns and rows respectively that the cell is to span. These attributes must not be used to overlap cells, as described in the description of the table model.

The td and th element may have a headers content attribute specified. The headers attribute, if specified, must contain a string consisting of an unordered set of unique space-separated tokens that are case-sensitive, each of which must have the value of an ID of a th element taking part in the same table as the td or th element (as defined by the table model).

A th element with ID id is said to be directly targeted by all td and th elements in the same table that have headers attributes whose values include as one of their tokens the ID id. A th element A is said to be targeted by a th or td element B if either A is directly targeted by B or if there exists an element C that is itself targeted by the element B and A is directly targeted by C.

A th element must not be targeted by itself.

The colspan, rowspan, and headers attributes take part in the table model.

The td and th elements implement interfaces that inherit from the HTMLTableCellElement interface:

interface HTMLTableCellElement : HTMLElement {
           attribute unsigned long colSpan;
           attribute unsigned long rowSpan;
  [PutForwards=value] readonly attribute DOMSettableTokenList headers;
  readonly attribute long cellIndex;
};

cell . cellIndex

Returns the position of the cell in the row's cells list. This does not necessarily correspond to the x-position of the cell in the table, since earlier cells might cover multiple rows or columns.

Returns −1 if the element isn't in a row.

The colSpan IDL attribute must reflect the colspan content attribute. Its default value is 1.

The rowSpan IDL attribute must reflect the rowspan content attribute. Its default value is 1.

The headers IDL attribute must reflect the content attribute of the same name.

The cellIndex IDL attribute must, if the element has a parent tr element, return the index of the cell's element in the parent element's cells collection. If there is no such parent element, then the attribute must return −1.

4.9.12 Processing model

The various table elements and their content attributes together define the table model.

A table consists of cells aligned on a two-dimensional grid of slots with coordinates (x, y). The grid is finite, and is either empty or has one or more slots. If the grid has one or more slots, then the x coordinates are always in the range 0 ≤ x < x_width, and the y coordinates are always in the range 0 ≤ y < y_height. If one or both of x_width and y_height are zero, then the table is empty (has no slots). Tables correspond to table elements.

A cell is a set of slots anchored at a slot (cell_x, cell_y), and with a particular width and height such that the cell covers all the slots with coordinates (x, y) where cell_x ≤ x < cell_x+width and cell_y ≤ y < cell_y+height. Cells can either be data cells or header cells. Data cells correspond to td elements, and header cells correspond to th elements. Cells of both types can have zero or more associated header cells.

It is possible, in certain error cases, for two cells to occupy the same slot.

A row is a complete set of slots from x=0 to x=x_width-1, for a particular value of y. Rows usually correspond to tr elements, though a row group can have some implied rows at the end in some cases involving cells spanning multiple rows.

A column is a complete set of slots from y=0 to y=y_height-1, for a particular value of x. Columns can correspond to col elements. In the absence of col elements, columns are implied.

A row group is a set of rows anchored at a slot (0, group_y) with a particular height such that the row group covers all the slots with coordinates (x, y) where 0 ≤ x < x_width and group_y ≤ y < group_y+height. Row groups correspond to tbody, thead, and tfoot elements. Not every row is necessarily in a row group.

A column group is a set of columns anchored at a slot (group_x, 0) with a particular width such that the column group covers all the slots with coordinates (x, y) where group_x ≤ x < group_x+width and 0 ≤ y < y_height. Column groups correspond to colgroup elements. Not every column is necessarily in a column group.

Row groups cannot overlap each other. Similarly, column groups cannot overlap each other.

A cell cannot cover slots that are from two or more row groups. It is, however, possible for a cell to be in multiple column groups. All the slots that form part of one cell are part of zero or one row groups and zero or more column groups.

In addition to cells, columns, rows, row groups, and column groups, tables can have a caption element associated with them. This gives the table a heading, or legend.

A table model error is an error with the data represented by table elements and their descendants. Documents must not have table model errors.

4.9.12.1 Forming a table

To determine which elements correspond to which slots in a table associated with a table element, to determine the dimensions of the table (x_width and y_height), and to determine if there are any table model errors, user agents must use the following algorithm:

Let x_width be zero.
Let y_height be zero.
Let pending tfoot elements be a list of tfoot elements, initially empty.
Let the table be the table represented by the table element. The x_width and y_height variables give the table's dimensions. The table is initially empty.
If the table element has no children elements, then return the table (which will be empty), and abort these steps.
Associate the first caption element child of the table element with the table. If there are no such children, then it has no associated caption element.
Let the current element be the first element child of the table element.

If a step in this algorithm ever requires the current element to be advanced to the next child of the table when there is no such next child, then the user agent must jump to the step labeled end, near the end of this algorithm.
While the current element is not one of the following elements, advance the current element to the next child of the table:
- colgroup
- thead
- tbody
- tfoot
- tr
If the current element is a colgroup, follow these substeps:
1. Column groups: Process the current element according to the appropriate case below:
  If the current element has any col element children
  Follow these steps:
  1. Let x_start have the value of x_width.
  2. Let the current column be the first col element child of the colgroup element.
  3. Columns: If the current column col element has a span attribute, then parse its value using the rules for parsing non-negative integers.
    
    If the result of parsing the value is not an error or zero, then let span be that value.
    
    Otherwise, if the col element has no span attribute, or if trying to parse the attribute's value resulted in an error or zero, then let span be 1.
  4. Increase x_width by span.
  5. Let the last span columns in the table correspond to the current column col element.
  6. If current column is not the last col element child of the colgroup element, then let the current column be the next col element child of the colgroup element, and return to the step labeled columns.
  7. Let all the last columns in the table from x=x_start to x=x_width-1 form a new column group, anchored at the slot (x_start, 0), with width x_width-x_start, corresponding to the colgroup element.
  If the current element has no col element children
  1. If the colgroup element has a span attribute, then parse its value using the rules for parsing non-negative integers.
    
    If the result of parsing the value is not an error or zero, then let span be that value.
    
    Otherwise, if the colgroup element has no span attribute, or if trying to parse the attribute's value resulted in an error or zero, then let span be 1.
  2. Increase x_width by span.
  3. Let the last span columns in the table form a new column group, anchored at the slot (x_width-span, 0), with width span, corresponding to the colgroup element.
2. Advance the current element to the next child of the table.
3. While the current element is not one of the following elements, advance the current element to the next child of the table:
  - colgroup
  - thead
  - tbody
  - tfoot
  - tr
4. If the current element is a colgroup element, jump to the step labeled column groups above.
Let y_current be zero.
Let the list of downward-growing cells be an empty list.
Rows: While the current element is not one of the following elements, advance the current element to the next child of the table:
- thead
- tbody
- tfoot
- tr
If the current element is a tr, then run the algorithm for processing rows, advance the current element to the next child of the table, and return to the step labeled rows.
Run the algorithm for ending a row group.
If the current element is a tfoot, then add that element to the list of pending tfoot elements, advance the current element to the next child of the table, and return to the step labeled rows.
The current element is either a thead or a tbody.

Run the algorithm for processing row groups.
Advance the current element to the next child of the table.
Return to the step labeled rows.
End: For each tfoot element in the list of pending tfoot elements, in tree order, run the algorithm for processing row groups.
If there exists a row or column in the table containing only slots that do not have a cell anchored to them, then this is a table model error.
Return the table.

The algorithm for processing row groups, which is invoked by the set of steps above for processing thead, tbody, and tfoot elements, is:

Let y_start have the value of y_height.
For each tr element that is a child of the element being processed, in tree order, run the algorithm for processing rows.
If y_height > y_start, then let all the last rows in the table from y=y_start to y=y_height-1 form a new row group, anchored at the slot with coordinate (0, y_start), with height y_height-y_start, corresponding to the element being processed.
Run the algorithm for ending a row group.

The algorithm for ending a row group, which is invoked by the set of steps above when starting and ending a block of rows, is:

While y_current is less than y_height, follow these steps:
1. Run the algorithm for growing downward-growing cells.
2. Increase y_current by 1.
Empty the list of downward-growing cells.

The algorithm for processing rows, which is invoked by the set of steps above for processing tr elements, is:

If y_height is equal to y_current, then increase y_height by 1. (y_current is never greater than y_height.)
Let x_current be 0.
Run the algorithm for growing downward-growing cells.
If the tr element being processed has no td or th element children, then increase y_current by 1, abort this set of steps, and return to the algorithm above.
Let current cell be the first td or th element child in the tr element being processed.
Cells: While x_current is less than x_width and the slot with coordinate (x_current, y_current) already has a cell assigned to it, increase x_current by 1.
If x_current is equal to x_width, increase x_width by 1. (x_current is never greater than x_width.)
If the current cell has a colspan attribute, then parse that attribute's value, and let colspan be the result.

If parsing that value failed, or returned zero, or if the attribute is absent, then let colspan be 1, instead.
If the current cell has a rowspan attribute, then parse that attribute's value, and let rowspan be the result.

If parsing that value failed or if the attribute is absent, then let rowspan be 1, instead.
If rowspan is zero and the table element's Document is not set to quirks mode, then let cell grows downward be true, and set rowspan to 1. Otherwise, let cell grows downward be false.
If x_width < x_current+colspan, then let x_width be x_current+colspan.
If y_height < y_current+rowspan, then let y_height be y_current+rowspan.
Let the slots with coordinates (x, y) such that x_current ≤ x < x_current+colspan and y_current ≤ y < y_current+rowspan be covered by a new cell c, anchored at (x_current, y_current), which has width colspan and height rowspan, corresponding to the current cell element.

If the current cell element is a th element, let this new cell c be a header cell; otherwise, let it be a data cell.

To establish which header cells apply to the current cell element, use the algorithm for assigning header cells described in the next section.

If any of the slots involved already had a cell covering them, then this is a table model error. Those slots now have two cells overlapping.
If cell grows downward is true, then add the tuple {c, x_current, colspan} to the list of downward-growing cells.
Increase x_current by colspan.
If current cell is the last td or th element child in the tr element being processed, then increase y_current by 1, abort this set of steps, and return to the algorithm above.
Let current cell be the next td or th element child in the tr element being processed.
Return to the step labeled cells.

When the algorithms above require the user agent to run the algorithm for growing downward-growing cells, the user agent must, for each {cell, cell_x, width} tuple in the list of downward-growing cells, if any, extend the cell cell so that it also covers the slots with coordinates (x, y_current), where cell_x ≤ x < cell_x+width.

4.9.12.2 Forming relationships between data cells and header cells

Each cell can be assigned zero or more header cells. The algorithm for assigning header cells to a cell principal cell is as follows.

Let header list be an empty list of cells.
Let (principal_x, principal_y) be the coordinate of the slot to which the principal cell is anchored.
If the principal cell has a headers attribute specified
1. Take the value of the principal cell's headers attribute and split it on spaces, letting id list be the list of tokens obtained.
2. For each token in the id list, if the first element in the Document with an ID equal to the token is a cell in the same table, and that cell is not the principal cell, then add that cell to header list.
If principal cell does not have a headers attribute specified
1. Let principal_width be the width of the principal cell.
2. Let principal_height be the height of the principal cell.
3. For each value of y from principal_y to principal_y+principal_height-1, run the internal algorithm for scanning and assigning header cells, with the principal cell, the header list, the initial coordinate (principal_x,y), and the increments Δx=−1 and Δy=0.
4. For each value of x from principal_x to principal_x+principal_width-1, run the internal algorithm for scanning and assigning header cells, with the principal cell, the header list, the initial coordinate (x,principal_y), and the increments Δx=0 and Δy=−1.
5. If the principal cell is anchored in a row group, then add all header cells that are row group headers and are anchored in the same row group with an x-coordinate less than or equal to principal_x+principal_width-1 and a y-coordinate less than or equal to principal_y+principal_height-1 to header list.
6. If the principal cell is anchored in a column group, then add all header cells that are column group headers and are anchored in the same column group with an x-coordinate less than or equal to principal_x+principal_width-1 and a y-coordinate less than or equal to principal_y+principal_height-1 to header list.
Remove all the empty cells from the header list.
Remove any duplicates from the header list.
Remove principal cell from the header list if it is there.
Assign the headers in the header list to the principal cell.

The internal algorithm for scanning and assigning header cells, given a principal cell, a header list, an initial coordinate (initial_x, initial_y), and Δx and Δy increments, is as follows:

Let x equal initial_x.
Let y equal initial_y.
Let opaque headers be an empty list of cells.
If principal cell is a header cell

Let in header block be true, and let headers from current header block be a list of cells containing just the principal cell.

Otherwise

Let in header block be false and let headers from current header block be an empty list of cells.
Loop: Increment x by Δx; increment y by Δy.

For each invocation of this algorithm, one of Δx and Δy will be −1, and the other will be 0.
If either x or y is less than 0, then abort this internal algorithm.
If there is no cell covering slot (x, y), or if there is more than one cell covering slot (x, y), return to the substep labeled loop.
Let current cell be the cell covering slot (x, y).
If current cell is a header cell
1. Set in header block to true.
2. Add current cell to headers from current header block.
3. Let blocked be false.
4. If Δx is 0
  
  If there are any cells in the opaque headers list anchored with the same x-coordinate as the current cell, and with the same width as current cell, then let blocked be true.
  
  If the current cell is not a column header, then let blocked be true.
  
  If Δy is 0
  
  If there are any cells in the opaque headers list anchored with the same y-coordinate as the current cell, and with the same height as current cell, then let blocked be true.
  
  If the current cell is not a row header, then let blocked be true.
5. If blocked is false, then add the current cell to the headers list.
If current cell is a data cell and in header block is true

Set in header block to false. Add all the cells in headers from current header block to the opaque headers list, and empty the headers from current header block list.
Return to the step labeled loop.

A header cell anchored at the slot with coordinate (x, y) with width width and height height is said to be a column header if any of the following conditions are true:

The cell's scope attribute is in the column state, or
The cell's scope attribute is in the auto state, and there are no data cells in any of the cells covering slots with y-coordinates y .. y+height-1.

A header cell anchored at the slot with coordinate (x, y) with width width and height height is said to be a row header if any of the following conditions are true:

The cell's scope attribute is in the row state, or
The cell's scope attribute is in the auto state, the cell is not a column header, and there are no data cells in any of the cells covering slots with x-coordinates x .. x+width-1.

A header cell is said to be a column group header if its scope attribute is in the column group state.

A header cell is said to be a row group header if its scope attribute is in the row group state.

A cell is said to be an empty cell if it contains no elements and its text content, if any, consists only of White_Space characters.

4.10 Forms

4.10.1 Introduction

This section is non-normative.

A form is a component of a Web page that has form controls, such as text fields, buttons, checkboxes, range controls, or color pickers. A user can interact with such a form, providing data that can then be sent to the server for further processing (e.g. returning the results of a search or calculation). No client-side scripting is needed in many cases, though an API is available so that scripts can augment the user experience or use forms for purposes other than submitting data to a server.

Writing a form consists of several steps, which can be performed in any order: writing the user interface, implementing the server-side processing, and configuring the user interface to communicate with the server.

4.10.1.1 Writing a form's user interface

This section is non-normative.

For the purposes of this brief introduction, we will create a pizza ordering form.

Any form starts with a form element, inside which are placed the controls. Most controls are represented by the input element, which by default provides a one-line text field. To label a control, the label element is used; the label text and the control itself go inside the label element. Each part of a form is considered a paragraph, and is typically separated from other parts using p elements. Putting this together, here is how one might ask for the customer's name:

<form>
 <p><label>Customer name: <input></label></p>
</form>

To let the user select the size of the pizza, we can use a set of radio buttons. Radio buttons also use the input element, this time with a type attribute with the value radio. To make the radio buttons work as a group, they are given a common name using the name attribute. To group a batch of controls together, such as, in this case, the radio buttons, one can use the fieldset element. The title of such a group of controls is given by the first element in the fieldset, which has to be a legend element.

<form>
 <p><label>Customer name: <input></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size> Small </label></p>
  <p><label> <input type=radio name=size> Medium </label></p>
  <p><label> <input type=radio name=size> Large </label></p>
 </fieldset>
</form>

Changes from the previous step are highlighted.

To pick toppings, we can use checkboxes. These use the input element with a type attribute with the value checkbox:

<form>
 <p><label>Customer name: <input></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size> Small </label></p>
  <p><label> <input type=radio name=size> Medium </label></p>
  <p><label> <input type=radio name=size> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox> Bacon </label></p>
  <p><label> <input type=checkbox> Extra Cheese </label></p>
  <p><label> <input type=checkbox> Onion </label></p>
  <p><label> <input type=checkbox> Mushroom </label></p>
 </fieldset>
</form>

The pizzeria for which this form is being written is always making mistakes, so it needs a way to contact the customer. For this purpose, we can use form controls specifically for telephone numbers (input elements with their type attribute set to tel) and e-mail addresses (input elements with their type attribute set to email):

<form>
 <p><label>Customer name: <input></label></p>
 <p><label>Telephone: <input type=tel></label></p>
 <p><label>E-mail address: <input type=email></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size> Small </label></p>
  <p><label> <input type=radio name=size> Medium </label></p>
  <p><label> <input type=radio name=size> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox> Bacon </label></p>
  <p><label> <input type=checkbox> Extra Cheese </label></p>
  <p><label> <input type=checkbox> Onion </label></p>
  <p><label> <input type=checkbox> Mushroom </label></p>
 </fieldset>
</form>

We can use an input element with its type attribute set to time to ask for a delivery time. Many of these form controls have attributes to control exactly what values can be specified; in this case, three attributes of particular interest are min, max, and step. These set the minimum time, the maximum time, and the interval between allowed values (in seconds). This pizzeria only delivers between 11am and 9pm, and doesn't promise anything better than 15 minute increments, which we can mark up as follows:

<form>
 <p><label>Customer name: <input></label></p>
 <p><label>Telephone: <input type=tel></label></p>
 <p><label>E-mail address: <input type=email></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size> Small </label></p>
  <p><label> <input type=radio name=size> Medium </label></p>
  <p><label> <input type=radio name=size> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox> Bacon </label></p>
  <p><label> <input type=checkbox> Extra Cheese </label></p>
  <p><label> <input type=checkbox> Onion </label></p>
  <p><label> <input type=checkbox> Mushroom </label></p>
 </fieldset>
 <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900"></label></p>
</form>

The textarea element can be used to provide a free-form text field. In this instance, we are going to use it to provide a space for the customer to give delivery instructions:

<form>
 <p><label>Customer name: <input></label></p>
 <p><label>Telephone: <input type=tel></label></p>
 <p><label>E-mail address: <input type=email></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size> Small </label></p>
  <p><label> <input type=radio name=size> Medium </label></p>
  <p><label> <input type=radio name=size> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox> Bacon </label></p>
  <p><label> <input type=checkbox> Extra Cheese </label></p>
  <p><label> <input type=checkbox> Onion </label></p>
  <p><label> <input type=checkbox> Mushroom </label></p>
 </fieldset>
 <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900"></label></p>
 <p><label>Delivery instructions: <textarea></textarea></label></p>
</form>

Finally, to make the form submittable we use the button element:

<form>
 <p><label>Customer name: <input></label></p>
 <p><label>Telephone: <input type=tel></label></p>
 <p><label>E-mail address: <input type=email></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size> Small </label></p>
  <p><label> <input type=radio name=size> Medium </label></p>
  <p><label> <input type=radio name=size> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox> Bacon </label></p>
  <p><label> <input type=checkbox> Extra Cheese </label></p>
  <p><label> <input type=checkbox> Onion </label></p>
  <p><label> <input type=checkbox> Mushroom </label></p>
 </fieldset>
 <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900"></label></p>
 <p><label>Delivery instructions: <textarea></textarea></label></p>
 <p><button>Submit order</button></p>
</form>

4.10.1.2 Implementing the server-side processing for a form

This section is non-normative.

The exact details for writing a server-side processor are out of scope for this specification. For the purposes of this introduction, we will assume that the script at https://pizza.example.com/order.cgi is configured to accept submissions using the application/x-www-form-urlencoded format, expecting the following parameters sent in an HTTP POST body:

custname: Customer's name
custtel: Customer's telephone number
custemail: Customer's e-mail address
size: The pizza size, either small, medium, or large
topping: A topping, specified once for each selected topping, with the allowed values being bacon, cheese, onion, and mushroom
delivery: The requested delivery time
comments: The delivery instructions

4.10.1.3 Configuring a form to communicate with a server

This section is non-normative.

Form submissions are exposed to servers in a variety of ways, most commonly as HTTP GET or POST requests. To specify the exact method used, the method attribute is specified on the form element. This doesn't specify how the form data is encoded, though; to specify that, you use the enctype attribute. You also have to specify the URL of the service that will handle the submitted data, using the action attribute.

For each form control you want submitted, you then have to give a name that will be used to refer to the data in the submission. We already specified the name for the group of radio buttons; the same attribute (name) also specifies the submission name. Radio buttons can be distinguished from each other in the submission by giving them different values, using the value attribute.

Multiple controls can have the same name; for example, here we give all the checkboxes the same name, and the server distinguishes which checkbox was checked by seeing which values are submitted with that name — like the radio buttons, they are also given unique values with the value attribute.

Given the settings in the previous section, this all becomes:

<form method="post"
      enctype="application/x-www-form-urlencoded"
      action="https://pizza.example.com/order.cgi">
 <p><label>Customer name: <input name="custname"></label></p>
 <p><label>Telephone: <input type=tel name="custtel"></label></p>
 <p><label>E-mail address: <input type=email name="custemail"></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size value="small"> Small </label></p>
  <p><label> <input type=radio name=size value="medium"> Medium </label></p>
  <p><label> <input type=radio name=size value="large"> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
  <p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
  <p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
  <p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
 </fieldset>
 <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery"></label></p>
 <p><label>Delivery instructions: <textarea name="comments"></textarea></label></p>
 <p><button>Submit order</button></p>
</form>

There is no particular significance to the way some of the attributes have their values quoted and others don't. The HTML syntax allows a variety of equally valid ways to specify attributes, as discussed in the syntax section.

For example, if the customer entered "Denise Lawrence" as their name, "555-321-8642" as their telephone number, did not specify an e-mail address, asked for a medium-sized pizza, selected the Extra Cheese and Mushroom toppings, entered a delivery time of 7pm, and left the delivery instructions text field blank, the user agent would submit the following to the online Web service:

custname=Denise+Lawrence&custtel=555-321-8624&custemail=&size=medium&topping=cheese&topping=mushroom&delivery=19%3A00&comments=

4.10.1.4 Client-side form validation

This section is non-normative.

Forms can be annotated in such a way that the user agent will check the user's input before the form is submitted. The server still has to verify the input is valid (since hostile users can easily bypass the form validation), but it allows the user to avoid the wait incurred by having the server be the sole checker of the user's input.

The simplest annotation is the required attribute, which can be specified on input elements to indicate that the form is not to be submitted until a value is given. By adding this attribute to the customer name, pizza size, and delivery time fields, we allow the user agent to notify the user when the user submits the form without filling in those fields:

<form method="post"
      enctype="application/x-www-form-urlencoded"
      action="https://pizza.example.com/order.cgi">
 <p><label>Customer name: <input name="custname" required></label></p>
 <p><label>Telephone: <input type=tel name="custtel"></label></p>
 <p><label>E-mail address: <input type=email name="custemail"></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size required value="small"> Small </label></p>
  <p><label> <input type=radio name=size required value="medium"> Medium </label></p>
  <p><label> <input type=radio name=size required value="large"> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
  <p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
  <p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
  <p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
 </fieldset>
 <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery" required></label></p>
 <p><label>Delivery instructions: <textarea name="comments"></textarea></label></p>
 <p><button>Submit order</button></p>
</form>

It is also possible to limit the length of the input, using the maxlength attribute. By adding this to the textarea element, we can limit users to 1000 characters, preventing them from writing huge essays to the busy delivery drivers instead of staying focused and to the point:

<form method="post"
      enctype="application/x-www-form-urlencoded"
      action="https://pizza.example.com/order.cgi">
 <p><label>Customer name: <input name="custname" required></label></p>
 <p><label>Telephone: <input type=tel name="custtel"></label></p>
 <p><label>E-mail address: <input type=email name="custemail"></label></p>
 <fieldset>
  <legend> Pizza Size </legend>
  <p><label> <input type=radio name=size required value="small"> Small </label></p>
  <p><label> <input type=radio name=size required value="medium"> Medium </label></p>
  <p><label> <input type=radio name=size required value="large"> Large </label></p>
 </fieldset>
 <fieldset>
  <legend> Pizza Toppings </legend>
  <p><label> <input type=checkbox name="topping" value="bacon"> Bacon </label></p>
  <p><label> <input type=checkbox name="topping" value="cheese"> Extra Cheese </label></p>
  <p><label> <input type=checkbox name="topping" value="onion"> Onion </label></p>
  <p><label> <input type=checkbox name="topping" value="mushroom"> Mushroom </label></p>
 </fieldset>
 <p><label>Preferred delivery time: <input type=time min="11:00" max="21:00" step="900" name="delivery" required></label></p>
 <p><label>Delivery instructions: <textarea name="comments" maxlength=1000></textarea></label></p>
 <p><button>Submit order</button></p>
</form>

When a form is submitted, invalid events are fired at each form control that is invalid, and then at the form element itself. This can be useful for displaying a summary of the problems with the form, since typically the browser itself will only report one problem at a time.

4.10.1.5 Date, time, and number formats

This section is non-normative.

In this pizza delivery example, the times are specified in the format "HH:MM": two digits for the hour, in 24-hour format, and two digits for the time. (Seconds could also be specified, though they are not necessary in this example.)

In some locales, however, times are often expressed differently when presented to users. For example, in the United States, it is still common to use the 12-hour clock with an am/pm indicator, as in "2pm". In France, it is common to separate the hours from the minutes using an "h" character, as in "14h00".

Similar issues exist with dates, with the added complication that even the order of the components is not always consistent — for example, in Cyprus the first of February 2003 would typically be written "1/2/03", while that same date in Japan would typically be written as "2003年02月01日" — and even with numbers, where locales differ, for example, in what punctuation is used as the decimal separator and the thousands separator.

It is therefore important to distinguish the time, date, and number formats used in HTML and in form submissions, which are always the formats defined in this specification (and based on the well-established ISO 8601 standard for computer-readable date and time formats), from the time, date, and number formats presented to the user by the browser and accepted as input from the user by the browser.

The format used "on the wire", i.e. in HTML markup and in form submissions, is intended to be computer-readable and consistent irrespective of the user's locale. Dates, for instance, are always written in the format "YYYY-MM-DD", as in "2003-02-01". Users are not expected to ever see this format.

The time, date, or number given by the page in the wire format is then translated to the user's preferred presentation (based on user preferences or on the locale of the page itself), before being displayed to the user. Similarly, after the user inputs a time, date, or number using their preferred format, the user agent converts it back to the wire format before putting it in the DOM or submitting it.

This allows scripts in pages and on servers to process times, dates, and numbers in a consistent manner without needing to support dozens of different formats, while still supporting the users' needs.

See also the implementation notes regarding localization of form controls.

4.10.2 Categories

Mostly for historical reasons, elements in this section fall into several overlapping (but subtly different) categories in addition to the usual ones like flow content, phrasing content, and interactive content.

A number of the elements are form-associated elements, which means they can have a form owner.

The form-associated elements fall into several subcategories:

Listed elements

Denotes elements that are listed in the form.elements and fieldset.elements APIs.

Submittable elements

Denotes elements that can be used for constructing the form data set when a form element is submitted.

Some submittable elements can be, depending on their attributes, buttons. The prose below defines when an element is a button. Some buttons are specifically submit buttons.

Resettable elements

Denotes elements that can be affected when a form element is reset.

Reassociateable elements

Denotes elements that have a form content attribute, and a matching form IDL attribute, that allow authors to specify an explicit form owner.

Some elements, not all of them form-associated, are categorized as labelable elements. These are elements that can be associated with a label element.

4.10.3 The `form` element

Where flow content is expected.

Flow content, but with no form element descendants.

accept-charset - Character encodings to use for form submission

action - URL to use for form submission

autocomplete - Default setting for autofill feature for controls in the form

enctype - Form data set encoding type to use for form submission

method - HTTP method to use for form submission

name - Name of form to use in the document.forms API

novalidate - Bypass form control validation for form submission

target - Browsing context for form submission

Tag omission in text/html:

Neither tag is omissible.

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

Reassociateable form-associated element.

[OverrideBuiltins]
interface HTMLFormElement : HTMLElement {
           attribute DOMString acceptCharset;
           attribute DOMString action;
           attribute DOMString autocomplete;
           attribute DOMString enctype;
           attribute DOMString encoding;
           attribute DOMString method;
           attribute DOMString name;
           attribute boolean noValidate;
           attribute DOMString target;

  readonly attribute HTMLFormControlsCollection elements;
  readonly attribute long length;
  getter Element (unsigned long index);
  getter (RadioNodeList or Element) (DOMString name);

  void submit();
  void reset();
  boolean checkValidity();
};

The form element represents a collection of form-associated elements, some of which can represent editable values that can be submitted to a server for processing.

The accept-charset attribute gives the character encodings that are to be used for the submission. If specified, the value must be an ordered set of unique space-separated tokens that are ASCII case-insensitive, and each token must be an ASCII case-insensitive match for one of the labels of an ASCII-compatible character encoding. [ENCODING]

The name attribute represents the form's name within the forms collection. The value must not be the empty string, and the value must be unique amongst the form elements in the forms collection that it is in, if any.

The autocomplete attribute is an enumerated attribute. The attribute has two states. The on keyword maps to the on state, and the off keyword maps to the off state. The attribute may also be omitted. The missing value default is the on state. The off state indicates that by default, form controls in the form will have their autofill field name set to "off"; the on state indicates that by default, form controls in the form will have their autofill field name set to "on".

The action, enctype, method, novalidate, and target attributes are attributes for form submission.

form . elements

Returns an HTMLCollection of the form controls in the form (excluding image buttons for historical reasons).

form . length

Returns the number of form controls in the form (excluding image buttons for historical reasons).

form[index]

Returns the indexth element in the form (excluding image buttons for historical reasons).

form[name]

Returns the form control (or, if there are several, a RadioNodeList of the form controls) in the form with the given ID or name (excluding image buttons for historical reasons); or, if there are none, returns the img element with the given ID.

Once an element has been referenced using a particular name, that name will continue being available as a way to reference that element in this method, even if the element's actual ID or name changes, for as long as the element remains in the Document.

If there are multiple matching items, then a RadioNodeList object containing all those elements is returned.

form . submit()

Submits the form.

form . reset()

Resets the form.

form . checkValidity()

Returns true if the form's controls are all valid; otherwise, returns false.

The autocomplete IDL attribute must reflect the content attribute of the same name, limited to only known values.

The name IDL attribute must reflect the content attribute of the same name.

The acceptCharset IDL attribute must reflect the accept-charset content attribute.

The elements IDL attribute must return an HTMLFormControlsCollection rooted at the Document node while the form element is in a Document and rooted at the form element itself when it is not, whose filter matches listed elements whose form owner is the form element, with the exception of input elements whose type attribute is in the Image Button state, which must, for historical reasons, be excluded from this particular collection.

The length IDL attribute must return the number of nodes represented by the elements collection.

The supported property indices at any instant are the indices supported by the object returned by the elements attribute at that instant.

When a form element is indexed for indexed property retrieval, the user agent must return the value returned by the item method on the elements collection, when invoked with the given index as its argument.

Each form element has a mapping of names to elements called the past names map. It is used to persist names of controls even when they change names.

The supported property names consist of the names obtained from the following algorithm, in the order obtained from this algorithm:

Let sourced names be an initially empty ordered list of tuples consisting of a string, an element, a source, where the source is either id, name, or past, and, if the source is past, an age.
For each listed element candidate whose form owner is the form element, with the exception of any input elements whose type attribute is in the Image Button state, run these substeps:
1. If candidate has an id attribute, add an entry to sourced names with that id attribute's value as the string, candidate as the element, and id as the source.
2. If candidate has a name attribute, add an entry to sourced names with that name attribute's value as the string, candidate as the element, and name as the source.
For each img element candidate whose form owner is the form element, run these substeps:
1. If candidate has an id attribute, add an entry to sourced names with that id attribute's value as the string, candidate as the element, and id as the source.
2. If candidate has a name attribute, add an entry to sourced names with that name attribute's value as the string, candidate as the element, and name as the source.
For each entry past entry in the past names map add an entry to sourced names with the past entry's name as the string, past entry's element as the element, past as the source, and the length of time past entry has been in the past names map as the age.
Sort sourced names by tree order of the element entry of each tuple, sorting entries with the same element by putting entries whose source is id first, then entries whose source is name, and finally entries whose source is past, and sorting entries with the same element and source by their age, oldest first.
Remove any entries in sourced names that have the empty string as their name.
Remove any entries in sourced names that have the same name as an earlier entry in the map.
Return the list of names from sourced names, maintaining their relative order.

The properties exposed in this way must not be enumerable.

When a form element is indexed for named property retrieval, the user agent must run the following steps:

Let candidates be a live RadioNodeList object containing all the listed elements whose form owner is the form element that have either an id attribute or a name attribute equal to name, with the exception of input elements whose type attribute is in the Image Button state, in tree order.
If candidates is empty, let candidates be a live RadioNodeList object containing all the img elements that are descendants of the form element and that have either an id attribute or a name attribute equal to name, in tree order.
If candidates is empty, name is the name of one of the entries in the form element's past names map: return the object associated with name in that map.
If candidates contains more than one node, return candidates and abort these steps.
Otherwise, candidates contains exactly one node. Add a mapping from name to the node in candidates in the form element's past names map, replacing the previous entry with the same name, if any.
Return the node in candidates.

If an element listed in a form element's past names map changes form owner, then its entries must be removed from that map.

The submit() method, when invoked, must submit the form element from the form element itself, with the submitted from submit() method flag set.

The reset() method, when invoked, must run the following steps:

If the form element is marked as locked for reset, then abort these steps.
Mark the form element as locked for reset.
Reset the form element.
Unmark the form element as locked for reset.

If the checkValidity() method is invoked, the user agent must statically validate the constraints of the form element, and return true if the constraint validation return a positive result, and false if it returned a negative result.

This example shows two search forms:

<form action="http://www.google.com/search" method="get">
 <label>Google: <input type="search" name="q"></label> <input type="submit" value="Search...">
</form>
<form action="http://www.bing.com/search" method="get">
 <label>Bing: <input type="search" name="q"></label> <input type="submit" value="Search...">
</form>

4.10.4 The `label` element

Where phrasing content is expected.

Phrasing content, but with no descendant labelable elements unless it is the element's labeled control, and no descendant label elements.

form - Associates the control with a form element

for - Associate the label with form control

Tag omission in text/html:

Neither tag is omissable

Allowed ARIA role attribute values:

None

interface HTMLLabelElement : HTMLElement {
  readonly attribute HTMLFormElement? form;
           attribute DOMString htmlFor;
  readonly attribute HTMLElement? control;
};

The label element represents a caption in a user interface. The caption can be associated with a specific form control, known as the label element's labeled control, either using the for attribute, or by putting the form control inside the label element itself.

Except where otherwise specified by the following rules, a label element has no labeled control.

The for attribute may be specified to indicate a form control with which the caption is to be associated. If the attribute is specified, the attribute's value must be the ID of a labelable element in the same Document as the label element. If the attribute is specified and there is an element in the Document whose ID is equal to the value of the for attribute, and the first such element is a labelable element, then that element is the label element's labeled control.

If the for attribute is not specified, but the label element has a labelable element descendant, then the first such descendant in tree order is the label element's labeled control.

The label element's exact default presentation and behavior, in particular what its activation behavior might be, if anything, should match the platform's label behavior. The activation behavior of a label element for events targeted at interactive content descendants of a label element, and any descendants of those interactive content descendants, must be to do nothing.

For example, on platforms where clicking a checkbox label checks the checkbox, clicking the label in the following snippet could trigger the user agent to run synthetic click activation steps on the input element, as if the element itself had been triggered by the user:

<label><input type=checkbox name=lost> Lost</label>

On other platforms, the behavior might be just to focus the control, or do nothing.

The form attribute is used to explicitly associate the label element with its form owner.

The following example shows three form controls each with a label, two of which have small text showing the right format for users to use.

<p><label>Full name: <input name=fn> <small>Format: First Last</small></label></p>
<p><label>Age: <input name=age type=number min=0></label></p>
<p><label>Post code: <input name=pc> <small>Format: AB12 3CD</small></label></p>

label . control: Returns the form control that is associated with this element.

The htmlFor IDL attribute must reflect the for content attribute.

The control IDL attribute must return the label element's labeled control, if any, or null if there isn't one.

The form IDL attribute is part of the element's forms API.

control . labels: Returns a NodeList of all the label elements that the form control is associated with.

Labelable elements have a NodeList object associated with them that represents the list of label elements, in tree order, whose labeled control is the element in question. The labels IDL attribute of labelable elements, on getting, must return that NodeList object.

4.10.5 The `input` element

If the type attribute is not in the Hidden state: Interactive content.

If the type attribute is not in the Hidden state: Listed, labelable, submittable, resettable, and reassociateable form-associated element.

If the type attribute is in the Hidden state: Listed, submittable, resettable, and reassociateable form-associated element.

If the type attribute is not in the Hidden state: Palpable content.

Where phrasing content is expected.

Empty.

accept - Hint for expected file type in file upload controls

alt - Replacement text for use when images are not available

autocomplete - Hint for form autofill feature

autofocus - Automatically focus the form control when the page is loaded

checked - Whether the command or control is checked

dirname - Name of form field to use for sending the element's directionality in form submission

disabled - Whether the form control is disabled

form - Associates the control with a form element

formaction - URL to use for form submission

formenctype - Form data set encoding type to use for form submission

formmethod - HTTP method to use for form submission

formnovalidate - Bypass form control validation for form submission

formtarget - Browsing context for form submission

height - Vertical dimension

inputmode - Hint for selecting an input modality

list - List of autocomplete options

max - Maximum value

maxlength - Maximum length of value

min - Minimum value

minlength - Minimum length of value

multiple - Whether to allow multiple values

name - Name of form control to use for form submission and in the form.elements API

pattern - Pattern to be matched by the form control's value

placeholder - User-visible label to be placed within the form control

readonly - Whether to allow the value to be edited by the user

required - Whether the control is required for form submission

size - Size of the control

src - Address of the resource

step - Granularity to be matched by the form control's value

type - Type of form control

value - Value of the form control

width - Horizontal dimension

Also, the title attribute has special semantics on this element when used in conjunction with the pattern attribute.

Tag omission in text/html:

Allowed ARIA role attribute values:

Depends upon state of the type attribute.

Any aria-* attributes applicable to the allowed roles.

interface HTMLInputElement : HTMLElement {
           attribute DOMString accept;
           attribute DOMString alt;
           attribute DOMString autocomplete;
           attribute boolean autofocus;
           attribute boolean defaultChecked;
           attribute boolean checked;
           attribute DOMString dirName;
           attribute boolean disabled;
  readonly attribute HTMLFormElement? form;
  readonly attribute FileList? files;
           attribute DOMString formAction;
           attribute DOMString formEnctype;
           attribute DOMString formMethod;
           attribute boolean formNoValidate;
           attribute DOMString formTarget;
           attribute unsigned long height;
           attribute boolean indeterminate;
  readonly attribute HTMLElement? list;
           attribute DOMString max;
           attribute long maxLength;
           attribute DOMString min;
           attribute long minLength;
           attribute boolean multiple;
           attribute DOMString name;
           attribute DOMString pattern;
           attribute DOMString placeholder;
           attribute boolean readOnly;
           attribute boolean required;
           attribute unsigned long size;
           attribute DOMString src;
           attribute DOMString step;
           attribute DOMString type;
           attribute DOMString defaultValue;
  [TreatNullAs=EmptyString] attribute DOMString value;
           attribute Date? valueAsDate;
           attribute unrestricted double valueAsNumber;
           attribute unsigned long width;

  void stepUp(optional long n = 1);
  void stepDown(optional long n = 1);

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);

  readonly attribute NodeList labels;

  void select();
           attribute unsigned long selectionStart;
           attribute unsigned long selectionEnd;
           attribute DOMString selectionDirection;
  void setRangeText(DOMString replacement);
  void setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode = "preserve");
  void setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
};

The input element represents a typed data field, usually with a form control to allow the user to edit the data.

The type attribute controls the data type (and associated control) of the element. It is an enumerated attribute. The following table lists the keywords and states for the attribute — the keywords in the left column map to the states in the cell in the second column on the same row as the keyword.

Keyword	State	Data type	Control type
`hidden`	Hidden	An arbitrary string	n/a
`text`	Text	Text with no line breaks	A text field
`search`	Search	Text with no line breaks	Search field
`tel`	Telephone	Text with no line breaks	A text field
`url`	URL	An absolute URL	A text field
`email`	E-mail	An e-mail address or list of e-mail addresses	A text field
`password`	Password	Text with no line breaks (sensitive information)	A text field that obscures data entry
`date`	Date	A date (year, month, day) with no time zone	A date control
`time`	Time	A time (hour, minute, seconds, fractional seconds) with no time zone	A time control
`number`	Number	A numerical value	A text field or spinner control
`range`	Range	A numerical value, with the extra semantic that the exact value is not important	A slider control or similar
`color`	Color	An sRGB color with 8-bit red, green, and blue components	A color well
`checkbox`	Checkbox	A set of zero or more values from a predefined list	A checkbox
`radio`	Radio Button	An enumerated value	A radio button
`file`	File Upload	Zero or more files each with a MIME type and optionally a file name	A label and a button
`submit`	Submit Button	An enumerated value, with the extra semantic that it must be the last value selected and initiates form submission	A button
`image`	Image Button	A coordinate, relative to a particular image's size, with the extra semantic that it must be the last value selected and initiates form submission	Either a clickable image, or a button
`reset`	Reset Button	n/a	A button
`button`	Button	n/a	A button

The missing value default is the Text state.

Which of the accept, alt, autocomplete, checked, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, list, max, maxlength, min, minlength, multiple, pattern, placeholder, readonly, required, size, src, step, and width content attributes, the checked, files, valueAsDate, valueAsNumber, and list IDL attributes, the select() method, the selectionStart, selectionEnd, and selectionDirection, IDL attributes, the setRangeText() and setSelectionRange() methods, the stepUp() and stepDown() methods, and the input and change events apply to an input element depends on the state of its type attribute. The subsections that define each type also clearly define in normative "bookkeeping" sections which of these feature apply, and which do not apply, to each type. The behavior of these features depends on whether they apply or not, as defined in their various sections (q.v. for content attributes, for APIs, for events).

The following table is non-normative and summarizes which of those content attributes, IDL attributes, methods, and events apply to each state:

	Hidden	Text, Search	URL, Telephone	E-mail	Password	Date, Time	Number	Range	Color	Checkbox, Radio Button	File Upload	Submit Button	Image Button	Reset Button, Button
Content attributes
`accept`	·	·	·	·	·	·	·	·	·	·	Yes	·	·	·
`alt`	·	·	·	·	·	·	·	·	·	·	·	·	Yes	·
`autocomplete`	·	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	·	·	·	·	·
`checked`	·	·	·	·	·	·	·	·	·	Yes	·	·	·	·
`dirname`	·	Yes	·	·	·	·	·	·	·	·	·	·	·	·
`formaction`	·	·	·	·	·	·	·	·	·	·	·	Yes	Yes	·
`formenctype`	·	·	·	·	·	·	·	·	·	·	·	Yes	Yes	·
`formmethod`	·	·	·	·	·	·	·	·	·	·	·	Yes	Yes	·
`formnovalidate`	·	·	·	·	·	·	·	·	·	·	·	Yes	Yes	·
`formtarget`	·	·	·	·	·	·	·	·	·	·	·	Yes	Yes	·
`height`	·	·	·	·	·	·	·	·	·	·	·	·	Yes	·
`inputmode`	·	Yes	·	·	Yes	·	·	·	·	·	·	·	·	·
`list`	·	Yes	Yes	Yes	·	Yes	Yes	Yes	Yes	·	·	·	·	·
`max`	·	·	·	·	·	Yes	Yes	Yes	·	·	·	·	·	·
`maxlength`	·	Yes	Yes	Yes	Yes	·	·	·	·	·	·	·	·	·
`min`	·	·	·	·	·	Yes	Yes	Yes	·	·	·	·	·	·
`minlength`	·	Yes	Yes	Yes	Yes	·	·	·	·	·	·	·	·	·
`multiple`	·	·	·	Yes	·	·	·	·	·	·	Yes	·	·	·
`pattern`	·	Yes	Yes	Yes	Yes	·	·	·	·	·	·	·	·	·
`placeholder`	·	Yes	Yes	Yes	Yes	·	Yes	·	·	·	·	·	·	·
`readonly`	·	Yes	Yes	Yes	Yes	Yes	Yes	·	·	·	·	·	·	·
`required`	·	Yes	Yes	Yes	Yes	Yes	Yes	·	·	Yes	Yes	·	·	·
`size`	·	Yes	Yes	Yes	Yes	·	·	·	·	·	·	·	·	·
`src`	·	·	·	·	·	·	·	·	·	·	·	·	Yes	·
`step`	·	·	·	·	·	Yes	Yes	Yes	·	·	·	·	·	·
`width`	·	·	·	·	·	·	·	·	·	·	·	·	Yes	·
IDL attributes and methods
`checked`	·	·	·	·	·	·	·	·	·	Yes	·	·	·	·
`files`	·	·	·	·	·	·	·	·	·	·	Yes	·	·	·
`value`	default	value	value	value	value	value	value	value	value	default/on	filename	default	default	default
`valueAsDate`	·	·	·	·	·	Yes	·	·	·	·	·	·	·	·
`valueAsNumber`	·	·	·	·	·	Yes	Yes	Yes	·	·	·	·	·	·
`list`	·	Yes	Yes	Yes	·	Yes	Yes	Yes	Yes	·	·	·	·	·
`select()`	·	Yes	Yes	·	Yes	·	·	·	·	·	·	·	·	·
`selectionStart`	·	Yes	Yes	·	Yes	·	·	·	·	·	·	·	·	·
`selectionEnd`	·	Yes	Yes	·	Yes	·	·	·	·	·	·	·	·	·
`selectionDirection`	·	Yes	Yes	·	Yes	·	·	·	·	·	·	·	·	·
`setRangeText()`	·	Yes	Yes	·	Yes	·	·	·	·	·	·	·	·	·
`setSelectionRange()`	·	Yes	Yes	·	Yes	·	·	·	·	·	·	·	·	·
`stepDown()`	·	·	·	·	·	Yes	Yes	Yes	·	·	·	·	·	·
`stepUp()`	·	·	·	·	·	Yes	Yes	Yes	·	·	·	·	·	·
Events
`input` event	·	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	·	·	·
`change` event	·	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	·	·	·

Some states of the type attribute define a value sanitization algorithm.

Each input element has a value, which is exposed by the value IDL attribute. Some states define an algorithm to convert a string to a number, an algorithm to convert a number to a string, an algorithm to convert a string to a Date object, and an algorithm to convert a Date object to a string, which are used by max, min, step, valueAsDate, valueAsNumber, stepDown(), and stepUp().

Each input element has a boolean dirty value flag. The dirty value flag must be initially set to false when the element is created, and must be set to true whenever the user interacts with the control in a way that changes the value. (It is also set to true when the value is programmatically changed, as described in the definition of the value IDL attribute.)

The value content attribute gives the default value of the input element. When the value content attribute is added, set, or removed, if the control's dirty value flag is false, the user agent must set the value of the element to the value of the value content attribute, if there is one, or the empty string otherwise, and then run the current value sanitization algorithm, if one is defined.

Each input element has a checkedness, which is exposed by the checked IDL attribute.

Each input element has a boolean dirty checkedness flag. When it is true, the element is said to have a dirty checkedness. The dirty checkedness flag must be initially set to false when the element is created, and must be set to true whenever the user interacts with the control in a way that changes the checkedness.

The checked content attribute is a boolean attribute that gives the default checkedness of the input element. When the checked content attribute is added, if the control does not have dirty checkedness, the user agent must set the checkedness of the element to true; when the checked content attribute is removed, if the control does not have dirty checkedness, the user agent must set the checkedness of the element to false.

The reset algorithm for input elements is to set the dirty value flag and dirty checkedness flag back to false, set the value of the element to the value of the value content attribute, if there is one, or the empty string otherwise, set the checkedness of the element to true if the element has a checked content attribute and false if it does not, empty the list of selected files, and then invoke the value sanitization algorithm, if the type attribute's current state defines one.

Each input element can be mutable. Except where otherwise specified, an input element is always mutable. Similarly, except where otherwise specified, the user agent should not allow the user to modify the element's value or checkedness.

When an input element is disabled, it is not mutable.

The readonly attribute can also in some cases (e.g. for the Date state, but not the Checkbox state) stop an input element from being mutable.

The cloning steps for input elements must propagate the value, dirty value flag, checkedness, and dirty checkedness flag from the node being cloned to the copy.

When an input element is first created, the element's rendering and behavior must be set to the rendering and behavior defined for the type attribute's state, and the value sanitization algorithm, if one is defined for the type attribute's state, must be invoked.

When an input element's type attribute changes state, the user agent must run the following steps:

If the previous state of the element's type attribute put the value IDL attribute in the value mode, and the element's value is not the empty string, and the new state of the element's type attribute puts the value IDL attribute in either the default mode or the default/on mode, then set the element's value content attribute to the element's value.
Otherwise, if the previous state of the element's type attribute put the value IDL attribute in any mode other than the value mode, and the new state of the element's type attribute puts the value IDL attribute in the value mode, then set the value of the element to the value of the value content attribute, if there is one, or the empty string otherwise, and then set the control's dirty value flag to false.
Update the element's rendering and behavior to the new state's.
Invoke the value sanitization algorithm, if one is defined for the type attribute's new state.

The name attribute represents the element's name. The dirname attribute controls how the element's directionality is submitted. The disabled attribute is used to make the control non-interactive and to prevent its value from being submitted. The form attribute is used to explicitly associate the input element with its form owner. The autofocus attribute controls focus. The autocomplete attribute controls how the user agent provides autofill behavior.

The indeterminate IDL attribute must initially be set to false. On getting, it must return the last value it was set to. On setting, it must be set to the new value. It has no effect except for changing the appearance of checkbox controls.

The accept, alt, max, min, multiple, pattern, placeholder, required, size, src, and step IDL attributes must reflect the respective content attributes of the same name. The dirName IDL attribute must reflect the dirname content attribute. The readOnly IDL attribute must reflect the readonly content attribute. The defaultChecked IDL attribute must reflect the checked content attribute. The defaultValue IDL attribute must reflect the value content attribute.

The type IDL attribute must reflect the respective content attribute of the same name, limited to only known values. The maxLength IDL attribute must reflect the maxlength content attribute, limited to only non-negative numbers. The minLength IDL attribute must reflect the minlength content attribute, limited to only non-negative numbers.

The IDL attributes width and height must return the rendered width and height of the image, in CSS pixels, if an image is being rendered, and is being rendered to a visual medium; or else the intrinsic width and height of the image, in CSS pixels, if an image is available but not being rendered to a visual medium; or else 0, if no image is available. When the input element's type attribute is not in the Image Button state, then no image is available. [CSS]

On setting, they must act as if they reflected the respective content attributes of the same name.

The willValidate, validity, and validationMessage IDL attributes, and the checkValidity(), and setCustomValidity() methods, are part of the constraint validation API. The labels IDL attribute provides a list of the element's labels. The select(), selectionStart, selectionEnd, selectionDirection, setRangeText(), and setSelectionRange() methods and IDL attributes expose the element's text selection. The autofocus, disabled, form, and name IDL attributes are part of the element's forms API.

4.10.5.1 States of the `type` attribute

4.10.5.1.1 Hidden state (`type=hidden`)

Allowed ARIA role attribute values:: None
Allowed ARIA state and property attributes:: Global aria-* attributes

When an input element's type attribute is in the Hidden state, the rules in this section apply.

The input element represents a value that is not intended to be examined or manipulated by the user.

Constraint validation: If an input element's type attribute is in the Hidden state, it is barred from constraint validation.

If the name attribute is present and has a value that is a case-sensitive match for the string "_charset_", then the element's value attribute must be omitted.

The value IDL attribute applies to this element and is in mode default.

The following content attributes must not be specified and do not apply to the element: accept, alt, autocomplete, checked, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, list, max, maxlength, min, minlength, multiple, pattern, placeholder, readonly, required, size, src, step, and width.

The following IDL attributes and methods do not apply to the element: checked, files, list, selectionStart, selectionEnd, selectionDirection, valueAsDate, and valueAsNumber IDL attributes; select(), setRangeText(), setSelectionRange(), stepDown(), and stepUp() methods.

The input and change events do not apply.

4.10.5.1.2 Text (`type=text`) state and Search state (`type=search`)

Allowed ARIA role attribute values:: textbox (default - do not set) or combobox.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Text state or the Search state, the rules in this section apply.

The input element represents a one line plain text edit control for the element's value.

The difference between the Text state and the Search state is primarily stylistic: on platforms where search fields are distinguished from regular text fields, the Search state might result in an appearance consistent with the platform's search fields rather than appearing like a regular text field.

If the element is mutable, its value should be editable by the user. User agents must not allow users to insert "LF" (U+000A) or "CR" (U+000D) characters into the element's value.

If the element is mutable, the user agent should allow the user to change the writing direction of the element, setting it either to a left-to-right writing direction or a right-to-left writing direction. If the user does so, the user agent must then run the following steps:

Set the element's dir attribute to "ltr" if the user selected a left-to-right writing direction, and "rtl" if the user selected a right-to-left writing direction.
Queue a task to fire a simple event that bubbles named input at the input element.

The value attribute, if specified, must have a value that contains no "LF" (U+000A) or "CR" (U+000D) characters.

The value sanitization algorithm is as follows: Strip line breaks from the value.

The following common input element content attributes, IDL attributes, and methods apply to the element: autocomplete, dirname, list, maxlength, minlength, pattern, placeholder, readonly, required, and size content attributes; list, selectionStart, selectionEnd, selectionDirection, and value IDL attributes; select(), setRangeText(), and setSelectionRange() methods.

The value IDL attribute is in mode value.

The input and change events apply.

The following content attributes must not be specified and do not apply to the element: accept, alt, checked, formaction, formenctype, formmethod, formnovalidate, formtarget, height, max, min, multiple, src, step, and width.

The following IDL attributes and methods do not apply to the element: checked, files, valueAsDate, and valueAsNumber IDL attributes; stepDown() and stepUp() methods.

4.10.5.1.3 Telephone state (`type=tel`)

Allowed ARIA role attribute values:: textbox (default - do not set) or combobox.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Telephone state, the rules in this section apply.

The input element represents a control for editing a telephone number given in the element's value.

If the element is mutable, its value should be editable by the user. User agents may change the spacing and, with care, the punctuation of values that the user enters. User agents must not allow users to insert "LF" (U+000A) or "CR" (U+000D) characters into the element's value.

The value attribute, if specified, must have a value that contains no "LF" (U+000A) or "CR" (U+000D) characters.

The value sanitization algorithm is as follows: Strip line breaks from the value.

Unlike the URL and E-mail types, the Telephone type does not enforce a particular syntax. This is intentional; in practice, telephone number fields tend to be free-form fields, because there are a wide variety of valid phone numbers. Systems that need to enforce a particular format are encouraged to use the pattern attribute or the setCustomValidity() method to hook into the client-side validation mechanism.

The following common input element content attributes, IDL attributes, and methods apply to the element: autocomplete, list, maxlength, minlength, pattern, placeholder, readonly, required, and size content attributes; list, selectionStart, selectionEnd, selectionDirection, and value IDL attributes; select(), setRangeText(), and setSelectionRange() methods.

The value IDL attribute is in mode value.

The input and change events apply.

The following content attributes must not be specified and do not apply to the element: accept, alt, checked, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, max, min, multiple, src, step, and width.

The following IDL attributes and methods do not apply to the element: checked, files, valueAsDate, and valueAsNumber IDL attributes; stepDown() and stepUp() methods.

4.10.5.1.4 URL state (`type=url`)

Allowed ARIA role attribute values:: textbox (default - do not set) or combobox.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the URL state, the rules in this section apply.

The input element represents a control for editing a single absolute URL given in the element's value.

If the element is mutable, the user agent should allow the user to change the URL represented by its value. User agents may allow the user to set the value to a string that is not a valid absolute URL, but may also or instead automatically escape characters entered by the user so that the value is always a valid absolute URL (even if that isn't the actual value seen and edited by the user in the interface). User agents should allow the user to set the value to the empty string. User agents must not allow users to insert "LF" (U+000A) or "CR" (U+000D) characters into the value.

The value attribute, if specified and not empty, must have a value that is a valid URL potentially surrounded by spaces that is also an absolute URL.

The value sanitization algorithm is as follows: Strip line breaks from the value, then strip leading and trailing whitespace from the value.

Constraint validation: While the value of the element is neither the empty string nor a valid absolute URL, the element is suffering from a type mismatch.

The value IDL attribute is in mode value.

The input and change events apply.

The following IDL attributes and methods do not apply to the element: checked, files, valueAsDate, and valueAsNumber IDL attributes; stepDown() and stepUp() methods.

If a document contained the following markup:

<input type="url" name="location" list="urls">
<datalist id="urls">
 <option label="MIME: Format of Internet Message Bodies" value="http://tools.ietf.org/html/rfc2045">
 <option label="HTML 4.01 Specification" value="http://www.w3.org/TR/html4/">
 <option label="Form Controls" value="http://www.w3.org/TR/xforms/slice8.html#ui-commonelems-hint">
 <option label="Scalable Vector Graphics (SVG) 1.1 Specification" value="http://www.w3.org/TR/SVG/">
 <option label="Feature Sets - SVG 1.1 - 20030114" value="http://www.w3.org/TR/SVG/feature.html">
 <option label="The Single UNIX Specification, Version 3" value="http://www.unix-systems.org/version3/">
</datalist>

...and the user had typed "www.w3", and the user agent had also found that the user had visited http://www.w3.org/Consortium/#membership and http://www.w3.org/TR/XForms/ in the recent past, then the rendering might look like this:

A text box with an icon on the left followed by the text "www.w3" and a cursor, with a drop down button on the right hand side; with, below, a drop down box containing a list of six URLs on the left, with the first four having grayed out labels on the right; and a scroll bar to the right of the drop down box, indicating further values are available.

The first four URLs in this sample consist of the four URLs in the author-specified list that match the text the user has entered, sorted in some UA-defined manner (maybe by how frequently the user refers to those URLs). Note how the UA is using the knowledge that the values are URLs to allow the user to omit the scheme part and perform intelligent matching on the domain name.

The last two URLs (and probably many more, given the scrollbar's indications of more values being available) are the matches from the user agent's session history data. This data is not made available to the page DOM. In this particular case, the UA has no titles to provide for those values.

4.10.5.1.5 E-mail state (`type=email`)

Allowed ARIA role attribute values:: textbox (default - do not set) or combobox.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the E-mail state, the rules in this section apply.

How the E-mail state operates depends on whether the multiple attribute is specified or not.

When the multiple attribute is not specified on the element

The input element represents a control for editing an e-mail address given in the element's value.

If the element is mutable, the user agent should allow the user to change the e-mail address represented by its value. User agents may allow the user to set the value to a string that is not a valid e-mail address. The user agent should act in a manner consistent with expecting the user to provide a single e-mail address. User agents should allow the user to set the value to the empty string. User agents must not allow users to insert "LF" (U+000A) or "CR" (U+000D) characters into the value. User agents may transform the value for display and editing; in particular, user agents should convert punycode in the value to IDN in the display and vice versa.

Constraint validation: While the user interface is representing input that the user agent cannot convert to punycode, the control is suffering from bad input.

The value attribute, if specified and not empty, must have a value that is a single valid e-mail address.

The value sanitization algorithm is as follows: Strip line breaks from the value, then strip leading and trailing whitespace from the value.

When the multiple attribute is removed, the user agent must run the value sanitization algorithm.

Constraint validation: While the value of the element is neither the empty string nor a single valid e-mail address, the element is suffering from a type mismatch.

When the multiple attribute is specified on the element

The element's values are the result of splitting on commas the element's value.

The input element represents a control for adding, removing, and editing the e-mail addresses given in the element's values.

If the element is mutable, the user agent should allow the user to add, remove, and edit the e-mail addresses represented by its values. User agents may allow the user to set any individual value in the list of values to a string that is not a valid e-mail address, but must not allow users to set any individual value to a string containing "," (U+002C), "LF" (U+000A), or "CR" (U+000D) characters. User agents should allow the user to remove all the addresses in the element's values. User agents may transform the values for display and editing; in particular, user agents should convert punycode in the value to IDN in the display and vice versa.

Constraint validation: While the user interface describes a situation where an individual value contains a "," (U+002C) or is representing input that the user agent cannot convert to punycode, the control is suffering from bad input.

Whenever the user changes the element's values, the user agent must run the following steps:

Let latest values be a copy of the element's values.
Strip leading and trailing whitespace from each value in latest values.
Let the element's value be the result of concatenating all the values in latest values, separating each value from the next by a single "," (U+002C) character, maintaining the list's order.

The value attribute, if specified, must have a value that is a valid e-mail address list.

The value sanitization algorithm is as follows:

Split on commas the element's value, strip leading and trailing whitespace from each resulting token, if any, and let the element's values be the (possibly empty) resulting list of (possibly empty) tokens, maintaining the original order.
Let the element's value be the result of concatenating the element's values, separating each value from the next by a single "," (U+002C) character, maintaining the list's order.

When the multiple attribute is set, the user agent must run the value sanitization algorithm.

Constraint validation: While the value of the element is not a valid e-mail address list, the element is suffering from a type mismatch.

A valid e-mail address is a string that matches the email production of the following ABNF, the character set for which is Unicode. This ABNF implements the extensions described in RFC 1123. [ABNF] [RFC5322] [RFC1034] [RFC1123]

email         = 1*( atext / "." ) "@" label *( "." label )
label         = let-dig [ [ ldh-str ] let-dig ]  ; limited to a length of 63 characters by RFC 1034 section 3.5
atext         = < as defined in RFC 5322 section 3.2.3 >
let-dig       = < as defined in RFC 1034 section 3.5 >
ldh-str       = < as defined in RFC 1034 section 3.5 >

This requirement is a willful violation of RFC 5322, which defines a syntax for e-mail addresses that is simultaneously too strict (before the "@" character), too vague (after the "@" character), and too lax (allowing comments, whitespace characters, and quoted strings in manners unfamiliar to most users) to be of practical use here.

The following JavaScript- and Perl-compatible regular expression is an implementation of the above definition.

/^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/

A valid e-mail address list is a set of comma-separated tokens, where each token is itself a valid e-mail address. To obtain the list of tokens from a valid e-mail address list, and implementation must split the string on commas.

The following common input element content attributes, IDL attributes, and methods apply to the element: autocomplete, list, maxlength, minlength, multiple, pattern, placeholder, readonly, required, and size content attributes; list and value IDL attributes.

The value IDL attribute is in mode value.

The input and change events apply.

The following IDL attributes and methods do not apply to the element: checked, files, selectionStart, selectionEnd, selectionDirection, valueAsDate, and valueAsNumber IDL attributes; select(), setRangeText(), setSelectionRange(), stepDown() and stepUp() methods.

4.10.5.1.6 Password state (`type=password`)

Allowed ARIA role attribute values:: textbox (default - do not set).
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Password state, the rules in this section apply.

The input element represents a one line plain text edit control for the element's value. The user agent should obscure the value so that people other than the user cannot see it.

If the element is mutable, its value should be editable by the user. User agents must not allow users to insert "LF" (U+000A) or "CR" (U+000D) characters into the value.

The value attribute, if specified, must have a value that contains no "LF" (U+000A) or "CR" (U+000D) characters.

The value sanitization algorithm is as follows: Strip line breaks from the value.

The following common input element content attributes, IDL attributes, and methods apply to the element: autocomplete, maxlength, minlength, pattern, placeholder, readonly, required, and size content attributes; selectionStart, selectionEnd, selectionDirection, and value IDL attributes; select(), setRangeText(), and setSelectionRange() methods.

The value IDL attribute is in mode value.

The input and change events apply.

The following content attributes must not be specified and do not apply to the element: accept, alt, checked, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, list, max, min, multiple, src, step, and width.

The following IDL attributes and methods do not apply to the element: checked, files, list, valueAsDate, and valueAsNumber IDL attributes; stepDown() and stepUp() methods.

4.10.5.1.7 Date state (`type=date`)

Allowed ARIA role attribute values:: None
Allowed ARIA state and property attributes:: Global aria-* attributes

When an input element's type attribute is in the Date state, the rules in this section apply.

The input element represents a control for setting the element's value to a string representing a specific date.

If the element is mutable, the user agent should allow the user to change the date represented by its value, as obtained by parsing a date from it. User agents must not allow the user to set the value to a non-empty string that is not a valid date string. If the user agent provides a user interface for selecting a date, then the value must be set to a valid date string representing the user's selection. User agents should allow the user to set the value to the empty string.

Constraint validation: While the user interface describes input that the user agent cannot convert to a valid date string, the control is suffering from bad input.

See the introduction section for a discussion of the difference between the input format and submission format for date, time, and number form controls, and the implementation notes regarding localization of form controls.

The value attribute, if specified and not empty, must have a value that is a valid date string.

The value sanitization algorithm is as follows: If the value of the element is not a valid date string, then set it to the empty string instead.

The min attribute, if specified, must have a value that is a valid date string. The max attribute, if specified, must have a value that is a valid date string.

The step attribute is expressed in days. The step scale factor is 86,400,000 (which converts the days to milliseconds, as used in the other algorithms). The default step is 1 day.

When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest date for which the element would not suffer from a step mismatch.

The algorithm to convert a string to a number, given a string input, is as follows: If parsing a date from input results in an error, then return an error; otherwise, return the number of milliseconds elapsed from midnight UTC on the morning of 1970-01-01 (the time represented by the value "1970-01-01T00:00:00.0Z") to midnight UTC on the morning of the parsed date, ignoring leap seconds.

The algorithm to convert a number to a string, given a number input, is as follows: Return a valid date string that represents the date that, in UTC, is current input milliseconds after midnight UTC on the morning of 1970-01-01 (the time represented by the value "1970-01-01T00:00:00.0Z").

The algorithm to convert a string to a Date object, given a string input, is as follows: If parsing a date from input results in an error, then return an error; otherwise, return a new Date object representing midnight UTC on the morning of the parsed date.

The algorithm to convert a Date object to a string, given a Date object input, is as follows: Return a valid date string that represents the date current at the time represented by input in the UTC time zone.

The following common input element content attributes, IDL attributes, and methods apply to the element: autocomplete, list, max, min, readonly, required, and step content attributes; list, value, valueAsDate, and valueAsNumber IDL attributes; stepDown() and stepUp() methods.

The value IDL attribute is in mode value.

The input and change events apply.

The following IDL attributes and methods do not apply to the element: checked, selectionStart, selectionEnd, and selectionDirection IDL attributes; select(), setRangeText(), and setSelectionRange() methods.

4.10.5.1.8 Time state (`type=time`)

Allowed ARIA role attribute values:: None
Allowed ARIA state and property attributes:: Global aria-* attributes

When an input element's type attribute is in the Time state, the rules in this section apply.

The input element represents a control for setting the element's value to a string representing a specific time.

If the element is mutable, the user agent should allow the user to change the time represented by its value, as obtained by parsing a time from it. User agents must not allow the user to set the value to a non-empty string that is not a valid time string. If the user agent provides a user interface for selecting a time, then the value must be set to a valid time string representing the user's selection. User agents should allow the user to set the value to the empty string.

Constraint validation: While the user interface describes input that the user agent cannot convert to a valid time string, the control is suffering from bad input.

The value attribute, if specified and not empty, must have a value that is a valid time string.

The value sanitization algorithm is as follows: If the value of the element is not a valid time string, then set it to the empty string instead.

The form control has a periodic domain.

The min attribute, if specified, must have a value that is a valid time string. The max attribute, if specified, must have a value that is a valid time string.

The step attribute is expressed in seconds. The step scale factor is 1000 (which converts the seconds to milliseconds, as used in the other algorithms). The default step is 60 seconds.

When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest time for which the element would not suffer from a step mismatch.

The algorithm to convert a string to a number, given a string input, is as follows: If parsing a time from input results in an error, then return an error; otherwise, return the number of milliseconds elapsed from midnight to the parsed time on a day with no time changes.

The algorithm to convert a number to a string, given a number input, is as follows: Return a valid time string that represents the time that is input milliseconds after midnight on a day with no time changes.

The algorithm to convert a string to a Date object, given a string input, is as follows: If parsing a time from input results in an error, then return an error; otherwise, return a new Date object representing the parsed time in UTC on 1970-01-01.

The algorithm to convert a Date object to a string, given a Date object input, is as follows: Return a valid time string that represents the UTC time component that is represented by input.

The value IDL attribute is in mode value.

The input and change events apply.

The following IDL attributes and methods do not apply to the element: checked, files, selectionStart, selectionEnd, and selectionDirection IDL attributes; select(), setRangeText(), and setSelectionRange() methods.

4.10.5.1.9 Number state (`type=number`)

Allowed ARIA role attribute values:: spinbutton (default - do not set).
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Number state, the rules in this section apply.

The input element represents a control for setting the element's value to a string representing a number.

If the element is mutable, the user agent should allow the user to change the number represented by its value, as obtained from applying the rules for parsing floating-point number values to it. User agents must not allow the user to set the value to a non-empty string that is not a valid floating-point number. If the user agent provides a user interface for selecting a number, then the value must be set to the best representation of the number representing the user's selection as a floating-point number. User agents should allow the user to set the value to the empty string.

Constraint validation: While the user interface describes input that the user agent cannot convert to a valid floating-point number, the control is suffering from bad input.

This specification does not define what user interface user agents are to use; user agent vendors are encouraged to consider what would best serve their users' needs. For example, a user agent in Persian or Arabic markets might support Persian and Arabic numeric input (converting it to the format required for submission as described above). Similarly, a user agent designed for Romans might display the value in Roman numerals rather than in decimal; or (more realistically) a user agent designed for the French market might display the value with apostrophes between thousands and commas before the decimals, and allow the user to enter a value in that manner, internally converting it to the submission format described above.

The value attribute, if specified and not empty, must have a value that is a valid floating-point number.

The value sanitization algorithm is as follows: If the value of the element is not a valid floating-point number, then set it to the empty string instead.

The min attribute, if specified, must have a value that is a valid floating-point number. The max attribute, if specified, must have a value that is a valid floating-point number.

The step scale factor is 1. The default step is 1 (allowing only integers to be selected by the user, unless the step base has a non-integer value).

When the element is suffering from a step mismatch, the user agent may round the element's value to the nearest number for which the element would not suffer from a step mismatch. If there are two such numbers, user agents are encouraged to pick the one nearest positive infinity.

The algorithm to convert a string to a number, given a string input, is as follows: If applying the rules for parsing floating-point number values to input results in an error, then return an error; otherwise, return the resulting number.

The algorithm to convert a number to a string, given a number input, is as follows: Return a valid floating-point number that represents input.

The following common input element content attributes, IDL attributes, and methods apply to the element: autocomplete, list, max, min, placeholder, readonly, required, and step content attributes; list, value, and valueAsNumber IDL attributes; stepDown() and stepUp() methods.

The value IDL attribute is in mode value.

The input and change events apply.

The following IDL attributes and methods do not apply to the element: checked, files, selectionStart, selectionEnd, selectionDirection, and valueAsDate IDL attributes; select(), setRangeText(), and setSelectionRange() methods.

Here is an example of using a numeric input control:

<label>How much do you want to charge? $<input type=number min=0 step=0.01 name=price></label>

As described above, a user agent might support numeric input in the user's local format, converting it to the format required for submission as described above. This might include handling grouping separators (as in "872,000,000,000") and various decimal separators (such as "3,99" vs "3.99") or using local digits (such as those in Arabic, Devanagari, Persian, and Thai).

The type=number state is not appropriate for input that happens to only consist of numbers but isn't strictly speaking a number. For example, it would be inappropriate for credit card numbers or US postal codes. A simple way of determining whether to use type=number is to consider whether it would make sense for the input control to have a spinbox interface (e.g. with "up" and "down" arrows). Getting a credit card number wrong by 1 in the last digit isn't a minor mistake, it's as wrong as getting every digit incorrect. So it would not make sense for the user to select a credit card number using "up" and "down" buttons. When a spinbox interface is not appropriate, type=text is probably the right choice (possibly with a pattern attribute).

4.10.5.1.10 Range state (`type=range`)

Allowed ARIA role attribute values:: slider (default - do not set).
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Range state, the rules in this section apply.

The input element represents a control for setting the element's value to a string representing a number, but with the caveat that the exact value is not important, letting UAs provide a simpler interface than they do for the Number state.

In this state, the range and step constraints are enforced even during user input, and there is no way to set the value to the empty string.

If the element is mutable, the user agent should allow the user to change the number represented by its value, as obtained from applying the rules for parsing floating-point number values to it. User agents must not allow the user to set the value to a string that is not a valid floating-point number. If the user agent provides a user interface for selecting a number, then the value must be set to a best representation of the number representing the user's selection as a floating-point number. User agents must not allow the user to set the value to the empty string.

Constraint validation: While the user interface describes input that the user agent cannot convert to a valid floating-point number, the control is suffering from bad input.

The value attribute, if specified, must have a value that is a valid floating-point number.

The value sanitization algorithm is as follows: If the value of the element is not a valid floating-point number, then set it to a valid floating-point number that represents the default value.

The min attribute, if specified, must have a value that is a valid floating-point number. The default minimum is 0. The max attribute, if specified, must have a value that is a valid floating-point number. The default maximum is 100.

The default value is the minimum plus half the difference between the minimum and the maximum, unless the maximum is less than the minimum, in which case the default value is the minimum.

When the element is suffering from an underflow, the user agent must set the element's value to a valid floating-point number that represents the minimum.

When the element is suffering from an overflow, if the maximum is not less than the minimum, the user agent must set the element's value to a valid floating-point number that represents the maximum.

The step scale factor is 1. The default step is 1 (allowing only integers, unless the min attribute has a non-integer value).

When the element is suffering from a step mismatch, the user agent must round the element's value to the nearest number for which the element would not suffer from a step mismatch, and which is greater than or equal to the minimum, and, if the maximum is not less than the minimum, which is less than or equal to the maximum, if there is a number that matches these constraints. If two numbers match these constraints, then user agents must use the one nearest to positive infinity.

For example, the markup <input type="range" min=0 max=100 step=20 value=50> results in a range control whose initial value is 60.

The algorithm to convert a number to a string, given a number input, is as follows: Return a valid floating-point number that represents input.

The following common input element content attributes, IDL attributes, and methods apply to the element: autocomplete, list, max, min, and step content attributes; list, value, and valueAsNumber IDL attributes; stepDown() and stepUp() methods.

The value IDL attribute is in mode value.

The input and change events apply.

Here is an example of a range control using an autocomplete list with the list attribute. This could be useful if there are values along the full range of the control that are especially important, such as preconfigured light levels or typical speed limits in a range control used as a speed control. The following markup fragment:

<input type="range" min="-100" max="100" value="0" step="10" name="power" list="powers">
<datalist id="powers">
 <option value="0">
 <option value="-30">
 <option value="30">
 <option value="++50">
</datalist>

...with the following style sheet applied:

input { height: 75px; width: 49px; background: #D5CCBB; color: black; }

...might render as:

A vertical slider control whose primary color is black and whose background color is beige, with the slider having five tick marks, one long one at each extremity, and three short ones clustered around the midpoint.

Note how the UA determined the orientation of the control from the ratio of the style-sheet-specified height and width properties. The colors were similarly derived from the style sheet. The tick marks, however, were derived from the markup. In particular, the step attribute has not affected the placement of tick marks, the UA deciding to only use the author-specified completion values and then adding longer tick marks at the extremes.

Note also how the invalid value ++50 was completely ignored.

For another example, consider the following markup fragment:

<input name=x type=range min=100 max=700 step=9.09090909 value=509.090909>

A user agent could display in a variety of ways, for instance:

As a dial.

Or, alternatively, for instance:

As a long horizontal slider with tick marks.

The user agent could pick which one to display based on the dimensions given in the style sheet. This would allow it to maintain the same resolution for the tick marks, despite the differences in width.

Finally, here is an example of a range control with two labeled values:

<input type="range" name="a" list="a-values">
<datalist id="a-values">
 <option value="10" label="Low">
 <option value="90" label="High">
</datalist>

With styles that make the control draw vertically, it might look as follows:

A vertical slider control with two tick marks, one near the top labeled 'High', and one near the bottom labeled 'Low'.

4.10.5.1.11 Color state (`type=color`)

Allowed ARIA role attribute values:: None
Allowed ARIA state and property attributes:: Global aria-* attributes

When an input element's type attribute is in the Color state, the rules in this section apply.

The input element represents a color well control, for setting the element's value to a string representing a simple color.

In this state, there is always a color picked, and there is no way to set the value to the empty string.

If the element is mutable, the user agent should allow the user to change the color represented by its value, as obtained from applying the rules for parsing simple color values to it. User agents must not allow the user to set the value to a string that is not a valid lowercase simple color. If the user agent provides a user interface for selecting a color, then the value must be set to the result of using the rules for serializing simple color values to the user's selection. User agents must not allow the user to set the value to the empty string.

Constraint validation: While the user interface describes input that the user agent cannot convert to a valid lowercase simple color, the control is suffering from bad input.

The value attribute, if specified and not empty, must have a value that is a valid simple color.

The value sanitization algorithm is as follows: If the value of the element is a valid simple color, then set it to the value of the element converted to ASCII lowercase; otherwise, set it to the string "#000000".

The following common input element content attributes and IDL attributes apply to the element: autocomplete and list content attributes; list and value IDL attributes.

The value IDL attribute is in mode value.

The input and change events apply.

4.10.5.1.12 Checkbox state (`type=checkbox`)

Allowed ARIA role attribute values:: checkbox (default - do not set) or menuitemcheckbox.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Checkbox state, the rules in this section apply.

The input element represents a two-state control that represents the element's checkedness state. If the element's checkedness state is true, the control represents a positive selection, and if it is false, a negative selection. If the element's indeterminate IDL attribute is set to true, then the control's selection should be obscured as if the control was in a third, indeterminate, state.

The control is never a true tri-state control, even if the element's indeterminate IDL attribute is set to true. The indeterminate IDL attribute only gives the appearance of a third state.

If the element is mutable, then: The pre-click activation steps consist of setting the element's checkedness to its opposite value (i.e. true if it is false, false if it is true), and of setting the element's indeterminate IDL attribute to false. The canceled activation steps consist of setting the checkedness and the element's indeterminate IDL attribute back to the values they had before the pre-click activation steps were run. The activation behavior is to fire a simple event that bubbles named input at the element and then fire a simple event that bubbles named change at the element.

If the element is not mutable, it has no activation behavior.

Constraint validation: If the element is required and its checkedness is false, then the element is suffering from being missing.

input . indeterminate [ = value ]: When set, overrides the rendering of checkbox controls so that the current value is not visible.

The following common input element content attributes and IDL attributes apply to the element: checked, and required content attributes; checked and value IDL attributes.

The value IDL attribute is in mode default/on.

The input and change events apply.

The following content attributes must not be specified and do not apply to the element: accept, alt, autocomplete, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, list, max, maxlength, min, minlength, multiple, pattern, placeholder, readonly, size, src, step, and width.

The following IDL attributes and methods do not apply to the element: files, list, selectionStart, selectionEnd, selectionDirection, valueAsDate, and valueAsNumber IDL attributes; select(), setRangeText(), setSelectionRange(), stepDown(), and stepUp() methods.

4.10.5.1.13 Radio Button state (`type=radio`)

Allowed ARIA role attribute values:: radio (default - do not set) or menuitemradio.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Radio Button state, the rules in this section apply.

The input element represents a control that, when used in conjunction with other input elements, forms a radio button group in which only one control can have its checkedness state set to true. If the element's checkedness state is true, the control represents the selected control in the group, and if it is false, it indicates a control in the group that is not selected.

The radio button group that contains an input element a also contains all the other input elements b that fulfill all of the following conditions:

The input element b's type attribute is in the Radio Button state.
Either a and b have the same form owner, or they both have no form owner.
Both a and b are in the same home subtree.
They both have a name attribute, their name attributes are not empty, and the value of a's name attribute is a compatibility caseless match for the value of b's name attribute.

A document must not contain an input element whose radio button group contains only that element.

When any of the following phenomena occur, if the element's checkedness state is true after the occurrence, the checkedness state of all the other elements in the same radio button group must be set to false:

The element's checkedness state is set to true (for whatever reason).
The element's name attribute is set, changed, or removed.
The element's form owner changes.

If the element is mutable, then: The pre-click activation steps consist of setting the element's checkedness to true. The canceled activation steps consist of setting the element's checkedness to false. The activation behavior is to fire a simple event that bubbles named input at the element and then fire a simple event that bubbles named change at the element. .

If the element is not mutable, it has no activation behavior.

Constraint validation: If an element in the radio button group is required, and all of the input elements in the radio button group have a checkedness that is false, then the element is suffering from being missing.

If none of the radio buttons in a radio button group are checked when they are inserted into the document, then they will all be initially unchecked in the interface, until such time as one of them is checked (either by the user or by script).

The following common input element content attributes and IDL attributes apply to the element: checked and required content attributes; checked and value IDL attributes.

The value IDL attribute is in mode default/on.

The input and change events apply.

4.10.5.1.14 File Upload state (`type=file`)

Allowed ARIA role attribute values:: None
Allowed ARIA state and property attributes:: Global aria-* attributes

When an input element's type attribute is in the File Upload state, the rules in this section apply.

The input element represents a list of selected files, each file consisting of a file name, a file type, and a file body (the contents of the file).

File names must not contain path components, even in the case that a user has selected an entire directory hierarchy or multiple files with the same name from different directories.

Unless the multiple attribute is set, there must be no more than one file in the list of selected files.

If the element is mutable, then the element's activation behavior is to run the following steps:

If the algorithm is not allowed to show a popup, then abort these steps without doing anything else.
Return, but continue running these steps asynchronously.
Optionally, wait until any prior execution of this algorithm has terminated.
Display a prompt to the user requesting that the user specify some files. If the multiple attribute is not set, there must be no more than one file selected; otherwise, any number may be selected. Files can be from the filesystem or created on the fly, e.g. a picture taken from a camera connected to the user's device.
Wait for the user to have made their selection.
Queue a task to first update the element's selected files so that it represents the user's selection, then fire a simple event that bubbles named input at the input element, and finally fire a simple event that bubbles named change at the input element.

If the element is mutable, the user agent should allow the user to change the files on the list in other ways also, e.g. adding or removing files by drag-and-drop. When the user does so, the user agent must queue a task to first update the element's selected files so that it represents the user's new selection, then fire a simple event that bubbles named input at the input element, and finally fire a simple event that bubbles named change at the input element.

If the element is not mutable, it has no activation behavior and the user agent must not allow the user to change the element's selection.

Constraint validation: If the element is required and the list of selected files is empty, then the element is suffering from being missing.

The accept attribute may be specified to provide user agents with a hint of what file types will be accepted.

If specified, the attribute must consist of a set of comma-separated tokens, each of which must be an ASCII case-insensitive match for one of the following:

The string audio/*: Indicates that sound files are accepted.
The string video/*: Indicates that video files are accepted.
The string image/*: Indicates that image files are accepted.
A valid MIME type with no parameters: Indicates that files of the specified type are accepted.
A string whose first character is a "." (U+002E) character: Indicates that files with the specified file extension are accepted.

The tokens must not be ASCII case-insensitive matches for any of the other tokens (i.e. duplicates are not allowed). To obtain the list of tokens from the attribute, the user agent must split the attribute value on commas.

User agents may use the value of this attribute to display a more appropriate user interface than a generic file picker. For instance, given the value image/*, a user agent could offer the user the option of using a local camera or selecting a photograph from their photo collection; given the value audio/*, a user agent could offer the user the option of recording a clip using a headset microphone.

User agents should prevent the user from selecting files that are not accepted by one (or more) of these tokens.

Authors are encouraged to specify both any MIME types and any corresponding extensions when looking for data in a specific format.

For example, consider an application that converts Microsoft Word documents to Open Document Format files. Since Microsoft Word documents are described with a wide variety of MIME types and extensions, the site can list several, as follows:

<input type="file" accept=".doc,.docx,.xml,application/msword,application/vnd.openxmlformats-officedocument.wordprocessingml.document">

On platforms that only use file extensions to describe file types, the extensions listed here can be used to filter the allowed documents, while the MIME types can be used with the system's type registration table (mapping MIME types to extensions used by the system), if any, to determine any other extensions to allow. Similarly, on a system that does not have file names or extensions but labels documents with MIME types internally, the MIME types can be used to pick the allowed files, while the extensions can be used if the system has an extension registration table that maps known extensions to MIME types used by the system.

Extensions tend to be ambiguous (e.g. there are an untold number of formats that use the ".dat" extension, and users can typically quite easily rename their files to have a ".doc" extension even if they are not Microsoft Word documents), and MIME types tend to be unreliable (e.g. many formats have no formally registered types, and many formats are in practice labeled using a number of different MIME types). Authors are reminded that, as usual, data received from a client should be treated with caution, as it may not be in an expected format even if the user is not hostile and the user agent fully obeyed the accept attribute's requirements.

For historical reasons, the value IDL attribute prefixes the file name with the string "C:\fakepath\". Some legacy user agents actually included the full path (which was a security vulnerability). As a result of this, obtaining the file name from the value IDL attribute in a backwards-compatible way is non-trivial. The following function extracts the file name in a suitably compatible manner:

function extractFilename(path) {
  if (path.substr(0, 12) == "C:\\fakepath\\")
    return path.substr(12); // modern browser
  var x;
  x = path.lastIndexOf('/');
  if (x >= 0) // Unix-based path
    return path.substr(x+1);
  x = path.lastIndexOf('\\');
  if (x >= 0) // Windows-based path
    return path.substr(x+1);
  return path; // just the file name
}

This can be used as follows:

<p><input type=file name=image onchange="updateFilename(this.value)"></p>
<p>The name of the file you picked is: <span id="filename">(none)</span></p>
<script>
 function updateFilename(path) {
   var name = extractFilename(path);
   document.getElementById('filename').textContent = name;
 }
</script>

The following common input element content attributes and IDL attributes apply to the element: accept, multiple, and required content attributes; files and value IDL attributes.

The value IDL attribute is in mode filename.

The input and change events apply.

The following content attributes must not be specified and do not apply to the element: alt, autocomplete, checked, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, list, max, maxlength, min, minlength, pattern, placeholder, readonly, size, src, step, and width.

The element's value attribute must be omitted.

The following IDL attributes and methods do not apply to the element: checked, list, selectionStart, selectionEnd, selectionDirection, valueAsDate, and valueAsNumber IDL attributes; select(), setRangeText(), setSelectionRange(), stepDown(), and stepUp() methods.

4.10.5.1.15 Submit Button state (`type=submit`)

Allowed ARIA role attribute values:: button (default - do not set).
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Submit Button state, the rules in this section apply.

The input element represents a button that, when activated, submits the form. If the element has a value attribute, the button's label must be the value of that attribute; otherwise, it must be an implementation-defined string that means "Submit" or some such. The element is a button, specifically a submit button.

If the element is mutable, then the element's activation behavior is as follows: if the element has a form owner, and the element's Document is fully active, submit the form owner from the input element; otherwise, do nothing.

If the element is not mutable, it has no activation behavior.

The formaction, formenctype, formmethod, formnovalidate, and formtarget attributes are attributes for form submission.

The formnovalidate attribute can be used to make submit buttons that do not trigger the constraint validation.

The following common input element content attributes and IDL attributes apply to the element: formaction, formenctype, formmethod, formnovalidate, and formtarget content attributes; value IDL attribute.

The value IDL attribute is in mode default.

The following content attributes must not be specified and do not apply to the element: accept, alt, autocomplete, checked, dirname, height, list, max, maxlength, min, minlength, multiple, pattern, placeholder, readonly, required, size, src, step, and width.

The input and change events do not apply.

4.10.5.1.16 Image Button state (`type=image`)

Allowed ARIA role attribute values:: button (default - do not set), link, menuitem, menuitemcheckbox, menuitemradio or radio.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Image Button state, the rules in this section apply.

The input element represents either an image from which a user can select a coordinate and submit the form, or alternatively a button from which the user can submit the form. The element is a button, specifically a submit button.

The coordinate is sent to the server during form submission by sending two entries for the element, derived from the name of the control but with ".x" and ".y" appended to the name with the x and y components of the coordinate respectively.

The image is given by the src attribute. The src attribute must be present, and must contain a valid non-empty URL potentially surrounded by spaces referencing a non-interactive, optionally animated, image resource that is neither paged nor scripted.

When any of the following events occur, unless the user agent cannot support images, or its support for images has been disabled, or the user agent only fetches elements on demand, or the src attribute's value is the empty string, the user agent must resolve the value of the src attribute, relative to the element, and if that is successful, must fetch the resulting absolute URL:

The input element's type attribute is first set to the Image Button state (possibly when the element is first created), and the src attribute is present.
The input element's type attribute is changed back to the Image Button state, and the src attribute is present, and its value has changed since the last time the type attribute was in the Image Button state.
The input element's type attribute is in the Image Button state, and the src attribute is set or changed.

Fetching the image must delay the load event of the element's document until the task that is queued by the networking task source once the resource has been fetched (defined below) has been run.

If the image was successfully obtained, with no network errors, and the image's type is a supported image type, and the image is a valid image of that type, then the image is said to be available. If this is true before the image is completely downloaded, each task that is queued by the networking task source while the image is being fetched must update the presentation of the image appropriately.

User agents must not support non-image resources with the input element. User agents must not run executable code embedded in the image resource. User agents must only display the first page of a multipage resource. User agents must not allow the resource to act in an interactive fashion, but should honor any animation in the resource.

The task that is queued by the networking task source once the resource has been fetched, must, if the download was successful and the image is available, queue a task to fire a simple event named load at the input element; and otherwise, if the fetching process fails without a response from the remote server, or completes but the image is not a valid or supported image, queue a task to fire a simple event named error on the input element.

The alt attribute provides the textual label for the button for users and user agents who cannot use the image. The alt attribute must be present, and must contain a non-empty string giving the label that would be appropriate for an equivalent button if the image was unavailable.

The input element supports dimension attributes.

If the src attribute is set, and the image is available and the user agent is configured to display that image, then: The element represents a control for selecting a coordinate from the image specified by the src attribute; if the element is mutable, the user agent should allow the user to select this coordinate, and the element's activation behavior is as follows: if the element has a form owner, and the element's Document is fully active, take the user's selected coordinate, and submit the input element's form owner from the input element. If the user activates the control without explicitly selecting a coordinate, then the coordinate (0,0) must be assumed.

Otherwise, the element represents a submit button whose label is given by the value of the alt attribute; if the element is mutable, then the element's activation behavior is as follows: if the element has a form owner, and the element's Document is fully active, set the selected coordinate to (0,0), and submit the input element's form owner from the input element.

In either case, if the element is mutable but has no form owner or the element's Document is not fully active, then its activation behavior must be to do nothing. If the element is not mutable, it has no activation behavior.

The selected coordinate must consist of an x-component and a y-component. The coordinates represent the position relative to the edge of the image, with the coordinate space having the positive x direction to the right, and the positive y direction downwards.

The x-component must be a valid integer representing a number x in the range −(border_left+padding_left) ≤ x ≤ width+border_right+padding_right, where width is the rendered width of the image, border_left is the width of the border on the left of the image, padding_left is the width of the padding on the left of the image, border_right is the width of the border on the right of the image, and padding_right is the width of the padding on the right of the image, with all dimensions given in CSS pixels.

The y-component must be a valid integer representing a number y in the range −(border_top+padding_top) ≤ y ≤ height+border_bottom+padding_bottom, where height is the rendered height of the image, border_top is the width of the border above the image, padding_top is the width of the padding above the image, border_bottom is the width of the border below the image, and padding_bottom is the width of the padding below the image, with all dimensions given in CSS pixels.

Where a border or padding is missing, its width is zero CSS pixels.

The formaction, formenctype, formmethod, formnovalidate, and formtarget attributes are attributes for form submission.

image . width [ = value ]

image . height [ = value ]

These attributes return the actual rendered dimensions of the image, or zero if the dimensions are not known.

They can be set, to change the corresponding content attributes.

The following common input element content attributes and IDL attributes apply to the element: alt, formaction, formenctype, formmethod, formnovalidate, formtarget, height, src, and width content attributes; value IDL attribute.

The value IDL attribute is in mode default.

The following content attributes must not be specified and do not apply to the element: accept, autocomplete, checked, dirname, list, max, maxlength, min, minlength, multiple, pattern, placeholder, readonly, required, size, and step.

The element's value attribute must be omitted.

The input and change events do not apply.

Many aspects of this state's behavior are similar to the behavior of the img element. Readers are encouraged to read that section, where many of the same requirements are described in more detail.

Take the following form:

<form action="process.cgi">
 <input type=image src=map.png name=where alt="Show location list">
</form>

If the user clicked on the image at coordinate (127,40) then the URL used to submit the form would be "process.cgi?where.x=127&where.y=40".

(In this example, it's assumed that for users who don't see the map, and who instead just see a button labeled "Show location list", clicking the button will cause the server to show a list of locations to pick from instead of the map.)

4.10.5.1.17 Reset Button state (`type=reset`)

Allowed ARIA role attribute values:: button (default - do not set).
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Reset Button state, the rules in this section apply.

The input element represents a button that, when activated, resets the form. If the element has a value attribute, the button's label must be the value of that attribute; otherwise, it must be an implementation-defined string that means "Reset" or some such. The element is a button.

If the element is mutable, then the element's activation behavior, if the element has a form owner and the element's Document is fully active, is to reset the form owner; otherwise, it is to do nothing.

If the element is not mutable, it has no activation behavior.

Constraint validation: The element is barred from constraint validation.

The value IDL attribute applies to this element and is in mode default.

The input and change events do not apply.

4.10.5.1.18 Button state (`type=button`)

Allowed ARIA role attribute values:: button (default - do not set), link, menuitem, menuitemcheckbox, menuitemradio or radio.
Allowed ARIA state and property attributes:: Global aria-* attributes; Any aria-* attributes applicable to the allowed roles.

When an input element's type attribute is in the Button state, the rules in this section apply.

The input element represents a button with no default behavior. A label for the button must be provided in the value attribute, though it may be the empty string. If the element has a value attribute, the button's label must be the value of that attribute; otherwise, it must be the empty string. The element is a button.

If the element is mutable, the element's activation behavior is to do nothing.

If the element is not mutable, it has no activation behavior.

Constraint validation: The element is barred from constraint validation.

The value IDL attribute applies to this element and is in mode default.

The input and change events do not apply.

4.10.5.2 Implemention notes regarding localization of form controls

This section is non-normative.

The formats shown to the user in date, time, and number controls is independent of the format used for form submission.

Browsers are encouraged to use user interfaces that present dates, times, and numbers according to the conventions of either the locale implied by the input element's language or the user's preferred locale. Using the page's locale will ensure consistency with page-provided data.

For example, it would be confusing to users if an American English page claimed that a Cirque De Soleil show was going to be showing on 02/03, but their browser, configured to use the British English locale, only showed the date 03/02 in the ticket purchase date picker. Using the page's locale would at least ensure that the date was presented in the same format everywhere. (There's still a risk that the user would end up arriving a month late, of course, but there's only so much that can be done about such cultural differences...)

4.10.5.3 Common `input` element attributes

These attributes only apply to an input element if its type attribute is in a state whose definition declares that the attribute applies. When an attribute doesn't apply to an input element, user agents must ignore the attribute, regardless of the requirements and definitions below.

4.10.5.3.1 The `maxlength` and `minlength` attributes

The maxlength attribute, when it applies, is a form control maxlength attribute controlled by the input element's dirty value flag.

The minlength attribute, when it applies, is a form control minlength attribute controlled by the input element's dirty value flag.

If the input element has a maximum allowed value length, then the code-unit length of the value of the element's value attribute must be equal to or less than the element's maximum allowed value length.

The following extract shows how a messaging client's text entry could be arbitrarily restricted to a fixed number of characters, thus forcing any conversation through this medium to be terse and discouraging intelligent discourse.

<label>What are you doing? <input name=status maxlength=140></label>

Here, a password is given a minimum length:

<p><label>Username: <input name=u required></label>
<p><label>Password: <input name=p required minlength=12></label>

4.10.5.3.2 The `size` attribute

The size attribute gives the number of characters that, in a visual rendering, the user agent is to allow the user to see while editing the element's value.

The size attribute, if specified, must have a value that is a valid non-negative integer greater than zero.

If the attribute is present, then its value must be parsed using the rules for parsing non-negative integers, and if the result is a number greater than zero, then the user agent should ensure that at least that many characters are visible.

The size IDL attribute is limited to only non-negative numbers greater than zero and has a default value of 20.

4.10.5.3.3 The `readonly` attribute

The readonly attribute is a boolean attribute that controls whether or not the user can edit the form control. When specified, the element is not mutable.

Constraint validation: If the readonly attribute is specified on an input element, the element is barred from constraint validation.

The difference between disabled and readonly is that read-only controls are still focusable, so the user can still select the text and interact with it, whereas disabled controls are entirely non-interactive. (For this reason, only text controls can be made read-only: it wouldn't make sense for checkboxes or buttons, for instances.)

In the following example, the existing product identifiers cannot be modified, but they are still displayed as part of the form, for consistency with the row representing a new product (where the identifier is not yet filled in).

<form action="products.cgi" method="post" enctype="multipart/form-data">
 <table>
  <tr> <th> Product ID <th> Product name <th> Price <th> Action
  <tr>
   <td> <input readonly="readonly" name="1.pid" value="H412">
   <td> <input required="required" name="1.pname" value="Floor lamp Ulke">
   <td> $<input required="required" type="number" min="0" step="0.01" name="1.pprice" value="49.99">
   <td> <button formnovalidate="formnovalidate" name="action" value="delete:1">Delete</button>
  <tr>
   <td> <input readonly="readonly" name="2.pid" value="FG28">
   <td> <input required="required" name="2.pname" value="Table lamp Ulke">
   <td> $<input required="required" type="number" min="0" step="0.01" name="2.pprice" value="24.99">
   <td> <button formnovalidate="formnovalidate" name="action" value="delete:2">Delete</button>
  <tr>
   <td> <input required="required" name="3.pid" value="" pattern="[A-Z0-9]+">
   <td> <input required="required" name="3.pname" value="">
   <td> $<input required="required" type="number" min="0" step="0.01" name="3.pprice" value="">
   <td> <button formnovalidate="formnovalidate" name="action" value="delete:3">Delete</button>
 </table>
 <p> <button formnovalidate="formnovalidate" name="action" value="add">Add</button> </p>
 <p> <button name="action" value="update">Save</button> </p>
</form>

4.10.5.3.4 The `required` attribute

The required attribute is a boolean attribute. When specified, the element is required.

Constraint validation: If the element is required, and its value IDL attribute applies and is in the mode value, and the element is mutable, and the element's value is the empty string, then the element is suffering from being missing.

The following form has two required fields, one for an e-mail address and one for a password. It also has a third field that is only considered valid if the user types the same password in the password field and this third field.

<h1>Create new account</h1>
<form action="/newaccount" method=post
      oninput="up2.setCustomValidity(up2.value != up.value ? 'Passwords do not match.' : '')">
 <p>
  <label for="username">E-mail address:</label>
  <input id="username" type=email required name=un>
 <p>
  <label for="password1">Password:</label>
  <input id="password1" type=password required name=up>
 <p>
  <label for="password2">Confirm password:</label>
  <input id="password2" type=password name=up2>
 <p>
  <input type=submit value="Create account">
</form>

For radio buttons, the required attribute is satisfied if any of the radio buttons in the group is selected. Thus, in the following example, any of the radio buttons can be checked, not just the one marked as required:

<fieldset>
 <legend>Did the movie pass the Bechdel test?</legend>
 <p><label><input type="radio" name="bechdel" value="no-characters"> No, there are not even two female characters in the movie. </label>
 <p><label><input type="radio" name="bechdel" value="no-names"> No, the female characters never talk to each other. </label>
 <p><label><input type="radio" name="bechdel" value="no-topic"> No, when female characters talk to each other it's always about a male character. </label>
 <p><label><input type="radio" name="bechdel" value="yes" required> Yes. </label>
 <p><label><input type="radio" name="bechdel" value="unknown"> I don't know. </label>
</fieldset>

To avoid confusion as to whether a radio button group is required or not, authors are encouraged to specify the attribute on all the radio buttons in a group. Indeed, in general, authors are encouraged to avoid having radio button groups that do not have any initially checked controls in the first place, as this is a state that the user cannot return to, and is therefore generally considered a poor user interface.

4.10.5.3.5 The `multiple` attribute

The multiple attribute is a boolean attribute that indicates whether the user is to be allowed to specify more than one value.

The following extract shows how an e-mail client's "Cc" field could accept multiple e-mail addresses.

<label>Cc: <input type=email multiple name=cc></label>

If the user had, amongst many friends in his user contacts database, two friends "Arthur Dent" (with address "art@example.net") and "Adam Josh" (with address "adamjosh@example.net"), then, after the user has typed "a", the user agent might suggest these two e-mail addresses to the user.

Form control group containing 'Send',
'Save now' and 'Discard' buttons, a 'To:' combo box with an 'a' displayed in the text box and 2 list items below.

The page could also link in the user's contacts database from the site:

<label>Cc: <input type=email multiple name=cc list=contacts></label>
...
<datalist id="contacts">
 <option value="hedral@damowmow.com">
 <option value="pillar@example.com">
 <option value="astrophy@cute.example">
 <option value="astronomy@science.example.org">
</datalist>

Suppose the user had entered "bob@example.net" into this text field, and then started typing a second e-mail address starting with "a". The user agent might show both the two friends mentioned earlier, as well as the "astrophy" and "astronomy" values given in the datalist element.

Form control group containing 'send',
'save now' and 'discard' buttons and a 'To:' combo box with 'bob@example.net,a' displayed in the text box and 4 list items below.

The following extract shows how an e-mail client's "Attachments" field could accept multiple files for upload.

<label>Attachments: <input type=file multiple name=att></label>

4.10.5.3.6 The `pattern` attribute

The pattern attribute specifies a regular expression against which the control's value, or, when the multiple attribute applies and is set, the control's values, are to be checked.

If specified, the attribute's value must match the JavaScript Pattern production. [ECMA262]

If an input element has a pattern attribute specified, and the attribute's value, when compiled as a JavaScript regular expression with the global, ignoreCase, and multiline flags disabled (see ECMA262 Edition 5, sections 15.10.7.2 through 15.10.7.4), compiles successfully, then the resulting regular expression is the element's compiled pattern regular expression. If the element has no such attribute, or if the value doesn't compile successfully, then the element has no compiled pattern regular expression. [ECMA262]

Constraint validation: If the element's value is not the empty string, and either the element's multiple attribute is not specified or it does not apply to the input element given its type attribute's current state, and the element has a compiled pattern regular expression but that regular expression does not match the entirety of the element's value, then the element is suffering from a pattern mismatch.

Constraint validation: If the element's value is not the empty string, and the element's multiple attribute is specified and applies to the input element, and the element has a compiled pattern regular expression but that regular expression does not match the entirety of each of the element's values, then the element is suffering from a pattern mismatch.

The compiled pattern regular expression, when matched against a string, must have its start anchored to the start of the string and its end anchored to the end of the string.

This implies that the regular expression language used for this attribute is the same as that used in JavaScript, except that the pattern attribute is matched against the entire value, not just any subset (somewhat as if it implied a ^(?: at the start of the pattern and a )$ at the end).

When an input element has a pattern attribute specified, authors should provide a description of the pattern in text near the control. Authors may also include a title attribute to give a description of the pattern. User agents may use the contents of this attribute, if it is present, when informing the user that the pattern is not matched, or at any other suitable time, such as in a tooltip or read out by assistive technology when the control gains focus.

Relying on the title attribute alone is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification (e.g. requiring a pointing device such as a mouse to cause a tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a modern phone or tablet).

For example, the following snippet includes the pattern description in text below the input, the pattern description is also included in the title attribute:

<label> Part number:
        <input pattern="[0-9][A-Z]{3}" name="part"
        data-x="A part number is a digit followed by three uppercase letters."/>
        </label>
        <p>A part number is a digit followed by three uppercase letters.</p>

The presence of the pattern description in text makes the advice available to any user regardless of device.

The presence of the pattern description in the title attribute, results in the description being announced by assistive technology such as screen readers when the input receives focus.

If the user has attempted to submit the form with incorrect information, the presence of the title attribute text could also cause the UA to display an alert such as:

A part number is a digit followed by three uppercase letters.
    You cannot submit this form when the field is incorrect.

In this example, the pattern description is in text below the input, but not in the title attribute. The aria-describedby attribute is used to explicitly associate the text description with the control, the description is announced by assistive technology such as screen readers when the input receives focus:

<label> Part number:
        <input pattern="[0-9][A-Z]{3}" name="part" aria-describedby="description">
        </label>
        <p id="description">A part number is a digit followed by three uppercase letters.</p>

When a control has a pattern attribute, the title attribute, if used, must describe the pattern. Additional information could also be included, so long as it assists the user in filling in the control. Otherwise, assistive technology would be impaired.

For instance, if the title attribute contained the caption of the control, assistive technology could end up saying something like The text you have entered does not match the required pattern. Birthday, which is not useful.

UAs may still show the title in non-error situations (for example, as a tooltip when hovering over the control), so authors should be careful not to word titles as if an error has necessarily occurred.

4.10.5.3.7 The `min` and `max` attributes

Some form controls can have explicit constraints applied limiting the allowed range of values that the user can provide. Normally, such a range would be linear and continuous. A form control can have a periodic domain, however, in which case the form control's broadest possible range is finite, and authors can specify explicit ranges within it that span the boundaries.

Specifically, the broadest range of a type=time control is midnight to midnight (24 hours), and authors can set both continuous linear ranges (such as 9pm to 11pm) and discontinuous ranges spanning midnight (such as 11pm to 1am).

The min and max attributes indicate the allowed range of values for the element.

Their syntax is defined by the section that defines the type attribute's current state.

If the element has a min attribute, and the result of applying the algorithm to convert a string to a number to the value of the min attribute is a number, then that number is the element's minimum; otherwise, if the type attribute's current state defines a default minimum, then that is the minimum; otherwise, the element has no minimum.

The min attribute also defines the step base.

If the element has a max attribute, and the result of applying the algorithm to convert a string to a number to the value of the max attribute is a number, then that number is the element's maximum; otherwise, if the type attribute's current state defines a default maximum, then that is the maximum; otherwise, the element has no maximum.

If the element does not have a periodic domain, the max attribute's value (the maximum) must not be less than the min attribute's value (its minimum).

If an element that does not have a periodic domain has a maximum that is less than its minimum, then so long as the element has a value, it will either be suffering from an underflow or suffering from an overflow.

An element has a reversed range if it has a periodic domain and its maximum is less than its minimum.

An element has range limitations if it has a defined minimum or a defined maximum.

Constraint validation: When the element has a minimum and does not have a reversed range, and the result of applying the algorithm to convert a string to a number to the string given by the element's value is a number, and the number obtained from that algorithm is less than the minimum, the element is suffering from an underflow.

Constraint validation: When the element has a maximum and does not have a reversed range, and the result of applying the algorithm to convert a string to a number to the string given by the element's value is a number, and the number obtained from that algorithm is more than the maximum, the element is suffering from an overflow.

Constraint validation: When an element has a reversed range, and the result of applying the algorithm to convert a string to a number to the string given by the element's value is a number, and the number obtained from that algorithm is more than the maximum and less than the minimum, the element is simultaneously suffering from an underflow and suffering from an overflow.

The following date control limits input to dates that are before the 1980s:

<input name=bday type=date max="1979-12-31">

The following number control limits input to whole numbers greater than zero:

<input name=quantity required="" type="number" min="1" value="1">

The following time control limits input to those minutes that occur between 9pm and 6am, defaulting to midnight:

<input name="sleepStart" type=time min="21:00" max="06:00" step="60" value="00:00">

4.10.5.3.8 The `step` attribute

The step attribute indicates the granularity that is expected (and required) of the value, by limiting the allowed values. The section that defines the type attribute's current state also defines the default step, the step scale factor, and in some cases the default step base, which are used in processing the attribute as described below.

The step attribute, if specified, must either have a value that is a valid floating-point number that parses to a number that is greater than zero, or must have a value that is an ASCII case-insensitive match for the string "any".

The attribute provides the allowed value step for the element, as follows:

If the attribute is absent, then the allowed value step is the default step multiplied by the step scale factor.
Otherwise, if the attribute's value is an ASCII case-insensitive match for the string "any", then there is no allowed value step.
Otherwise, if the rules for parsing floating-point number values, when they are applied to the attribute's value, return an error, zero, or a number less than zero, then the allowed value step is the default step multiplied by the step scale factor.
Otherwise, the allowed value step is the number returned by the rules for parsing floating-point number values when they are applied to the attribute's value, multiplied by the step scale factor.

The step base is the value returned by the following algorithm:

If the element has a min content attribute, and the result of applying the algorithm to convert a string to a number to the value of the min content attribute is not an error, then return that result and abort these steps.
If the element has a value content attribute, and the result of applying the algorithm to convert a string to a number to the value of the value content attribute is not an error, then return that result and abort these steps.
If a default step base is defined for this element given its type attribute's state, then return it and abort these steps.
Return zero.

Constraint validation: When the element has an allowed value step, and the result of applying the algorithm to convert a string to a number to the string given by the element's value is a number, and that number subtracted from the step base is not an integral multiple of the allowed value step, the element is suffering from a step mismatch.

The following range control only accepts values in the range 0..1, and allows 256 steps in that range:

<input name=opacity type=range min=0 max=1 step=0.00392156863>

The following control allows any time in the day to be selected, with any accuracy (e.g. thousandth-of-a-second accuracy or more):

<input name=favtime type=time step=any>

Normally, time controls are limited to an accuracy of one minute.

4.10.5.3.9 The `list` attribute

The list attribute is used to identify an element that lists predefined options suggested to the user.

If present, its value must be the ID of a datalist element in the same document.

The suggestions source element is the first element in the document in tree order to have an ID equal to the value of the list attribute, if that element is a datalist element. If there is no list attribute, or if there is no element with that ID, or if the first element with that ID is not a datalist element, then there is no suggestions source element.

If there is a suggestions source element, then, when the user agent is allowing the user to edit the input element's value, the user agent should offer the suggestions represented by the suggestions source element to the user in a manner suitable for the type of control used. The user agent may use the suggestion's label to identify the suggestion if appropriate.

How user selections of suggestions are handled depends on whether the element is a control accepting a single value only, or whether it accepts multiple values:

If the element does not have a multiple attribute specified or if the multiple attribute does not apply: When the user selects a suggestion, the input element's value must be set to the selected suggestion's value, as if the user had written that value himself.
If the element does have a multiple attribute specified, and the multiple attribute does apply: When the user selects a suggestion, the user agent must either add a new entry to the input element's values, whose value is the selected suggestion's value, or change an existing entry in the input element's values to have the value given by the selected suggestion's value, as if the user had himself added an entry with that value, or edited an existing entry to be that value. Which behavior is to be applied depends on the user interface in a user-agent-defined manner.

If the list attribute does not apply, there is no suggestions source element.

This URL field offers some suggestions.

<label>Homepage: <input name=hp type=url list=hpurls></label>
<datalist id=hpurls>
 <option value="http://www.google.com/" label="Google">
 <option value="http://www.reddit.com/" label="Reddit">
</datalist>

Other URLs from the user's history might show also; this is up to the user agent.

This example demonstrates how to design a form that uses the autocompletion list feature while still degrading usefully in legacy user agents.

If the autocompletion list is merely an aid, and is not important to the content, then simply using a datalist element with children option elements is enough. To prevent the values from being rendered in legacy user agents, they need to be placed inside the value attribute instead of inline.

<p>
 <label>
  Enter a breed:
  <input type="text" name="breed" list="breeds">
  <datalist id="breeds">
   <option value="Abyssinian">
   <option value="Alpaca">
   <!-- ... -->
  </datalist>
 </label>
</p>

However, if the values need to be shown in legacy UAs, then fallback content can be placed inside the datalist element, as follows:

<p>
 <label>
  Enter a breed:
  <input type="text" name="breed" list="breeds">
 </label>
 <datalist id="breeds">
  <label>
   or select one from the list:
   <select name="breed">
    <option value=""> (none selected)
    <option>Abyssinian
    <option>Alpaca
    <!-- ... -->
   </select>
  </label>
 </datalist>
</p>

The fallback content will only be shown in UAs that don't support datalist. The options, on the other hand, will be detected by all UAs, even though they are not children of the datalist element.

Note that if an option element used in a datalist is selected, it will be selected by default by legacy UAs (because it affects the select), but it will not have any effect on the input element in UAs that support datalist.

4.10.5.3.10 The `placeholder` attribute

The placeholder attribute represents a short hint (a word or short phrase) intended to aid the user with data entry when the control has no value. A hint could be a sample value or a brief description of the expected format. The attribute, if specified, must have a value that contains no "LF" (U+000A) or "CR" (U+000D) characters.

The placeholder attribute should not be used as a replacement for a label. For a longer hint or other advisory text, place the text next to the control.

Use of the placeholder attribute as a replacement for a label can reduce the accessibility and usability of the control for a range of users including older users and users with cognitive, mobility, fine motor skill or vision impairments. While the hint given by the control's label is shown at all times, the short hint given in the placeholder attribute is only shown before the user enters a value. Furthermore, placeholder text may be mistaken for a pre-filled value, and as commonly implemented the default color of the placeholder text provides insufficient contrast and the lack of a separate visible label reduces the size of the hit region available for setting focus on the control.

User agents should present this hint to the user, after having stripped line breaks from it, when the element's value is the empty string and the control is not focused (i.e., by displaying it inside a blank unfocused control).

Here is an example of a mail configuration user interface that uses the placeholder attribute:

<fieldset>
 <legend>Mail Account</legend>
 <p><label>Name: <input type="text" name="fullname" placeholder="John Ratzenberger"></label></p>
 <p><label>Address: <input type="email" name="address" placeholder="john@example.net"></label></p>
 <p><label>Password: <input type="password" name="password"></label></p>
 <p><label>Description: <input type="text" name="desc" placeholder="My Email Account"></label></p>
</fieldset>

In situations where the control's content has one directionality but the placeholder needs to have a different directionality, Unicode's bidirectional-algorithm formatting characters can be used in the attribute value:

<input name=t1 type=tel placeholder="&#x202B; رقم الهاتف 1 &#x202E;">
<input name=t2 type=tel placeholder="&#x202B; رقم الهاتف 2 &#x202E;">

For slightly more clarity, here's the same example using numeric character references instead of inline Arabic:

<input name=t1 type=tel placeholder="&#x202B;&#1585;&#1602;&#1605; &#1575;&#1604;&#1607;&#1575;&#1578;&#1601; 1&#x202E;">
<input name=t2 type=tel placeholder="&#x202B;&#1585;&#1602;&#1605; &#1575;&#1604;&#1607;&#1575;&#1578;&#1601; 2&#x202E;">

4.10.5.4 Common `input` element APIs

input . value [ = value ]

Returns the current value of the form control.

Can be set, to change the value.

Throws an InvalidStateError exception if it is set to any value other than the empty string when the control is a file upload control.

input . checked [ = value ]

Returns the current checkedness of the form control.

Can be set, to change the checkedness.

input . files

Returns a FileList object listing the selected files of the form control.

Returns null if the control isn't a file control.

input . valueAsDate [ = value ]

Returns a Date object representing the form control's value, if applicable; otherwise, returns null.

Can be set, to change the value.

Throws an InvalidStateError exception if the control isn't date- or time-based.

input . valueAsNumber [ = value ]

Returns a number representing the form control's value, if applicable; otherwise, returns NaN.

Can be set, to change the value. Setting this to NaN will set the underlying value to the empty string.

Throws an InvalidStateError exception if the control is neither date- or time-based nor numeric.

input . stepUp( [ n ] )

input . stepDown( [ n ] )

Changes the form control's value by the value given in the step attribute, multiplied by n. The default value for n is 1.

Throws InvalidStateError exception if the control is neither date- or time-based nor numeric, or if the step attribute's value is "any".

input . list

Returns the datalist element indicated by the list attribute.

The value IDL attribute allows scripts to manipulate the value of an input element. The attribute is in one of the following modes, which define its behavior:

value

On getting, it must return the current value of the element. On setting, it must set the element's value to the new value, set the element's dirty value flag to true, invoke the value sanitization algorithm, if the element's type attribute's current state defines one, and then, if the element has a text entry cursor position, should move the text entry cursor position to the end of the text field, unselecting any selected text and resetting the selection direction to none.

default

On getting, if the element has a value attribute, it must return that attribute's value; otherwise, it must return the empty string. On setting, it must set the element's value attribute to the new value.

default/on

On getting, if the element has a value attribute, it must return that attribute's value; otherwise, it must return the string "on". On setting, it must set the element's value attribute to the new value.

filename

On getting, it must return the string "C:\fakepath\" followed by the name of the first file in the list of selected files, if any, or the empty string if the list is empty. On setting, if the new value is the empty string, it must empty the list of selected files; otherwise, it must throw an InvalidStateError exception.

This "fakepath" requirement is a sad accident of history. See the example in the File Upload state section for more information.

The checked IDL attribute allows scripts to manipulate the checkedness of an input element. On getting, it must return the current checkedness of the element; and on setting, it must set the element's checkedness to the new value and set the element's dirty checkedness flag to true.

The files IDL attribute allows scripts to access the element's selected files. On getting, if the IDL attribute applies, it must return a FileList object that represents the current selected files. The same object must be returned until the list of selected files changes. If the IDL attribute does not apply, then it must instead return null. [FILEAPI]

The valueAsDate IDL attribute represents the value of the element, interpreted as a date.

On getting, if the valueAsDate attribute does not apply, as defined for the input element's type attribute's current state, then return null. Otherwise, run the algorithm to convert a string to a Date object defined for that state; if the algorithm returned a Date object, then return it, otherwise, return null.

On setting, if the valueAsDate attribute does not apply, as defined for the input element's type attribute's current state, then throw an InvalidStateError exception; otherwise, if the new value is null or a Date object representing the NaN time value, then set the value of the element to the empty string; otherwise, run the algorithm to convert a Date object to a string, as defined for that state, on the new value, and set the value of the element to the resulting string.

The valueAsNumber IDL attribute represents the value of the element, interpreted as a number.

On getting, if the valueAsNumber attribute does not apply, as defined for the input element's type attribute's current state, then return a Not-a-Number (NaN) value. Otherwise, if the valueAsDate attribute applies, run the algorithm to convert a string to a Date object defined for that state; if the algorithm returned a Date object, then return the time value of the object (the number of milliseconds from midnight UTC the morning of 1970-01-01 to the time represented by the Date object), otherwise, return a Not-a-Number (NaN) value. Otherwise, run the algorithm to convert a string to a number defined for that state; if the algorithm returned a number, then return it, otherwise, return a Not-a-Number (NaN) value.

On setting, if the new value is infinite, then throw a TypeError exception. Otherwise, if the valueAsNumber attribute does not apply, as defined for the input element's type attribute's current state, then throw an InvalidStateError exception. Otherwise, if the new value is a Not-a-Number (NaN) value, then set the value of the element to the empty string. Otherwise, if the valueAsDate attribute applies, run the algorithm to convert a Date object to a string defined for that state, passing it a Date object whose time value is the new value, and set the value of the element to the resulting string. Otherwise, run the algorithm to convert a number to a string, as defined for that state, on the new value, and set the value of the element to the resulting string.

The stepDown(n) and stepUp(n) methods, when invoked, must run the following algorithm:

If the stepDown() and stepUp() methods do not apply, as defined for the input element's type attribute's current state, then throw an InvalidStateError exception, and abort these steps.
If the element has no allowed value step, then throw an InvalidStateError exception, and abort these steps.
If the element has a minimum and a maximum and the minimum is greater than the maximum, then abort these steps.
If the element has a minimum and a maximum and there is no value greater than or equal to the element's minimum and less than or equal to the element's maximum that, when subtracted from the step base, is an integral multiple of the allowed value step, then abort these steps.
If applying the algorithm to convert a string to a number to the string given by the element's value does not result in an error, then let value be the result of that algorithm. Otherwise, let value be zero.
If value subtracted from the step base is not an integral multiple of the allowed value step, then set value to the nearest value that, when subtracted from the step base, is an integral multiple of the allowed value step, and that is less than value if the method invoked was the stepDown() and more than value otherwise.

Otherwise (value subtracted from the step base is an integral multiple of the allowed value step), run the following substeps:
1. Let n be the argument.
2. Let delta be the allowed value step multiplied by n.
3. If the method invoked was the stepDown() method, negate delta.
4. Let value be the result of adding delta to value.
If the element has a minimum, and value is less than that minimum, then set value to the smallest value that, when subtracted from the step base, is an integral multiple of the allowed value step, and that is more than or equal to minimum.
If the element has a maximum, and value is greater than that maximum, then set value to the largest value that, when subtracted from the step base, is an integral multiple of the allowed value step, and that is less than or equal to maximum.
Let value as string be the result of running the algorithm to convert a number to a string, as defined for the input element's type attribute's current state, on value.
Set the value of the element to value as string.

The list IDL attribute must return the current suggestions source element, if any, or null otherwise.

4.10.5.5 Common event behaviors

When the input and change events apply (which is the case for all input controls other than buttons and those with the type attribute in the Hidden state), the events are fired to indicate that the user has interacted with the control. The input event fires whenever the user has modified the data of the control. The change event fires when the value is committed, if that makes sense for the control, or else when the control loses focus. In all cases, the input event comes before the corresponding change event (if any).

When an input element has a defined activation behavior, the rules for dispatching these events, if they apply, are given in the section above that defines the type attribute's state. (This is the case for all input controls with the type attribute in the Checkbox state, the Radio Button state, or the File Upload state.)

For input elements without a defined activation behavior, but to which these events apply, and for which the user interface involves both interactive manipulation and an explicit commit action, then when the user changes the element's value, the user agent must queue a task to fire a simple event that bubbles named input at the input element, and any time the user commits the change, the user agent must queue a task to fire a simple event that bubbles named change at the input element.

An example of a user interface involving both interactive manipulation and a commit action would be a Range controls that use a slider, when manipulated using a pointing device. While the user is dragging the control's knob, input events would fire whenever the position changed, whereas the change event would only fire when the user let go of the knob, committing to a specific value.

For input elements without a defined activation behavior, but to which these events apply, and for which the user interface involves an explicit commit action but no intermediate manipulation, then any time the user commits a change to the element's value, the user agent must queue a task to first fire a simple event that bubbles named input at the input element, and then fire a simple event that bubbles named change at the input element.

An example of a user interface with a commit action would be a Color control that consists of a single button that brings up a color wheel: if the value only changes when the dialog is closed, then that would be the explicit commit action. On the other hand, if the control can be focused and manipulating the control changes the color interactively, then there might be no commit action.

Another example of a user interface with a commit action would be a Date control that allows both text-based user input and user selection from a drop-down calendar: while text input might not have an explicit commit step, selecting a date from the drop down calendar and then dismissing the drop down would be a commit action.

For input elements without a defined activation behavior, but to which these events apply, any time the user causes the element's value to change without an explicit commit action, the user agent must queue a task to fire a simple event that bubbles named input at the input element. The corresponding change event, if any, will be fired when the control loses focus.

Examples of a user changing the element's value would include the user typing into a text field, pasting a new value into the field, or undoing an edit in that field. Some user interactions do not cause changes to the value, e.g. hitting the "delete" key in an empty text field, or replacing some text in the field with text from the clipboard that happens to be exactly the same text.

A Range control in the form of a slider that the user has focused and is interacting with using a keyboard would be another example of the user changing the element's value without a commit step.

In the case of tasks that just fire an input event, user agents may wait for a suitable break in the user's interaction before queuing the tasks; for example, a user agent could wait for the user to have not hit a key for 100ms, so as to only fire the event when the user pauses, instead of continuously for each keystroke.

When the user agent is to change an input element's value on behalf of the user (e.g. as part of a form prefilling feature), the user agent must queue a task to first update the value accordingly, then fire a simple event that bubbles named input at the input element, then fire a simple event that bubbles named change at the input element.

These events are not fired in response to changes made to the values of form controls by scripts. (This is to make it easier to update the values of form controls in response to the user manipulating the controls, without having to then filter out the script's own changes to avoid an infinite loop.)

The task source for these tasks is the user interaction task source.

4.10.6 The `button` element

Listed, labelable, submittable, and reassociateable form-associated element.

Where phrasing content is expected.

Phrasing content, but there must be no interactive content descendant.

autofocus - Automatically focus the form control when the page is loaded

disabled - Whether the form control is disabled

form - Associates the control with a form element

formaction - URL to use for form submission

formenctype - Form data set encoding type to use for form submission

formmethod - HTTP method to use for form submission

formnovalidate - Bypass form control validation for form submission

formtarget - Browsing context for form submission

menu - Specifies the element's designated pop-up menu

name - Name of form control to use for form submission and in the form.elements API

type - Type of button

value - Value to be used for form submission

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

button (default - do not set), link, menuitem, menuitemcheckbox, menuitemradio or radio.

Any aria-* attributes applicable to the allowed roles.

interface HTMLButtonElement : HTMLElement {
           attribute boolean autofocus;
           attribute boolean disabled;
  readonly attribute HTMLFormElement? form;
           attribute DOMString formAction;
           attribute DOMString formEnctype;
           attribute DOMString formMethod;
           attribute boolean formNoValidate;
           attribute DOMString formTarget;
           attribute DOMString name;
           attribute DOMString type;
           attribute DOMString value;

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);

  readonly attribute NodeList labels;
};

The button element represents a button labeled by its contents.

The element is a button.

The type attribute controls the behavior of the button when it is activated. It is an enumerated attribute. The following table lists the keywords and states for the attribute — the keywords in the left column map to the states in the cell in the second column on the same row as the keyword.

Keyword	State	Brief description
`submit`	Submit Button	Submits the form.
`reset`	Reset Button	Resets the form.
`button`	Button	Does nothing.

The missing value default is the Submit Button state.

If the type attribute is in the Submit Button state, the element is specifically a submit button.

Constraint validation: If the type attribute is in the Reset Button state, or the Button state, the element is barred from constraint validation.

When a button element is not disabled, its activation behavior element is to run the steps defined in the following list for the current state of the element's type attribute:

Submit Button: If the element has a form owner and the element's Document is fully active, the element must submit the form owner from the button element.
Reset Button: If the element has a form owner and the element's Document is fully active, the element must reset the form owner.
Button: Do nothing.

The form attribute is used to explicitly associate the button element with its form owner. The name attribute represents the element's name. The disabled attribute is used to make the control non-interactive and to prevent its value from being submitted. The autofocus attribute controls focus. The formaction, formenctype, formmethod, formnovalidate, and formtarget attributes are attributes for form submission.

The formnovalidate attribute can be used to make submit buttons that do not trigger the constraint validation.

The formaction, formenctype, formmethod, formnovalidate, and formtarget must not be specified if the element's type attribute is not in the Submit Button state.

The value attribute gives the element's value for the purposes of form submission. The element's value is the value of the element's value attribute, if there is one, or the empty string otherwise.

A button (and its value) is only included in the form submission if the button itself was used to initiate the form submission.

The type IDL attribute must reflect the content attribute of the same name, limited to only known values.

The following button is labeled "Show hint" and pops up a dialog box when activated:

<button type=button
        onclick="alert('This 15-20 minute piece was composed by George Gershwin.')">
 Show hint
</button>

4.10.7 The `select` element

Listed, labelable, submittable, resettable, and reassociateable form-associated element.

Where phrasing content is expected.

Zero or more option, optgroup, and script-supporting elements.

autofocus - Automatically focus the form control when the page is loaded

disabled - Whether the form control is disabled

form - Associates the control with a form element

multiple - Whether to allow multiple values

name - Name of form control to use for form submission and in the form.elements API

required - Whether the control is required for form submission

size - Size of the control

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

listbox (default - do not set) or menu.

Any aria-* attributes applicable to the allowed roles.

interface HTMLSelectElement : HTMLElement {
           attribute boolean autofocus;
           attribute boolean disabled;
  readonly attribute HTMLFormElement? form;
           attribute boolean multiple;
           attribute DOMString name;
           attribute boolean required;
           attribute unsigned long size;

  readonly attribute DOMString type;

  readonly attribute HTMLOptionsCollection options;
           attribute unsigned long length;
  getter Element? item(unsigned long index);
  HTMLOptionElement? namedItem(DOMString name);
  void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
  void remove(); // ChildNode overload
  void remove(long index);
  setter creator void (unsigned long index, HTMLOptionElement? option);

  readonly attribute HTMLCollection selectedOptions;
           attribute long selectedIndex;
           attribute DOMString value;

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);

  readonly attribute NodeList labels;
};

The select element represents a control for selecting amongst a set of options.

The multiple attribute is a boolean attribute. If the attribute is present, then the select element represents a control for selecting zero or more options from the list of options. If the attribute is absent, then the select element represents a control for selecting a single option from the list of options.

The size attribute gives the number of options to show to the user. The size attribute, if specified, must have a value that is a valid non-negative integer greater than zero.

The display size of a select element is the result of applying the rules for parsing non-negative integers to the value of element's size attribute, if it has one and parsing it is successful. If applying those rules to the attribute's value is not successful, or if the size attribute is absent, then the element's display size is 4 if the element's multiple content attribute is present, and 1 otherwise.

The list of options for a select element consists of all the option element children of the select element, and all the option element children of all the optgroup element children of the select element, in tree order.

The required attribute is a boolean attribute. When specified, the user will be required to select a value before submitting the form.

If a select element has a required attribute specified, does not have a multiple attribute specified, and has a display size of 1; and if the value of the first option element in the select element's list of options (if any) is the empty string, and that option element's parent node is the select element (and not an optgroup element), then that option is the select element's placeholder label option.

If a select element has a required attribute specified, does not have a multiple attribute specified, and has a display size of 1, then the select element must have a placeholder label option.

Constraint validation: If the element has its required attribute specified, and either none of the option elements in the select element's list of options have their selectedness set to true, or the only option element in the select element's list of options with its selectedness set to true is the placeholder label option, then the element is suffering from being missing.

If the multiple attribute is absent, and the element is not disabled, then the user agent should allow the user to pick an option element in its list of options that is itself not disabled. Upon this option element being picked (either through a click, or through unfocusing the element after changing its value, or through any other mechanism), and before the relevant user interaction event is queued (e.g. before the click event), the user agent must set the selectedness of the picked option element to true, set its dirtiness to true, and then send select update notifications.

If the multiple attribute is absent, whenever an option element in the select element's list of options has its selectedness set to true, and whenever an option element with its selectedness set to true is added to the select element's list of options, the user agent must set the selectedness of all the other option elements in its list of options to false.

If the multiple attribute is absent and the element's display size is greater than 1, then the user agent should also allow the user to request that the option whose selectedness is true, if any, be unselected. Upon this request being conveyed to the user agent, and before the relevant user interaction event is queued (e.g. before the click event), the user agent must set the selectedness of that option element to false, set its dirtiness to true, and then send select update notifications.

If nodes are inserted or nodes are removed causing the list of options to gain or lose one or more option elements, or if an option element in the list of options asks for a reset, then, if the select element's multiple attribute is absent, the select element's display size is 1, and no option elements in the select element's list of options have their selectedness set to true, the user agent must set the selectedness of the first option element in the list of options in tree order that is not disabled, if any, to true.

If the multiple attribute is present, and the element is not disabled, then the user agent should allow the user to toggle the selectedness of the option elements in its list of options that are themselves not disabled. Upon such an element being toggled (either through a click, or any other mechanism), and before the relevant user interaction event is queued (e.g. before a related click event), the selectedness of the option element must be changed (from true to false or false to true), the dirtiness of the element must be set to true, and the user agent must send select update notifications.

When the user agent is to send select update notifications, queue a task to first fire a simple event that bubbles named input at the select element, and then fire a simple event that bubbles named change at the select element, using the user interaction task source as the task source.

The reset algorithm for select elements is to go through all the option elements in the element's list of options, set their selectedness to true if the option element has a selected attribute, and false otherwise, set their dirtiness to false, and then have the option elements ask for a reset.

The form attribute is used to explicitly associate the select element with its form owner. The name attribute represents the element's name. The disabled attribute is used to make the control non-interactive and to prevent its value from being submitted. The autofocus attribute controls focus.

A select element that is not disabled is mutable.

select . type

Returns "select-multiple" if the element has a multiple attribute, and "select-one" otherwise.

select . options

Returns an HTMLOptionsCollection of the list of options.

select . length [ = value ]

Returns the number of elements in the list of options.

When set to a smaller number, truncates the number of option elements in the select.

When set to a greater number, adds new blank option elements to the select.

element = select . item(index)

select[index]

Returns the item with index index from the list of options. The items are sorted in tree order.

element = select . namedItem(name)

Returns the first item with ID or name name from the list of options.

Returns null if no element with that ID could be found.

select . add(element [, before ])

Inserts element before the node given by before.

The before argument can be a number, in which case element is inserted before the item with that number, or an element from the list of options, in which case element is inserted before that element.

If before is omitted, null, or a number out of range, then element will be added at the end of the list.

This method will throw a HierarchyRequestError exception if element is an ancestor of the element into which it is to be inserted.

select . selectedOptions

Returns an HTMLCollection of the list of options that are selected.

select . selectedIndex [ = value ]

Returns the index of the first selected item, if any, or −1 if there is no selected item.

Can be set, to change the selection.

select . value [ = value ]

Returns the value of the first selected item, if any, or the empty string if there is no selected item.

Can be set, to change the selection.

The type IDL attribute, on getting, must return the string "select-one" if the multiple attribute is absent, and the string "select-multiple" if the multiple attribute is present.

The options IDL attribute must return an HTMLOptionsCollection rooted at the select node, whose filter matches the elements in the list of options.

The options collection is also mirrored on the HTMLSelectElement object. The supported property indices at any instant are the indices supported by the object returned by the options attribute at that instant.

The length IDL attribute must return the number of nodes represented by the options collection. On setting, it must act like the attribute of the same name on the options collection.

The item(index) method must return the value returned by the method of the same name on the options collection, when invoked with the same argument.

The namedItem(name) method must return the value returned by the method of the same name on the options collection, when invoked with the same argument.

When the user agent is to set the value of a new indexed property for a given property index index to a new value value, it must instead set the value of a new indexed property with the given property index index to the new value value on the options collection.

Similarly, the add() method must act like its namesake method on that same options collection.

The remove() method must act like its namesake method on that same options collection when it has arguments, and like its namesake method on the ChildNode interface implemented by the HTMLSelectElement ancestor interface Element when it has no arguments.

The selectedOptions IDL attribute must return an HTMLCollection rooted at the select node, whose filter matches the elements in the list of options that have their selectedness set to true.

The selectedIndex IDL attribute, on getting, must return the index of the first option element in the list of options in tree order that has its selectedness set to true, if any. If there isn't one, then it must return −1.

On setting, the selectedIndex attribute must set the selectedness of all the option elements in the list of options to false, and then the option element in the list of options whose index is the given new value, if any, must have its selectedness set to true and its dirtiness set to true.

This can result in no element having a selectedness set to true even in the case of the select element having no multiple attribute and a display size of 1.

The value IDL attribute, on getting, must return the value of the first option element in the list of options in tree order that has its selectedness set to true, if any. If there isn't one, then it must return the empty string.

On setting, the value attribute must set the selectedness of all the option elements in the list of options to false, and then the first option element in the list of options, in tree order, whose value is equal to the given new value, if any, must have its selectedness set to true and its dirtiness set to true.

This can result in no element having a selectedness set to true even in the case of the select element having no multiple attribute and a display size of 1.

The multiple, required, and size IDL attributes must reflect the respective content attributes of the same name. The size IDL attribute has a default value of zero.

For historical reasons, the default value of the size IDL attribute does not return the actual size used, which, in the absence of the size content attribute, is either 1 or 4 depending on the presence of the multiple attribute.

The following example shows how a select element can be used to offer the user with a set of options from which the user can select a single option. The default option is preselected.

<p>
 <label for="unittype">Select unit type:</label>
 <select id="unittype" name="unittype">
  <option value="1"> Miner </option>
  <option value="2"> Puffer </option>
  <option value="3" selected> Snipey </option>
  <option value="4"> Max </option>
  <option value="5"> Firebot </option>
 </select>
</p>

When there is no default option, a placeholder can be used instead:

<select name="unittype" required>
 <option value=""> Select unit type </option>
 <option value="1"> Miner </option>
 <option value="2"> Puffer </option>
 <option value="3"> Snipey </option>
 <option value="4"> Max </option>
 <option value="5"> Firebot </option>
</select>

Here, the user is offered a set of options from which he can select any number. By default, all five options are selected.

<p>
 <label for="allowedunits">Select unit types to enable on this map:</label>
 <select id="allowedunits" name="allowedunits" multiple>
  <option value="1" selected> Miner </option>
  <option value="2" selected> Puffer </option>
  <option value="3" selected> Snipey </option>
  <option value="4" selected> Max </option>
  <option value="5" selected> Firebot </option>
 </select>
</p>

Sometimes, a user has to select one or more items. This example shows such an interface.

<p>Select the songs from that you would like on your Act II Mix Tape:</p>
<select multiple required name="act2">
 <option value="s1">It Sucks to Be Me (Reprise)
 <option value="s2">There is Life Outside Your Apartment
 <option value="s3">The More You Ruv Someone
 <option value="s4">Schadenfreude
 <option value="s5">I Wish I Could Go Back to College
 <option value="s6">The Money Song
 <option value="s7">School for Monsters
 <option value="s8">The Money Song (Reprise)
 <option value="s9">There's a Fine, Fine Line (Reprise)
 <option value="s10">What Do You Do With a B.A. in English? (Reprise)
 <option value="s11">For Now
</select>

4.10.8 The `datalist` element

Where phrasing content is expected.

Either: phrasing content (with zero or more option elements descendants).

Or: Zero or more option elements.

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

listbox (default - do not set).

Any aria-* attributes applicable to the allowed roles.

interface HTMLDataListElement : HTMLElement {
  readonly attribute HTMLCollection options;
};

The datalist element represents a set of option elements that represent predefined options for other controls. In the rendering, the datalist element represents nothing and it, along with its children, should be hidden.

The datalist element can be used in two ways. In the simplest case, the datalist element has just option element children.

<label>
 Sex:
 <input name=sex list=sexes>
 <datalist id=sexes>
  <option value="Female">
  <option value="Male">
 </datalist>
</label>

In the more elaborate case, the datalist element can be given contents that are to be displayed for down-level clients that don't support datalist. In this case, the option elements are provided inside a select element inside the datalist element.

<label>
 Sex:
 <input name=sex list=sexes>
</label>
<datalist id=sexes>
 <label>
  or select from the list:
  <select name=sex>
   <option value="">
   <option>Female
   <option>Male
  </select>
 </label>
</datalist>

The datalist element is hooked up to an input element using the list attribute on the input element.

Each option element that is a descendant of the datalist element, that is not disabled, and whose value is a string that isn't the empty string, represents a suggestion. Each suggestion has a value and a label.

datalist . options: Returns an HTMLCollection of the options elements of the datalist element.

The options IDL attribute must return an HTMLCollection rooted at the datalist node, whose filter matches option elements.

Constraint validation: If an element has a datalist element ancestor, it is barred from constraint validation.

4.10.9 The `optgroup` element

None.

As a child of a select element.

Zero or more option and script-supporting elements.

disabled - Whether the form control is disabled

label - User-visible label

Tag omission in text/html:

An optgroup element's end tag may be omitted if the optgroup element is immediately followed by another optgroup element, or if there is no more content in the parent element.

Allowed ARIA role attribute values:

None

interface HTMLOptGroupElement : HTMLElement {
           attribute boolean disabled;
           attribute DOMString label;
};

The optgroup element represents a group of option elements with a common label.

The element's group of option elements consists of the option elements that are children of the optgroup element.

When showing option elements in select elements, user agents should show the option elements of such groups as being related to each other and separate from other option elements.

The disabled attribute is a boolean attribute and can be used to disable a group of option elements together.

The label attribute must be specified. Its value gives the name of the group, for the purposes of the user interface. User agents should use this attribute's value when labeling the group of option elements in a select element.

The disabled and label attributes must reflect the respective content attributes of the same name.

The following snippet shows how a set of lessons from three courses could be offered in a select drop-down widget:

<form action="courseselector.dll" method="get">
 <p>Which course would you like to watch today?
 <p><label>Course:
  <select name="c">
   <optgroup label="8.01 Physics I: Classical Mechanics">
    <option value="8.01.1">Lecture 01: Powers of Ten
    <option value="8.01.2">Lecture 02: 1D Kinematics
    <option value="8.01.3">Lecture 03: Vectors
   <optgroup label="8.02 Electricity and Magnestism">
    <option value="8.02.1">Lecture 01: What holds our world together?
    <option value="8.02.2">Lecture 02: Electric Field
    <option value="8.02.3">Lecture 03: Electric Flux
   <optgroup label="8.03 Physics III: Vibrations and Waves">
    <option value="8.03.1">Lecture 01: Periodic Phenomenon
    <option value="8.03.2">Lecture 02: Beats
    <option value="8.03.3">Lecture 03: Forced Oscillations with Damping
  </select>
 </label>
 <p><input type=submit value="▶ Play">
</form>

4.10.10 The `option` element

None.

As a child of a select element.

As a child of a datalist element.

As a child of an optgroup element.

If the element has a label attribute and a value attribute: Empty.

If the element has a label attribute but no value attribute: Text.

If the element has no label attribute: Text that is not inter-element whitespace.

disabled - Whether the form control is disabled

label - User-visible label

selected - Whether the option is selected by default

value - Value to be used for form submission

Tag omission in text/html:

An option element's end tag may be omitted if the option element is immediately followed by another option element, or if it is immediately followed by an optgroup element, or if there is no more content in the parent element.

Allowed ARIA role attribute values:

option (default - do not set), menuitem, menuitemradio or separator.

Any aria-* attributes applicable to the allowed roles.

[NamedConstructor=Option(optional DOMString text = "", optional DOMString value, optional boolean defaultSelected = false, optional boolean selected = false)]
interface HTMLOptionElement : HTMLElement {
           attribute boolean disabled;
  readonly attribute HTMLFormElement? form;
           attribute DOMString label;
           attribute boolean defaultSelected;
           attribute boolean selected;
           attribute DOMString value;

           attribute DOMString text;
  readonly attribute long index;
};

The option element represents an option in a select element or as part of a list of suggestions in a datalist element.

In certain circumstances described in the definition of the select element, an option element can be a select element's placeholder label option. A placeholder label option does not represent an actual option, but instead represents a label for the select control.

The disabled attribute is a boolean attribute. An option element is disabled if its disabled attribute is present or if it is a child of an optgroup element whose disabled attribute is present.

An option element that is disabled must prevent any click events that are queued on the user interaction task source from being dispatched on the element.

The label attribute provides a label for element. The label of an option element is the value of the label content attribute, if there is one, or, if there is not, the value of the element's text IDL attribute.

The label content attribute, if specified, must not be empty.

The value attribute provides a value for element. The value of an option element is the value of the value content attribute, if there is one, or, if there is not, the value of the element's text IDL attribute.

The selected attribute is a boolean attribute. It represents the default selectedness of the element.

The dirtiness of an option element is a boolean state, initially false. It controls whether adding or removing the selected content attribute has any effect.

The selectedness of an option element is a boolean state, initially false. Except where otherwise specified, when the element is created, its selectedness must be set to true if the element has a selected attribute. Whenever an option element's selected attribute is added, if its dirtiness is false, its selectedness must be set to true. Whenever an option element's selected attribute is removed, if its dirtiness is false, its selectedness must be set to false.

The Option() constructor, when called with three or fewer arguments, overrides the initial state of the selectedness state to always be false even if the third argument is true (implying that a selected attribute is to be set). The fourth argument can be used to explicitly set the initial selectedness state when using the constructor.

A select element whose multiple attribute is not specified must not have more than one descendant option element with its selected attribute set.

An option element's index is the number of option elements that are in the same list of options but that come before it in tree order. If the option element is not in a list of options, then the option element's index is zero.

option . selected

Returns true if the element is selected, and false otherwise.

Can be set, to override the current state of the element.

option . index

Returns the index of the element in its select element's options list.

option . form

Returns the element's form element, if any, or null otherwise.

option . text

Same as textContent, except that spaces are collapsed and script elements are skipped.

option = new Option( [ text [, value [, defaultSelected [, selected ] ] ] ] )

Returns a new option element.

The text argument sets the contents of the element.

The value argument sets the value attribute.

The defaultSelected argument sets the selected attribute.

The selected argument sets whether or not the element is selected. If it is omitted, even if the defaultSelected argument is true, the element is not selected.

The disabled IDL attribute must reflect the content attribute of the same name. The defaultSelected IDL attribute must reflect the selected content attribute.

The label IDL attribute, on getting, must return the element's label. On setting, the element's label content attribute must be set to the new value.

The value IDL attribute, on getting, must return the element's value. On setting, the element's value content attribute must be set to the new value.

The selected IDL attribute, on getting, must return true if the element's selectedness is true, and false otherwise. On setting, it must set the element's selectedness to the new value, set its dirtiness to true, and then cause the element to ask for a reset.

The index IDL attribute must return the element's index.

The text IDL attribute, on getting, must return the result of stripping and collapsing whitespace from the concatenation of data of all the Text node descendants of the option element, in tree order, excluding any that are descendants of descendants of the option element that are themselves script elements in the HTML namespace or script elements in the SVG namespace.

On setting, the text attribute must act as if the textContent IDL attribute on the element had been set to the new value.

The form IDL attribute's behavior depends on whether the option element is in a select element or not. If the option has a select element as its parent, or has an optgroup element as its parent and that optgroup element has a select element as its parent, then the form IDL attribute must return the same value as the form IDL attribute on that select element. Otherwise, it must return null.

A constructor is provided for creating HTMLOptionElement objects (in addition to the factory methods from DOM such as createElement()): Option(text, value, defaultSelected, selected). When invoked as a constructor, it must return a new HTMLOptionElement object (a new option element). If the first argument is not the empty string, the new object must have as its only child a Text node whose data is the value of that argument. Otherwise, it must have no children. If the value argument is present, the new object must have a value attribute set with the value of the argument as its value. If the defaultSelected argument is true, the new object must have a selected attribute set with no value. If the selected argument is true, the new object must have its selectedness set to true; otherwise the selectedness must be set to false, even if the defaultSelected argument is true. The element's document must be the active document of the browsing context of the Window object on which the interface object of the invoked constructor is found.

4.10.11 The `textarea` element

Listed, labelable, submittable, resettable, and reassociateable form-associated element.

Where phrasing content is expected.

Text.

autocomplete - - Hint for form autofill feature

autofocus - Automatically focus the form control when the page is loaded

cols - Maximum number of characters per line

dirname - Name of form field to use for sending the element's directionality in form submission

disabled - Whether the form control is disabled

form - Associates the control with a form element

inputmode - Hint for selecting an input modality

maxlength - Maximum length of value

minlength - Minimum length of value

name - Name of form control to use for form submission and in the form.elements API

placeholder - User-visible label to be placed within the form control

readonly - Whether to allow the value to be edited by the user

required - Whether the control is required for form submission

rows - Number of lines to show

wrap - How the value of the form control is to be wrapped for form submission

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

textbox (default - do not set).

Any aria-* attributes applicable to the allowed roles.

interface HTMLTextAreaElement : HTMLElement {
           attribute DOMString autocomplete;
           attribute boolean autofocus;
           attribute unsigned long cols;
           attribute DOMString dirName;
           attribute boolean disabled;
  readonly attribute HTMLFormElement? form;
           attribute long maxLength;
           attribute long minLength;
           attribute DOMString name;
           attribute DOMString placeholder;
           attribute boolean readOnly;
           attribute boolean required;
           attribute unsigned long rows;
           attribute DOMString wrap;

  readonly attribute DOMString type;
           attribute DOMString defaultValue;
  [TreatNullAs=EmptyString] attribute DOMString value;
  readonly attribute unsigned long textLength;

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);

  readonly attribute NodeList labels;

  void select();
           attribute unsigned long selectionStart;
           attribute unsigned long selectionEnd;
           attribute DOMString selectionDirection;
  void setRangeText(DOMString replacement);
  void setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode = "preserve");
  void setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
};

The textarea element represents a multiline plain text edit control for the element's raw value. The contents of the control represent the control's default value.

The raw value of a textarea control must be initially the empty string.

This element has rendering requirements involving the bidirectional algorithm.

The readonly attribute is a boolean attribute used to control whether the text can be edited by the user or not.

In this example, a text field is marked read-only because it represents a read-only file:

Filename: <code>/etc/bash.bashrc</code>
<textarea name="buffer" readonly>
# System-wide .bashrc file for interactive bash(1) shells.

# To enable the settings / commands in this file for login shells as well,
# this file has to be sourced in /etc/profile.

# If not running interactively, don't do anything
[ -z "$PS1" ] &amp;&amp; return

...</textarea>

Constraint validation: If the readonly attribute is specified on a textarea element, the element is barred from constraint validation.

A textarea element is mutable if it is neither disabled nor has a readonly attribute specified.

When a textarea is mutable, its raw value should be editable by the user: the user agent should allow the user to edit, insert, and remove text, and to insert and remove line breaks in the form of "LF" (U+000A) characters. Any time the user causes the element's raw value to change, the user agent must queue a task to fire a simple event that bubbles named input at the textarea element. User agents may wait for a suitable break in the user's interaction before queuing the task; for example, a user agent could wait for the user to have not hit a key for 100ms, so as to only fire the event when the user pauses, instead of continuously for each keystroke.

A textarea element has a dirty value flag, which must be initially set to false, and must be set to true whenever the user interacts with the control in a way that changes the raw value.

When the textarea element's textContent IDL attribute changes value, if the element's dirty value flag is false, then the element's raw value must be set to the value of the element's textContent IDL attribute.

The reset algorithm for textarea elements is to set the element's value to the value of the element's textContent IDL attribute.

If the element is mutable, the user agent should allow the user to change the writing direction of the element, setting it either to a left-to-right writing direction or a right-to-left writing direction. If the user does so, the user agent must then run the following steps:

Set the element's dir attribute to "ltr" if the user selected a left-to-right writing direction, and "rtl" if the user selected a right-to-left writing direction.
Queue a task to fire a simple event that bubbles named input at the textarea element.

The cols attribute specifies the expected maximum number of characters per line. If the cols attribute is specified, its value must be a valid non-negative integer greater than zero. If applying the rules for parsing non-negative integers to the attribute's value results in a number greater than zero, then the element's character width is that value; otherwise, it is 20.

The user agent may use the textarea element's character width as a hint to the user as to how many characters the server prefers per line (e.g. for visual user agents by making the width of the control be that many characters). In visual renderings, the user agent should wrap the user's input in the rendering so that each line is no wider than this number of characters.

The rows attribute specifies the number of lines to show. If the rows attribute is specified, its value must be a valid non-negative integer greater than zero. If applying the rules for parsing non-negative integers to the attribute's value results in a number greater than zero, then the element's character height is that value; otherwise, it is 2.

Visual user agents should set the height of the control to the number of lines given by character height.

The wrap attribute is an enumerated attribute with two keywords and states: the soft keyword which maps to the Soft state, and the hard keyword which maps to the Hard state. The missing value default is the Soft state.

The Soft state indicates that the text in the textarea is not to be wrapped when it is submitted (though it can still be wrapped in the rendering).

The Hard state indicates that the text in the textarea is to have newlines added by the user agent so that the text is wrapped when it is submitted.

If the element's wrap attribute is in the Hard state, the cols attribute must be specified.

For historical reasons, the element's value is normalised in three different ways for three different purposes. The raw value is the value as it was originally set. It is not normalized. The API value is the value used in the value IDL attribute. It is normalized so that line breaks use "LF" (U+000A) characters. Finally, there is the form submission value. It is normalized so that line breaks use U+000D CARRIAGE RETURN "CRLF" (U+000A) character pairs, and in addition, if necessary given the element's wrap attribute, additional line breaks are inserted to wrap the text at the given width.

The element's API value is defined to be the element's raw value with the following transformation applied:

Replace every U+000D CARRIAGE RETURN "CRLF" (U+000A) character pair from the raw value with a single "LF" (U+000A) character.
Replace every remaining U+000D CARRIAGE RETURN character from the raw value with a single "LF" (U+000A) character.

The element's value is defined to be the element's raw value with the following transformation applied:

Replace every occurrence of a "CR" (U+000D) character not followed by a "LF" (U+000A) character, and every occurrence of a "LF" (U+000A) character not preceded by a "CR" (U+000D) character, by a two-character string consisting of a U+000D CARRIAGE RETURN "CRLF" (U+000A) character pair.
If the element's wrap attribute is in the Hard state, insert U+000D CARRIAGE RETURN "CRLF" (U+000A) character pairs into the string using a UA-defined algorithm so that each line has no more than character width characters. For the purposes of this requirement, lines are delimited by the start of the string, the end of the string, and U+000D CARRIAGE RETURN "CRLF" (U+000A) character pairs.

The maxlength attribute is a form control maxlength attribute controlled by the textarea element's dirty value flag.

If the textarea element has a maximum allowed value length, then the element's children must be such that the code-unit length of the value of the element's textContent IDL attribute is equal to or less than the element's maximum allowed value length.

The minlength attribute is a form control minlength attribute controlled by the textarea element's dirty value flag.

The required attribute is a boolean attribute. When specified, the user will be required to enter a value before submitting the form.

Constraint validation: If the element has its required attribute specified, and the element is mutable, and the element's value is the empty string, then the element is suffering from being missing.

The placeholder attribute should not be used as a replacement for a label. For a longer hint or other advisory text, place the text next to the control.

User agents should present this hint to the user when the element's value is the empty string and the control is not focused (e.g. by displaying it inside a blank unfocused control). All U+000D CARRIAGE RETURN U+000A LINE FEED character pairs (CRLF) in the hint, as well as all other "CR" (U+000D) and "LF" (U+000A) characters in the hint, must be treated as line breaks when rendering the hint.

The name attribute represents the element's name. The dirname attribute controls how the element's directionality is submitted. The disabled attribute is used to make the control non-interactive and to prevent its value from being submitted. The form attribute is used to explicitly associate the textarea element with its form owner. The autofocus attribute controls focus. The autocomplete attribute controls how the user agent provides autofill behavior.

textarea . type

Returns the string "textarea".

textarea . value

Returns the current value of the element.

Can be set, to change the value.

The cols, placeholder, required, rows, and wrap attributes must reflect the respective content attributes of the same name. The cols and rows attributes are limited to only non-negative numbers greater than zero. The cols attribute's default value is 20. The rows attribute's default value is 2. The dirName IDL attribute must reflect the dirname content attribute. The maxLength IDL attribute must reflect the maxlength content attribute, limited to only non-negative numbers. The minLength IDL attribute must reflect the minlength content attribute, limited to only non-negative numbers. The readOnly IDL attribute must reflect the readonly content attribute.

The type IDL attribute must return the value "textarea".

The defaultValue IDL attribute must act like the element's textContent IDL attribute.

The value attribute must, on getting, return the element's API value; on setting, it must set the element's raw value to the new value, set the element's dirty value flag to true, and should then move the text entry cursor position to the end of the text field, unselecting any selected text and resetting the selection direction to none.

The textLength IDL attribute must return the code-unit length of the element's API value.

Here is an example of a textarea being used for unrestricted free-form text input in a form:

<p>If you have any comments, please let us know: <textarea cols=80 name=comments></textarea></p>

To specify a maximum length for the comments, one can use the maxlength attribute:

<p>If you have any short comments, please let us know: <textarea cols=80 name=comments maxlength=200></textarea></p>

To give a default value, text can be included inside the element:

<p>If you have any comments, please let us know: <textarea cols=80 name=comments>You rock!</textarea></p>

You can also give a minimum length. Here, a letter needs to be filled out by the user; a template (which is shorter than the minimum length) is provided, but is insufficient to submit the form:

<textarea required minlength="500">Dear Madam Speaker,

Regarding your letter dated ...

...

Yours Sincerely,

...</textarea>

A placeholder can be given as well, to suggest the basic form to the user, without providing an explicit template:

<textarea placeholder="Dear Francine,

They closed the parks this week, so we won't be able to
meet your there. Should we just have dinner?

Love,
Daddy"></textarea>

To have the browser submit the directionality of the element along with the value, the dirname attribute can be specified:

<p>If you have any comments, please let us know (you may use either English or Hebrew for your comments):
<textarea cols=80 name=comments dirname=comments.dir></textarea></p>

4.10.12 The `keygen` element

Listed, labelable, submittable, resettable, and reassociateable form-associated element.

Where phrasing content is expected.

Empty.

autofocus - Automatically focus the form control when the page is loaded

challenge - String to package with the generated and signed public key

disabled - Whether the form control is disabled

form - Associates the control with a form element

keytype - The type of cryptographic key to generate

name - Name of form control to use for form submission and in the form.elements API

Tag omission in text/html:

No end tag.

Allowed ARIA role attribute values:

None

interface HTMLKeygenElement : HTMLElement {
           attribute boolean autofocus;
           attribute DOMString challenge;
           attribute boolean disabled;
  readonly attribute HTMLFormElement? form;
           attribute DOMString keytype;
           attribute DOMString name;

  readonly attribute DOMString type;

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);

  readonly attribute NodeList labels;
};

The keygen element represents a key pair generator control. When the control's form is submitted, the private key is stored in the local keystore, and the public key is packaged and sent to the server.

The challenge attribute may be specified. Its value will be packaged with the submitted key.

The keytype attribute is an enumerated attribute. The following table lists the keywords and states for the attribute — the keywords in the left column map to the states listed in the cell in the second column on the same row as the keyword. User agents are not required to support these values, and must only recognize values whose corresponding algorithms they support.

Keyword	State
`rsa`	RSA

The invalid value default state is the unknown state. The missing value default state is the RSA state, if it is supported, or the unknown state otherwise.

This specification does not specify what key types user agents are to support — it is possible for a user agent to not support any key types at all.

The user agent may expose a user interface for each keygen element to allow the user to configure settings of the element's key pair generator, e.g. the key length.

The reset algorithm for keygen elements is to set these various configuration settings back to their defaults.

The element's value is the string returned from the following algorithm:

Use the appropriate step from the following list:

If the keytype attribute is in the RSA state

Generate an RSA key pair using the settings given by the user, if appropriate, using the md5WithRSAEncryption RSA signature algorithm (the signature algorithm with MD5 and the RSA encryption algorithm) referenced in section 2.2.1 ("RSA Signature Algorithm") of RFC 3279, and defined in RFC 3447. [RFC3279] [RFC3447]

Otherwise, the keytype attribute is in the unknown state

The given key type is not supported. Return the empty string and abort this algorithm.

Let private key be the generated private key.

Let public key be the generated public key.

Let signature algorithm be the selected signature algorithm.
If the element has a challenge attribute, then let challenge be that attribute's value. Otherwise, let challenge be the empty string.
Let algorithm be an ASN.1 AlgorithmIdentifier structure as defined by RFC 5280, with the algorithm field giving the ASN.1 OID used to identify signature algorithm, using the OIDs defined in section 2.2 ("Signature Algorithms") of RFC 3279, and the parameters field set up as required by RFC 3279 for AlgorithmIdentifier structures for that algorithm. [X690] [RFC5280] [RFC3279]
Let spki be an ASN.1 SubjectPublicKeyInfo structure as defined by RFC 5280, with the algorithm field set to the algorithm structure from the previous step, and the subjectPublicKey field set to the BIT STRING value resulting from ASN.1 DER encoding the public key. [X690] [RFC5280]
Let publicKeyAndChallenge be an ASN.1 PublicKeyAndChallenge structure as defined below, with the spki field set to the spki structure from the previous step, and the challenge field set to the string challenge obtained earlier. [X690]
Let signature be the BIT STRING value resulting from ASN.1 DER encoding the signature generated by applying the signature algorithm to the byte string obtained by ASN.1 DER encoding the publicKeyAndChallenge structure, using private key as the signing key. [X690]
Let signedPublicKeyAndChallenge be an ASN.1 SignedPublicKeyAndChallenge structure as defined below, with the publicKeyAndChallenge field set to the publicKeyAndChallenge structure, the signatureAlgorithm field set to the algorithm structure, and the signature field set to the BIT STRING signature from the previous step. [X690]
Return the result of base64 encoding the result of ASN.1 DER encoding the signedPublicKeyAndChallenge structure. [RFC4648] [X690]

The data objects used by the above algorithm are defined as follows. These definitions use the same "ASN.1-like" syntax defined by RFC 5280. [RFC5280]

PublicKeyAndChallenge ::= SEQUENCE {
    spki SubjectPublicKeyInfo,
    challenge IA5STRING
}

SignedPublicKeyAndChallenge ::= SEQUENCE {
    publicKeyAndChallenge PublicKeyAndChallenge,
    signatureAlgorithm AlgorithmIdentifier,
    signature BIT STRING
}

Constraint validation: The keygen element is barred from constraint validation.

The form attribute is used to explicitly associate the keygen element with its form owner. The name attribute represents the element's name. The disabled attribute is used to make the control non-interactive and to prevent its value from being submitted. The autofocus attribute controls focus.

keygen . type: Returns the string "keygen".

The challenge IDL attribute must reflect the content attribute of the same name.

The keytype IDL attribute must reflect the content attribute of the same name, limited to only known values.

The type IDL attribute must return the value "keygen".

This specification does not specify how the private key generated is to be used. It is expected that after receiving the SignedPublicKeyAndChallenge (SPKAC) structure, the server will generate a client certificate and offer it back to the user for download; this certificate, once downloaded and stored in the key store along with the private key, can then be used to authenticate to services that use TLS and certificate authentication.

To generate a key pair, add the private key to the user's key store, and submit the public key to the server, markup such as the following can be used:

<form action="processkey.cgi" method="post" enctype="multipart/form-data">
 <p><keygen name="key"></p>
 <p><input type=submit value="Submit key..."></p>
</form>

The server will then receive a form submission with a packaged RSA public key as the value of "key". This can then be used for various purposes, such as generating a client certificate, as mentioned above.

4.10.13 The `output` element

Listed, labelable, resettable, and reassociateable form-associated element.

Where phrasing content is expected.

for - Specifies controls from which the output was calculated

form - Associates the control with a form element

name - Name of form control to use for form submission and in the form.elements API

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

status (default - do not set), any role value.

Any aria-* attributes applicable to the allowed roles.

interface HTMLOutputElement : HTMLElement {
  [PutForwards=value] readonly attribute DOMSettableTokenList htmlFor;
  readonly attribute HTMLFormElement? form;
           attribute DOMString name;

  readonly attribute DOMString type;
           attribute DOMString defaultValue;
           attribute DOMString value;

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);

  readonly attribute NodeList labels;
};

The output element represents the result of a calculation or user action.

The for content attribute allows an explicit relationship to be made between the result of a calculation and the elements that represent the values that went into the calculation or that otherwise influenced the calculation. The for attribute, if specified, must contain a string consisting of an unordered set of unique space-separated tokens that are case-sensitive, each of which must have the value of an ID of an element in the same Document.

The form attribute is used to explicitly associate the output element with its form owner. The name attribute represents the element's name.

The element has a value mode flag which is either value or default. Initially, the value mode flag must be set to default.

The element also has a default value. Initially, the default value must be the empty string.

When the value mode flag is in mode default, the contents of the element represent both the value of the element and its default value. When the value mode flag is in mode value, the contents of the element represent the value of the element only, and the default value is only accessible using the defaultValue IDL attribute.

Whenever the element's descendants are changed in any way, if the value mode flag is in mode default, the element's default value must be set to the value of the element's textContent IDL attribute.

The reset algorithm for output elements is to set the element's value mode flag to default and then to set the element's textContent IDL attribute to the value of the element's default value (thus replacing the element's child nodes).

output . value [ = value ]

Returns the element's current value.

Can be set, to change the value.

output . defaultValue [ = value ]

Returns the element's current default value.

Can be set, to change the default value.

output . type

Returns the string "output".

The value IDL attribute must act like the element's textContent IDL attribute, except that on setting, in addition, before the child nodes are changed, the element's value mode flag must be set to value.

The defaultValue IDL attribute, on getting, must return the element's default value. On setting, the attribute must set the element's default value, and, if the element's value mode flag is in the mode default, set the element's textContent IDL attribute as well.

The type attribute must return the string "output".

The htmlFor IDL attribute must reflect the for content attribute.

A simple calculator could use output for its display of calculated results:

<form onsubmit="return false" oninput="o.value = a.valueAsNumber + b.valueAsNumber">
 <input name=a type=number step=any> +
 <input name=b type=number step=any> =
 <output name=o for="a b"></output>
</form>

In this example, an output element is used to report the results from a remote server, as they come in:

<output id="result"></output>
<script>
 var primeSource = new WebSocket('ws://primes.example.net/');
 primeSource.onmessage = function (event) {
   document.getElementById('result').value = event.data;
 }
</script>

4.10.14 The `progress` element

Where phrasing content is expected.

Phrasing content, but there must be no progress element descendants.

value - Current value of the element

max - Upper bound of range

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

progressbar (default - do not set).

Any aria-* attributes applicable to the allowed roles.

interface HTMLProgressElement : HTMLElement {
           attribute double value;
           attribute double max;
  readonly attribute double position;
  readonly attribute NodeList labels;
};

The progress element represents the completion progress of a task. The progress is either indeterminate, indicating that progress is being made but that it is not clear how much more work remains to be done before the task is complete (e.g. because the task is waiting for a remote host to respond), or the progress is a number in the range zero to a maximum, giving the fraction of work that has so far been completed.

There are two attributes that determine the current task completion represented by the element. The value attribute specifies how much of the task has been completed, and the max attribute specifies how much work the task requires in total. The units are arbitrary and not specified.

To make a determinate progress bar, add a value attribute with the current progress (either a number from 0.0 to 1.0, or, if the max attribute is specified, a number from 0 to the value of the max attribute). To make an indeterminate progress bar, remove the value attribute.

Authors are encouraged to also include the current value and the maximum value inline as text inside the element, so that the progress is made available to users of legacy user agents.

Here is a snippet of a Web application that shows the progress of some automated task:

<section>
 <h2>Task Progress</h2>
 <p>Progress: <progress id="p" max=100><span>0</span>%</progress></p>
 <script>
  var progressBar = document.getElementById('p');
  function updateProgress(newValue) {
    progressBar.value = newValue;
    progressBar.getElementsByTagName('span')[0].textContent = newValue;
  }
 </script>
</section>

(The updateProgress() method in this example would be called by some other code on the page to update the actual progress bar as the task progressed.)

The value and max attributes, when present, must have values that are valid floating-point numbers. The value attribute, if present, must have a value equal to or greater than zero, and less than or equal to the value of the max attribute, if present, or 1.0, otherwise. The max attribute, if present, must have a value greater than zero.

The progress element is the wrong element to use for something that is just a gauge, as opposed to task progress. For instance, indicating disk space usage using progress would be inappropriate. Instead, the meter element is available for such use cases.

User agent requirements: If the value attribute is omitted, then the progress bar is an indeterminate progress bar. Otherwise, it is a determinate progress bar.

If the progress bar is a determinate progress bar and the element has a max attribute, the user agent must parse the max attribute's value according to the rules for parsing floating-point number values. If this does not result in an error, and if the parsed value is greater than zero, then the maximum value of the progress bar is that value. Otherwise, if the element has no max attribute, or if it has one but parsing it resulted in an error, or if the parsed value was less than or equal to zero, then the maximum value of the progress bar is 1.0.

If the progress bar is a determinate progress bar, user agents must parse the value attribute's value according to the rules for parsing floating-point number values. If this does not result in an error, and if the parsed value is less than the maximum value and greater than zero, then the current value of the progress bar is that parsed value. Otherwise, if the parsed value was greater than or equal to the maximum value, then the current value of the progress bar is the maximum value of the progress bar. Otherwise, if parsing the value attribute's value resulted in an error, or a number less than or equal to zero, then the current value of the progress bar is zero.

UA requirements for showing the progress bar: When representing a progress element to the user, the UA should indicate whether it is a determinate or indeterminate progress bar, and in the former case, should indicate the relative position of the current value relative to the maximum value.

progress . position

For a determinate progress bar (one with known current and maximum values), returns the result of dividing the current value by the maximum value.

For an indeterminate progress bar, returns −1.

If the progress bar is an indeterminate progress bar, then the position IDL attribute must return −1. Otherwise, it must return the result of dividing the current value by the maximum value.

If the progress bar is an indeterminate progress bar, then the value IDL attribute, on getting, must return 0. Otherwise, it must return the current value. On setting, the given value must be converted to the best representation of the number as a floating-point number and then the value content attribute must be set to that string.

Setting the value IDL attribute to itself when the corresponding content attribute is absent would change the progress bar from an indeterminate progress bar to a determinate progress bar with no progress.

The max IDL attribute must reflect the content attribute of the same name, limited to numbers greater than zero. The default value for max is 1.0.

The labels IDL attribute provides a list of the element's labels.

4.10.15 The `meter` element

Where phrasing content is expected.

Phrasing content, but there must be no meter element descendants.

value - Current value of the element

min - Lower bound of range

max - Upper bound of range

low - High limit of low range

high - Low limit of high range

optimum - Optimum value in gauge

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

None

interface HTMLMeterElement : HTMLElement {
           attribute double value;
           attribute double min;
           attribute double max;
           attribute double low;
           attribute double high;
           attribute double optimum;
  readonly attribute NodeList labels;
};

The meter element represents a scalar measurement within a known range, or a fractional value; for example disk usage, the relevance of a query result, or the fraction of a voting population to have selected a particular candidate.

This is also known as a gauge.

The meter element should not be used to indicate progress (as in a progress bar). For that role, HTML provides a separate progress element.

The meter element also does not represent a scalar value of arbitrary range — for example, it would be wrong to use this to report a weight, or height, unless there is a known maximum value.

There are six attributes that determine the semantics of the gauge represented by the element.

The min attribute specifies the lower bound of the range, and the max attribute specifies the upper bound. The value attribute specifies the value to have the gauge indicate as the "measured" value.

The other three attributes can be used to segment the gauge's range into "low", "medium", and "high" parts, and to indicate which part of the gauge is the "optimum" part. The low attribute specifies the range that is considered to be the "low" part, and the high attribute specifies the range that is considered to be the "high" part. The optimum attribute gives the position that is "optimum"; if that is higher than the "high" value then this indicates that the higher the value, the better; if it's lower than the "low" mark then it indicates that lower values are better, and naturally if it is in between then it indicates that neither high nor low values are good.

Authoring requirements: The value attribute must be specified. The value, min, low, high, max, and optimum attributes, when present, must have values that are valid floating-point numbers.

In addition, the attributes' values are further constrained:

Let value be the value attribute's number.

If the min attribute is specified, then let minimum be that attribute's value; otherwise, let it be zero.

If the max attribute is specified, then let maximum be that attribute's value; otherwise, let it be 1.0.

The following inequalities must hold, as applicable:

minimum ≤ value ≤ maximum
minimum ≤ low ≤ maximum (if low is specified)
minimum ≤ high ≤ maximum (if high is specified)
minimum ≤ optimum ≤ maximum (if optimum is specified)
low ≤ high (if both low and high are specified)

If no minimum or maximum is specified, then the range is assumed to be 0..1, and the value thus has to be within that range.

Authors are encouraged to include a textual representation of the gauge's state in the element's contents, for users of user agents that do not support the meter element.

The following examples show three gauges that would all be three-quarters full:

Storage space usage: <meter value=6 max=8>6 blocks used (out of 8 total)</meter>
Voter turnout: <meter value=0.75><img alt="75%" src="graph75.png"></meter>
Tickets sold: <meter min="0" max="100" value="75"></meter>

The following example is incorrect use of the element, because it doesn't give a range (and since the default maximum is 1, both of the gauges would end up looking maxed out):

<p>The grapefruit pie had a radius of <meter value=12>12cm</meter>
and a height of <meter value=2>2cm</meter>.</p> <!-- BAD! -->

Instead, one would either not include the meter element, or use the meter element with a defined range to give the dimensions in context compared to other pies:

<p>The grapefruit pie had a radius of 12cm and a height of
2cm.</p>
<dl>
 <dt>Radius: <dd> <meter min=0 max=20 value=12>12cm</meter>
 <dt>Height: <dd> <meter min=0 max=10 value=2>2cm</meter>
</dl>

There is no explicit way to specify units in the meter element, but the units may be specified in the title attribute in free-form text.

The example above could be extended to mention the units:

<dl>
 <dt>Radius: <dd> <meter min=0 max=20 value=12 title="centimeters">12cm</meter>
 <dt>Height: <dd> <meter min=0 max=10 value=2 title="centimeters">2cm</meter>
</dl>

User agent requirements: User agents must parse the min, max, value, low, high, and optimum attributes using the rules for parsing floating-point number values.

User agents must then use all these numbers to obtain values for six points on the gauge, as follows. (The order in which these are evaluated is important, as some of the values refer to earlier ones.)

The minimum value

If the min attribute is specified and a value could be parsed out of it, then the minimum value is that value. Otherwise, the minimum value is zero.

The maximum value

If the max attribute is specified and a value could be parsed out of it, then the candidate maximum value is that value. Otherwise, the candidate maximum value is 1.0.

If the candidate maximum value is greater than or equal to the minimum value, then the maximum value is the candidate maximum value. Otherwise, the maximum value is the same as the minimum value.

The actual value

If the value attribute is specified and a value could be parsed out of it, then that value is the candidate actual value. Otherwise, the candidate actual value is zero.

If the candidate actual value is less than the minimum value, then the actual value is the minimum value.

Otherwise, if the candidate actual value is greater than the maximum value, then the actual value is the maximum value.

Otherwise, the actual value is the candidate actual value.

The low boundary

If the low attribute is specified and a value could be parsed out of it, then the candidate low boundary is that value. Otherwise, the candidate low boundary is the same as the minimum value.

If the candidate low boundary is less than the minimum value, then the low boundary is the minimum value.

Otherwise, if the candidate low boundary is greater than the maximum value, then the low boundary is the maximum value.

Otherwise, the low boundary is the candidate low boundary.

The high boundary

If the high attribute is specified and a value could be parsed out of it, then the candidate high boundary is that value. Otherwise, the candidate high boundary is the same as the maximum value.

If the candidate high boundary is less than the low boundary, then the high boundary is the low boundary.

Otherwise, if the candidate high boundary is greater than the maximum value, then the high boundary is the maximum value.

Otherwise, the high boundary is the candidate high boundary.

The optimum point

If the optimum attribute is specified and a value could be parsed out of it, then the candidate optimum point is that value. Otherwise, the candidate optimum point is the midpoint between the minimum value and the maximum value.

If the candidate optimum point is less than the minimum value, then the optimum point is the minimum value.

Otherwise, if the candidate optimum point is greater than the maximum value, then the optimum point is the maximum value.

Otherwise, the optimum point is the candidate optimum point.

All of which will result in the following inequalities all being true:

minimum value ≤ actual value ≤ maximum value
minimum value ≤ low boundary ≤ high boundary ≤ maximum value
minimum value ≤ optimum point ≤ maximum value

UA requirements for regions of the gauge: If the optimum point is equal to the low boundary or the high boundary, or anywhere in between them, then the region between the low and high boundaries of the gauge must be treated as the optimum region, and the low and high parts, if any, must be treated as suboptimal. Otherwise, if the optimum point is less than the low boundary, then the region between the minimum value and the low boundary must be treated as the optimum region, the region from the low boundary up to the high boundary must be treated as a suboptimal region, and the remaining region must be treated as an even less good region. Finally, if the optimum point is higher than the high boundary, then the situation is reversed; the region between the high boundary and the maximum value must be treated as the optimum region, the region from the high boundary down to the low boundary must be treated as a suboptimal region, and the remaining region must be treated as an even less good region.

UA requirements for showing the gauge: When representing a meter element to the user, the UA should indicate the relative position of the actual value to the minimum and maximum values, and the relationship between the actual value and the three regions of the gauge.

The following markup:

<h3>Suggested groups</h3>
<ul>
 <li>
  <p><a href="/group/comp.infosystems.www.authoring.stylesheets/view">comp.infosystems.www.authoring.stylesheets</a> -
     <a href="/group/comp.infosystems.www.authoring.stylesheets/subscribe">join</a></p>
  <p>Group description: <strong>Layout/presentation on the WWW.</strong></p>
  <p><meter value="0.5">Moderate activity,</meter> Usenet, 618 subscribers</p>
 </li>
 <li>
  <p><a href="/group/netscape.public.mozilla.xpinstall/view">netscape.public.mozilla.xpinstall</a> -
     <a href="/group/netscape.public.mozilla.xpinstall/subscribe">join</a></p>
  <p>Group description: <strong>Mozilla XPInstall discussion.</strong></p>
  <p><meter value="0.25">Low activity,</meter> Usenet, 22 subscribers</p>
 </li>
 <li>
  <p><a href="/group/mozilla.dev.general/view">mozilla.dev.general</a> -
     <a href="/group/mozilla.dev.general/subscribe">join</a></p>
  <p><meter value="0.25">Low activity,</meter> Usenet, 66 subscribers</p>
 </li>
</ul>

Might be rendered as follows:

With the <meter> elements rendered as inline green bars of varying lengths.

User agents may combine the value of the title attribute and the other attributes to provide context-sensitive help or inline text detailing the actual values.

For example, the following snippet:

<meter min=0 max=60 value=23.2 title=seconds></meter>

...might cause the user agent to display a gauge with a tooltip saying "Value: 23.2 out of 60." on one line and "seconds" on a second line.

The value IDL attribute, on getting, must return the actual value. On setting, the given value must be converted to the best representation of the number as a floating-point number and then the value content attribute must be set to that string.

The min IDL attribute, on getting, must return the minimum value. On setting, the given value must be converted to the best representation of the number as a floating-point number and then the min content attribute must be set to that string.

The max IDL attribute, on getting, must return the maximum value. On setting, the given value must be converted to the best representation of the number as a floating-point number and then the max content attribute must be set to that string.

The low IDL attribute, on getting, must return the low boundary. On setting, the given value must be converted to the best representation of the number as a floating-point number and then the low content attribute must be set to that string.

The high IDL attribute, on getting, must return the high boundary. On setting, the given value must be converted to the best representation of the number as a floating-point number and then the high content attribute must be set to that string.

The optimum IDL attribute, on getting, must return the optimum value. On setting, the given value must be converted to the best representation of the number as a floating-point number and then the optimum content attribute must be set to that string.

The labels IDL attribute provides a list of the element's labels.

The following example shows how a gauge could fall back to localized or pretty-printed text.

<p>Disk usage: <meter min=0 value=170261928 max=233257824>170 261 928 bytes used
out of 233 257 824 bytes available</meter></p>

4.10.16 The `fieldset` element

Sectioning root.

Listed and reassociateable form-associated element.

Where flow content is expected.

Optionally a legend element, followed by flow content.

disabled - Whether the form control is disabled

form - Associates the control with a form element

name - Name of form control to use for form submission and in the form.elements API

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

group (default - do not set) or presentation.

Any aria-* attributes applicable to the allowed roles.

interface HTMLFieldSetElement : HTMLElement {
           attribute boolean disabled;
  readonly attribute HTMLFormElement? form;
           attribute DOMString name;

  readonly attribute DOMString type;

  readonly attribute HTMLFormControlsCollection elements;

  readonly attribute boolean willValidate;
  readonly attribute ValidityState validity;
  readonly attribute DOMString validationMessage;
  boolean checkValidity();
  void setCustomValidity(DOMString error);
};

The fieldset element represents a set of form controls optionally grouped under a common name.

The name of the group is given by the first legend element that is a child of the fieldset element, if any. The remainder of the descendants form the group.

The disabled attribute, when specified, causes all the form control descendants of the fieldset element, excluding those that are descendants of the fieldset element's first legend element child, if any, to be disabled.

The form attribute is used to explicitly associate the fieldset element with its form owner. The name attribute represents the element's name.

fieldset . type: Returns the string "fieldset".
fieldset . elements: Returns an HTMLFormControlsCollection of the form controls in the element.

The disabled IDL attribute must reflect the content attribute of the same name.

The type IDL attribute must return the string "fieldset".

The elements IDL attribute must return an HTMLFormControlsCollection rooted at the fieldset element, whose filter matches listed elements.

This example shows a fieldset element being used to group a set of related controls:

<fieldset>
 <legend>Display</legend>
 <div><label><input type=radio name=c value=0 checked> Black on White</label></div>
 <div><label><input type=radio name=c value=1> White on Black</label></div>
 <div><label><input type=checkbox name=g> Use grayscale</label></div>
 <div><label>Enhance contrast <input type=range name=e list=contrast min=0 max=100 value=0 step=1></label></div>
 <datalist id=contrast>
  <option label=Normal value=0>
  <option label=Maximum value=100>
 </datalist>
</fieldset>

The div elements used in the code samples above and below are not intended to convey any semantic meaning and are used only to create a non-inline rendering of the grouped fieldset controls.

The following snippet shows a fieldset with a checkbox in the legend that controls whether or not the fieldset is enabled. The contents of the fieldset consist of two required text fields and an optional year/month control.

<fieldset name="clubfields" disabled>
 <legend> <label>
  <input type=checkbox name=club onchange="form.clubfields.disabled = !checked">
  Use Club Card
 </label> </legend>
 <div><label>Name on card: <input name=clubname required></label></div>
 <div><label>Card number: <input name=clubnum required pattern="[-0-9]+"></label></div>
 <div><label>Expiry date: <input name=clubexp type=date></label></div>
</fieldset>

You can also nest fieldset elements. Here is an example expanding on the previous one that does so:

<fieldset name="clubfields" disabled>
 <legend> <label>
  <input type=checkbox name=club onchange="form.clubfields.disabled = !checked">
  Use Club Card
 </label> </legend>
 <div><label>Name on card: <input name=clubname required></label></div>
 <fieldset name="numfields">
  <legend> <label>
   <input type=radio checked name=clubtype onchange="form.numfields.disabled = !checked">
   My card has numbers on it
  </label> </legend>
  <div><label>Card number: <input name=clubnum required pattern="[-0-9]+"></label></div>
 </fieldset>
 <fieldset name="letfields" disabled>
  <legend> <label>
   <input type=radio name=clubtype onchange="form.letfields.disabled = !checked">
   My card has letters on it
  </label> </legend>
  <div><label>Card code: <input name=clublet required pattern="[A-Za-z]+"></label></>
 </fieldset>
</fieldset>

In this example, if the outer "Use Club Card" checkbox is not checked, everything inside the outer fieldset, including the two radio buttons in the legends of the two nested fieldsets, will be disabled. However, if the checkbox is checked, then the radio buttons will both be enabled and will let you select which of the two inner fieldsets is to be enabled.

4.10.17 The `legend` element

None.

As the first child of a fieldset element.

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

Script-supporting element.

interface HTMLLegendElement : HTMLElement {
  readonly attribute HTMLFormElement? form;
};

The legend element represents a caption for the rest of the contents of the legend element's parent fieldset element, if any.

legend . form: Returns the element's form element, if any, or null otherwise.

The form IDL attribute's behavior depends on whether the legend element is in a fieldset element or not. If the legend has a fieldset element as its parent, then the form IDL attribute must return the same value as the form IDL attribute on that fieldset element. Otherwise, it must return null.

4.10.18 Form control infrastructure

4.10.18.1 A form control's value

Form controls have a value and a checkedness. (The latter is only used by input elements.) These are used to describe how the user interacts with the control.

To define the behaviour of constraint validation in the face of the input element's multiple attribute, input elements can also have separately defined values.

4.10.18.2 Mutability

A form control can be designated as mutable.

This determines (by means of definitions and requirements in this specification that rely on whether an element is so designated) whether or not the user can modify the value or checkedness of a form control, or whether or not a control can be automatically prefilled.

4.10.18.3 Association of controls and forms

A form-associated element can have a relationship with a form element, which is called the element's form owner. If a form-associated element is not associated with a form element, its form owner is said to be null.

A form-associated element is, by default, associated with its nearest ancestor form element (as described below), but, if it is reassociateable, may have a form attribute specified to override this.

This feature allows authors to work around the lack of support for nested form elements.

If a reassociateable form-associated element has a form attribute specified, then that attribute's value must be the ID of a form element in the element's owner Document.

The rules in this section are complicated by the fact that although conforming documents will never contain nested form elements, it is quite possible (e.g. using a script that performs DOM manipulation) to generate documents that have such nested elements. They are also complicated by rules in the HTML parser that, for historical reasons, can result in a form-associated element being associated with a form element that is not its ancestor.

When a form-associated element is created, its form owner must be initialized to null (no owner).

When a form-associated element is to be associated with a form, its form owner must be set to that form.

When a form-associated element or one of its ancestors is inserted into a Document, then the user agent must reset the form owner of that form-associated element. The HTML parser overrides this requirement when inserting form controls.

When an element is removed from a Document resulting in a form-associated element and its form owner (if any) no longer being in the same home subtree, then the user agent must reset the form owner of that form-associated element.

When a reassociateable form-associated element's form attribute is set, changed, or removed, then the user agent must reset the form owner of that element.

When a reassociateable form-associated element has a form attribute and the ID of any of the elements in the Document changes, then the user agent must reset the form owner of that form-associated element.

When a reassociateable form-associated element has a form attribute and an element with an ID is inserted into or removed from the Document, then the user agent must reset the form owner of that form-associated element.

When the user agent is to reset the form owner of a form-associated element, it must run the following steps:

If the element's form owner is not null, and either the element is not reassociateable or its form content attribute is not present, and the element's form owner is its nearest form element ancestor after the change to the ancestor chain, then do nothing, and abort these steps.
Let the element's form owner be null.
If the element is reassociateable, has a form content attribute, and is itself in a Document, then run these substeps:
1. If the first element in the Document to have an ID that is case-sensitively equal to the element's form content attribute's value is a form element, then associate the form-associated element with that form element.
2. Abort the "reset the form owner" steps.
Otherwise, if the form-associated element in question has an ancestor form element, then associate the form-associated element with the nearest such ancestor form element.
Otherwise, the element is left unassociated.

In the following non-conforming snippet:

...
 <form id="a">
  <div id="b"></div>
 </form>
 <script>
  document.getElementById('b').innerHTML =
     '<table><tr><td><form id="c"><input id="d"></table>' +
     '<input id="e">';
 </script>
...

The form owner of "d" would be the inner nested form "c", while the form owner of "e" would be the outer form "a".

This happens as follows: First, the "e" node gets associated with "c" in the HTML parser. Then, the innerHTML algorithm moves the nodes from the temporary document to the "b" element. At this point, the nodes see their ancestor chain change, and thus all the "magic" associations done by the parser are reset to normal ancestor associations.

This example is a non-conforming document, though, as it is a violation of the content models to nest form elements.

element . form

Returns the element's form owner.

Returns null if there isn't one.

Reassociateable form-associated elements have a form IDL attribute, which, on getting, must return the element's form owner, or null if there isn't one.

4.10.19 Attributes common to form controls

4.10.19.1 Naming form controls: the `name` attribute

The name content attribute gives the name of the form control, as used in form submission and in the form element's elements object. If the attribute is specified, its value must not be the empty string.

Any non-empty value for name is allowed, but the names "_charset_" and "isindex" are special:

isindex: This value, if used as the name of a Text control that is the first control in a form that is submitted using the application/x-www-form-urlencoded mechanism, causes the submission to only include the value of this control, with no name.
_charset_: This value, if used as the name of a Hidden control with no value attribute, is automatically given a value during submission consisting of the submission character encoding.

The name IDL attribute must reflect the name content attribute.

4.10.19.2 Submitting element directionality: the `dirname` attribute

The dirname attribute on a form control element enables the submission of the directionality of the element, and gives the name of the field that contains this value during form submission. If such an attribute is specified, its value must not be the empty string.

In this example, a form contains a text field and a submission button:

<form action="addcomment.cgi" method=post>
 <p><label>Comment: <input type=text name="comment" dirname="comment.dir" required></label></p>
 <p><button name="mode" type=submit value="add">Post Comment</button></p>
</form>

When the user submits the form, the user agent includes three fields, one called "comment", one called "comment.dir", and one called "mode"; so if the user types "Hello", the submission body might be something like:

comment=Hello&comment.dir=ltr&mode=add

If the user manually switches to a right-to-left writing direction and enters "مرحبا", the submission body might be something like:

comment=%D9%85%D8%B1%D8%AD%D8%A8%D8%A7&comment.dir=rtl&mode=add

4.10.19.3 Limiting user input length: the `maxlength` attribute

A form control maxlength attribute, controlled by a dirty value flag, declares a limit on the number of characters a user can input.

If an element has its form control maxlength attribute specified, the attribute's value must be a valid non-negative integer. If the attribute is specified and applying the rules for parsing non-negative integers to its value results in a number, then that number is the element's maximum allowed value length. If the attribute is omitted or parsing its value results in an error, then there is no maximum allowed value length.

Constraint validation: If an element has a maximum allowed value length, its dirty value flag is true, its value was last changed by a user edit (as opposed to a change made by a script), and the code-unit length of the element's value is greater than the element's maximum allowed value length, then the element is suffering from being too long.

User agents may prevent the user from causing the element's value to be set to a value whose code-unit length is greater than the element's maximum allowed value length.

4.10.19.4 Setting minimum input length requirements: the `minlength` attribute

A form control minlength attribute, controlled by a dirty value flag, declares a lower bound on the number of characters a user can input.

The minlength attribute does not imply the required attribute. If the form control has no minlength attribute, then the value can still be omitted; the minlength attribute only kicks in once the user has entered a value at all. If the empty string is not allowed, then the required attribute also needs to be set.

If an element has its form control minlength attribute specified, the attribute's value must be a valid non-negative integer. If the attribute is specified and applying the rules for parsing non-negative integers to its value results in a number, then that number is the element's minimum allowed value length. If the attribute is omitted or parsing its value results in an error, then there is no minimum allowed value length.

If an element has both a maximum allowed value length and a minimum allowed value length, the minimum allowed value length must be smaller than or equal to the maximum allowed value length.

Constraint validation: If an element has a minimum allowed value length, its value is not the empty string, and the code-unit length of the element's value is less than the element's minimum allowed value length, then the element is suffering from being too short.

In this example, there are four text fields. The first is required, and has to be at least 5 characters long. The other three are optional, but if the user fills one in, the user has to enter at least 10 characters.

<form action="/events/menu.cgi" method="post">
 <p><label>Name of Event: <input required minlength=5 maxlength=50 name=event></label></p>
 <p><label>Describe what you would like for breakfast, if anything:
    <textarea name="breakfast" minlength="10"></textarea></label></p>
 <p><label>Describe what you would like for lunch, if anything:
    <textarea name="lunch" minlength="10"></textarea></label></p>
 <p><label>Describe what you would like for dinner, if anything:
    <textarea name="dinner" minlength="10"></textarea></label></p>
 <p><input type=submit value="Submit Request"></p>
</form>

4.10.19.5 Enabling and disabling form controls: the `disabled` attribute

The disabled content attribute is a boolean attribute.

A form control is disabled if its disabled attribute is set, or if it is a descendant of a fieldset element whose disabled attribute is set and is not a descendant of that fieldset element's first legend element child, if any.

A form control that is disabled must prevent any click events that are queued on the user interaction task source from being dispatched on the element.

Constraint validation: If an element is disabled, it is barred from constraint validation.

The disabled IDL attribute must reflect the disabled content attribute.

4.10.19.6 Form submission

Attributes for form submission can be specified both on form elements and on submit buttons (elements that represent buttons that submit forms, e.g. an input element whose type attribute is in the Submit Button state).

The attributes for form submission that may be specified on form elements are action, enctype, method, novalidate, and target.

The corresponding attributes for form submission that may be specified on submit buttons are formaction, formenctype, formmethod, formnovalidate, and formtarget. When omitted, they default to the values given on the corresponding attributes on the form element.

The action and formaction content attributes, if specified, must have a value that is a valid non-empty URL potentially surrounded by spaces.

The action of an element is the value of the element's formaction attribute, if the element is a submit button and has such an attribute, or the value of its form owner's action attribute, if it has one, or else the empty string.

The method and formmethod content attributes are enumerated attributes with the following keywords and states:

The keyword get, mapping to the state GET, indicating the HTTP GET method.
The keyword post, mapping to the state POST, indicating the HTTP POST method.

The invalid value default for these attributes is the GET state. The missing value default for the method attribute is also the GET state. (There is no missing value default for the formmethod attribute.)

The method of an element is one of those states. If the element is a submit button and has a formmethod attribute, then the element's method is that attribute's state; otherwise, it is the form owner's method attribute's state.

Here the method attribute is used to explicitly specify the default value, "get", so that the search query is submitted in the URL:

<form method="get" action="/search.cgi">
 <p><label>Search terms: <input type=search name=q></label></p>
 <p><input type=submit></p>
</form>

On the other hand, here the method attribute is used to specify the value "post", so that the user's message is submitted in the HTTP request's body:

<form method="post" action="/post-message.cgi">
 <p><label>Message: <input type=text name=m></label></p>
 <p><input type=submit value="Submit message"></p>
</form>

The enctype and formenctype content attributes are enumerated attributes with the following keywords and states:

The "application/x-www-form-urlencoded" keyword and corresponding state.
The "multipart/form-data" keyword and corresponding state.
The "text/plain" keyword and corresponding state.

The invalid value default for these attributes is the application/x-www-form-urlencoded state. The missing value default for the enctype attribute is also the application/x-www-form-urlencoded state. (There is no missing value default for the formenctype attribute.)

The enctype of an element is one of those three states. If the element is a submit button and has a formenctype attribute, then the element's enctype is that attribute's state; otherwise, it is the form owner's enctype attribute's state.

The target and formtarget content attributes, if specified, must have values that are valid browsing context names or keywords.

The target of an element is the value of the element's formtarget attribute, if the element is a submit button and has such an attribute; or the value of its form owner's target attribute, if it has such an attribute; or, if the Document contains a base element with a target attribute, then the value of the target attribute of the first such base element; or, if there is no such element, the empty string.

The novalidate and formnovalidate content attributes are boolean attributes. If present, they indicate that the form is not to be validated during submission.

The no-validate state of an element is true if the element is a submit button and the element's formnovalidate attribute is present, or if the element's form owner's novalidate attribute is present, and false otherwise.

This attribute is useful to include "save" buttons on forms that have validation constraints, to allow users to save their progress even though they haven't fully entered the data in the form. The following example shows a simple form that has two required fields. There are three buttons: one to submit the form, which requires both fields to be filled in; one to save the form so that the user can come back and fill it in later; and one to cancel the form altogether.

<form action="editor.cgi" method="post">
 <p><label>Name: <input required name=fn></label></p>
 <p><label>Essay: <textarea required name=essay></textarea></label></p>
 <p><input type=submit name=submit value="Submit essay"></p>
 <p><input type=submit formnovalidate name=save value="Save essay"></p>
 <p><input type=submit formnovalidate name=cancel value="Cancel"></p>
</form>

The action IDL attribute must reflect the content attribute of the same name, except that on getting, when the content attribute is missing or its value is the empty string, the document's address must be returned instead. The target IDL attribute must reflect the content attribute of the same name. The method and enctype IDL attributes must reflect the respective content attributes of the same name, limited to only known values. The encoding IDL attribute must reflect the enctype content attribute, limited to only known values. The noValidate IDL attribute must reflect the novalidate content attribute. The formAction IDL attribute must reflect the formaction content attribute, except that on getting, when the content attribute is missing or its value is the empty string, the document's address must be returned instead. The formEnctype IDL attribute must reflect the formenctype content attribute, limited to only known values. The formMethod IDL attribute must reflect the formmethod content attribute, limited to only known values. The formNoValidate IDL attribute must reflect the formnovalidate content attribute. The formTarget IDL attribute must reflect the formtarget content attribute.

4.10.19.7 Autofocusing a form control: the `autofocus` attribute

The autofocus content attribute allows the author to indicate that a control is to be focused as soon as the page is loaded, allowing the user to just start typing without having to manually focus the main control.

The autofocus attribute is a boolean attribute.

An element's nearest ancestor autofocus scoping root element is the element's root element.

There must not be two elements with the same nearest ancestor autofocus scoping root element that both have the autofocus attribute specified.

When an element with the autofocus attribute specified is inserted into a document, user agents should run the following steps:

Let target be the element's Document.
If target has no browsing context, abort these steps.
If target's browsing context has no top-level browsing context (e.g. it is a nested browsing context with no parent browsing context), abort these steps.
If target's active sandboxing flag set has the sandboxed automatic features browsing context flag, abort these steps.
If target's origin is not the same as the origin of the Document of the currently focused element in target's top-level browsing context, abort these steps.
If target's origin is not the same as the origin of the active document of target's top-level browsing context, abort these steps.
If the user agent has already reached the last step of this list of steps in response to an element being inserted into a Document whose top-level browsing context's active document is the same as target's top-level browsing context's active document, abort these steps.
If the user has indicated (for example, by starting to type in a form control) that he does not wish focus to be changed, then optionally abort these steps.
Queue a task that checks to see if the element is focusable, and if so, runs the focusing steps for that element. User agents may also change the scrolling position of the document, or perform some other action that brings the element to the user's attention. The task source for this task is the user interaction task source.

This handles the automatic focusing during document load.

Focusing the control does not imply that the user agent must focus the browser window if it has lost focus.

The autofocus IDL attribute must reflect the content attribute of the same name.

In the following snippet, the text control would be focused when the document was loaded.

<input maxlength="256" name="q" value="" autofocus>
<input type="submit" value="Search">

4.10.19.8 Autofilling form controls: the `autocomplete` attribute

User agents sometimes have features for helping users fill forms in, for example prefilling the user's address based on earlier user input. The autocomplete content attribute can be used to hint to the user agent how to, or indeed whether to, provide such a feature.

The attribute, if present, must have a value that is an ASCII case-insensitive match for the string "off", or a single token that is an ASCII case-insensitive match for the string "on"

The "off" keyword indicates either that the control's input data is particularly sensitive (for example the activation code for a nuclear weapon); or that it is a value that will never be reused (for example a one-time-key for a bank login) and the user will therefore have to explicitly enter the data each time, instead of being able to rely on the UA to prefill the value for him; or that the document provides its own autocomplete mechanism and does not want the user agent to provide autocompletion values.

The "on" keyword indicates that the user agent is allowed to provide the user with autocompletion values, but does not provide any further information about what kind of data the user might be expected to enter. User agents would have to use heuristics to decide what autocompletion values to suggest.

If the autocomplete attribute is omitted, the default value corresponding to the state of the element's form owner's autocomplete attribute is used instead (either "on" or "off"). If there is no form owner, then the value "on" is used.

When an element's autofill field name is "off", the user agent should not remember the control's value, and should not offer past values to the user.

In addition, when an element's autofill field name is "off", values are reset when traversing the history.

Banks frequently do not want UAs to prefill login information:

<p><label>Account: <input type="text" name="ac" autocomplete="off"></label></p>
<p><label>PIN: <input type="password" name="pin" autocomplete="off"></label></p>

When an element's autofill field name is not "off", the user agent may store the control's value, and may offer previously stored values to the user.

When the autofill field name is "on", the user agent should attempt to use heuristics to determine the most appropriate values to offer the user, e.g. based on the element's name value, the position of the element in the document's DOM, what other fields exist in the form, and so forth.

The autocompletion mechanism must be implemented by the user agent acting as if the user had modified the element's value, and must be done at a time where the element is mutable (e.g. just after the element has been inserted into the document, or when the user agent stops parsing). User agents must only prefill controls using values that the user could have entered.

A user agent prefilling a form control's value must not cause that control to suffer from a type mismatch, suffer from a pattern mismatch, suffer from being too long, suffer from being too short, suffer from an underflow, suffer from an overflow, or suffer from a step mismatch. Where possible given the control's constraints, user agents must use the format given as canonical in the aforementioned table. Where it's not possible for the canonical format to be used, user agents should use heuristics to attempt to convert values so that they can be used.

A user agent may allow the user to override an element's autofill field name, e.g. to change it from "off" to "on" to allow values to be remembered and prefilled despite the page author's objections, or to always "off", never remembering values. However, user agents should not allow users to trivially override the autofill field name from "off" to "on" or other values, as there are significant security implications for the user if all values are always remembered, regardless of the site's preferences.

The autocomplete IDL attribute, on getting, must return the element's IDL-exposed autofill value, and on setting, must reflect the content attribute of the same name.

4.10.20 APIs for the text field selections

The input and textarea elements define the following members in their DOM interfaces for handling their selection:

  void select();
           attribute unsigned long selectionStart;
           attribute unsigned long selectionEnd;
           attribute DOMString selectionDirection;
  void setRangeText(DOMString replacement);
  void setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode = "preserve");
  void setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction = "preserve");

The setRangeText method uses the following enumeration:

enum SelectionMode {
  "select",
  "start",
  "end",
  "preserve" // default
};

These methods and attributes expose and control the selection of input and textarea text fields.

element . select()

Selects everything in the text field.

element . selectionStart [ = value ]

Returns the offset to the start of the selection.

Can be set, to change the start of the selection.

element . selectionEnd [ = value ]

Returns the offset to the end of the selection.

Can be set, to change the end of the selection.

element . selectionDirection [ = value ]

Returns the current direction of the selection.

Can be set, to change the direction of the selection.

The possible values are "forward", "backward", and "none".

element . setSelectionRange(start, end [, direction] )

Changes the selection to cover the given substring in the given direction. If the direction is omitted, it will be reset to be the platform default (none or forward).

element . setRangeText(replacement [, start, end [, selectionMode ] ] )

Replaces a range of text with the new text. If the start and end arguments are not provided, the range is assumed to be the selection.

The final argument determines how the selection should be set after the text has been replaced. The possible values are:

"select": Selects the newly inserted text.
"start": Moves the selection to just before the inserted text.
"end": Moves the selection to just after the selected text.
"preserve": Attempts to preserve the selection. This is the default.

For input elements, calling these methods while they don't apply, and getting or setting these attributes while they don't apply, must throw an InvalidStateError exception. Otherwise, they must act as described below.

For input elements, these methods and attributes must operate on the element's value. For textarea elements, these methods and attributes must operate on the element's raw value.

Where possible, user interface features for changing the text selection in input and textarea elements must be implemented in terms of the DOM API described in this section, so that, e.g., all the same events fire.

The selections of input and textarea elements have a direction, which is either forward, backward, or none. This direction is set when the user manipulates the selection. The exact meaning of the selection direction depends on the platform.

On Windows, the direction indicates the position of the caret relative to the selection: a forward selection has the caret at the end of the selection and a backward selection has the caret at the start of the selection. Windows has no none direction. On Mac, the direction indicates which end of the selection is affected when the user adjusts the size of the selection using the arrow keys with the Shift modifier: the forward direction means the end of the selection is modified, and the backwards direction means the start of the selection is modified. The none direction is the default on Mac, it indicates that no particular direction has yet been selected. The user sets the direction implicitly when first adjusting the selection, based on which directional arrow key was used.

The select() method must cause the contents of the text field to be fully selected, with the selection direction being none, if the platform support selections with the direction none, or otherwise forward. The user agent must then queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

The selectionStart attribute must, on getting, return the offset (in logical order) to the character that immediately follows the start of the selection. If there is no selection, then it must return the offset (in logical order) to the character that immediately follows the text entry cursor.

On setting, it must act as if the setSelectionRange() method had been called, with the new value as the first argument; the current value of the selectionEnd attribute as the second argument, unless the current value of the selectionEnd is less than the new value, in which case the second argument must also be the new value; and the current value of the selectionDirection as the third argument.

The selectionEnd attribute must, on getting, return the offset (in logical order) to the character that immediately follows the end of the selection. If there is no selection, then it must return the offset (in logical order) to the character that immediately follows the text entry cursor.

On setting, it must act as if the setSelectionRange() method had been called, with the current value of the selectionStart attribute as the first argument, the new value as the second argument, and the current value of the selectionDirection as the third argument.

The selectionDirection attribute must, on getting, return the string corresponding to the current selection direction: if the direction is forward, "forward"; if the direction is backward, "backward"; and otherwise, "none".

On setting, it must act as if the setSelectionRange() method had been called, with the current value of the selectionStart attribute as the first argument, the current value of the selectionEnd attribute as the second argument, and the new value as the third argument.

The setSelectionRange(start, end, direction) method must set the selection of the text field to the sequence of characters starting with the character at the startth position (in logical order) and ending with the character at the (end-1)th position. Arguments greater than the length of the value of the text field must be treated as pointing at the end of the text field. If end is less than or equal to start then the start of the selection and the end of the selection must both be placed immediately before the character with offset end. In UAs where there is no concept of an empty selection, this must set the cursor to be just before the character with offset end. The direction of the selection must be set to backward if direction is a case-sensitive match for the string "backward", forward if direction is a case-sensitive match for the string "forward" or if the platform does not support selections with the direction none, and none otherwise (including if the argument is omitted). The user agent must then queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

The setRangeText(replacement, start, end, selectMode) method must run the following steps:

If the method has only one argument, then let start and end have the values of the selectionStart attribute and the selectionEnd attribute respectively.

Otherwise, let start, end have the values of the second and third arguments respectively.
If start is greater than end, then throw an IndexSizeError exception and abort these steps.
If start is greater than the length of the value of the text field, then set it to the length of the value of the text field.
If end is greater than the length of the value of the text field, then set it to the length of the value of the text field.
Let selection start be the current value of the selectionStart attribute.
Let selection end be the current value of the selectionEnd attribute.
If start is less than end, delete the sequence of characters starting with the character at the startth position (in logical order) and ending with the character at the (end-1)th position.
Insert the value of the first argument into the text of the value of the text field, immediately before the startth character.
Let new length be the length of the value of the first argument.
Let new end be the sum of start and new length.
Run the appropriate set of substeps from the following list:
If the fourth argument's value is "select"

Let selection start be start.

Let selection end be new end.

If the fourth argument's value is "start"

Let selection start and selection end be start.

If the fourth argument's value is "end"

Let selection start and selection end be new end.

If the fourth argument's value is "preserve" (the default)
1. Let old length be end minus start.
2. Let delta be new length minus old length.
3. If selection start is greater than end, then increment it by delta. (If delta is negative, i.e. the new text is shorter than the old text, then this will decrease the value of selection start.)
  
  Otherwise: if selection start is greater than start, then set it to start. (This snaps the start of the selection to the start of the new text if it was in the middle of the text that it replaced.)
4. If selection end is greater than end, then increment it by delta in the same way.
  
  Otherwise: if selection end is greater than start, then set it to new end. (This snaps the end of the selection to the end of the new text if it was in the middle of the text that it replaced.)
Set the selection of the text field to the sequence of characters starting with the character at the selection startth position (in logical order) and ending with the character at the (selection end-1)th position. In UAs where there is no concept of an empty selection, this must set the cursor to be just before the character with offset end. The direction of the selection must be set to forward if the platform does not support selections with the direction none, and none otherwise.
Queue a task to fire a simple event that bubbles named select at the element, using the user interaction task source as the task source.

All elements to which this API applies have either a selection or a text entry cursor position at all times (even for elements that are not being rendered). User agents should follow platform conventions to determine their initial state.

Characters with no visible rendering, such as U+200D ZERO WIDTH JOINER, still count as characters. Thus, for instance, the selection can include just an invisible character, and the text insertion cursor can be placed to one side or another of such a character.

To obtain the currently selected text, the following JavaScript suffices:

var selectionText = control.value.substring(control.selectionStart, control.selectionEnd);

...where control is the input or textarea element.

To add some text at the start of a text control, while maintaining the text selection, the three attributes must be preserved:

var oldStart = control.selectionStart;
var oldEnd = control.selectionEnd;
var oldDirection = control.selectionDirection;
var prefix = "http://";
control.value = prefix + control.value;
control.setSelectionRange(oldStart + prefix.length, oldEnd + prefix.length, oldDirection);

...where control is the input or textarea element.

4.10.21 Constraints

4.10.21.1 Definitions

A submittable element is a candidate for constraint validation except when a condition has barred the element from constraint validation. (For example, an element is barred from constraint validation if it is an object element.)

An element can have a custom validity error message defined. Initially, an element must have its custom validity error message set to the empty string. When its value is not the empty string, the element is suffering from a custom error. It can be set using the setCustomValidity() method. The user agent should use the custom validity error message when alerting the user to the problem with the control.

An element can be constrained in various ways. The following is the list of validity states that a form control can be in, making the control invalid for the purposes of constraint validation. (The definitions below are non-normative; other parts of this specification define more precisely when each state applies or does not.)

Suffering from being missing: When a control has no value but has a required attribute (input required, select required, textarea required), or, in the case of an element in a radio button group, any of the other elements in the group has a required attribute.
Suffering from a type mismatch: When a control that allows arbitrary user input has a value that is not in the correct syntax (E-mail, URL).
Suffering from a pattern mismatch: When a control has a value that doesn't satisfy the pattern attribute.
Suffering from being too long: When a control has a value that is too long for the form control maxlength attribute (input maxlength, textarea maxlength).
Suffering from being too short: When a control has a value that is too short for the form control minlength attribute (input minlength, textarea minlength).
Suffering from an underflow: When a control has a value that is too low for the min attribute.
Suffering from an overflow: When a control has a value that is too high for the max attribute.
Suffering from a step mismatch: When a control has a value that doesn't fit the rules given by the step attribute.
Suffering from bad input: When a control has incomplete input and the user agent does not think the user ought to be able to submit the form in its current state.
Suffering from a custom error: When a control's custom validity error message (as set by the element's setCustomValidity() method) is not the empty string.

An element can still suffer from these states even when the element is disabled; thus these states can be represented in the DOM even if validating the form during submission wouldn't indicate a problem to the user.

An element satisfies its constraints if it is not suffering from any of the above validity states.

4.10.21.2 Constraint validation

When the user agent is required to statically validate the constraints of form element form, it must run the following steps, which return either a positive result (all the controls in the form are valid) or a negative result (there are invalid controls) along with a (possibly empty) list of elements that are invalid and for which no script has claimed responsibility:

Let controls be a list of all the submittable elements whose form owner is form, in tree order.
Let invalid controls be an initially empty list of elements.
For each element field in controls, in tree order, run the following substeps:
1. If field is not a candidate for constraint validation, then move on to the next element.
2. Otherwise, if field satisfies its constraints, then move on to the next element.
3. Otherwise, add field to invalid controls.
If invalid controls is empty, then return a positive result and abort these steps.
Let unhandled invalid controls be an initially empty list of elements.
For each element field in invalid controls, if any, in tree order, run the following substeps:
1. Fire a simple event named invalid that is cancelable at field.
2. If the event was not canceled, then add field to unhandled invalid controls.
Return a negative result with the list of elements in the unhandled invalid controls list.

If a user agent is to interactively validate the constraints of form element form, then the user agent must run the following steps:

Statically validate the constraints of form, and let unhandled invalid controls be the list of elements returned if the result was negative.
If the result was positive, then return that result and abort these steps.
Report the problems with the constraints of at least one of the elements given in unhandled invalid controls to the user. User agents may focus one of those elements in the process, by running the focusing steps for that element, and may change the scrolling position of the document, or perform some other action that brings the element to the user's attention. User agents may report more than one constraint violation. User agents may coalesce related constraint violation reports if appropriate (e.g. if multiple radio buttons in a group are marked as required, only one error need be reported). If one of the controls is not being rendered (e.g. it has the hidden attribute set) then user agents may report a script error.
Return a negative result.

4.10.21.3 The constraint validation API

element . willValidate

Returns true if the element will be validated when the form is submitted; false otherwise.

element . setCustomValidity(message)

Sets a custom error, so that the element would fail to validate. The given message is the message to be shown to the user when reporting the problem to the user.

If the argument is the empty string, clears the custom error.

element . validity . valueMissing

Returns true if the element has no value but is a required field; false otherwise.

element . validity . typeMismatch

Returns true if the element's value is not in the correct syntax; false otherwise.

element . validity . patternMismatch

Returns true if the element's value doesn't match the provided pattern; false otherwise.

element . validity . tooLong

Returns true if the element's value is longer than the provided maximum length; false otherwise.

element . validity . tooShort

Returns true if the element's value, if it is not the empty string, is shorter than the provided minimum length; false otherwise.

element . validity . rangeUnderflow

Returns true if the element's value is lower than the provided minimum; false otherwise.

element . validity . rangeOverflow

Returns true if the element's value is higher than the provided maximum; false otherwise.

element . validity . stepMismatch

Returns true if the element's value doesn't fit the rules given by the step attribute; false otherwise.

element . validity . badInput

Returns true if the user has provided input in the user interface that the user agent is unable to convert to a value; false otherwise.

element . validity . customError

Returns true if the element has a custom error; false otherwise.

element . validity . valid

Returns true if the element's value has no validity problems; false otherwise.

valid = element . checkValidity()

Returns true if the element's value has no validity problems; false otherwise. Fires an invalid event at the element in the latter case.

element . validationMessage

Returns the error message that would be shown to the user if the element was to be checked for validity.

The willValidate attribute must return true if an element is a candidate for constraint validation, and false otherwise (i.e. false if any conditions are barring it from constraint validation).

The setCustomValidity(message), when invoked, must set the custom validity error message to the value of the given message argument.

In the following example, a script checks the value of a form control each time it is edited, and whenever it is not a valid value, uses the setCustomValidity() method to set an appropriate message.

<label>Feeling: <input name=f type="text" oninput="check(this)"></label>
<script>
 function check(input) {
   if (input.value == "good" ||
       input.value == "fine" ||
       input.value == "tired") {
     input.setCustomValidity('"' + input.value + '" is not a feeling.');
   } else {
     // input is fine -- reset the error message
     input.setCustomValidity('');
   }
 }
</script>

The validity attribute must return a ValidityState object that represents the validity states of the element. This object is live, and the same object must be returned each time the element's validity attribute is retrieved.

interface ValidityState {
  readonly attribute boolean valueMissing;
  readonly attribute boolean typeMismatch;
  readonly attribute boolean patternMismatch;
  readonly attribute boolean tooLong;
  readonly attribute boolean tooShort;
  readonly attribute boolean rangeUnderflow;
  readonly attribute boolean rangeOverflow;
  readonly attribute boolean stepMismatch;
  readonly attribute boolean badInput;
  readonly attribute boolean customError;
  readonly attribute boolean valid;
};

A ValidityState object has the following attributes. On getting, they must return true if the corresponding condition given in the following list is true, and false otherwise.

valueMissing: The control is suffering from being missing.
typeMismatch: The control is suffering from a type mismatch.
patternMismatch: The control is suffering from a pattern mismatch.
tooLong: The control is suffering from being too long.
tooShort: The control is suffering from being too short.
rangeUnderflow: The control is suffering from an underflow.
rangeOverflow: The control is suffering from an overflow.
stepMismatch: The control is suffering from a step mismatch.
badInput: The control is suffering from bad input.
customError: The control is suffering from a custom error.
valid: None of the other conditions are true.

When the checkValidity() method is invoked, if the element is a candidate for constraint validation and does not satisfy its constraints, the user agent must fire a simple event named invalid that is cancelable (but in this case has no default action) at the element and return false. Otherwise, it must only return true without doing anything else.

The validationMessage attribute must return the empty string if the element is not a candidate for constraint validation or if it is one but it satisfies its constraints; otherwise, it must return a suitably localized message that the user agent would show the user if this were the only form control with a validity constraint problem. If the user agent would not actually show a textual message in such a situation (e.g. it would show a graphical cue instead), then the attribute must return a suitably localized message that expresses (one or more of) the validity constraint(s) that the control does not satisfy. If the element is a candidate for constraint validation and is suffering from a custom error, then the custom validity error message should be present in the return value.

4.10.21.4 Security

Servers should not rely on client-side validation. Client-side validation can be intentionally bypassed by hostile users, and unintentionally bypassed by users of older user agents or automated tools that do not implement these features. The constraint validation features are only intended to improve the user experience, not to provide any kind of security mechanism.

4.10.22 Form submission

4.10.22.1 Introduction

This section is non-normative.

When a form is submitted, the data in the form is converted into the structure specified by the enctype, and then sent to the destination specified by the action using the given method.

For example, take the following form:

<form action="/find.cgi" method=get>
 <input type=text name=t>
 <input type=search name=q>
 <input type=submit>
</form>

If the user types in "cats" in the first field and "fur" in the second, and then hits the submit button, then the user agent will load /find.cgi?t=cats&q=fur.

On the other hand, consider this form:

<form action="/find.cgi" method=post enctype="multipart/form-data">
 <input type=text name=t>
 <input type=search name=q>
 <input type=submit>
</form>

Given the same user input, the result on submission is quite different: the user agent instead does an HTTP POST to the given URL, with as the entity body something like the following text:

------kYFrd4jNJEgCervE
Content-Disposition: form-data; name="t"

cats
------kYFrd4jNJEgCervE
Content-Disposition: form-data; name="q"

fur
------kYFrd4jNJEgCervE--

4.10.22.2 Implicit submission

A form element's default button is the first submit button in tree order whose form owner is that form element.

If the user agent supports letting the user submit a form implicitly (for example, on some platforms hitting the "enter" key while a text field is focused implicitly submits the form), then doing so for a form whose default button has a defined activation behavior must cause the user agent to run synthetic click activation steps on that default button.

Consequently, if the default button is disabled, the form is not submitted when such an implicit submission mechanism is used. (A button has no activation behavior when disabled.)

There are pages on the Web that are only usable if there is a way to implicitly submit forms, so user agents are strongly encouraged to support this.

If the form has no submit button, then the implicit submission mechanism must do nothing if the form has more than one field that blocks implicit submission, and must submit the form element from the form element itself otherwise.

For the purpose of the previous paragraph, an element is a field that blocks implicit submission of a form element if it is an input element whose form owner is that form element and whose type attribute is in one of the following states: Text, Search, URL, Telephone, E-mail, Password, Date, Time, Number

4.10.22.3 Form submission algorithm

When a form element form is submitted from an element submitter (typically a button), optionally with a submitted from submit() method flag set, the user agent must run the following steps:

Let form document be the form's Document.
If form document has no associated browsing context or its active sandboxing flag set has its sandboxed forms browsing context flag set, then abort these steps without doing anything.
Let form browsing context be the browsing context of form document.
If the submitted from submit() method flag is not set, and the submitter element's no-validate state is false, then interactively validate the constraints of form and examine the result: if the result is negative (the constraint validation concluded that there were invalid fields and probably informed the user of this) then fire a simple event named invalid at the form element and then abort these steps.
If the submitted from submit() method flag is not set, then fire a simple event that bubbles and is cancelable named submit, at form. If the event's default action is prevented (i.e. if the event is canceled) then abort these steps. Otherwise, continue (effectively the default action is to perform the submission).
Let form data set be the result of constructing the form data set for form in the context of submitter.
Let action be the submitter element's action.
If action is the empty string, let action be the document's address of the form document.
Resolve the URL action, relative to the submitter element. If this fails, abort these steps.
Let action be the resulting absolute URL.
Let action components be the resulting parsed URL.
Let scheme be the scheme of the resulting parsed URL.
Let enctype be the submitter element's enctype.
Let method be the submitter element's method.
Let target be the submitter element's target.
If the user indicated a specific browsing context to use when submitting the form, then let target browsing context be that browsing context. Otherwise, apply the rules for choosing a browsing context given a browsing context name using target as the name and form browsing context as the context in which the algorithm is executed, and let target browsing context be the resulting browsing context.
If target browsing context was created in the previous step, or, alternatively, if the form document has not yet completely loaded and the submitted from submit() method flag is set, then let replace be true. Otherwise, let it be false.

Otherwise, select the appropriate row in the table below based on the value of scheme as given by the first cell of each row. Then, select the appropriate cell on that row based on the value of method as given in the first cell of each column. Then, jump to the steps named in that cell and defined below the table.

	GET	POST
`http`	Mutate action URL	Submit as entity body
`https`	Mutate action URL	Submit as entity body
`ftp`	Get action URL	Get action URL
`javascript`	Get action URL	Get action URL
`data`	Get action URL	Post to data:
`mailto`	Mail with headers	Mail as body

If scheme is not one of those listed in this table, then the behavior is not defined by this specification. User agents should, in the absence of another specification defining this, act in a manner analogous to that defined in this specification for similar schemes.

Each form element has a planned navigation, which is either null or a task; when the form is first created, its planned navigation must be set to null. In the behaviours described below, when the user agent is required to plan to navigate to a particular resource destination, it must run the following steps:

If the form has a non-null planned navigation, remove it from its task queue.
Let the form's planned navigation be a new task that consists of running the following steps:
1. Let the form's planned navigation be null.
2. Navigate target browsing context to the particular resource destination. If replace is true, then target browsing context must be navigated with replacement enabled.
For the purposes of this task, target browsing context and replace are the variables that were set up when the overall form submission algorithm was run, with their values as they stood when this planned navigation was queued.
Queue the task that is the form's new planned navigation.

The task source for this task is the DOM manipulation task source.

The behaviors are as follows:

Mutate action URL

Let query be the result of encoding the form data set using the application/x-www-form-urlencoded encoding algorithm, interpreted as a US-ASCII string.

Set parsed action's query component to query.

Let destination be a new URL formed by applying the URL serializer algorithm to parsed action.

Plan to navigate to destination.

Submit as entity body

Let entity body be the result of encoding the form data set using the appropriate form encoding algorithm.

Let MIME type be determined as follows:

If enctype is application/x-www-form-urlencoded: Let MIME type be "application/x-www-form-urlencoded".
If enctype is multipart/form-data: Let MIME type be the concatenation of the string "multipart/form-data;", a U+0020 SPACE character, the string "boundary=", and the multipart/form-data boundary string generated by the multipart/form-data encoding algorithm.
If enctype is text/plain: Let MIME type be "text/plain".

Otherwise, plan to navigate to action using the HTTP method given by method and with entity body as the entity body, of type MIME type.

Get action URL

Plan to navigate to action.

The form data set is discarded.

Post to data:

Let data be the result of encoding the form data set using the appropriate form encoding algorithm.

If action contains the string "%%%%" (four U+0025 PERCENT SIGN characters), then percent encode all bytes in data that, if interpreted as US-ASCII, are not characters in the URL default encode set, and then, treating the result as a US-ASCII string, UTF-8 percent encode all the U+0025 PERCENT SIGN characters in the resulting string and replace the first occurrence of "%%%%" in action with the resulting doubly-escaped string. [URL]

Otherwise, if action contains the string "%%" (two U+0025 PERCENT SIGN characters in a row, but not four), then UTF-8 percent encode all characters in data that, if interpreted as US-ASCII, are not characters in the URL default encode set, and then, treating the result as a US-ASCII string, replace the first occurrence of "%%" in action with the resulting escaped string. [URL]

Plan to navigate to the potentially modified action (which will be a data: URL).

Mail with headers

Let headers be the resulting encoding the form data set using the application/x-www-form-urlencoded encoding algorithm, interpreted as a US-ASCII string.

Replace occurrences of "+" (U+002B) characters in headers with the string "%20".

Let destination consist of all the characters from the first character in action to the character immediately before the first "?" (U+003F) character, if any, or the end of the string if there are none.

Append a single "?" (U+003F) character to destination.

Append headers to destination.

Plan to navigate to destination.

Mail as body

Let body be the resulting of encoding the form data set using the appropriate form encoding algorithm and then percent encoding all the bytes in the resulting byte string that, when interpreted as US-ASCII, are not characters in the URL default encode set. [URL]

Let destination have the same value as action.

If destination does not contain a "?" (U+003F) character, append a single "?" (U+003F) character to destination. Otherwise, append a single U+0026 AMPERSAND character (&).

Append the string "body=" to destination.

Append body, interpreted as a US-ASCII string, to destination.

Plan to navigate to destination.

The appropriate form encoding algorithm is determined as follows:

If enctype is application/x-www-form-urlencoded: Use the application/x-www-form-urlencoded encoding algorithm.
If enctype is multipart/form-data: Use the multipart/form-data encoding algorithm.
If enctype is text/plain: Use the text/plain encoding algorithm.

4.10.22.4 Constructing the form data set

The algorithm to construct the form data set for a form form optionally in the context of a submitter submitter is as follows. If not specified otherwise, submitter is null.

Let controls be a list of all the submittable elements whose form owner is form, in tree order.
Let the form data set be a list of name-value-type tuples, initially empty.
Loop: For each element field in controls, in tree order, run the following substeps:
1. If any of the following conditions are met, then skip these substeps for this element:
  - The field element has a datalist element ancestor.
  - The field element is disabled.
  - The field element is a button but it is not submitter.
  - The field element is an input element whose type attribute is in the Checkbox state and whose checkedness is false.
  - The field element is an input element whose type attribute is in the Radio Button state and whose checkedness is false.
  - The field element is not an input element whose type attribute is in the Image Button state, and either the field element does not have a name attribute specified, or its name attribute's value is the empty string.
  - The field element is an object element that is not using a plugin.
  Otherwise, process field as follows:
2. Let type be the value of the type IDL attribute of field.
3. If the field element is an input element whose type attribute is in the Image Button state, then run these further nested substeps:
  1. If the field element has a name attribute specified and its value is not the empty string, let name be that value followed by a single "." (U+002E) character. Otherwise, let name be the empty string.
  2. Let name_x be the string consisting of the concatenation of name and a single U+0078 LATIN SMALL LETTER X character (x).
  3. Let name_y be the string consisting of the concatenation of name and a single U+0079 LATIN SMALL LETTER Y character (y).
  4. The field element is submitter, and before this algorithm was invoked the user indicated a coordinate. Let x be the x-component of the coordinate selected by the user, and let y be the y-component of the coordinate selected by the user.
  5. Append an entry to the form data set with the name name_x, the value x, and the type type.
  6. Append an entry to the form data set with the name name_y and the value y, and the type type.
  7. Skip the remaining substeps for this element: if there are any more elements in controls, return to the top of the loop step, otherwise, jump to the end step below.
4. Let name be the value of the field element's name attribute.
5. If the field element is a select element, then for each option element in the select element's list of options whose selectedness is true and that is not disabled, append an entry to the form data set with the name as the name, the value of the option element as the value, and type as the type.
6. Otherwise, if the field element is an input element whose type attribute is in the Checkbox state or the Radio Button state, then run these further nested substeps:
  1. If the field element has a value attribute specified, then let value be the value of that attribute; otherwise, let value be the string "on".
  2. Append an entry to the form data set with name as the name, value as the value, and type as the type.
7. Otherwise, if the field element is an input element whose type attribute is in the File Upload state, then for each file selected in the input element, append an entry to the form data set with the name as the name, the file (consisting of the name, the type, and the body) as the value, and type as the type. If there are no selected files, then append an entry to the form data set with the name as the name, the empty string as the value, and application/octet-stream as the type.
8. Otherwise, if the field element is an object element: try to obtain a form submission value from the plugin, and if that is successful, append an entry to the form data set with name as the name, the returned form submission value as the value, and the string "object" as the type.
9. Otherwise, append an entry to the form data set with name as the name, the value of the field element as the value, and type as the type.
10. If the element has a dirname attribute, and that attribute's value is not the empty string, then run these substeps:
  1. Let dirname be the value of the element's dirname attribute.
  2. Let dir be the string "ltr" if the directionality of the element is 'ltr', and "rtl" otherwise (i.e. when the directionality of the element is 'rtl').
  3. Append an entry to the form data set with dirname as the name, dir as the value, and the string "direction" as the type.
  An element can only have a dirname attribute if it is a textarea element or an input element whose type attribute is in either the Text state or the Search state.
End: For the name of each entry in the form data set, and for the value of each entry in the form data set whose type is not "file" or "textarea", replace every occurrence of a "CR" (U+000D) character not followed by a "LF" (U+000A) character, and every occurrence of a "LF" (U+000A) character not preceded by a "CR" (U+000D) character, by a two-character string consisting of a U+000D CARRIAGE RETURN "CRLF" (U+000A) character pair.

In the case of the value of textarea elements, this newline normalization is already performed during the conversion of the control's raw value into the control's value (which also performs any necessary line wrapping). In the case of input elements type attributes in the File Upload state, the value is not normalized.
Return the form data set.

4.10.22.5 Selecting a form submission encoding

If the user agent is to pick an encoding for a form, optionally with an allow non-ASCII-compatible encodings flag set, it must run the following substeps:

Let input be the value of the form element's accept-charset attribute.
Let candidate encoding labels be the result of splitting input on spaces.
Let candidate encodings be an empty list of character encodings.
For each token in candidate encoding labels in turn (in the order in which they were found in input), get an encoding for the token and, if this does not result in failure, append the encoding to candidate encodings.
If the allow non-ASCII-compatible encodings flag is not set, remove any encodings that are not ASCII-compatible character encodings from candidate encodings.
If candidate encodings is empty, return UTF-8 and abort these steps.
Each character encoding in candidate encodings can represent a finite number of characters. (For example, UTF-8 can represent all 1.1 million or so Unicode code points, while Windows-1252 can only represent 256.)

For each encoding in candidate encodings, determine how many of the characters in the names and values of the entries in the form data set the encoding can represent (without ignoring duplicates). Let max be the highest such count. (For UTF-8, max would equal the number of characters in the names and values of the entries in the form data set.)

Return the first encoding in candidate encodings that can encode max characters in the names and values of the entries in the form data set.

4.10.22.6 URL-encoded form data

This form data set encoding is in many ways an aberrant monstrosity, the result of many years of implementation accidents and compromises leading to a set of requirements necessary for interoperability, but in no way representing good design practices. In particular, readers are cautioned to pay close attention to the twisted details involving repeated (and in some cases nested) conversions between character encodings and byte sequences.

The application/x-www-form-urlencoded encoding algorithm is as follows:

Let result be the empty string.
If the form element has an accept-charset attribute, let the selected character encoding be the result of picking an encoding for the form.

Otherwise, if the form element has no accept-charset attribute, but the document's character encoding is an ASCII-compatible character encoding, then that is the selected character encoding.

Otherwise, let the selected character encoding be UTF-8.
Let charset be the name of the selected character encoding.
For each entry in the form data set, perform these substeps:
1. If the entry's name is "_charset_" and its type is "hidden", replace its value with charset.
2. If the entry's type is "file", replace its value with the file's name only.
3. For each character in the entry's name and value that cannot be expressed using the selected character encoding, replace the character by a string consisting of a U+0026 AMPERSAND character (&), a "#" (U+0023) character, one or more ASCII digits representing the Unicode code point of the character in base ten, and finally a ";" (U+003B) character.
4. Encode the entry's name and value using the encoder for the selected character encoding. The entry's name and value are now byte strings.
5. For each byte in the entry's name and value, apply the appropriate subsubsteps from the following list:
  If the byte is 0x20 (U+0020 SPACE if interpreted as ASCII)
  
  Replace the byte with a single 0x2B byte ("+" (U+002B) character if interpreted as ASCII).
  
  If the byte is in the range 0x2A, 0x2D, 0x2E, 0x30 to 0x39, 0x41 to 0x5A, 0x5F, 0x61 to 0x7A
  
  Leave the byte as is.
  
  Otherwise
  1. Let s be a string consisting of a U+0025 PERCENT SIGN character (%) followed by uppercase ASCII hex digits representing the hexadecimal value of the byte in question (zero-padded if necessary).
  2. Encode the string s as US-ASCII, so that it is now a byte string.
  3. Replace the byte in question in the name or value being processed by the bytes in s, preserving their relative order.
6. Interpret the entry's name and value as Unicode strings encoded in US-ASCII. (All of the bytes in the string will be in the range 0x00 to 0x7F; the high bit will be zero throughout.) The entry's name and value are now Unicode strings again.
7. If the entry's name is "isindex", its type is "text", and this is the first entry in the form data set, then append the value to result and skip the rest of the substeps for this entry, moving on to the next entry, if any, or the next step in the overall algorithm otherwise.
8. If this is not the first entry, append a single U+0026 AMPERSAND character (&) to result.
9. Append the entry's name to result.
10. Append a single "=" (U+003D) character to result.
11. Append the entry's value to result.
Encode result as US-ASCII and return the resulting byte stream.

To decode application/x-www-form-urlencoded payloads, the following algorithm should be used. This algorithm uses as inputs the payload itself, payload, consisting of a Unicode string using only characters in the range U+0000 to U+007F; a default character encoding encoding; and optionally an isindex flag indicating that the payload is to be processed as if it had been generated for a form containing an isindex control. The output of this algorithm is a sorted list of name-value pairs. If the isindex flag is set and the first control really was an isindex control, then the first name-value pair will have as its name the empty string.

Which default character encoding to use can only be determined on a case-by-case basis, but generally the best character encoding to use as a default is the one that was used to encode the page on which the form used to create the payload was itself found. In the absence of a better default, UTF-8 is suggested.

The isindex flag is for legacy use only. Forms in conforming HTML documents will not generate payloads that need to be decoded with this flag set.

Let strings be the result of strictly splitting the string payload on U+0026 AMPERSAND characters (&).
If the isindex flag is set and the first string in strings does not contain a "=" (U+003D) character, insert a "=" (U+003D) character at the start of the first string in strings.
Let pairs be an empty list of name-value pairs.
For each string string in strings, run these substeps:
1. If string contains a "=" (U+003D) character, then let name be the substring of string from the start of string up to but excluding its first "=" (U+003D) character, and let value be the substring from the first character, if any, after the first "=" (U+003D) character up to the end of string. If the first "=" (U+003D) character is the first character, then name will be the empty string. If it is the last character, then value will be the empty string.
  
  Otherwise, string contains no "=" (U+003D) characters. Let name have the value of string and let value be the empty string.
2. Replace any "+" (U+002B) characters in name and value with U+0020 SPACE characters.
3. Replace any escape in name and value with the character represented by the escape. This replacement must not be recursive.
  
  An escape is a "%" (U+0025) character followed by two ASCII hex digits.
  
  The character represented by an escape is the Unicode character whose code point is equal to the value of the two characters after the "%" (U+0025) character, interpreted as a hexadecimal number (in the range 0..255).
  
  So for instance the string "A%2BC" would become "A+C". Similarly, the string "100%25AA%21" becomes the string "100%AA!".
4. Convert the name and value strings to their byte representation in ISO-8859-1 (i.e. convert the Unicode string to a byte string, mapping code points to byte values directly).
5. Add a pair consisting of name and value to pairs.
If any of the name-value pairs in pairs have a name component consisting of the string "_charset_" encoded in US-ASCII, and the value component of the first such pair, when decoded as US-ASCII, is the name of a supported character encoding, then let encoding be that character encoding (replacing the default passed to the algorithm).
Convert the name and value components of each name-value pair in pairs to Unicode by interpreting the bytes according to the encoding encoding.
Return pairs.

Parameters on the application/x-www-form-urlencoded MIME type are ignored. In particular, this MIME type does not support the charset parameter.

4.10.22.7 Multipart form data

The multipart/form-data encoding algorithm is as follows:

Let result be the empty string.
If the algorithm was invoked with an explicit character encoding, let the selected character encoding be that encoding. (This algorithm is used by other specifications, which provide an explicit character encoding to avoid the dependency on the form element described in the next paragraph.)

Otherwise, if the form element has an accept-charset attribute, let the selected character encoding be the result of picking an encoding for the form.

Otherwise, if the form element has no accept-charset attribute, but the document's character encoding is an ASCII-compatible character encoding, then that is the selected character encoding.

Otherwise, let the selected character encoding be UTF-8.
Let charset be the name of the selected character encoding.
For each entry in the form data set, perform these substeps:
1. If the entry's name is "_charset_" and its type is "hidden", replace its value with charset.
2. For each character in the entry's name and value that cannot be expressed using the selected character encoding, replace the character by a string consisting of a U+0026 AMPERSAND character (&), a "#" (U+0023) character, one or more ASCII digits representing the Unicode code point of the character in base ten, and finally a ";" (U+003B) character.
Encode the (now mutated) form data set using the rules described by RFC 2388, Returning Values from Forms: multipart/form-data, and return the resulting byte stream. [RFC2388]

Each entry in the form data set is a field, the name of the entry is the field name and the value of the entry is the field value.

The order of parts must be the same as the order of fields in the form data set. Multiple entries with the same name must be treated as distinct fields.

In particular, this means that multiple files submitted as part of a single <input type=file multiple> element will result in each file having its own field; the "sets of files" feature ("multipart/mixed") of RFC 2388 is not used.

The parts of the generated multipart/form-data resource that correspond to non-file fields must not have a Content-Type header specified. Their names and values must be encoded using the character encoding selected above (field names in particular do not get converted to a 7-bit safe encoding as suggested in RFC 2388).

File names included in the generated multipart/form-data resource (as part of file fields) must use the character encoding selected above, though the precise name may be approximated if necessary (e.g. newlines could be removed from file names, quotes could be changed to "%22", and characters not expressible in the selected character encoding could be replaced by other characters). User agents must not use the RFC 2231 encoding suggested by RFC 2388.

The boundary used by the user agent in generating the return value of this algorithm is the multipart/form-data boundary string. (This value is used to generate the MIME type of the form submission payload generated by this algorithm.)

For details on how to interpret multipart/form-data payloads, see RFC 2388. [RFC2388]

4.10.22.8 Plain text form data

The text/plain encoding algorithm is as follows:

Let result be the empty string.
If the form element has an accept-charset attribute, let the selected character encoding be the result of picking an encoding for the form, with the allow non-ASCII-compatible encodings flag unset.

Otherwise, if the form element has no accept-charset attribute, then that is the selected character encoding.
Let charset be the name of the selected character encoding.
If the entry's name is "_charset_" and its type is "hidden", replace its value with charset.
If the entry's type is "file", replace its value with the file's name only.
For each entry in the form data set, perform these substeps:
1. Append the entry's name to result.
2. Append a single "=" (U+003D) character to result.
3. Append the entry's value to result.
4. Append a "CR" (U+000D) "LF" (U+000A) character pair to result.
Encode result using the encoder for the selected character encoding and return the resulting byte stream.

Payloads using the text/plain format are intended to be human readable. They are not reliably interpretable by computer, as the format is ambiguous (for example, there is no way to distinguish a literal newline in a value from the newline at the end of the value).

4.10.23 Resetting a form

When a form element form is reset, the user agent must fire a simple event named reset, that bubbles and is cancelable, at form, and then, if that event is not canceled, must invoke the reset algorithm of each resettable element whose form owner is form.

Each resettable element defines its own reset algorithm. Changes made to form controls as part of these algorithms do not count as changes caused by the user (and thus, e.g., do not cause input events to fire).

4.11 Scripting

Scripts allow authors to add interactivity to their documents.

Authors are encouraged to use declarative alternatives to scripting where possible, as declarative mechanisms are often more maintainable, and many users disable scripting.

Authors are also encouraged to make their applications degrade gracefully in the absence of scripting support.

4.11.1 The `script` element

Where metadata content is expected.

Where phrasing content is expected.

Where script-supporting elements are expected.

If there is no src attribute, depends on the value of the type attribute, but must match script content restrictions.

If there is a src attribute, the element must be either empty or contain only script documentation that also matches script content restrictions.

src - Address of the resource

type - Type of embedded resource

charset - Character encoding of the external script resource

async - Execute script asynchronously

defer - Defer script execution

crossorigin - How the element handles crossorigin requests

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

None

Script-supporting element.

interface HTMLScriptElement : HTMLElement {
           attribute DOMString src;
           attribute DOMString type;
           attribute DOMString charset;
           attribute boolean async;
           attribute boolean defer;
           attribute DOMString crossOrigin;
           attribute DOMString text;
};

The script element allows authors to include dynamic script and data blocks in their documents. The element does not represent content for the user.

When used to include dynamic scripts, the scripts may either be embedded inline or may be imported from an external file using the src attribute. If the language is not that described by "text/javascript", then the type attribute must be present, as described below. Whatever language is used, the contents of the script element must conform with the requirements of that language's specification.

When used to include data blocks (as opposed to scripts), the data must be embedded inline, the format of the data must be given using the type attribute, the src attribute must not be specified, and the contents of the script element must conform to the requirements defined for the format used.

The type attribute gives the language of the script or format of the data. If the attribute is present, its value must be a valid MIME type. The charset parameter must not be specified. The default, which is used if the attribute is absent, is "text/javascript".

The src attribute, if specified, gives the address of the external script resource to use. The value of the attribute must be a valid non-empty URL potentially surrounded by spaces identifying a script resource of the type given by the type attribute, if the attribute is present, or of the type "text/javascript", if the attribute is absent. A resource is a script resource of a given type if that type identifies a scripting language and the resource conforms with the requirements of that language's specification.

The charset attribute gives the character encoding of the external script resource. The attribute must not be specified if the src attribute is not present. If the attribute is set, its value must be an ASCII case-insensitive match for one of the labels of an encoding, and must specify the same encoding as the charset parameter of the Content-Type metadata of the external file, if any. [ENCODING]

The async and defer attributes are boolean attributes that indicate how the script should be executed. The defer and async attributes must not be specified if the src attribute is not present.

There are three possible modes that can be selected using these attributes. If the async attribute is present, then the script will be executed asynchronously, as soon as it is available. If the async attribute is not present but the defer attribute is present, then the script is executed when the page has finished parsing. If neither attribute is present, then the script is fetched and executed immediately, before the user agent continues parsing the page.

The exact processing details for these attributes are, for mostly historical reasons, somewhat non-trivial, involving a number of aspects of HTML. The implementation requirements are therefore by necessity scattered throughout the specification. The algorithms below (in this section) describe the core of this processing, but these algorithms reference and are referenced by the parsing rules for script start and end tags in HTML, in foreign content, and in XML, the rules for the document.write() method, the handling of scripting, etc.

The defer attribute may be specified even if the async attribute is specified, to cause legacy Web browsers that only support defer (and not async) to fall back to the defer behavior instead of the synchronous blocking behavior that is the default.

The crossorigin attribute is a CORS settings attribute. It controls, for scripts that are obtained from other origins, whether error information will be exposed.

Changing the src, type, charset, async, defer, and crossorigin attributes dynamically has no direct effect; these attribute are only used at specific times described below.

A script element has several associated pieces of state.

The first is a flag indicating whether or not the script block has been "already started". Initially, script elements must have this flag unset (script blocks, when created, are not "already started"). The cloning steps for script elements must set the "already started" flag on the copy if it is set on the element being cloned.

The second is a flag indicating whether the element was "parser-inserted". Initially, script elements must have this flag unset. It is set by the HTML parser and the XML parser on script elements they insert and affects the processing of those elements.

The third is a flag indicating whether the element will "force-async". Initially, script elements must have this flag set. It is unset by the HTML parser and the XML parser on script elements they insert. In addition, whenever a script element whose "force-async" flag is set has a async content attribute added, the element's "force-async" flag must be unset.

The fourth is a flag indicating whether or not the script block is "ready to be parser-executed". Initially, script elements must have this flag unset (script blocks, when created, are not "ready to be parser-executed"). This flag is used only for elements that are also "parser-inserted", to let the parser know when to execute the script.

The last few pieces of state are the script block's type, the script block's character encoding, and the script block's fallback character encoding. They are determined when the script is prepared, based on the attributes on the element at that time, and the Document of the script element.

When a script element that is not marked as being "parser-inserted" experiences one of the events listed in the following list, the user agent must synchronously prepare the script element:

The script element gets inserted into a document, at the time the node is inserted according to the DOM, after any other script elements inserted at the same time that are earlier in the Document in tree order.
The script element is in a Document and a node or document fragment is inserted into the script element, after any script elements inserted at that time.
The script element is in a Document and has a src attribute set where previously the element had no such attribute.

To prepare a script, the user agent must act as follows:

If the script element is marked as having "already started", then the user agent must abort these steps at this point. The script is not executed.
If the element has its "parser-inserted" flag set, then set was-parser-inserted to true and unset the element's "parser-inserted" flag. Otherwise, set was-parser-inserted to false.

This is done so that if parser-inserted script elements fail to run when the parser tries to run them, e.g. because they are empty or specify an unsupported scripting language, another script can later mutate them and cause them to run again.
If was-parser-inserted is true and the element does not have an async attribute, then set the element's "force-async" flag to true.

This is done so that if a parser-inserted script element fails to run when the parser tries to run it, but it is later executed after a script dynamically updates it, it will execute asynchronously even if the async attribute isn't set.
If the element has no src attribute, and its child nodes, if any, consist only of comment nodes and empty Text nodes, then the user agent must abort these steps at this point. The script is not executed.
If the element is not in a Document, then the user agent must abort these steps at this point. The script is not executed.
If either:
- the script element has a type attribute and its value is the empty string, or
- the script element has no type attribute but it has a language attribute and that attribute's value is the empty string, or
- the script element has neither a type attribute nor a language attribute, then
...let the script block's type for this script element be "text/javascript".

Otherwise, if the script element has a type attribute, let the script block's type for this script element be the value of that attribute with any leading or trailing sequences of space characters removed.

Otherwise, the element has a non-empty language attribute; let the script block's type for this script element be the concatenation of the string "text/" followed by the value of the language attribute.

The language attribute is never conforming, and is always ignored if there is a type attribute present.
If the user agent does not support the scripting language given by the script block's type for this script element, then the user agent must abort these steps at this point. The script is not executed.
If was-parser-inserted is true, then flag the element as "parser-inserted" again, and set the element's "force-async" flag to false.
The user agent must set the element's "already started" flag.

The state of the element at this moment is later used to determine the script source.
If the element is flagged as "parser-inserted", but the element's Document is not the Document of the parser that created the element, then abort these steps.
If scripting is disabled for the script element, then the user agent must abort these steps at this point. The script is not executed.

The definition of scripting is disabled means that, amongst others, the following scripts will not execute: scripts in XMLHttpRequest's responseXML documents, scripts in DOMParser-created documents, scripts in documents created by XSLTProcessor's transformToDocument feature, and scripts that are first inserted by a script into a Document that was created using the createDocument() API. [XHR] [DOMPARSING] [DOM]
If the script element has an event attribute and a for attribute, then run these substeps:
1. Let for be the value of the for attribute.
2. Let event be the value of the event attribute.
3. Strip leading and trailing whitespace from event and for.
4. If for is not an ASCII case-insensitive match for the string "window", then the user agent must abort these steps at this point. The script is not executed.
5. If event is not an ASCII case-insensitive match for either the string "onload" or the string "onload()", then the user agent must abort these steps at this point. The script is not executed.
If the script element has a charset attribute, then let the script block's character encoding for this script element be the result of getting an encoding from the value of the charset attribute.

Otherwise, let the script block's fallback character encoding for this script element be the same as the encoding of the document itself.

Only one of these two pieces of state is set.
If the element has a src content attribute, run these substeps:
1. Let src be the value of the element's src attribute.
2. If src is the empty string, queue a task to fire a simple event named error at the element, and abort these steps.
3. Resolve src relative to the element.
4. If the previous step failed, queue a task to fire a simple event named error at the element, and abort these steps.
5. Do a potentially CORS-enabled fetch of the resulting absolute URL, with the mode being the current state of the element's crossorigin content attribute, the origin being the origin of the script element's Document, and the default origin behaviour set to taint.
  
  The resource obtained in this fashion can be either CORS-same-origin or CORS-cross-origin. This only affects how error reporting happens.
  
  For performance reasons, user agents may start fetching the script (as defined above) as soon as the src attribute is set, instead, in the hope that the element will be inserted into the document (and that the crossorigin attribute won't change value in the meantime). Either way, once the element is inserted into the document, the load must have started as described in this step. If the UA performs such prefetching, but the element is never inserted in the document, or the src attribute is dynamically changed, or the crossorigin attribute is dynamically changed, then the user agent will not execute the script so obtained, and the fetching process will have been effectively wasted.
Then, the first of the following options that describes the situation must be followed:
If the element has a src attribute, and the element has a defer attribute, and the element has been flagged as "parser-inserted", and the element does not have an async attribute

The element must be added to the end of the list of scripts that will execute when the document has finished parsing associated with the Document of the parser that created the element.

The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

If the element has a src attribute, and the element has been flagged as "parser-inserted", and the element does not have an async attribute

The element is the pending parsing-blocking script of the Document of the parser that created the element. (There can only be one such script per Document at a time.)

The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

If the element does not have a src attribute, and the element has been flagged as "parser-inserted", and either the parser that created the script is an XML parser or it's an HTML parser whose script nesting level is not greater than one, and the Document of the HTML parser or XML parser that created the script element has a style sheet that is blocking scripts

The element is the pending parsing-blocking script of the Document of the parser that created the element. (There can only be one such script per Document at a time.)

Set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

If the element has a src attribute, does not have an async attribute, and does not have the "force-async" flag set
The element must be added to the end of the list of scripts that will execute in order as soon as possible associated with the Document of the script element at the time the prepare a script algorithm started.

The task that the networking task source places on the task queue once the fetching algorithm has completed must run the following steps:
1. If the element is not now the first element in the list of scripts that will execute in order as soon as possible to which it was added above, then mark the element as ready but abort these steps without executing the script yet.
2. Execution: Execute the script block corresponding to the first script element in this list of scripts that will execute in order as soon as possible.
3. Remove the first element from this list of scripts that will execute in order as soon as possible.
4. If this list of scripts that will execute in order as soon as possible is still not empty and the first entry has already been marked as ready, then jump back to the step labeled execution.
If the element has a src attribute

The element must be added to the set of scripts that will execute as soon as possible of the Document of the script element at the time the prepare a script algorithm started.

The task that the networking task source places on the task queue once the fetching algorithm has completed must execute the script block and then remove the element from the set of scripts that will execute as soon as possible.

Otherwise

The user agent must immediately execute the script block, even if other scripts are already executing.

Fetching an external script must delay the load event of the element's document until the task that is queued by the networking task source once the resource has been fetched (defined above) has been run.

The pending parsing-blocking script of a Document is used by the Document's parser(s).

If a script element that blocks a parser gets moved to another Document before it would normally have stopped blocking that parser, it nonetheless continues blocking that parser until the condition that causes it to be blocking the parser no longer applies (e.g. if the script is a pending parsing-blocking script because there was a style sheet that is blocking scripts when it was parsed, but then the script is moved to another Document before the style sheet loads, the script still blocks the parser until the style sheets are all loaded, at which time the script executes and the parser is unblocked).

When the user agent is required to execute a script block, it must run the following steps:

If the element is flagged as "parser-inserted", but the element's Document is not the Document of the parser that created the element, then abort these steps.
Jump to the appropriate set of steps from the list below:
If the load resulted in an error (for example a DNS error, or an HTTP 404 error)

Executing the script block must just consist of firing a simple event named error at the element.

If the load was successful
Executing the script block must consist of running the following steps. For the purposes of these steps, the script is considered to be from an external file if, while the prepare a script algorithm above was running for this script, the script element had a src attribute specified.
1. Initialize the script block's source as follows:
  If the script is from an external file and the script block's type is a text-based language
  
  The contents of that file, interpreted as a Unicode string, are the script source.
  
  To obtain the Unicode string, the user agent run the following steps:
  
  If the resource's Content Type metadata, if any, specifies a character encoding, and the user agent supports that encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.
  
  If the algorithm above set the script block's character encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.
  
  Let character encoding be the script block's fallback character encoding.
  
  If the specification for the script block's type gives specific rules for decoding files in that format to Unicode, follow them, using character encoding as the character encoding specified by higher-level protocols, if necessary.
  
  Otherwise, decode the file to Unicode, using character encoding as the fallback encoding.
  
  The decode algorithm overrides character encoding if the file contains a BOM.
  
  If the script is from an external file and the script block's type is an XML-based language
  
  The external file is the script source. When it is later executed, it must be interpreted in a manner consistent with the specification defining the language given by the script block's type.
  
  If the script is inline and the script block's type is a text-based language
  
  The value of the text IDL attribute at the time the element's "already started" flag was last set is the script source.
  
  If the script is inline and the script block's type is an XML-based language
  
  The child nodes of the script element at the time the element's "already started" flag was last set are the script source.
2. Fire a simple event named beforescriptexecute that bubbles and is cancelable at the script element.
  
  If the event is canceled, then abort these steps.
3. If the script is from an external file, then increment the ignore-destructive-writes counter of the script element's Document. Let neutralized doc be that Document.
4. Create a script, using the script block's source, the URL from which the script was obtained, the script block's type as the scripting language, and the script settings object of the script element's Document's Window object.
  
  If the script came from a resource that was fetched in the steps above, and the resource was CORS-cross-origin, then pass the muted errors flag to the create a script algorithm as well.
  
  This is where the script is compiled and actually executed.
5. Decrement the ignore-destructive-writes counter of neutralized doc, if it was incremented in the earlier step.
6. Fire a simple event named afterscriptexecute that bubbles (but is not cancelable) at the script element.
7. If the script is from an external file, fire a simple event named load at the script element.
  
  Otherwise, the script is internal; queue a task to fire a simple event named load at the script element.

The IDL attributes src, type, charset, defer, each must reflect the respective content attributes of the same name.

The crossOrigin IDL attribute must reflect the crossorigin content attribute, limited to only known values.

The async IDL attribute controls whether the element will execute asynchronously or not. If the element's "force-async" flag is set, then, on getting, the async IDL attribute must return true, and on setting, the "force-async" flag must first be unset, and then the content attribute must be removed if the IDL attribute's new value is false, and must be set to the empty string if the IDL attribute's new value is true. If the element's "force-async" flag is not set, the IDL attribute must reflect the async content attribute.

script . text [ = value ]

Returns the contents of the element, ignoring child nodes that aren't Text nodes.

Can be set, to replace the element's children with the given value.

The IDL attribute text must return a concatenation of the contents of all the Text nodes that are children of the script element (ignoring any other nodes such as comments or elements), in tree order. On setting, it must act the same way as the textContent IDL attribute.

When inserted using the document.write() method, script elements execute (typically synchronously), but when inserted using innerHTML and outerHTML attributes, they do not execute at all.

In this example, two script elements are used. One embeds an external script, and the other includes some data.

<script src="game-engine.js"></script>
<script type="text/x-game-map">
........U.........e
o............A....e
.....A.....AAA....e
.A..AAA...AAAAA...e
</script>

The data in this case might be used by the script to generate the map of a video game. The data doesn't have to be used that way, though; maybe the map data is actually embedded in other parts of the page's markup, and the data block here is just used by the site's search engine to help users who are looking for particular features in their game maps.

The following sample shows how a script element can be used to define a function that is then used by other parts of the document. It also shows how a script element can be used to invoke script while the document is being parsed, in this case to initialize the form's output.

<script>
 function calculate(form) {
   var price = 52000;
   if (form.elements.brakes.checked)
     price += 1000;
   if (form.elements.radio.checked)
     price += 2500;
   if (form.elements.turbo.checked)
     price += 5000;
   if (form.elements.sticker.checked)
     price += 250;
   form.elements.result.value = price;
 }
</script>
<form name="pricecalc" onsubmit="return false" onchange="calculate(this)">
 <fieldset>
  <legend>Work out the price of your car</legend>
  <p>Base cost: £52000.</p>
  <p>Select additional options:</p>
  <ul>
   <li><label><input type=checkbox name=brakes> Ceramic brakes (£1000)</label></li>
   <li><label><input type=checkbox name=radio> Satellite radio (£2500)</label></li>
   <li><label><input type=checkbox name=turbo> Turbo charger (£5000)</label></li>
   <li><label><input type=checkbox name=sticker> "XZ" sticker (£250)</label></li>
  </ul>
  <p>Total: £<output name=result></output></p>
 </fieldset>
 <script>
  calculate(document.forms.pricecalc);
 </script>
</form>

4.11.1.1 Scripting languages

A user agent is said to support the scripting language if each component of the script block's type is an ASCII case-insensitive match for the corresponding component in the MIME type string of a scripting language that the user agent implements.

The following lists the MIME type strings that user agents must recognize, and the languages to which they refer:

"application/ecmascript"
"application/javascript"
"application/x-ecmascript"
"application/x-javascript"
"text/ecmascript"
"text/javascript"
"text/javascript1.0"
"text/javascript1.1"
"text/javascript1.2"
"text/javascript1.3"
"text/javascript1.4"
"text/javascript1.5"
"text/jscript"
"text/livescript"
"text/x-ecmascript"
"text/x-javascript": JavaScript. [ECMA262]

User agents may support other MIME types for other languages, but must not support other MIME types for the languages in the list above. User agents are not required to support the languages listed above.

The following MIME types (with or without parameters) must not be interpreted as scripting languages:

"text/plain"
"text/xml"
"application/octet-stream"
"application/xml"

These types are explicitly listed here because they are poorly-defined types that are nonetheless likely to be used as formats for data blocks, and it would be problematic if they were suddenly to be interpreted as script by a user agent.

When examining types to determine if they represent supported languages, user agents must not ignore MIME parameters. Types are to be compared including all parameters.

For example, types that include the charset parameter will not be recognized as referencing any of the scripting languages listed above.

4.11.1.2 Restrictions for contents of `script` elements

The easiest and safest way to avoid the rather strange restrictions described in this section is to always escape "<!--" as "<\!--", "<script" as "<\script", and "</script" as "<\/script" when these sequences appear in literals in scripts (e.g. in strings, regular expressions, or comments), and to avoid writing code that uses such constructs in expressions. Doing so avoids the pitfalls that the restrictions in this section are prone to triggering: namely, that, for historical reasons, parsing of script blocks in HTML is a strange and exotic practice that acts unintuitively in the face of these sequences.

The textContent of a script element must match the script production in the following ABNF, the character set for which is Unicode. [ABNF]

script        = outer *( comment-open inner comment-close outer )

outer         = < any string that doesn't contain a substring that matches not-in-outer >
not-in-outer  = comment-open
inner         = < any string that doesn't contain a substring that matches not-in-inner >
not-in-inner  = comment-close / script-open

comment-open  = "<!--"
comment-close = "-->"
script-open   = "<" s c r i p t tag-end

s             =  %x0053 ; U+0053 LATIN CAPITAL LETTER S
s             =/ %x0073 ; U+0073 LATIN SMALL LETTER S
c             =  %x0043 ; U+0043 LATIN CAPITAL LETTER C
c             =/ %x0063 ; U+0063 LATIN SMALL LETTER C
r             =  %x0052 ; U+0052 LATIN CAPITAL LETTER R
r             =/ %x0072 ; U+0072 LATIN SMALL LETTER R
i             =  %x0049 ; U+0049 LATIN CAPITAL LETTER I
i             =/ %x0069 ; U+0069 LATIN SMALL LETTER I
p             =  %x0050 ; U+0050 LATIN CAPITAL LETTER P
p             =/ %x0070 ; U+0070 LATIN SMALL LETTER P
t             =  %x0054 ; U+0054 LATIN CAPITAL LETTER T
t             =/ %x0074 ; U+0074 LATIN SMALL LETTER T

tag-end       =  %x0009 ; "tab" (U+0009)
tag-end       =/ %x000A ; "LF" (U+000A)
tag-end       =/ %x000C ; "FF" (U+000C)
tag-end       =/ %x0020 ; U+0020 SPACE
tag-end       =/ %x002F ; "/" (U+002F)
tag-end       =/ %x003E ; ">" (U+003E)

When a script element contains script documentation, there are further restrictions on the contents of the element, as described in the section below.

The following script illustrates this issue. Suppose you have a script that contains a string, as in:

var example = 'Consider this string: <!-- <script>';
console.log(example);

If one were to put this string directly in a script block, it would violate the restrictions above:

<script>
  var example = 'Consider this string: <!-- <script>';
  console.log(example);
</script>

The bigger problem, though, and the reason why it would violate those restrictions, is that actually the script would get parsed weirdly: the script block above is not terminated. That is, what looks like a "</script>" end tag in this snippet is actually still part of the script block. The script doesn't execute (since it's not terminated); if it somehow were to execute, as it might if the markup looked as follows, it would fail because the script (highlighted here) is not valid JavaScript:

<script>
  var example = 'Consider this string: <!-- <script>';
  console.log(example);
</script>
<!-- despite appearances, this is actually part of the script still! -->
<script>
 ... // this is the same script block still...
</script>

What is going on here is that for legacy reasons, "<!--" and "<script" strings in script elements in HTML need to be balanced in order for the parser to consider closing the block.

By escaping the problematic strings as mentioned at the top of this section, the problem is avoided entirely:

<script>
  var example = 'Consider this string: <\!-- <\script>';
  console.log(example);
</script>
<!-- this is just a comment between script blocks -->
<script>
 ... // this is a new script block
</script>

It is possible for these sequences to naturally occur in script expressions, as in the following examples:

if (x<!--y) { ... }
if ( player<script ) { ... }

In such cases the characters cannot be escaped, but the expressions can be rewritten so that the sequences don't occur, as in:

if (x < !--y) { ... }
if (!--y > x) { ... }
if (!(--y) > x) { ... }
if (player < script) { ... }
if (script > player) { ... }

Doing this also avoids a different pitfall as well: for related historical reasons, the string "<!--" in JavaScript is actually treated as a line comment start, just like "//".

4.11.1.3 Inline documentation for external scripts

If a script element's src attribute is specified, then the contents of the script element, if any, must be such that the value of the text IDL attribute, which is derived from the element's contents, matches the documentation production in the following ABNF, the character set for which is Unicode. [ABNF]

documentation = *( *( space / tab / comment ) [ line-comment ] newline )
comment       = slash star *( not-star / star not-slash ) 1*star slash
line-comment  = slash slash *not-newline

; characters
tab           = %x0009 ; "tab" (U+0009)
newline       = %x000A ; "LF" (U+000A)
space         = %x0020 ; U+0020 SPACE
star          = %x002A ; "*" (U+002A)
slash         = %x002F ; "/" (U+002F)
not-newline   = %x0000-0009 / %x000B-10FFFF
                ; a Unicode character other than "LF" (U+000A)
not-star      = %x0000-0029 / %x002B-10FFFF
                ; a Unicode character other than "*" (U+002A)
not-slash     = %x0000-002E / %x0030-10FFFF
                ; a Unicode character other than "/" (U+002F)

This corresponds to putting the contents of the element in JavaScript comments.

This requirement is in addition to the earlier restrictions on the syntax of contents of script elements.

This allows authors to include documentation, such as license information or API information, inside their documents while still referring to external script files. The syntax is constrained so that authors don't accidentally include what looks like valid script while also providing a src attribute.

<script src="cool-effects.js">
 // create new instances using:
 //    var e = new Effect();
 // start the effect using .play, stop using .stop:
 //    e.play();
 //    e.stop();
</script>

4.11.1.4 Interaction of `script` elements and XSLT

This section is non-normative.

This specification does not define how XSLT interacts with the script element. However, in the absence of another specification actually defining this, here are some guidelines for implementors, based on existing implementations:

When an XSLT transformation program is triggered by an <?xml-stylesheet?> processing instruction and the browser implements a direct-to-DOM transformation, script elements created by the XSLT processor need to be marked "parser-inserted" and run in document order (modulo scripts marked defer or async), asynchronously while the transformation is occurring.
The XSLTProcessor.transformToDocument() method adds elements to a Document that is not in a browsing context, and, accordingly, any script elements they create need to have their "already started" flag set in the prepare a script algorithm and never get executed (scripting is disabled). Such script elements still need to be marked "parser-inserted", though, such that their async IDL attribute will return false in the absence of an async content attribute.
The XSLTProcessor.transformToFragment() method needs to create a fragment that is equivalent to one built manually by creating the elements using document.createElementNS(). For instance, it needs to create script elements that aren't "parser-inserted" and that don't have their "already started" flag set, so that they will execute when the fragment is inserted into a document.

The main distinction between the first two cases and the last case is that the first two operate on Documents and the last operates on a fragment.

4.11.2 The `noscript` element

Categories:: Metadata content.; Flow content.; Phrasing content.
Contexts in which this element can be used:: In a head element of an HTML document, if there are no ancestor noscript elements.; Where phrasing content is expected in HTML documents, if there are no ancestor noscript elements.
Content model:: When scripting is disabled, in a head element: in any order, zero or more link elements, zero or more style elements, and zero or more meta elements.; When scripting is disabled, not in a head element: transparent, but there must be no noscript element descendants.; Otherwise: text that conforms to the requirements given in the prose.
Content attributes:: Global attributes
Tag omission in text/html:: Neither tag is omissible
Allowed ARIA role attribute values:: None
Allowed ARIA state and property attributes:: Global aria-* attributes
DOM interface:: Uses HTMLElement.

The noscript element represents nothing if scripting is enabled, and represents its children if scripting is disabled. It is used to present different markup to user agents that support scripting and those that don't support scripting, by affecting how the document is parsed.

When used in HTML documents, the allowed content model is as follows:

In a head element, if scripting is disabled for the noscript element

The noscript element must contain only link, style, and meta elements.

In a head element, if scripting is enabled for the noscript element

The noscript element must contain only text, except that invoking the HTML fragment parsing algorithm with the noscript element as the context element and the text contents as the input must result in a list of nodes that consists only of link, style, and meta elements that would be conforming if they were children of the noscript element, and no parse errors.

Outside of head elements, if scripting is disabled for the noscript element

The noscript element's content model is transparent, with the additional restriction that a noscript element must not have a noscript element as an ancestor (that is, noscript can't be nested).

Outside of head elements, if scripting is enabled for the noscript element

The noscript element must contain only text, except that the text must be such that running the following algorithm results in a conforming document with no noscript elements and no script elements, and such that no step in the algorithm causes an HTML parser to flag a parse error:

Remove every script element from the document.
Make a list of every noscript element in the document. For every noscript element in that list, perform the following steps:
1. Let the parent element be the parent element of the noscript element.
2. Take all the children of the parent element that come before the noscript element, and call these elements the before children.
3. Take all the children of the parent element that come after the noscript element, and call these elements the after children.
4. Let s be the concatenation of all the Text node children of the noscript element.
5. Set the innerHTML attribute of the parent element to the value of s. (This, as a side-effect, causes the noscript element to be removed from the document.)
6. Insert the before children at the start of the parent element, preserving their original relative order.
7. Insert the after children at the end of the parent element, preserving their original relative order.

All these contortions are required because, for historical reasons, the noscript element is handled differently by the HTML parser based on whether scripting was enabled or not when the parser was invoked.

The noscript element must not be used in XML documents.

The noscript element is only effective in the HTML syntax, it has no effect in the XHTML syntax. This is because the way it works is by essentially "turning off" the parser when scripts are enabled, so that the contents of the element are treated as pure text and not as real elements. XML does not define a mechanism by which to do this.

The noscript element has no other requirements. In particular, children of the noscript element are not exempt from form submission, scripting, and so forth, even when scripting is enabled for the element.

In the following example, a noscript element is used to provide fallback for a script.

<form action="calcSquare.php">
 <p>
  <label for=x>Number</label>:
  <input id="x" name="x" type="number">
 </p>
 <script>
  var x = document.getElementById('x');
  var output = document.createElement('p');
  output.textContent = 'Type a number; it will be squared right then!';
  x.form.appendChild(output);
  x.form.onsubmit = function () { return false; }
  x.oninput = function () {
    var v = x.valueAsNumber;
    output.textContent = v + ' squared is ' + v * v;
  };
 </script>
 <noscript>
  <input type=submit value="Calculate Square">
 </noscript>
</form>

When script is disabled, a button appears to do the calculation on the server side. When script is enabled, the value is computed on-the-fly instead.

The noscript element is a blunt instrument. Sometimes, scripts might be enabled, but for some reason the page's script might fail. For this reason, it's generally better to avoid using noscript, and to instead design the script to change the page from being a scriptless page to a scripted page on the fly, as in the next example:

<form action="calcSquare.php">
 <p>
  <label for=x>Number</label>:
  <input id="x" name="x" type="number">
 </p>
 <input id="submit" type=submit value="Calculate Square">
 <script>
  var x = document.getElementById('x');
  var output = document.createElement('p');
  output.textContent = 'Type a number; it will be squared right then!';
  x.form.appendChild(output);
  x.form.onsubmit = function () { return false; }
  x.oninput = function () {
    var v = x.valueAsNumber;
    output.textContent = v + ' squared is ' + v * v;
  };
  var submit = document.getElementById('submit');
  submit.parentNode.removeChild(submit);
 </script>
</form>

The above technique is also useful in XHTML, since noscript is not supported in the XHTML syntax.

4.11.3 The `template` element

Where metadata content is expected.

Where phrasing content is expected.

Where script-supporting elements are expected.

As a child of a colgroup element that doesn't have a span attribute.

Either: Metadata content.

Or: Flow content.

Or: The content model of ol and ul elements.

Or: The content model of dl elements.

Or: The content model of figure elements.

Or: The content model of ruby elements.

Or: The content model of object elements.

Or: The content model of video and audio elements.

Or: The content model of table elements.

Or: The content model of colgroup elements.

Or: The content model of thead, tbody, and tfoot elements.

Or: The content model of tr elements.

Or: The content model of fieldset elements.

Or: The content model of select elements.

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

None

interface HTMLTemplateElement : HTMLElement {
  readonly attribute DocumentFragment content;
};

The template element is used to declare fragments of HTML that can be cloned and inserted in the document by script.

Templates provide a method for declaring inert DOM subtrees and manipulating them to instantiate document fragments with identical contents.

When web pages dynamically alter the contents of their documents (e.g. in response to user interaction or new data arriving from the server), it is common that they require fragments of HTML which may require further modification before use, such as the insertion of values appropriate for the usage context.

The template element allows for the declaration of document fragments which are unused by the document when loaded, but are parsed as HTML and are available at runtime for use by the web page.

In a rendering, the template element represents nothing.

template . content: Returns the contents of the template, which are stored in a DocumentFragment associated with a different Document so as to avoid the template contents interfering with the main Document. (For example, this avoids form controls from being submitted, scripts from executing, and so forth.)

Each template element has an associated DocumentFragment object that is its template contents. When a template element is created, the user agent must run the following steps to establish the template contents:

Let doc be the template element's ownerDocument's appropriate template contents owner document.
Create a DocumentFragment object whose ownerDocument is doc.
Set the template element's template contents to the newly created DocumentFragment object.

A Document doc's appropriate template contents owner document is the Document returned by the following algorithm:

If doc is not a Document created by this algorithm, run these substeps:
1. If doc does not yet have an associated inert template document then run these substeps:
  1. Let new doc be a new Document (that does not have a browsing context). This is "a Document created by this algorithm" for the purposes of the step above.
  2. If doc is an HTML document, mark new doc as an HTML document also.
  3. Let doc's associated inert template document be new doc.
2. Set doc to doc's associated inert template document.
Each Document not created by this algorithm thus gets a single Document to act as its proxy for owning the template contents of all its template elements, so that they aren't in a browsing context and thus remain inert (e.g. scripts do not run). Meanwhile, template elements inside Document objects that are created by this algorithm just reuse the same Document owner for their contents.
Return doc.

When a template element changes ownerDocument, the user agent must run the following steps:

Let doc be the template element's new ownerDocument's appropriate template contents owner document.
Adopt the template element's template contents (a DocumentFragment object) into doc.

The content IDL attribute must return the template element's template contents.

The cloning steps for a template element node being cloned to a copy copy must run the following steps:

If the clone children flag is not set in the calling clone algorithm, abort these steps.
Let copied contents be the result of cloning all the children of node's template contents, with ownerDocument set to copy's template contents's ownerDocument, and with the clone children flag set.
Append copied contents to copy's template contents.

In this example, a script populates a table with data from a data structure, using a template to provide the element structure instead of manually generating the structure from markup.

<!DOCTYPE html>
<title>Cat data</title>
<script>
 // Data is hard-coded here, but could come from the server
 var data = [
   { name: 'Pillar', color: 'Ticked Tabby', sex: 'Female (neutered)', legs: 3 },
   { name: 'Hedral', color: 'Tuxedo', sex: 'Male (neutered)', legs: 4 },
 ];
</script>
<table>
 <thead>
  <tr>
   <th>Name <th>Color <th>Sex <th>Legs
 <tbody>
  <template id="row">
   <tr><td><td><td><td>
  </template>
</table>
<script>
 var template = document.querySelector('#row');
 for (var i = 0; i < data.length; i += 1) {
   var cat = data[i];
   var clone = template.content.cloneNode(true);
   var cells = clone.querySelectorAll('td');
   cells[0].textContent = cat.name;
   cells[1].textContent = cat.color;
   cells[2].textContent = cat.sex;
   cells[3].textContent = cat.legs;
   template.parentNode.appendChild(clone);
 }
</script>

4.11.3.1 Interaction of `template` elements with XSLT and XPath

This section is non-normative.

This specification does not define how XSLT and XPath interact with the template element. However, in the absence of another specification actually defining this, here are some guidelines for implementors, which are intended to be consistent with other processing described in this specification:

An XSLT processor based on an XML parser that acts as described in this specification needs to act as if template elements contain as descendants their template contents for the purposes of the transform.
An XSLT processor that outputs a DOM needs to ensure that nodes that would go into a template element are instead placed into the element's template contents.
XPath evaluation using the XPath DOM API when applied to a Document parsed using the HTML parser or the XML parser described in this specification needs to ignore template contents.

4.11.4 The `canvas` element

Where embedded content is expected.

Transparent.

width - Horizontal dimension

height - Vertical dimension

Tag omission in text/html:

Neither tag is omissible

Allowed ARIA role attribute values:

Any aria-* attributes applicable to the allowed roles.

width

height

View this example online.

typedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext;

interface HTMLCanvasElement : HTMLElement {
           attribute unsigned long width;
           attribute unsigned long height;

  RenderingContext? getContext(DOMString contextId, any... arguments);

  DOMString toDataURL(optional DOMString type, any... arguments);
  void toBlob(FileCallback? _callback, optional DOMString type, any... arguments);
};

The canvas element provides scripts with a resolution-dependent bitmap canvas, which can be used for rendering graphs, game graphics, art, or other visual images on the fly.

Authors should not use the canvas element in a document when a more suitable element is available. For example, it is inappropriate to use a canvas element to render a page heading: if the desired presentation of the heading is graphically intense, it should be marked up using appropriate elements (typically h1) and then styled using CSS and supporting technologies such as XBL.

When authors use the canvas element, they must also provide content that, when presented to the user, conveys essentially the same function or purpose as the canvas' bitmap. This content may be placed as content of the canvas element. The contents of the canvas element, if any, are the element's fallback content.

In interactive visual media, if scripting is enabled for the canvas element, and if support for canvas elements has been enabled, the canvas element represents embedded content consisting of a dynamically created image, the element's bitmap.

In non-interactive, static, visual media, if the canvas element has been previously associated with a rendering context (e.g. if the page was viewed in an interactive visual medium and is now being printed, or if some script that ran during the page layout process painted on the element), then the canvas element represents embedded content with the element's current bitmap and size. Otherwise, the element represents its fallback content instead.

In non-visual media, and in visual media if scripting is disabled for the canvas element or if support for canvas elements has been disabled, the canvas element represents its fallback content instead.

When a canvas element represents embedded content, the user can still focus descendants of the canvas element (in the fallback content). When an element is focused, it is the target of keyboard interaction events (even though the element itself is not visible). This allows authors to make an interactive canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable elements in the fallback content. (Focus has no effect on mouse interaction events.) [DOMEVENTS]

The canvas element has two attributes to control the size of the element's bitmap: width and height. These attributes, when specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their numeric values. If an attribute is missing, or if parsing its value returns an error, then the default value must be used instead. The width attribute defaults to 300, and the height attribute defaults to 150.

The intrinsic dimensions of the canvas element when it represents embedded content are equal to the dimensions of the element's bitmap.

A canvas element can be sized arbitrarily by a style sheet, its bitmap is then subject to the 'object-fit' CSS property. [CSSIMAGES]

The bitmaps of canvas elements, as well as some of the bitmaps of rendering contexts, such as those described in the HTML Canvas 2D Context specification [CANVAS2D], have an origin-clean flag, which can be set to true or false. Initially, when the canvas element is created, its bitmap's origin-clean flag must be set to true.

A canvas bitmap can also have a hit region list, as described in the CanvasRenderingContext2D section below.

A canvas element can have a rendering context bound to it. Initially, it does not have a bound rendering context. To keep track of whether it has a rendering context or not, and what kind of rendering context it is, a canvas also has a canvas context mode, which is initially none but can be changed to either direct-2d, direct-webgl, indirect, or proxied by algorithms defined in this specification.

When its canvas context mode is none, a canvas element has no rendering context, and its bitmap must be fully transparent black with an intrinsic width equal to the numeric value of the element's width attribute and an intrinsic height equal to the numeric value of the element's height attribute, those values being interpreted in CSS pixels, and being updated as the attributes are set, changed, or removed.

When a canvas element represents embedded content, it provides a paint source whose width is the element's intrinsic width, whose height is the element's intrinsic height, and whose appearance is the element's bitmap.

Whenever the width and height content attributes are set, removed, changed, or redundantly set to the value they already have, if the canvas context mode is direct-2d, the user agent must set bitmap dimensions to the numeric values of the width and height content attributes.

The width and height IDL attributes must reflect the respective content attributes of the same name, with the same defaults.

The bitmaps used with canvas elements can have arbitrary pixel densities. Typically, the density will match that of the user's screen.

context = canvas . getContext(contextId [, ... ])

Returns an object that exposes an API for drawing on the canvas. The first argument specifies the desired API, either "2d" or "webgl". Subsequent arguments are handled by that API.

The list of defined contexts is given on the WHATWG Wiki CanvasContexts page. [WHATWGWIKI]

Example contexts are the "2d" [CANVAS2D] and the "webgl" context [WEBGL].

Returns null if the given context ID is not supported or if the canvas has already been initialized with some other (incompatible) context type (e.g. trying to get a "2d" context after getting a "webgl" context).

Each rendering context has a context bitmap mode, which is one of fixed, unbound, or bound. Initially, rendering contexts must be in the unbound mode.

The getContext(contextId, arguments...) method of the canvas element, when invoked, must run the steps in the cell of the following table whose column header describes the canvas element's canvas context mode and whose row header describes the method's first argument.

	none	direct-2d	direct-webgl	indirect	proxied
"`2d`"	Set the `canvas` element's context mode to direct-2d, obtain a `CanvasRenderingContext2D` object as defined in the HTML Canvas 2D Context specification [CANVAS2D], set the obtained `CanvasRenderingContext2D` object's context bitmap mode to fixed, and return the `CanvasRenderingContext2D` object	Return the same object as was return the last time the method was invoked with this same argument.	Return null.	Throw an `InvalidStateError` exception.	Throw an `InvalidStateError` exception.
"`webgl`", if the user agent supports the WebGL feature in its current configuration	Follow the instructions given in the WebGL specification's Context Creation section to obtain either a `WebGLRenderingContext` or null; if the returned value is null, then return null and abort these steps, otherwise, set the `canvas` element's context mode to direct-webgl, set the new `WebGLRenderingContext` object's context bitmap mode to fixed, and return the `WebGLRenderingContext` object‡ [WEBGL]	Return null.	Return the same object as was return the last time the method was invoked with this same argument.	Throw an `InvalidStateError` exception.	Throw an `InvalidStateError` exception.
A vendor-specific extension*	Behave as defined for the extension.	Behave as defined for the extension.	Behave as defined for the extension.	Throw an `InvalidStateError` exception.	Throw an `InvalidStateError` exception.
An unsupported value†	Return null.	Return null.	Return null.	Throw an `InvalidStateError` exception.	Throw an `InvalidStateError` exception.

* Vendors may define experimental contexts using the syntax vendorname-context, for example, moz-3d.

† For example, the "webgl" value in the case of a user agent having exhausted the graphics hardware's abilities and having no software fallback implementation.

‡ The second (and subsequent) argument(s) to the method, if any, are ignored in all cases except this one. See the WebGL specification for details.

url = canvas . toDataURL( [ type, ... ] )

Returns a data: URL for the image in the canvas.

The first argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is image/png; that type is also used if the given type isn't supported. The other arguments are specific to the type, and control the way that the image is generated, as given in the table below.

When trying to use types other than "image/png", authors can check if the image was really returned in the requested format by checking to see if the returned string starts with one of the exact strings "data:image/png," or "data:image/png;". If it does, the image is PNG, and thus the requested type was not supported. (The one exception to this is if the canvas has either no height or no width, in which case the result might simply be "data:,".)

The toDataURL() method returns the data at a resolution of 96dpi.

canvas . toBlob(callback [, type, ... ])

Creates a Blob object representing a file containing the image in the canvas, and invokes a callback with a handle to that object.

The second argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is image/png; that type is also used if the given type isn't supported. The other arguments are specific to the type, and control the way that the image is generated, as given in the table below.

The toBlob() method provides the data at a resolution of 96dpi.

The toDataURL() method must run the following steps:

If the canvas element's bitmap's origin-clean flag is set to false, throw a SecurityError exception and abort these steps.
If the canvas element's bitmap has no pixels (i.e. either its horizontal dimension or its vertical dimension is zero) then return the string "data:," and abort these steps. (This is the shortest data: URL; it represents the empty string in a text/plain resource.)
Let file be a serialization of the canvas element's bitmap as a file, using the method's arguments (if any) as the arguments.
Return a data: URL representing file. [RFC2397]

The toBlob() method must run the following steps:

If the canvas element's bitmap's origin-clean flag is set to false, throw a SecurityError exception and abort these steps.
Let callback be the first argument.
Let arguments be the second and subsequent arguments to the method, if any.
If the canvas element's bitmap has no pixels (i.e. either its horizontal dimension or its vertical dimension is zero) then let result be null.

Otherwise, let result be a Blob object representing a serialization of the canvas element's bitmap as a file, using arguments. [FILEAPI]
Return, but continue running these steps asynchronously.
If callback is null, abort these steps.
Queue a task to invoke the FileCallback callback with result as its argument. The task source for this task is the canvas blob serialization task source.

4.11.4.1 Color spaces and color correction

The canvas APIs must perform color correction at only two points: when rendering images with their own gamma correction and color space information onto a bitmap, to convert the image to the color space used by the bitmaps (e.g. using the 2D Context's drawImage() method with an HTMLImageElement object), and when rendering the actual canvas bitmap to the output device.

Thus, in the 2D context, colors used to draw shapes onto the canvas will exactly match colors obtained through the getImageDataHD() method.

The toDataURL() method must not include color space information in the resources it returns. Where the output format allows it, the color of pixels in resources created by toDataURL() must match those returned by the getImageData() method.

In user agents that support CSS, the color space used by a canvas element must match the color space used for processing any colors for that element in CSS.

The gamma correction and color space information of images must be handled in such a way that an image rendered directly using an img element would use the same colors as one painted on a canvas element that is then itself rendered. Furthermore, the rendering of images that have no color correction information (such as those returned by the toDataURL() method) must be rendered with no color correction.

4.11.4.2 Serializing bitmaps to a file

When a user agent is to create a serialization of the bitmap as a file, optionally with some given arguments, it must create an image file in the format given by the first value of arguments, or, if there are no arguments, in the PNG format. [PNG]

If if the bitmap has one pixel per coordinate space unit then the image file must have the same pixel data (before compression, if applicable) as the bitmap, and if the file format used supports encoding resolution metadata, the resolution of that bitmap (device pixels per coordinate space units being interpreted as image pixels per CSS pixel) must be given as well.

Otherwise, the image file's pixel data must be the bitmap's pixel data scaled to one image pixel per coordinate space unit, and if the file format used supports encoding resolution metadata, the resolution must be given as 96dpi (one image pixel per CSS pixel).

If arguments is not empty, the first value must be interpreted as a MIME type giving the format to use. If the type has any parameters, it must be treated as not supported.

For example, the value "image/png" would mean to generate a PNG image, the value "image/jpeg" would mean to generate a JPEG image, and the value "image/svg+xml" would mean to generate an SVG image (which would require that the user agent track how the bitmap was generated, an unlikely, though potentially awesome, feature).

User agents must support PNG ("image/png"). User agents may support other types. If the user agent does not support the requested type, it must create the file using the PNG format. [PNG]

User agents must convert the provided type to ASCII lowercase before establishing if they support that type.

For image types that do not support an alpha channel, the serialized image must be the bitmap image composited onto a solid black background using the source-over operator.

If the first argument in arguments gives a type corresponding to one of the types given in the first column of the following table, and the user agent supports that type, then the subsequent arguments, if any, must be treated as described in the second cell of that row.

Arguments for serialization methods
Type	Other arguments	Reference
`image/jpeg`	The second argument, if it is a number in the range 0.0 to 1.0 inclusive, must be treated as the desired quality level. If it is not a number or is outside that range, the user agent must use its default value, as if the argument had been omitted.	[JPEG]

For the purposes of these rules, an argument is considered to be a number if it is converted to an IDL double value by the rules for handling arguments of type any in the Web IDL specification. [WEBIDL]

Other arguments must be ignored and must not cause the user agent to throw an exception. A future version of this specification will probably define other parameters to be passed to these methods to allow authors to more carefully control compression settings, image metadata, etc.

4.11.4.3 Security with `canvas` elements

This section is non-normative.

Information leakage can occur if scripts from one origin can access information (e.g. read pixels) from images from another origin (one that isn't the same).

To mitigate this, bitmaps used with canvas elements are defined to have a flag indicating whether they are origin-clean. All bitmaps start with their origin-clean set to true. The flag is set to false when cross-origin images or fonts are used.

The toDataURL(), toBlob(), getImageData(), and getImageDataHD() methods check the flag and will throw a SecurityError exception rather than leak cross-origin data.

The flag can be reset in certain situations; for example, when a CanvasRenderingContext2D is bound to a new canvas, the bitmap is cleared and its flag reset.

4.12 Common idioms without dedicated elements

4.12.1 Subheadings, subtitles, alternative titles and taglines

HTML does not have a dedicated mechanism for marking up subheadings, alternative titles or taglines. Here are the suggested alternatives.

h1–h6 elements must not be used to markup subheadings, subtitles, alternative titles and taglines unless intended to be the heading for a new section or subsection.

In the following example the title and subtitles of a web page are grouped using a header element. As the author does not want the subtitles to be included the table of contents and they are not intended to signify the start of a new section, they are marked up using p elements. A sample CSS styled rendering of the title and subtitles is provided below the code example.

   <header>
   <h1>HTML 5.1 Nightly</h1>
   <p>A vocabulary and associated APIs for HTML and XHTML</p>
   <p>Editor's Draft 9 May 2013</p>
   </header>

Title:'HTML 5.1 Nightly' in a mid blue Sans Serif font.
Subtitle 1:'A vocabulary and associated APIs for HTML and XHTML' on a new line, same style smaller font size.
Subtitle 2:'Editor's Draft 9 May 2013' on a new line, same style and size as subtitle 1.

In the following example the subtitle of a book is on the same line as the title separated by a colon. A sample CSS styled rendering of the title and subtitle is provided below the code example.

<h1>The Lord of the Rings: The Two Towers</h1>

In the following example part of an album title is included in a span element, allowing it to be styled differently from the rest of the title. A br element is used to place the album title on a new line. A sample CSS styled rendering of the heading is provided below the code example.

   <h1>Ramones <br>
   <span>Hey! Ho! Let's Go</span> 
   </h1>

Line 1:'Ramones' displayed in a large bold angular hand writing style font with a spray can paint effect. Line 2:'Hey! Ho! Let's Go' displayed in a smaller, standard sans serif style font.

In the following example the title and tagline for a news article are grouped using a header element. The title is marked up using a h2 element and the tagline is in a p element. A sample CSS styled rendering of the title and tagline is provided below the code example.

   <header>
   <h2>3D films set for popularity slide </h2>
   <p>First drop in 3D box office projected for this year despite hotly tipped summer blockbusters,
    according to Fitch Ratings report</p>
   </header>

Title:'3D films set for popularity slide' in a large, bold, dark blue Serif font style. Paragraph: 'First drop in 3D box office projected for this year despite...' in a smaller, dark grey, Sans Serif font style.

In this last example the title and taglines for a news magazine are grouped using a header element. The title is marked up using a h1 element and the taglines are each in a p element. A sample CSS styled rendering of the title and taglines is provided below the code example.

   <header>
   <p>Magazine of the Decade</p>
   <h1>THE MONTH</h1>
   <p>The Best of UK and Foreign Media</p>
   </header>

Tagline above the heading:'Magazine of the Decade'. Tagline below the heading 'The Best of UK and Foreign Media' both in a small,all caps, sans-serif font style. Heading:'The Month' in a large, Serif font style. All text is black against a red background.

4.12.2 Bread crumb navigation

This specification does not provide a machine-readable way of describing bread-crumb navigation menus. Authors are encouraged to markup bread-crumb navigation as a list. The nav element can be used to mark the list containing links as being a navigation block.

In the following example, the current page can be reached via the path indicated. The path is indicated using the right arrow symbol "→". A text label is provided to give the user context. The links are structured as a list, which provides users with an indication of item number.

   <nav>
   <h2>You are here:</h2>
   <ul id="navlist">
   <li><a href="/">Main</a> →</li> 
   <li><a href="/products/">Products</a> →</li> 
   <li><a href="/products/dishwashers/">Dishwashers</a> →</li> 
   <li><a>Second hand</a></li> 
   </ul>
   </nav>

The breadcrumb code example could be styled as a horizonatal list using CSS:

The use of the right angle bracket symbol ">" to indicate path direction is discouraged as its meaning, in the context used, is not clearly conveyed to all users.

4.12.3 Tag clouds

This specification does not define any markup specifically for marking up lists of keywords that apply to a group of pages (also known as tag clouds). In general, authors are encouraged to either mark up such lists using ul elements with explicit inline counts that are then hidden and turned into a presentational effect using a style sheet, or to use SVG.

Here, three tags are included in a short tag cloud:

<style>
@media screen, print, handheld, tv {
  /* should be ignored by non-visual browsers */
  .tag-cloud > li > span { display: none; }
  .tag-cloud > li { display: inline; }
  .tag-cloud-1 { font-size: 0.7em; }
  .tag-cloud-2 { font-size: 0.9em; }
  .tag-cloud-3 { font-size: 1.1em; }
  .tag-cloud-4 { font-size: 1.3em; }
  .tag-cloud-5 { font-size: 1.5em; }
}
</style>
...
<ul class="tag-cloud">
 <li class="tag-cloud-4"><a title="28 instances" href="/t/apple">apple</a> <span>(popular)</span>
 <li class="tag-cloud-2"><a title="6 instances"  href="/t/kiwi">kiwi</a> <span>(rare)</span>
 <li class="tag-cloud-5"><a title="41 instances" href="/t/pear">pear</a> <span>(very popular)</span>
</ul>

The actual frequency of each tag is given using the title attribute. A CSS style sheet is provided to convert the markup into a cloud of differently-sized words, but for user agents that do not support CSS or are not visual, the markup contains annotations like "(popular)" or "(rare)" to categorize the various tags by frequency, thus enabling all users to benefit from the information.

The ul element is used (rather than ol) because the order is not particularly important: while the list is in fact ordered alphabetically, it would convey the same information if ordered by, say, the length of the tag.

The tag rel-keyword is not used on these a elements because they do not represent tags that apply to the page itself; they are just part of an index listing the tags themselves.

4.12.4 Conversations

This specification does not define a specific element for marking up conversations, meeting minutes, chat transcripts, dialogues in screenplays, instant message logs, and other situations where different players take turns in discourse.

Instead, authors are encouraged to mark up conversations using p elements and punctuation. Authors who need to mark the speaker for styling purposes are encouraged to use span or b. Paragraphs with their text wrapped in the i element can be used for marking up stage directions.

This example demonstrates this using an extract from Abbot and Costello's famous sketch, Who's on first:

<p> Costello: Look, you gotta first baseman?
<p> Abbott: Certainly.
<p> Costello: Who's playing first?
<p> Abbott: That's right.
<p> Costello becomes exasperated.
<p> Costello: When you pay off the first baseman every month, who gets the money?
<p> Abbott: Every dollar of it.

The following extract shows how an IM conversation log could be marked up, using the data element to provide Unix timestamps for each line. Note that the timestamps are provided in a format that the time element does not support, so the data element is used instead (namely, Unix time_t timestamps). Had the author wished to mark up the data using one of the date and time formats supported by the time element, that element could have been used instead of data. This could be advantageous as it would allow data analysis tools to detect the timestamps unambiguously, without coordination with the page author.

<p> <data value="1319898155">14:22</data> <b>egof</b> I'm not that nerdy, I've only seen 30% of the star trek episodes
<p> <data value="1319898192">14:23</data> <b>kaj</b> if you know what percentage of the star trek episodes you have seen, you are inarguably nerdy
<p> <data value="1319898200">14:23</data> <b>egof</b> it's unarguably
<p> <data value="1319898228">14:23</data> <i>* kaj blinks</i>
<p> <data value="1319898260">14:24</data> <b>kaj</b> you are not helping your case

HTML does not have a good way to mark up graphs, so descriptions of interactive conversations from games are more difficult to mark up. This example shows one possible convention using dl elements to list the possible responses at each point in the conversation. Another option to consider is describing the conversation in the form of a DOT file, and outputting the result as an SVG image to place in the document. [DOT]

<p> Next, you meet a fisherman. You can say one of several greetings:
<dl>
 <dt> "Hello there!"
 <dd>
  <p> He responds with "Hello, how may I help you?"; you can respond with:
  <dl>
   <dt> "I would like to buy a fish."
   <dd> <p> He sells you a fish and the conversation finishes.
   <dt> "Can I borrow your boat?"
   <dd>
    <p> He is surprised and asks "What are you offering in return?".
    <dl>
     <dt> "Five gold." (if you have enough)
     <dt> "Ten gold." (if you have enough)
     <dt> "Fifteen gold." (if you have enough)
     <dd> <p> He lends you his boat. The conversation ends.
     <dt> "A fish." (if you have one)
     <dt> "A newspaper." (if you have one)
     <dt> "A pebble." (if you have one)
     <dd> <p> "No thanks", he replies. Your conversation options        
     at this point are the same as they were after asking to borrow
     his boat, minus any options you've suggested before.
    </dl>
   </dd>
  </dl>
 </dd>
 <dt> "Vote for me in the next election!"
 <dd> <p> He turns away. The conversation finishes.
 <dt> "Sir, are you aware that your fish are running away?"
 <dd>
  <p> He looks at you skeptically and says "Fish cannot run, sir".
  <dl>
   <dt> "You got me!"
   <dd> <p> The fisherman sighs and the conversation ends.
   <dt> "Only kidding."
   <dd> <p> "Good one!" he retorts. Your conversation options at this
   point are the same as those following "Hello there!" above.
   <dt> "Oh, then what are they doing?"
   <dd> <p> He looks at his fish, giving you an opportunity to steal
   his boat, which you do. The conversation ends.
  </dl>
 </dd>
</dl>

In some games, conversations are simpler: each character merely has a fixed set of lines that they say. In this example, a game FAQ/walkthrough lists some of the known possible responses for each character:

<section>
 <h1>Dialogue</h1>
 <p><small>Some characters repeat their lines in order each time you interact
 with them, others randomly pick from amongst their lines. Those who respond in
 order have numbered entries in the lists below.</small>
 <h2>The Shopkeeper</h2>
 <ul>
  <li>How may I help you?
  <li>Fresh apples!
  <li>A loaf of bread for madam?
 </ul>
 <h2>The pilot</h2>
 <p>Before the accident:
 <ul>
  </li>I'm about to fly out, sorry!
  </li>Sorry, I'm just waiting for flight clearance and then I'll be off!
 </ul>
 <p>After the accident:
 <ol>
  <li>I'm about to fly out, sorry!
  <li>Ok, I'm not leaving right now, my plane is being cleaned.
  <li>Ok, it's not being cleaned, it needs a minor repair first.
  <li>Ok, ok, stop bothering me! Truth is, I had a crash.
 </ol>
 <h2>Clan Leader</h2>
 <p>During the first clan meeting:
 <ul>
  <li>Hey, have you seen my daughter? I bet she's up to something nefarious again...
  <li>Nice weather we're having today, eh?
  <li>The name is Bailey, Jeff Bailey. How can I help you today?
  <li>A glass of water? Fresh from the well!
 </ul>
 <p>After the earthquake:
 <ol>
  <li>Everyone is safe in the shelter, we just have to put out the fire!
  <li>I'll go and tell the fire brigade, you keep hosing it down!
 </ol>
</section>

4.12.5 Footnotes

HTML does not have a dedicated mechanism for marking up footnotes. Here are the suggested alternatives.

For annotations, the a element should be used, pointing to an element later in the document. The convention is that the contents of the link be a number in square brackets.

In this example, a footnote in the dialogue links to a paragraph below the dialogue. The paragraph then reciprocally links back to the dialogue, allowing the user to return to the location of the footnote.

<p> Announcer: Number 16: The <i>hand</i>.
<p> Interviewer: Good evening. I have with me in the studio tonight
Mr Norman St John Polevaulter, who for the past few years has been
contradicting people. Mr Polevaulter, why <em>do</em> you
contradict people?
<p> Norman: I don't. <sup><a href="#fn1" id="r1">[1]</a></sup>
<p> Interviewer: You told me you did!
...
<section>
 <p id="fn1"><a href="#r1">[1]</a> This is, naturally, a lie,
 but paradoxically if it were true he could not say so without
 contradicting the interviewer and thus making it false.</p>
</section>

For side notes, longer annotations that apply to entire sections of the text rather than just specific words or sentences, the aside element should be used.

In this example, a sidebar is given after a dialogue, giving it some context.

<p> <span class="speaker">Customer</span>: I will not buy this record, it is scratched.
<p> <span class="speaker">Shopkeeper</span>: I'm sorry?
<p> <span class="speaker">Customer</span>: I will not buy this record, it is scratched.
<p> <span class="speaker">Shopkeeper</span>: No no no, this's'a tobacconist's.
<aside>
 <p>In 1970, the British Empire lay in ruins, and foreign
 nationalists frequented the streets — many of them Hungarians
 (not the streets — the foreign nationals). Sadly, Alexander
 Yalt has been publishing incompetently-written phrase books.
</aside>

For figures or tables, footnotes can be included in the relevant figcaption or caption element, or in surrounding prose.

In this example, a table has cells with footnotes that are given in prose. A figure element is used to give a single legend to the combination of the table and its footnotes.

<figure>
 <figcaption>Table 1. Alternative activities for knights.</figcaption>
 <table>
  <tr>
   <th> Activity
   <th> Location
   <th> Cost
  <tr>
   <td> Dance
   <td> Wherever possible
   <td> £0<sup><a href="#fn1">1</a></sup>
  <tr>
   <td> Routines, chorus scenes<sup><a href="#fn2">2</a></sup>
   <td> Undisclosed
   <td> Undisclosed
  <tr>
   <td> Dining<sup><a href="#fn3">3</a></sup>
   <td> Camelot
   <td> Cost of ham, jam, and spam<sup><a href="#fn4">4</a></sup>
 </table>
 <p id="fn1">1. Assumed.</p>
 <p id="fn2">2. Footwork impeccable.</p>
 <p id="fn3">3. Quality described as "well".</p>
 <p id="fn4">4. A lot.</p>
</figure>

4.13 Disabled elements

An element is said to be actually disabled if it falls into one of the following categories:

button elements that are disabled
input elements that are disabled
select elements that are disabled
textarea elements that are disabled
optgroup elements that have a disabled attribute
option elements that are disabled
fieldset elements that have a disabled attribute

This definition is used to determine what elements can be focused and which elements match the :disabled pseudo-class.

4.14 Matching HTML elements using selectors

4.14.1 Case-sensitivity

The Selectors specification leaves the case-sensitivity of IDs, classes, element names, attribute names, and attribute values to be defined by the host language. [SELECTORS]

The unique identifier of HTML elements in documents that are in quirks mode must be treated as ASCII case-insensitive for the purposes of selector matching.

Classes from the class attribute of HTML elements in documents that are in quirks mode must be treated as ASCII case-insensitive for the purposes of selector matching.

When comparing a CSS element type selector to the names of HTML elements in HTML documents, the CSS element type selector must first be converted to ASCII lowercase. The same selector when compared to other elements must be compared according to its original case. In both cases, the comparison is case-sensitive.

When comparing the name part of a CSS attribute selector to the names of namespace-less attributes on HTML elements in HTML documents, the name part of the CSS attribute selector must first be converted to ASCII lowercase. The same selector when compared to other attributes must be compared according to its original case. In both cases, the comparison is case-sensitive.

Everything else (attribute values on HTML elements, IDs and classes in no-quirks mode and limited-quirks mode, and element names, attribute names, and attribute values in XML documents) must be treated as case-sensitive for the purposes of selector matching.

4.14.2 Pseudo-classes

There are a number of dynamic selectors that can be used with HTML. This section defines when these selectors match HTML elements. [SELECTORS]

:link

:visited

All a elements that have an href attribute, all area elements that have an href attribute, and all link elements that have an href attribute, must match one of :link and :visited.

Other specifications might apply more specific rules regarding how these elements are to match these pseudo-classes, to mitigate some privacy concerns that apply with straightforward implementations of this requirement.

:active

The :active pseudo-class is defined to match an element while an element is being activated by the user. For the purposes of defining the :active pseudo-class only, an HTML user agent must consider an element as being activated if it is:

An element falling into one of the following categories between the time the user begins to indicate an intent to trigger the element's activation behavior and either the time the user stops indicating an intent to trigger the element's activation behavior, or the time the element's activation behavior has finished running, which ever comes first:
- a elements that have an href attribute
- area elements that have an href attribute
- link elements that have an href attribute
- button elements that are not disabled
- input elements whose type attribute is in the Submit Button, Image Button, Reset Button, or Button state
- elements that have their tabindex focus flag set
For example, if the user is using a keyboard to push a button element by pressing the space bar, the element would match this pseudo-class in between the time that the element received the keydown event and the time the element received the keyup event.
An element that the user indicates using a pointing device while that pointing device is in the "down" state (e.g. for a mouse, between the time the mouse button is pressed and the time it is depressed).
An element that has a descendant that is currently matching the :active pseudo-class.

:hover

The :hover pseudo-class is defined to match an element while the user designates an element with a pointing device. For the purposes of defining the :hover pseudo-class only, an HTML user agent must consider an element as being one that the user designates if it is:

An element that the user indicates using a pointing device.
An element that has a descendant that the user indicates using a pointing device.
An element that is the labeled control of a label element that is currently matching :hover.

Consider in particular a fragment such as:

<p> <label for=c> <input id=a> </label> <span id=b> <input id=c> </span> </p>

If the user designates the element with ID "a" with their pointing device, then the p element (and all its ancestors not shown in the snippet above), the label element, the element with ID "a", and the element with ID "c" will match the :hover pseudo-class. The element with ID "a" matches it from condition 1, the label and p elements match it because of condition 2 (one of their descendants is designated), and the element with ID "c" matches it through condition 3 (its label element matches :hover). However, the element with ID "b" does not match :hover: its descendant is not designated, even though it matches :hover.

:enabled

The :enabled pseudo-class must match any element falling into one of the following categories:

a elements that have an href attribute
area elements that have an href attribute
link elements that have an href attribute
button elements that are not disabled
input elements that are not disabled
select elements that are not disabled
textarea elements that are not disabled
optgroup elements that do not have a disabled attribute
option elements that are not disabled
fieldset elements that do not have a disabled attribute

:disabled

The :disabled pseudo-class must match any element that is actually disabled.

:checked

The :checked pseudo-class must match any element falling into one of the following categories:

input elements whose type attribute is in the Checkbox state and whose checkedness state is true
input elements whose type attribute is in the Radio Button state and whose checkedness state is true
option elements whose selectedness is true

:indeterminate

The :indeterminate pseudo-class must match any element falling into one of the following categories:

input elements whose type attribute is in the Checkbox state and whose indeterminate IDL attribute is set to true
input elements whose type attribute is in the Radio Button state and whose radio button group contains no input elements whose checkedness state is true.
progress elements with no value content attribute

:default

The :default pseudo-class must match any element falling into one of the following categories:

button elements that are their form's default button
input elements whose type attribute is in the Submit Button or Image Button state, and that are their form's default button
input elements to which the checked attribute applies and that have a checked attribute
option elements that have a selected attribute

:valid

The :valid pseudo-class must match any element falling into one of the following categories:

elements that are candidates for constraint validation and that satisfy their constraints
form elements that are not the form owner of any elements that themselves are candidates for constraint validation but do not satisfy their constraints
fieldset elements that have no descendant elements that themselves are candidates for constraint validation but do not satisfy their constraints

:invalid

The :invalid pseudo-class must match any element falling into one of the following categories:

elements that are candidates for constraint validation but that do not satisfy their constraints
form elements that are the form owner of one or more elements that themselves are candidates for constraint validation but do not satisfy their constraints
fieldset elements that have of one or more descendant elements that themselves are candidates for constraint validation but do not satisfy their constraints

:in-range

The :in-range pseudo-class must match all elements that are candidates for constraint validation, have range limitations, and that are neither suffering from an underflow nor suffering from an overflow.

:out-of-range

The :out-of-range pseudo-class must match all elements that are candidates for constraint validation, have range limitations, and that are either suffering from an underflow or suffering from an overflow.

:required

The :required pseudo-class must match any element falling into one of the following categories:

input elements that are required
select elements that have a required attribute
textarea elements that have a required attribute

:optional

The :optional pseudo-class must match any element falling into one of the following categories:

input elements to which the required attribute applies that are not required
select elements that do not have a required attribute
textarea elements that do not have a required attribute

:read-only

:read-write

The :read-write pseudo-class must match any element falling into one of the following categories, which for the purposes of Selectors are thus considered user-alterable: [SELECTORS]

input elements to which the readonly attribute applies, and that are mutable (i.e. that do not have the readonly attribute specified and that are not disabled)
textarea elements that do not have a readonly attribute, and that are not disabled
elements that are editing hosts or editable and are neither input elements nor textarea elements

The :read-only pseudo-class must match all other HTML elements.

Another section of this specification defines the target element used with the :target pseudo-class.

This specification does not define when an element matches the :focus or :lang() dynamic pseudo-classes, as those are all defined in sufficient detail in a language-agnostic fashion in the Selectors specification. [SELECTORS]

5 Loading Web pages

This section describes features that apply most directly to Web browsers. Having said that, except where specified otherwise, the requirements defined in this section do apply to all user agents, whether they are Web browsers or not.

5.1 Browsing contexts

A browsing context is an environment in which Document objects are presented to the user.

A tab or window in a Web browser typically contains a browsing context, as does an iframe or frames in a frameset.

Each browsing context has a corresponding WindowProxy object.

A browsing context has a session history, which lists the Document objects that that browsing context has presented, is presenting, or will present. At any time, one Document in each browsing context is designated the active document. A Document's browsing context is that browsing context whose session history contains the Document, if any. (A Document created using an API such as createDocument() has no browsing context.)

Each Document in a browsing context is associated with a Window object. A browsing context's WindowProxy object forwards everything to the browsing context's active document's Window object.

In general, there is a 1-to-1 mapping from the Window object to the Document object. There are two exceptions. First, a Window can be reused for the presentation of a second Document in the same browsing context, such that the mapping is then 1-to-2. This occurs when a browsing context is navigated from the initial about:blank Document to another, with replacement enabled. Second, a Document can end up being reused for several Window objects when the document.open() method is used, such that the mapping is then many-to-1.

A Document does not necessarily have a browsing context associated with it. In particular, data mining tools are likely to never instantiate browsing contexts.

A browsing context can have a creator browsing context, the browsing context that was responsible for its creation. If a browsing context has a parent browsing context, then that is its creator browsing context. Otherwise, if the browsing context has an opener browsing context, then that is its creator browsing context. Otherwise, the browsing context has no creator browsing context.

If a browsing context A has a creator browsing context, then the Document that was the active document of that creator browsing context at the time A was created is the creator Document.

When a browsing context is first created, it must be created with a single Document in its session history, whose address is about:blank, which is marked as being an HTML document, whose character encoding is UTF-8, and which is both ready for post-load tasks and completely loaded immediately, along with a new Window object that the Document is associated with. The Document must have a single child html node, which itself has two empty child nodes: a head element, and a body element. As soon as this Document is created, the user agent must implement the sandboxing for it. If the browsing context has a creator Document, then the browsing context's Document's referrer must be set to the address of that creator Document at the time of the browsing context's creation.

If the browsing context is created specifically to be immediately navigated, then that initial navigation will have replacement enabled.

The origin and effective script origin of the about:blank Document are set when the Document is created. If the new browsing context has a creator browsing context, then the origin of the about:blank Document is an alias to the origin of the creator Document and the effective script origin of the about:blank Document is initially an alias to the effective script origin of the creator Document. Otherwise, the origin of the about:blank Document is a globally unique identifier assigned when the new browsing context is created and the effective script origin of the about:blank Document is initially an alias to its origin.

5.1.1 Nested browsing contexts

Certain elements (for example, iframe elements) can instantiate further browsing contexts. These are called nested browsing contexts. If a browsing context P has a Document D with an element E that nests another browsing context C inside it, then C is said to be nested through D, and E is said to be the browsing context container of C. If the browsing context container element E is in the Document D, then P is said to be the parent browsing context of C and C is said to be a child browsing context of P. Otherwise, the nested browsing context C has no parent browsing context.

A browsing context A is said to be an ancestor of a browsing context B if there exists a browsing context A' that is a child browsing context of A and that is itself an ancestor of B, or if the browsing context A is the parent browsing context of B.

A browsing context that is not a nested browsing context has no parent browsing context, and is the top-level browsing context of all the browsing contexts for which it is an ancestor browsing context.

The transitive closure of parent browsing contexts for a nested browsing context gives the list of ancestor browsing contexts.

The list of the descendant browsing contexts of a Document d is the (ordered) list returned by the following algorithm:

Let list be an empty list.
For each child browsing context of d that is nested through an element that is in the Document d, in the tree order of the elements nesting those browsing contexts, run these substeps:
1. Append that child browsing context to the list list.
2. Append the list of the descendant browsing contexts of the active document of that child browsing context to the list list.
Return the constructed list.

A Document is said to be fully active when it is the active document of its browsing context, and either its browsing context is a top-level browsing context, or it has a parent browsing context and the Document through which it is nested is itself fully active.

Because they are nested through an element, child browsing contexts are always tied to a specific Document in their parent browsing context. User agents must not allow the user to interact with child browsing contexts of elements that are in Documents that are not themselves fully active.

A nested browsing context can be put into a delaying load events mode. This is used when it is navigated, to delay the load event of the browsing context container iframe element before the new Document is created.

The document family of a browsing context consists of the union of all the Document objects in that browsing context's session history and the document families of all those Document objects. The document family of a Document object consists of the union of all the document families of the browsing contexts that are nested through the Document object.

5.1.1.1 Navigating nested browsing contexts in the DOM

window . top

Returns the WindowProxy for the top-level browsing context.

window . parent

Returns the WindowProxy for the parent browsing context.

window . frameElement

Returns the Element for the browsing context container.

Returns null if there isn't one.

Throws a SecurityError exception in cross-origin situations.

The top IDL attribute on the Window object of a Document in a browsing context b must return the WindowProxy object of its top-level browsing context (which would be its own WindowProxy object if it was a top-level browsing context itself), if it has one, or its own WindowProxy object otherwise (e.g. if it was a detached nested browsing context).

The parent IDL attribute on the Window object of a Document in a browsing context b must return the WindowProxy object of the parent browsing context, if there is one (i.e. if b is a child browsing context), or the WindowProxy object of the browsing context b itself, otherwise (i.e. if it is a top-level browsing context or a detached nested browsing context).

The frameElement IDL attribute on the Window object of a Document d, on getting, must run the following algorithm:

If d is not a Document in a nested browsing context, return null and abort these steps.
If the browsing context container's Document does not have the same effective script origin as the effective script origin specified by the entry settings object, then throw a SecurityError exception and abort these steps.
Return the browsing context container for b.

5.1.2 Auxiliary browsing contexts

It is possible to create new browsing contexts that are related to a top-level browsing context without being nested through an element. Such browsing contexts are called auxiliary browsing contexts. Auxiliary browsing contexts are always top-level browsing contexts.

An auxiliary browsing context has an opener browsing context, which is the browsing context from which the auxiliary browsing context was created.

5.1.2.1 Navigating auxiliary browsing contexts in the DOM

The opener IDL attribute on the Window object, on getting, must return the WindowProxy object of the browsing context from which the current browsing context was created (its opener browsing context), if there is one, if it is still available, and if the current browsing context has not disowned its opener; otherwise, it must return null. On setting, if the new value is null then the current browsing context must disown its opener; if the new value is anything else then the user agent must call the [[DefineOwnProperty]] internal method of the Window object, passing the property name "opener" as the property key, and the Property Descriptor { [[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true } as the property descriptor, where value is the new value.

5.1.3 Secondary browsing contexts

User agents may support secondary browsing contexts, which are browsing contexts that form part of the user agent's interface, apart from the main content area.

5.1.4 Security

A browsing context A is familiar with a second browsing context B if one of the following conditions is true:

Either the origin of the active document of A is the same as the origin of the active document of B, or
The browsing context A is a nested browsing context with a top-level browsing context, and its top-level browsing context is B, or
The browsing context B is an auxiliary browsing context and A is familiar with B's opener browsing context, or
The browsing context B is not a top-level browsing context, but there exists an ancestor browsing context of B whose active document has the same origin as the active document of A (possibly in fact being A itself).

A browsing context A is allowed to navigate a second browsing context B if the following algorithm terminates positively:

If A is not the same browsing context as B, and A is not one of the ancestor browsing contexts of B, and B is not a top-level browsing context, and A's active document's active sandboxing flag set has its sandboxed navigation browsing context flag set, then abort these steps negatively.
Otherwise, if B is a top-level browsing context, and is one of the ancestor browsing contexts of A, and A's Document's active sandboxing flag set has its sandboxed top-level navigation browsing context flag set, then abort these steps negatively.
Otherwise, if B is a top-level browsing context, and is neither A nor one of the ancestor browsing contexts of A, and A's Document's active sandboxing flag set has its sandboxed navigation browsing context flag set, and A is not the one permitted sandboxed navigator of B, then abort these steps negatively.
Otherwise, terminate positively!

An element has a browsing context scope origin if its Document's browsing context is a top-level browsing context or if all of its Document's ancestor browsing contexts all have active documents whose origin are the same origin as the element's Document's origin. If an element has a browsing context scope origin, then its value is the origin of the element's Document.

5.1.5 Groupings of browsing contexts

Each browsing context is defined as having a list of one or more directly reachable browsing contexts. These are:

The browsing context itself.
All the browsing context's child browsing contexts.
The browsing context's parent browsing context.
All the browsing contexts that have the browsing context as their opener browsing context.
The browsing context's opener browsing context.

The transitive closure of all the browsing contexts that are directly reachable browsing contexts forms a unit of related browsing contexts.

Each unit of related browsing contexts is then further divided into the smallest number of groups such that every member of each group has an active document with an effective script origin that, through appropriate manipulation of the document.domain attribute, could be made to be the same as other members of the group, but could not be made the same as members of any other group. Each such group is a unit of related similar-origin browsing contexts.

There is also at most one event loop per unit of related similar-origin browsing contexts (though several units of related similar-origin browsing contexts can have a shared event loop).

5.1.6 Browsing context names

Browsing contexts can have a browsing context name. By default, a browsing context has no name (its name is not set).

A valid browsing context name is any string with at least one character that does not start with a U+005F LOW LINE character. (Names starting with an underscore are reserved for special keywords.)

A valid browsing context name or keyword is any string that is either a valid browsing context name or that is an ASCII case-insensitive match for one of: _blank, _self, _parent, or _top.

These values have different meanings based on whether the page is sandboxed or not, as summarized in the following (non-normative) table. In this table, "current" means the browsing context that the link or script is in, "parent" means the parent browsing context of the one the link or script is in, "master" means the nearest ancestor browsing context of the one the link or script is in, "top" means the top-level browsing context of the one the link or script is in, "new" means a new top-level browsing context or auxiliary browsing context is to be created, subject to various user preferences and user agent policies, "none" means that nothing will happen, and "maybe new" means the same as "new" if the "allow-popups" keyword is also specified on the sandbox attribute (or if the user overrode the sandboxing), and the same as "none" otherwise.

Keyword	Ordinary effect	Effect in an `iframe` with...
Keyword	Ordinary effect	`sandbox=""`	`sandbox="allow-top-navigation"`
none specified, for links and form submissions	current	current	current
empty string	current	current	current
`_blank`	new	maybe new	maybe new
`_self`	current	current	current
`_parent` if there isn't a parent	current	current	current
`_parent` if parent is also top	parent/top	none	parent/top
`_parent` if there is one and it's not top	parent	none	none
`_top` if top is current	current	current	current
`_top` if top is not current	top	none	top
name that doesn't exist	new	maybe new	maybe new
name that exists and is a descendant	specified descendant	specified descendant	specified descendant
name that exists and is current	current	current	current
name that exists and is an ancestor that is top	specified ancestor	none	specified ancestor/top
name that exists and is an ancestor that is not top	specified ancestor	none	none
other name that exists with common top	specified	none	none
name that exists with different top, if familiar and one permitted sandboxed navigator	specified	specified	specified
name that exists with different top, if familiar but not one permitted sandboxed navigator	specified	none	none
name that exists with different top, not familiar	new	maybe new	maybe new

Most of the restrictions on sandboxed browsing contexts are applied by other algorithms, e.g. the navigation algorithm, not the rules for choosing a browsing context given a browsing context name given below.

An algorithm is allowed to show a popup if any of the following conditions is true:

The task in which the algorithm is running is currently processing an activation behavior whose click event was trusted.
The task in which the algorithm is running is currently running the event listener for a trusted event whose type is in the following list:
- change
- click
- dblclick
- mouseup
- reset
- submit
The task in which the algorithm is running was queued by an algorithm that was allowed to show a popup, and the chain of such algorithms started within a user-agent defined timeframe.

For example, if a user clicked a button, it might be acceptable for a popup to result from that after 4 seconds, but it would likely not be acceptable for a popup to result from that after 4 hours.

The rules for choosing a browsing context given a browsing context name are as follows. The rules assume that they are being applied in the context of a browsing context, as part of the execution of a task.

If the given browsing context name is the empty string or _self, then the chosen browsing context must be the current one.

If the given browsing context name is _self, then this is an explicit self-navigation override.
If the given browsing context name is _parent, then the chosen browsing context must be the parent browsing context of the current one, unless there isn't one, in which case the chosen browsing context must be the current browsing context.
If the given browsing context name is _top, then the chosen browsing context must be the top-level browsing context of the current one, if there is one, or else the current browsing context.
If the given browsing context name is not _blank and there exists a browsing context whose name is the same as the given browsing context name, and the current browsing context is familiar with that browsing context, and the user agent determines that the two browsing contexts are related enough that it is ok if they reach each other, then that browsing context must be the chosen one. If there are multiple matching browsing contexts, the user agent should select one in some arbitrary consistent manner, such as the most recently opened, most recently focused, or more closely related.

If the browsing context is chosen by this step to be the current browsing context, then this is also an explicit self-navigation override.
Otherwise, a new browsing context is being requested, and what happens depends on the user agent's configuration and abilities — it is determined by the rules given for the first applicable option from the following list:

If the algorithm is not allowed to show a popup and the user agent has been configured to not show popups (i.e. the user agent has a "popup blocker" enabled)

There is no chosen browsing context. The user agent may inform the user that a popup has been blocked.

If the current browsing context's active document's active sandboxing flag set has the sandboxed auxiliary navigation browsing context flag set.

Typically, there is no chosen browsing context.

The user agent may offer to create a new top-level browsing context or reuse an existing top-level browsing context. If the user picks one of those options, then the designated browsing context must be the chosen one (the browsing context's name isn't set to the given browsing context name). The default behaviour (if the user agent doesn't offer the option to the user, or if the user declines to allow a browsing context to be used) must be that there must not be a chosen browsing context.

If this case occurs, it means that an author has explicitly sandboxed the document that is trying to open a link.

If the user agent has been configured such that in this instance it will create a new browsing context, and the browsing context is being requested as part of following a hyperlink whose link types include the noreferrer keyword

A new top-level browsing context must be created. If the given browsing context name is not _blank, then the new top-level browsing context's name must be the given browsing context name (otherwise, it has no name). The chosen browsing context must be this new browsing context. The creation of such a browsing context is a new start for session storage.

If it is immediately navigated, then the navigation will be done with replacement enabled.

If the user agent has been configured such that in this instance it will create a new browsing context, and the noreferrer keyword doesn't apply

A new auxiliary browsing context must be created, with the opener browsing context being the current one. If the given browsing context name is not _blank, then the new auxiliary browsing context's name must be the given browsing context name (otherwise, it has no name). The chosen browsing context must be this new browsing context.

If it is immediately navigated, then the navigation will be done with replacement enabled.

If the user agent has been configured such that in this instance it will reuse the current browsing context

The chosen browsing context is the current browsing context.

If the user agent has been configured such that in this instance it will not find a browsing context

There must not be a chosen browsing context.

User agent implementors are encouraged to provide a way for users to configure the user agent to always reuse the current browsing context.

If the current browsing context's active document's active sandboxing flag set has the sandboxed navigation browsing context flag set and the chosen browsing context picked above, if any, is a new browsing context (whether top-level or auxiliary), then all the flags that are set in the current browsing context's active document's active sandboxing flag set when the new browsing context is created must be set in the new browsing context's popup sandboxing flag set, and the current browsing context must be set as the new browsing context's one permitted sandboxed navigator.

5.2 The `Window` object

[Global]
/*sealed*/ interface Window : EventTarget {
  // the current browsing context
  [Unforgeable] readonly attribute WindowProxy window;
  [Replaceable] readonly attribute WindowProxy self;
  [Unforgeable] readonly attribute Document document;
           attribute DOMString name; 
  [PutForwards=href, Unforgeable] readonly attribute Location location;
  readonly attribute History history;
  [Replaceable] readonly attribute BarProp locationbar;
  [Replaceable] readonly attribute BarProp menubar;
  [Replaceable] readonly attribute BarProp personalbar;
  [Replaceable] readonly attribute BarProp scrollbars;
  [Replaceable] readonly attribute BarProp statusbar;
  [Replaceable] readonly attribute BarProp toolbar;
           attribute DOMString status;
  void close();
  readonly attribute boolean closed;
  void stop();
  void focus();
  void blur();

  // other browsing contexts
  [Replaceable] readonly attribute WindowProxy frames;
  [Replaceable] readonly attribute unsigned long length;
  [Unforgeable] readonly attribute WindowProxy top;
           attribute WindowProxy? opener;
  readonly attribute WindowProxy parent;
  readonly attribute Element? frameElement;
  WindowProxy open(optional DOMString url = "about:blank", optional DOMString target = "_blank", optional DOMString features = "", optional boolean replace = false);
  getter WindowProxy (unsigned long index);
  getter object (DOMString name);

  // the user agent
  readonly attribute Navigator navigator; 
  readonly attribute External external;
  readonly attribute ApplicationCache applicationCache;

  // user prompts
  void alert(optional DOMString message = "");
  boolean confirm(optional DOMString message = "");
  DOMString? prompt(optional DOMString message = "", optional DOMString default = "");
  void print();


};
Window implements GlobalEventHandlers;
Window implements WindowEventHandlers;

window . window
window . frames
window . self: These attributes all return window.
window . document: Returns the Document associated with window.
document . defaultView: Returns the Window object of the active document.

The Window interface must only be exposed to JavaScript if the JavaScript global environment is a document environment.

The window, frames, and self IDL attributes must all return the Window object's browsing context's WindowProxy object.

The document IDL attribute must return the Window object's newest Document object.

The defaultView IDL attribute of the Document interface must return the Document's browsing context's WindowProxy object, if there is one, or null otherwise.

For historical reasons, Window objects must also have a writable, configurable, non-enumerable property named HTMLDocument whose value is the Document interface object.

5.2.1 Security

This section describes a security model that is underdefined, imperfect, and does not match implementations. Work is ongoing to attempt to resolve this, but in the meantime, please do not rely on this section for precision. Implementors are urged to send their feedback on how cross-origin cross-global access to Window and Location objects should work. See bug 20701.

User agents must throw a SecurityError exception whenever any properties of a Window object are accessed when the incumbent settings object specifies an effective script origin that is not the same as the Window object's Document's effective script origin, with the following exceptions:

The location attribute
The postMessage() method
The window attribute
The frames attribute
The self attribute
The top attribute
The parent attribute
The opener attribute
The closed attribute
The close() method
The blur() method
The focus() method
The dynamic nested browsing context properties

When the incumbent settings object specifies an effective script origin that is different than a Window object's Document's effective script origin, the user agent must act as if any changes to that Window object's properties, getters, setters, etc, were not present, and as if all the properties of that Window object had their [[Enumerable]] attribute set to false.

For members that return objects (including function objects), each distinct effective script origin that is not the same as the Window object's Document's effective script origin must be provided with a separate set of objects. These objects must have the prototype chain appropriate for the script for which the objects are created (not those that would be appropriate for scripts whose global object, as specified by their settings object, is the Window object in question).

For instance, if two frames containing Documents from different origins access the same Window object's postMessage() method, they will get distinct objects that are not equal.

5.2.2 APIs for creating and navigating browsing contexts by name

window = window . open( [ url [, target [, features [, replace ] ] ] ] )

Opens a window to show url (defaults to about:blank), and returns it. The target argument gives the name of the new window. If a window exists with that name already, it is reused. The replace attribute, if true, means that whatever page is currently open in that window will be removed from the window's session history. The features argument is ignored.

window . name [ = value ]

Returns the name of the window.

Can be set, to change the name.

window . close()

Closes the window.

window . closed

Returns true if the window has been closed, false otherwise.

window . stop()

Cancels the document load.

The open() method on Window objects provides a mechanism for navigating an existing browsing context or opening and navigating an auxiliary browsing context.

The method has four arguments, though they are all optional.

The first argument, url, must be a valid non-empty URL for a page to load in the browsing context. If the first argument is the empty string, then the url argument must be interpreted as "about:blank". Otherwise, the argument must be resolved to an absolute URL (or an error), relative to the API base URL specified by the entry settings object when the method was invoked.

The second argument, target, specifies the name of the browsing context that is to be navigated. It must be a valid browsing context name or keyword.

The third argument, features, has no defined effect and is mentioned for historical reasons only. User agents may interpret this argument as instructions to set the size and position of the browsing context, but are encouraged to instead ignore the argument entirely.

The fourth argument, replace, specifies whether or not the new page will replace the page currently loaded in the browsing context, when target identifies an existing browsing context (as opposed to leaving the current page in the browsing context's session history).

When the method is invoked, the user agent must first select a browsing context to navigate by applying the rules for choosing a browsing context given a browsing context name using the target argument as the name and the browsing context of the script as the context in which the algorithm is executed, unless the user has indicated a preference, in which case the browsing context to navigate may instead be the one indicated by the user.

For example, suppose there is a user agent that supports control-clicking a link to open it in a new tab. If a user clicks in that user agent on an element whose onclick handler uses the window.open() API to open a page in an iframe, but, while doing so, holds the control key down, the user agent could override the selection of the target browsing context to instead target a new tab.

If applying the rules for choosing a browsing context given a browsing context name using the target argument would result in there not being a chosen browsing context, then throw an InvalidAccessError exception and abort these steps.

Otherwise, if url is not "about:blank", the user agent must navigate the selected browsing context to the absolute URL obtained from resolving url earlier, with exceptions enabled. If the replace is true or if the browsing context was just created as part of the rules for choosing a browsing context given a browsing context name, then replacement must be enabled also. The navigation must be done with the responsible browsing context specified by the incumbent settings object as the source browsing context. If the resolve a URL algorithm failed, then the user agent may either instead navigate to an inline error page, with exceptions enabled and using the same replacement behavior and source browsing context behavior as described earlier in this paragraph; or treat the url as "about:blank", acting as described in the next paragraph.

If url is "about:blank", and the browsing context was just created as part of the rules for choosing a browsing context given a browsing context name, then the user agent must instead queue a task to fire a simple event named load at the selected browsing context's Window object, but with its target set to the selected browsing context's Window object's Document object (and the currentTarget set to the Window object).

The method must return the WindowProxy object of the browsing context that was navigated, or null if no browsing context was navigated.

The name attribute of the Window object must, on getting, return the current name of the browsing context, and, on setting, set the name of the browsing context to the new value.

The name gets reset when the browsing context is navigated to another domain.

The close() method on Window objects should, if all the following conditions are met, close the browsing context A:

The corresponding browsing context A is script-closable.
The responsible browsing context specified by the incumbent settings object is familiar with the browsing context A.
The responsible browsing context specified by the incumbent settings object is allowed to navigate the browsing context A.

A browsing context is script-closable if it is an auxiliary browsing context that was created by a script (as opposed to by an action of the user), or if it is a browsing context whose session history contains only one Document.

The closed attribute on Window objects must return true if the Window object's browsing context has been discarded, and false otherwise.

The stop() method on Window objects should, if there is an existing attempt to navigate the browsing context and that attempt is not currently running the unload a document algorithm, cancel that navigation; then, it must abort the active document of the browsing context of the Window object on which it was invoked.

5.2.3 Accessing other browsing contexts

window . length: Returns the number of child browsing contexts.
window[index]: Returns the indicated child browsing context.

The length IDL attribute on the Window interface must return the number of child browsing contexts that are nested through elements that are in the Document that is the active document of that Window object, if that Window's browsing context shares the same event loop as the responsible document specified by the entry settings object accessing the IDL attribute; otherwise, it must return zero.

The supported property indices on the Window object at any instant are the numbers in the range 0 .. n-1, where n is the number returned by the length IDL attribute. If n is zero then there are no supported property indices.

To determine the value of an indexed property index of a Window object, the user agent must return the WindowProxy object of the indexth child browsing context of the Document that is nested through an element that is in the Document, sorted in the tree order of the elements nesting those browsing contexts.

These properties are the dynamic nested browsing context properties.

5.2.4 Named access on the `Window` object

window[name]: Returns the indicated element or collection of elements.

The Window interface supports named properties. The supported property names at any moment consist of the following, in tree order, ignoring later duplicates:

the browsing context name of any child browsing context of the active document whose name is not the empty string,
the value of the name content attribute for all a, applet, area, embed, form, frameset, img, and object elements in the active document that have a non-empty name content attribute, and
the value of the id content attribute of any HTML element in the active document with a non-empty id content attribute.

To determine the value of a named property name when the Window object is indexed for property retrieval, the user agent must return the value obtained using the following steps:

Let objects be the list of named objects with the name name in the active document.
There will be at least one such object, by definition.
If objects contains a nested browsing context, then return the WindowProxy object of the nested browsing context corresponding to the first browsing context container in tree order whose browsing context is in objects, and abort these steps.
Otherwise, if objects has only one element, return that element and abort these steps.
Otherwise return an HTMLCollection rooted at the Document node, whose filter matches only named objects with the name name. (By definition, these will all be elements.)

Named objects with the name name, for the purposes of the above algorithm, are those that are either:

child browsing contexts of the active document whose name is name,
a, applet, area, embed, form, frameset, img, or object elements that have a name content attribute whose value is name, or
HTML elements that have an id content attribute whose value is name.

5.2.5 Garbage collection and browsing contexts

A browsing context has a strong reference to each of its Documents and its WindowProxy object, and the user agent itself has a strong reference to its top-level browsing contexts.

A Document has a strong reference to its Window object.

A Window object has a strong reference to its Document object through its document attribute. Thus, references from other scripts to either of those objects will keep both alive. Similarly, both Document and Window objects have implied strong references to the WindowProxy object.

Each script has a strong reference to its settings object, and each script settings object has strong references to its global object, responsible browsing context, and responsible document.

When a browsing context is to discard a Document, the user agent must run the following steps:

Set the Document's salvageable state to false.
Run any unloading document cleanup steps for the Document that are defined by this specification and other applicable specifications.
Abort the Document.
Remove any tasks associated with the Document in any task source, without running those tasks.
Discard all the child browsing contexts of the Document.
Lose the strong reference from the Document's browsing context to the Document.

Whenever a Document object is discarded, it is also removed from the list of the worker's Documents of each worker whose list contains that Document.

When a browsing context is discarded, the strong reference from the user agent itself to the browsing context must be severed, and all the Document objects for all the entries in the browsing context's session history must be discarded as well.

User agents may discard top-level browsing contexts at any time (typically, in response to user requests, e.g. when a user force-closes a window containing one or more top-level browsing contexts). Other browsing contexts must be discarded once their WindowProxy object is eligible for garbage collection.

5.2.6 Closing browsing contexts

When the user agent is required to close a browsing context, it must run the following steps:

Let specified browsing context be the browsing context being closed.
Prompt to unload the active document of the specified browsing context. If the user refused to allow the document to be unloaded, then abort these steps.
Unload the active document of the specified browsing context with the recycle parameter set to false.
Remove the specified browsing context from the user interface (e.g. close or hide its tab in a tabbed browser).
Discard the specified browsing context.

User agents should offer users the ability to arbitrarily close any top-level browsing context.

5.2.7 Browser interface elements

To allow Web pages to integrate with Web browsers, certain Web browser interface elements are exposed in a limited way to scripts in Web pages.

Each interface element is represented by a BarProp object:

interface BarProp {
           attribute boolean visible;
};

window . locationbar . visible: Returns true if the location bar is visible; otherwise, returns false.
window . menubar . visible: Returns true if the menu bar is visible; otherwise, returns false.
window . personalbar . visible: Returns true if the personal bar is visible; otherwise, returns false.
window . scrollbars . visible: Returns true if the scroll bars are visible; otherwise, returns false.
window . statusbar . visible: Returns true if the status bar is visible; otherwise, returns false.
window . toolbar . visible: Returns true if the toolbar is visible; otherwise, returns false.

The visible attribute, on getting, must return either true or a value determined by the user agent to most accurately represent the visibility state of the user interface element that the object represents, as described below. On setting, the new value must be discarded.

The following BarProp objects exist for each Document object in a browsing context. Some of the user interface elements represented by these objects might have no equivalent in some user agents; for those user agents, except when otherwise specified, the object must act as if it was present and visible (i.e. its visible attribute must return true).

The location bar BarProp object: Represents the user interface element that contains a control that displays the URL of the active document, or some similar interface concept.
The menu bar BarProp object: Represents the user interface element that contains a list of commands in menu form, or some similar interface concept.
The personal bar BarProp object: Represents the user interface element that contains links to the user's favorite pages, or some similar interface concept.
The scrollbar BarProp object: Represents the user interface element that contains a scrolling mechanism, or some similar interface concept.
The status bar BarProp object: Represents a user interface element found immediately below or after the document, as appropriate for the user's media. If the user agent has no such user interface element, then the object may act as if the corresponding user interface element was absent (i.e. its visible attribute may return false).
The toolbar BarProp object: Represents the user interface element found immediately above or before the document, as appropriate for the user's media. If the user agent has no such user interface element, then the object may act as if the corresponding user interface element was absent (i.e. its visible attribute may return false).

The locationbar attribute must return the location bar BarProp object.

The menubar attribute must return the menu bar BarProp object.

The personalbar attribute must return the personal bar BarProp object.

The scrollbars attribute must return the scrollbar BarProp object.

The statusbar attribute must return the status bar BarProp object.

The toolbar attribute must return the toolbar BarProp object.

For historical reasons, the status attribute on the Window object must, on getting, return the last string it was set to, and on setting, must set itself to the new value. When the Window object is created, the attribute must be set to the empty string. It does not do anything else.

5.2.8 The `WindowProxy` object

As mentioned earlier, each browsing context has a WindowProxy object. This object is unusual in that all operations that would be performed on it must be performed on the Window object of the browsing context's active document instead. It is thus indistinguishable from that Window object in every way until the browsing context is navigated.

There is no WindowProxy interface object.

The WindowProxy object allows scripts to act as if each browsing context had a single Window object, while still keeping separate Window objects for each Document.

In the following example, the variable x is set to the WindowProxy object returned by the window accessor on the global object. All of the expressions following the assignment return true, because in every respect, the WindowProxy object acts like the underlying Window object.

var x = window;
x instanceof Window; // true
x === this; // true

5.3 Origin

Origins are the fundamental currency of the Web's security model. Two actors in the Web platform that share an origin are assumed to trust each other and to have the same authority. Actors with differing origins are considered potentially hostile versus each other, and are isolated from each other to varying degrees.

For example, if Example Bank's Web site, hosted at bank.example.com, tries to examine the DOM of Example Charity's Web site, hosted at charity.example.org, a SecurityError exception will be raised.

The origin of a resource and the effective script origin of a resource are both either opaque identifiers or tuples consisting of a scheme component, a host component, a port component, and optionally extra data.

The extra data could include the certificate of the site when using encrypted connections, to ensure that if the site's secure certificate changes, the origin is considered to change as well.

An origin or effective script origin can be defined as an alias to another origin or effective script origin. The value of the origin or effective script origin is then the value of the origin or effective script origin to which it is an alias.

These characteristics are defined as follows:

For URLs

The origin and effective script origin of the URL are the origin defined in The Web Origin Concept. [ORIGIN]

For Document objects

If a Document's active sandboxing flag set has its sandboxed origin browsing context flag set

The origin is a globally unique identifier assigned when the Document is created.

The effective script origin is initially an alias to the origin of the Document.

If a Document was served over the network and has an address that uses a URL scheme with a server-based naming authority

The origin is an alias to the origin of the Document's address.

The effective script origin is initially an alias to the origin of the Document.

If a Document was generated from a data: URL found in another Document or in a script

The origin is an alias to the origin specified by the incumbent settings object when the navigate algorithm was invoked, or, if no script was involved, of the Document of the element that initiated the navigation to that URL.

The effective script origin is initially an alias to the effective script origin of that same script settings object or Document.

If a Document is the initial "about:blank" document

The origin and effective script origin of the Document are those it was assigned when its browsing context was created.

If a Document was created as part of the processing for javascript: URLs

The origin is an alias to the origin of the active document of the browsing context being navigated when the navigate algorithm was invoked.

The effective script origin is initially an alias to the effective script origin of that same Document.

If a Document is an iframe srcdoc document

The origin of the Document is an alias to the origin of the Document's browsing context's browsing context container's Document.

The effective script origin is initially an alias to the effective script origin of the Document's browsing context's browsing context container's Document.

If a Document was obtained in some other manner (e.g. a data: URL typed in by the user or that was returned as the location of an HTTP redirect (or equivalent in other protocols), a Document created using the createDocument() API, etc)

The default behavior as defined in the DOM standard applies. [DOM].

The origin is a globally unique identifier assigned when the Document is created, and the effective script origin is initially an alias to the origin of the Document.

The effective script origin of a Document can be manipulated using the document.domain IDL attribute.

For images of img elements

If the image data is CORS-cross-origin: The origin is a globally unique identifier assigned when the image is created.
If the image data is CORS-same-origin: The origin is an alias to the origin of the img element's Document.

Images do not have an effective script origin.

For audio and video elements

If the media data is CORS-cross-origin: The origin is a globally unique identifier assigned when the media data is fetched.
If the media data is CORS-same-origin: The origin is an alias to the origin of the media element's Document.

Media elements do not have an effective script origin.

For fonts

The origin of a downloadable Web font is an alias to the origin of the absolute URL used to obtain the font (after any redirects). [CSSFONTS] [CSSFONTLOAD]

The origin of a locally installed system font is an alias to the origin of the Document in which that font is being used.

Fonts do not have an effective script origin.

Other specifications can override the above definitions by themselves specifying the origin of a particular URL, Document, image, media element, or font.

The Unicode serialization of an origin is the string obtained by applying the following algorithm to the given origin:

If the origin in question is not a scheme/host/port tuple, then return the literal string "null" and abort these steps.
Otherwise, let result be the scheme part of the origin tuple.
Append the string "://" to result.
Apply the IDNA ToUnicode algorithm to each component of the host part of the origin tuple, and append the results — each component, in the same order, separated by "." (U+002E) characters — to result. [RFC5890]
If the port part of the origin tuple gives a port that is different from the default port for the protocol given by the scheme part of the origin tuple, then append a ":" (U+003A) character and the given port, in base ten, to result.
Return result.

The ASCII serialization of an origin is the string obtained by applying the following algorithm to the given origin:

If the origin in question is not a scheme/host/port tuple, then return the literal string "null" and abort these steps.
Otherwise, let result be the scheme part of the origin tuple.
Append the string "://" to result.
Apply the IDNA ToASCII algorithm to the host part of the origin tuple, with both the AllowUnassigned and UseSTD3ASCIIRules flags set, and append the results to result.

If ToASCII fails to convert one of the components of the string, e.g. because it is too long or because it contains invalid characters, then return the empty string and abort these steps. [RFC5890]
If the port part of the origin tuple gives a port that is different from the default port for the protocol given by the scheme part of the origin tuple, then append a ":" (U+003A) character and the given port, in base ten, to result.
Return result.

Two origins are said to be the same origin if the following algorithm returns true:

Let A be the first origin being compared, and B be the second origin being compared.
If A and B are both opaque identifiers, and their value is equal, then return true.
Otherwise, if either A or B or both are opaque identifiers, return false.
If A and B have scheme components that are not identical, return false.
If A and B have host components that are not identical, return false.
If A and B have port components that are not identical, return false.
If either A or B have additional data, but that data is not identical for both, return false.
Return true.

5.3.1 Relaxing the same-origin restriction

document . domain [ = domain ]

Returns the current domain used for security checks.

Can be set to a value that removes subdomains, to change the effective script origin to allow pages on other subdomains of the same domain (if they do the same thing) to access each other. (Can't be set in sandboxed iframes.)

The domain attribute on Document objects must be initialized to the document's domain, if it has one, and the empty string otherwise. If the document's domain starts with a "[" (U+005B) character and ends with a "]" (U+005D) character, it is an IPv6 address; these square brackets must be omitted when initializing the attribute's value.

On getting, the attribute must return its current value, unless the Document has no browsing context, in which case it must return the empty string.

On setting, the user agent must run the following algorithm:

If the Document has no browsing context, throw a SecurityError exception and abort these steps.
If the Document's active sandboxing flag set has its sandboxed document.domain browsing context flag set, throw a SecurityError exception and abort these steps.
If the new value is an IPv4 or IPv6 address, let new value be the new value. Otherwise, apply the IDNA ToASCII algorithm to the new value, with both the AllowUnassigned and UseSTD3ASCIIRules flags set, and let new value be the result of the ToASCII algorithm.

If ToASCII fails to convert one of the components of the string, e.g. because it is too long or because it contains invalid characters, then throw a SecurityError exception and abort these steps. [RFC5890]
If new value is not exactly equal to the current value of the document.domain attribute, then run these substeps:
1. If the current value is an IPv4 or IPv6 address, throw a SecurityError exception and abort these steps.
2. If new value, prefixed by a "." (U+002E), does not exactly match the end of the current value, throw a SecurityError exception and abort these steps.
  
  If the new value is an IPv4 or IPv6 address, it cannot match the new value in this way and thus an exception will be thrown here.
3. If new value matches a suffix in the Public Suffix List, or, if new value, prefixed by a "." (U+002E), matches the end of a suffix in the Public Suffix List, then throw a SecurityError exception and abort these steps. [PSL]
  
  Suffixes must be compared after applying the IDNA ToASCII algorithm to them, with both the AllowUnassigned and UseSTD3ASCIIRules flags set, in an ASCII case-insensitive manner. [RFC5890]
Release the storage mutex.
Set the attribute's value to new value.
If the effective script origin of the Document is an alias, set it to the value of the effective script origin (essentially de-aliasing the effective script origin).
If new value is not the empty string, then run these substeps:
1. Set the host part of the effective script origin tuple of the Document to new value.
2. Set the port part of the effective script origin tuple of the Document to "manual override" (a value that, for the purposes of comparing origins, is identical to "manual override" but not identical to any other value).

The domain of a Document is the host part of the document's origin, if the value of that origin is a scheme/host/port tuple. If it isn't, then the document does not have a domain.

The domain attribute is used to enable pages on different hosts of a domain to access each others' DOMs.

Do not use the document.domain attribute when using shared hosting. If an untrusted third party is able to host an HTTP server at the same IP address but on a different port, then the same-origin protection that normally protects two different sites on the same host will fail, as the ports are ignored when comparing origins after the document.domain attribute has been used.

5.4 Sandboxing

A sandboxing flag set is a set of zero or more of the following flags, which are used to restrict the abilities that potentially untrusted resources have:

The sandboxed navigation browsing context flag

This flag prevents content from navigating browsing contexts other than the sandboxed browsing context itself (or browsing contexts further nested inside it), auxiliary browsing contexts (which are protected by the sandboxed auxiliary navigation browsing context flag defined next), and the top-level browsing context (which is protected by the sandboxed top-level navigation browsing context flag defined below).

If the sandboxed auxiliary navigation browsing context flag is not set, then in certain cases the restrictions nonetheless allow popups (new top-level browsing contexts) to be opened. These browsing contexts always have one permitted sandboxed navigator, set when the browsing context is created, which allows the browsing context that created them to actually navigate them. (Otherwise, the sandboxed navigation browsing context flag would prevent them from being navigated even if they were opened.)

The sandboxed auxiliary navigation browsing context flag

This flag prevents content from creating new auxiliary browsing contexts, e.g. using the target attribute, or the window.open() method.

The sandboxed top-level navigation browsing context flag

This flag prevents content from navigating their top-level browsing context and prevents content from closing their top-level browsing context.

When the sandboxed top-level navigation browsing context flag is not set, content can navigate its top-level browsing context, but other browsing contexts are still protected by the sandboxed navigation browsing context flag and possibly the sandboxed auxiliary navigation browsing context flag.

The sandboxed plugins browsing context flag

This flag prevents content from instantiating plugins, whether using the embed element, the object element, the applet element, or through navigation of a nested browsing context, unless those plugins can be secured.

The sandboxed origin browsing context flag

This flag forces content into a unique origin, thus preventing it from accessing other content from the same origin.

This flag also prevents script from reading from or writing to the document.cookie IDL attribute, and blocks access to localStorage. [WEBSTORAGE]

The sandboxed forms browsing context flag

This flag blocks form submission.

The sandboxed pointer lock browsing context flag

This flag disables the Pointer Lock API. [POINTERLOCK]

The sandboxed scripts browsing context flag

This flag blocks script execution.

The sandboxed automatic features browsing context flag

This flag blocks features that trigger automatically, such as automatically playing a video or automatically focusing a form control.

The sandboxed fullscreen browsing context flag

This flag prevents content from using the requestFullscreen() method.

The sandboxed document.domain browsing context flag

This flag prevents content from using the document.domain feature to change the effective script origin.

When the user agent is to parse a sandboxing directive, given a string input, a sandboxing flag set output, and optionally an allow fullscreen flag, it must run the following steps:

Split input on spaces, to obtain tokens.
Let output be empty.
Add the following flags to output:
- The sandboxed navigation browsing context flag.
- The sandboxed auxiliary navigation browsing context flag, unless tokens contains the allow-popups keyword.
- The sandboxed top-level navigation browsing context flag, unless tokens contains the allow-top-navigation keyword.
- The sandboxed plugins browsing context flag.
- The sandboxed origin browsing context flag, unless the tokens contains the allow-same-origin keyword.
  
  The allow-same-origin keyword is intended for two cases.
  
  First, it can be used to allow content from the same site to be sandboxed to disable scripting, while still allowing access to the DOM of the sandboxed content.
  
  Second, it can be used to embed content from a third-party site, sandboxed to prevent that site from opening pop-up windows, etc, without preventing the embedded page from communicating back to its originating site, using the database APIs to store data, etc.
- The sandboxed forms browsing context flag, unless tokens contains the allow-forms keyword.
- The sandboxed pointer lock browsing context flag, unless tokens contains the allow-pointer-lock keyword.
- The sandboxed scripts browsing context flag, unless tokens contains the allow-scripts keyword.
- The sandboxed automatic features browsing context flag, unless tokens contains the allow-scripts keyword (defined above).
  
  This flag is relaxed by the same keyword as scripts, because when scripts are enabled these features are trivially possible anyway, and it would be unfortunate to force authors to use script to do them when sandboxed rather than allowing them to use the declarative features.
- The sandboxed fullscreen browsing context flag, unless the allow fullscreen flag was passed to the parse a sandboxing directive flag.
- The sandboxed document.domain browsing context flag.

Every top-level browsing context has a popup sandboxing flag set, which is a sandboxing flag set. When a browsing context is created, its popup sandboxing flag set must be empty. It is populated by the rules for choosing a browsing context given a browsing context name.

Every nested browsing context has an iframe sandboxing flag set, which is a sandboxing flag set. Which flags in a nested browsing context's iframe sandboxing flag set are set at any particular time is determined by the iframe element's sandbox attribute.

Every Document has an active sandboxing flag set, which is a sandboxing flag set. When the Document is created, its active sandboxing flag set must be empty. It is populated by the navigation algorithm.

Every resource that is obtained by the navigation algorithm has a forced sandboxing flag set, which is a sandboxing flag set. A resource by default has no flags set in its forced sandboxing flag set, but other specifications can define that certain flags are set.

In particular, the forced sandboxing flag set is used by the Content Security Policy specification. [CSP]

When a user agent is to implement the sandboxing for a Document, it must populate Document's active sandboxing flag set with the union of the flags that are present in the following sandboxing flag sets at the time the Document object is created:

If the Document's browsing context is a top-level browsing context, then: the flags set on the browsing context's popup sandboxing flag set.
If the Document's browsing context is a nested browsing context, then: the flags set on the browsing context's iframe sandboxing flag set.
If the Document's browsing context is a nested browsing context, then: the flags set on the browsing context's parent browsing context's active document's active sandboxing flag set.
The flags set on the Document's resource's forced sandboxing flag set, if it has one.

5.5 Session history and navigation

5.5.1 The session history of browsing contexts

The sequence of Documents in a browsing context is its session history. Each browsing context, including nested browsing contexts, has a distinct session history. A browsing context's session history consists of a flat list of session history entries. Each session history entry consists, at a minimum, of a URL, and each entry may in addition have a state object, a title, a Document object, form data, a scroll position, and other information associated with it.

Each entry, when first created, has a Document. However, when a Document is not active, it's possible for it to be discarded to free resources. The URL and other data in a session history entry is then used to bring a new Document into being to take the place of the original, should the user agent find itself having to reactivate that Document.

Titles associated with session history entries need not have any relation with the current title of the Document. The title of a session history entry is intended to explain the state of the document at that point, so that the user can navigate the document's history.

URLs without associated state objects are added to the session history as the user (or script) navigates from page to page.

Each Document object in a browsing context's session history is associated with a unique History object which must all model the same underlying session history.

The history attribute of the Window interface must return the object implementing the History interface for that Window object's newest Document.

A state object is an object representing a user interface state.

Pages can add state objects to the session history. These are then returned to the script when the user (or script) goes back in the history, thus enabling authors to use the "navigation" metaphor even in one-page applications.

State objects are intended to be used for two main purposes: first, storing a preparsed description of the state in the URL so that in the simple case an author doesn't have to do the parsing (though one would still need the parsing for handling URLs passed around by users, so it's only a minor optimization), and second, so that the author can store state that one wouldn't store in the URL because it only applies to the current Document instance and it would have to be reconstructed if a new Document were opened.

An example of the latter would be something like keeping track of the precise coordinate from which a pop-up div was made to animate, so that if the user goes back, it can be made to animate to the same location. Or alternatively, it could be used to keep a pointer into a cache of data that would be fetched from the server based on the information in the URL, so that when going back and forward, the information doesn't have to be fetched again.

At any point, one of the entries in the session history is the current entry. This is the entry representing the active document of the browsing context. Which entry is the current entry is changed by the algorithms defined in this specification, e.g. during session history traversal.

The current entry is usually an entry for the address of the Document. However, it can also be one of the entries for state objects added to the history by that document.

An entry with persisted user state is one that also has user-agent defined state. This specification does not specify what kind of state can be stored.

For example, some user agents might want to persist the scroll position, or the values of form controls.

User agents that persist the value of form controls are encouraged to also persist their directionality (the value of the element's dir attribute). This prevents values from being displayed incorrectly after a history traversal when the user had originally entered the values with an explicit, non-default directionality.

Entries that consist of state objects share the same Document as the entry for the page that was active when they were added.

Contiguous entries that differ just by fragment identifier also share the same Document.

All entries that share the same Document (and that are therefore merely different states of one particular document) are contiguous by definition.

Each Document in a browsing context can also have a latest entry. This is the entry or that Document that was most the recently traversed to. When a Document is created, it initially has no latest entry.

User agents may discard the Document objects of entries other than the current entry that are not referenced from any script, reloading the pages afresh when the user or script navigates back to such pages. This specification does not specify when user agents should discard Document objects and when they should cache them.

Entries that have had their Document objects discarded must, for the purposes of the algorithms given below, act as if they had not. When the user or script navigates back or forwards to a page which has no in-memory DOM objects, any other entries that shared the same Document object with it must share the new object as well.

5.5.2 The `History` interface

interface History {
  readonly attribute long length;
  readonly attribute any state;
  void go(optional long delta);
  void back();
  void forward();
  void pushState(any data, DOMString title, optional DOMString? url = null);
  void replaceState(any data, DOMString title, optional DOMString? url = null);
};

window . history . length

Returns the number of entries in the joint session history.

window . history . state

Returns the current state object.

window . history . go( [ delta ] )

Goes back or forward the specified number of steps in the joint session history.

A zero delta will reload the current page.

If the delta is out of range, does nothing.

window . history . back()

Goes back one step in the joint session history.

If there is no previous page, does nothing.

window . history . forward()

Goes forward one step in the joint session history.

If there is no next page, does nothing.

window . history . pushState(data, title [, url ] )

Pushes the given data onto the session history, with the given title, and, if provided and not null, the given URL.

window . history . replaceState(data, title [, url ] )

Updates the current entry in the session history to have the given data, title, and, if provided and not null, URL.

The joint session history of a top-level browsing context is the union of all the session histories of all browsing contexts of all the fully active Document objects that share that top-level browsing context, with all the entries that are current entries in their respective session histories removed except for the current entry of the joint session history.

The current entry of the joint session history is the entry that most recently became a current entry in its session history.

Entries in the joint session history are ordered chronologically by the time they were added to their respective session histories. Each entry has an index; the earliest entry has index 0, and the subsequent entries are numbered with consecutively increasing integers (1, 2, 3, etc).

Since each Document in a browsing context might have a different event loop, the actual state of the joint session history can be someone nebulous. For example, two sibling iframe elements could both traverse from one unique origin to another at the same time, so their precise order might not be well-defined; similarly, since they might only find out about each other later, they might disagree about the length of the joint session history.

All the getters and setters for attributes, and all the methods, defined on the History interface, when invoked on a History object associated with a Document that is not fully active, must throw a SecurityError exception instead of operating as described below.

The length attribute of the History interface must return the number of entries in the top-level browsing context's joint session history.

The actual entries are not accessible from script.

The state attribute of the History interface must return the last value it was set to by the user agent. Initially, its value must be null.

When the go(delta) method is invoked, if the argument to the method was omitted or has the value zero, the user agent must act as if the location.reload() method was called instead. Otherwise, the user agent must traverse the history by a delta whose value is the value of the method's argument.

When the back() method is invoked, the user agent must traverse the history by a delta −1.

When the forward()method is invoked, the user agent must traverse the history by a delta +1.

Each top-level browsing context has a session history traversal queue, initially empty, to which tasks can be added.

Each top-level browsing context, when created, must asynchronously begin running the following algorithm, known as the session history event loop for that top-level browsing context:

Wait until this top-level browsing context's session history traversal queue is not empty.
Pull the first task from this top-level browsing context's session history traversal queue, and execute it.
Return to the first step of this algorithm.

The session history event loop helps coordinate cross-browsing-context transitions of the joint session history: since each browsing context might, at any particular time, have a different event loop (this can happen if the user agent has more than one event loop per unit of related browsing contexts), transitions would otherwise have to involve cross-event-loop synchronisation.

To traverse the history by a delta delta, the user agent must append a task to this top-level browsing context's session history traversal queue, the task consisting of running the following steps:

Let delta be the argument to the method.
If the index of the current entry of the joint session history plus delta is less than zero or greater than or equal to the number of items in the joint session history, then abort these steps.
Let specified entry be the entry in the joint session history whose index is the sum of delta and the index of the current entry of the joint session history.
Let specified browsing context be the browsing context of the specified entry.
If the specified browsing context's active document's unload a document algorithm is currently running, abort these steps.
Queue a task that consists of running the following substeps. The relevant event loop is that of the specified browsing context's active document. The task source for the queued task is the history traversal task source.
1. If there is an ongoing attempt to navigate specified browsing context that has not yet matured (i.e. it has not passed the point of making its Document the active document), then cancel that attempt to navigate the browsing context.
2. If the specified browsing context's active document is not the same Document as the Document of the specified entry, then run these substeps:
  1. Fully exit fullscreen.
  2. Prompt to unload the active document of the specified browsing context. If the user refused to allow the document to be unloaded, then abort these steps.
  3. Unload the active document of the specified browsing context with the recycle parameter set to false.
3. Traverse the history of the specified browsing context to the specified entry.

When the user navigates through a browsing context, e.g. using a browser's back and forward buttons, the user agent must traverse the history by a delta equivalent to the action specified by the user.

The pushState(data, title, url) method adds a state object entry to the history.

The replaceState(data, title, url) method updates the state object, title, and optionally the URL of the current entry in the history.

When either of these methods is invoked, the user agent must run the following steps:

Let cloned data be a structured clone of the specified data. If this throws an exception, then rethrow that exception and abort these steps.
If the third argument is not null, run these substeps:
1. Resolve the value of the third argument, relative to the API base URL specified by the entry settings object.
2. If that fails, throw a SecurityError exception and abort these steps.
3. Compare the resulting parsed URL to the result of applying the URL parser algorithm to the document's address. If any component of these two URLs differ other than the path, query, and fragment components, then throw a SecurityError exception and abort these steps.
4. If the origin of the resulting absolute URL is not the same as the origin of the responsible document specified by the entry settings object, and either the path or query components of the two parsed URLs compared in the previous step differ, throw a SecurityError exception and abort these steps. (This prevents sandboxed content from spoofing other pages on the same origin.)
5. Let new URL be the resulting absolute URL.
For the purposes of the comparisons in the above substeps, the path and query components can only be the same if the scheme component of both parsed URLs are relative schemes.
If the third argument is null, then let new URL be the URL of the current entry.
If the method invoked was the pushState() method:
1. Remove all the entries in the browsing context's session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.
  
  This doesn't necessarily have to affect the user agent's user interface.
2. Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context's document family.
3. If appropriate, update the current entry to reflect any state that the user agent wishes to persist. The entry is then said to be an entry with persisted user state.
4. Add a state object entry to the session history, after the current entry, with cloned data as the state object, the given title as the title, and new URL as the URL of the entry.
5. Update the current entry to be this newly added entry.
Otherwise, if the method invoked was the replaceState() method:
1. Update the current entry in the session history so that cloned data is the entry's new state object, the given title is the new title, and new URL is the entry's new URL.
If the current entry in the session history represents a non-GET request (e.g. it was the result of a POST submission) then update it to instead represent a GET request (or equivalent).
Set the document's address to new URL.

Since this is neither a navigation of the browsing context nor a history traversal, it does not cause a hashchange event to be fired.
Set history.state to a structured clone of cloned data.
Let the latest entry of the Document of the current entry be the current entry.

The title is purely advisory. User agents might use the title in the user interface.

User agents may limit the number of state objects added to the session history per page. If a page hits the UA-defined limit, user agents must remove the entry immediately after the first entry for that Document object in the session history after having added the new entry. (Thus the state history acts as a FIFO buffer for eviction, but as a LIFO buffer for navigation.)

Consider a game where the user can navigate along a line, such that the user is always at some coordinate, and such that the user can bookmark the page corresponding to a particular coordinate, to return to it later.

A static page implementing the x=5 position in such a game could look like the following:

<!DOCTYPE HTML>
<!-- this is http://example.com/line?x=5 -->
<title>Line Game - 5</title>
<p>You are at coordinate 5 on the line.</p>
<p>
 <a href="?x=6">Advance to 6</a> or
 <a href="?x=4">retreat to 4</a>?
</p>

The problem with such a system is that each time the user clicks, the whole page has to be reloaded. Here instead is another way of doing it, using script:

<!DOCTYPE HTML>
<!-- this starts off as http://example.com/line?x=5 -->
<title>Line Game - 5</title>
<p>You are at coordinate <span id="coord">5</span> on the line.</p>
<p>
 <a href="?x=6" onclick="go(1); return false;">Advance to 6</a> or
 <a href="?x=4" onclick="go(-1); return false;">retreat to 4</a>?
</p>
<script>
 var currentPage = 5; // prefilled by server
 function go(d) {
   setupPage(currentPage + d);
   history.pushState(currentPage, document.title, '?x=' + currentPage);
 }
 onpopstate = function(event) {
   setupPage(event.state);
 }
 function setupPage(page) {
   currentPage = page;
   document.title = 'Line Game - ' + currentPage;
   document.getElementById('coord').textContent = currentPage;
   document.links[0].href = '?x=' + (currentPage+1);
   document.links[0].textContent = 'Advance to ' + (currentPage+1);
   document.links[1].href = '?x=' + (currentPage-1);
   document.links[1].textContent = 'retreat to ' + (currentPage-1);
 }
</script>

In systems without script, this still works like the previous example. However, users that do have script support can now navigate much faster, since there is no network access for the same experience. Furthermore, contrary to the experience the user would have with just a naïve script-based approach, bookmarking and navigating the session history still work.

In the example above, the data argument to the pushState() method is the same information as would be sent to the server, but in a more convenient form, so that the script doesn't have to parse the URL each time the user navigates.

Applications might not use the same title for a session history entry as the value of the document's title element at that time. For example, here is a simple page that shows a block in the title element. Clearly, when navigating backwards to a previous state the user does not go back in time, and therefore it would be inappropriate to put the time in the session history title.

<!DOCTYPE HTML>
<TITLE>Line</TITLE>
<SCRIPT>
 setInterval(function () { document.title = 'Line - ' + new Date(); }, 1000);
 var i = 1;
 function inc() {
   set(i+1);
   history.pushState(i, 'Line - ' + i);
 }
 function set(newI) {
   i = newI;
   document.forms.F.I.value = newI;
 }
</SCRIPT>
<BODY ONPOPSTATE="set(event.state)">
<FORM NAME=F>
State: <OUTPUT NAME=I>1</OUTPUT> <INPUT VALUE="Increment" TYPE=BUTTON ONCLICK="inc()">
</FORM>

5.5.3 The `Location` interface

Each Document object in a browsing context's session history is associated with a unique instance of a Location object.

document . location [ = value ]

window . location [ = value ]

Returns a Location object with the current page's location.

Can be set, to navigate to another page.

The location attribute of the Document interface must return the Location object for that Document object, if it is in a browsing context, and null otherwise.

The location attribute of the Window interface must return the Location object for that Window object's Document.

Location objects provide a representation of the address of the active document of their Document's browsing context, and allow the current entry of the browsing context's session history to be changed, by adding or replacing entries in the history object.

[Unforgeable] interface Location {
  void assign(DOMString url);
  void replace(DOMString url);
  void reload();
};
Location implements URLUtils;

location . assign(url): Navigates to the given page.
location . replace(url): Removes the current page from the session history and navigates to the given page.
location . reload(): Reloads the current page.

The relevant Document is the Location object's associated Document object's browsing context's active document.

When the assign(url) method is invoked, the UA must resolve the argument, relative to the API base URL specified by the entry settings object, and if that is successful, must navigate the browsing context to the specified url, with exceptions enabled. If the browsing context's session history contains only one Document, and that was the about:blank Document created when the browsing context was created, then the navigation must be done with replacement enabled.

When the replace(url) method is invoked, the UA must resolve the argument, relative to the API base URL specified by the entry settings object, and if that is successful, navigate the browsing context to the specified url with replacement enabled and exceptions enabled.

Navigation for the assign() and replace() methods must be done with the responsible browsing context specified by the incumbent settings object as the source browsing context.

If the resolving step of the assign() and replace() methods is not successful, then the user agent must instead throw a SyntaxError exception.

When the reload() method is invoked, the user agent must run the appropriate steps from the following list:

If the currently executing task is the dispatch of a resize event in response to the user resizing the browsing context: Repaint the browsing context and abort these steps.
If the browsing context's active document is an iframe srcdoc document: Reprocess the iframe attributes of the browsing context's browsing context container.
If the browsing context's active document has its reload override flag set: Perform an overridden reload, with the browsing context being navigated as the responsible browsing context.
Otherwise: Navigate the browsing context to the document's address with replacement enabled and exceptions enabled. The source browsing context must be the browsing context being navigated.

When a user requests that the active document of a browsing context be reloaded through a user interface element, the user agent should navigate the browsing context to the same resource as that Document, with replacement enabled. In the case of non-idempotent methods (e.g. HTTP POST), the user agent should prompt the user to confirm the operation first, since otherwise transactions (e.g. purchases or database modifications) could be repeated. User agents may allow the user to explicitly override any caches when reloading. If browsing context's active document's reload override flag is set, then the user agent may instead perform an overridden reload rather than the navigation described in this paragraph (with the browsing context being reloaded as the source browsing context).

The Location interface also supports the URLUtils interface. [URL]

When the object is created, and whenever the the address of the relevant Document changes, the user agent must invoke the object's URLUtils interface's set the input algorithm with the address of the relevant Document as the given value.

The object's URLUtils interface's get the base algorithm must return the API base URL specified by the entry settings object, if there is one, or null otherwise.

The object's URLUtils interface's query encoding is the document's character encoding.

When the object's URLUtils interface invokes its update steps with the string value, the user agent must run the following steps:

If any of the following conditions are met, let mode be normal navigation; otherwise, let it be replace navigation:
- The Location object's relevant Document has completely loaded, or
- In the task in which the algorithm is running, an activation behavior is currently being processed whose click event was trusted, or
- In the task in which the algorithm is running, the event listener for a trusted click event is being handled.
If mode is normal navigation, then act as if the assign() method had been called with value as its argument. Otherwise, act as if the replace() method had been called with value as its argument.

5.5.3.1 Security

User agents must throw a SecurityError exception whenever any properties of a Location object are accessed when the entry settings object specifies an effective script origin that is not the same as the Location object's associated Document's browsing context's active document's effective script origin, with the following exceptions:

The href setter, if the responsible browsing context specified by the entry settings object is familiar with the browsing context with which the Location object is associated
The replace() method, if the responsible browsing context specified by the entry settings object is familiar with the browsing context with which the Location object is associated
Any properties not defined in the IDL for the Location object or indirectly via one of those properties (e.g. toString(), which is defined via the stringifier keyword), if the effective script origin specified by the entry settings object is the same origin as the Location object's associated Document's effective script origin

When the effective script origin specified by the entry settings object is different than a Location object's associated Document's effective script origin, the user agent must act as if any changes to that Location object's properties, getters, setters, etc, were not present, and as if all the properties of that Location object had their [[Enumerable]] attribute set to false.

For members that return objects (including function objects), each distinct effective script origin that is not the same origin as the Location object's Document's effective script origin must be provided with a separate set of objects. These objects must have the prototype chain appropriate for the script for which the objects are created (not those that would be appropriate for scripts whose settings object specifies a global object that is the Location object's Document's Window object).

5.5.4 Implementation notes for session history

This section is non-normative.

The History interface is not meant to place restrictions on how implementations represent the session history to the user.

For example, session history could be implemented in a tree-like manner, with each page having multiple "forward" pages. This specification doesn't define how the linear list of pages in the history object are derived from the actual session history as seen from the user's perspective.

Similarly, a page containing two iframes has a history object distinct from the iframes' history objects, despite the fact that typical Web browsers present the user with just one "Back" button, with a session history that interleaves the navigation of the two inner frames and the outer page.

Security: It is suggested that to avoid letting a page "hijack" the history navigation facilities of a UA by abusing pushState(), the UA provide the user with a way to jump back to the previous page (rather than just going back to the previous state). For example, the back button could have a drop down showing just the pages in the session history, and not showing any of the states. Similarly, an aural browser could have two "back" commands, one that goes back to the previous state, and one that jumps straight back to the previous page.

In addition, a user agent could ignore calls to pushState() that are invoked on a timer, or from event listeners that are not triggered in response to a clear user action, or that are invoked in rapid succession.

5.6 Browsing the Web

Features described in this section can at times prove difficult to test in order to assess interoperability. As a result, at present, their test coverage is less than much of the other features with all the stability implications this may have.

5.6.1 Navigating across documents

Certain actions cause the browsing context to navigate to a new resource. A user agent may provide various ways for the user to explicitly cause a browsing context to navigate, in addition to those defined in this specification.

For example, following a hyperlink, form submission, and the window.open() and location.assign() methods can all cause a browsing context to navigate.

A resource has a URL, but that might not be the only information necessary to identify it. For example, a form submission that uses HTTP POST would also have the HTTP method and payload. Similarly, an iframe srcdoc document needs to know the data it is to use.

Navigation always involves source browsing context, which is the browsing context which was responsible for starting the navigation.

When a browsing context is navigated to a new resource, the user agent must run the following steps:

Release the storage mutex.
If the source browsing context is not allowed to navigate the browsing context being navigated, then abort these steps.

If these steps are aborted here, the user agent may instead offer to open the new resource in a new top-level browsing context or in the top-level browsing context of the source browsing context, at the user's option, in which case the user agent must navigate that designated top-level browsing context to the new resource as if the user had requested it independently.

Doing so, however, can be dangerous, as it means that the user is overriding the author's explicit request to sandbox the content.

If the navigate algorithm was invoked with exceptions enabled, and it is aborted on this step, then in addition to aborting this algorithm, the user agent must also throw a SecurityError exception.
If there is a preexisting attempt to navigate the browsing context, and the source browsing context is the same as the browsing context being navigated, and that attempt is currently running the unload a document algorithm, and the origin of the URL of the resource being loaded in that navigation is not the same origin as the origin of the URL of the resource being loaded in this navigation, then abort these steps without affecting the preexisting attempt to navigate the browsing context.
If a task queued by the traverse the history by a delta algorithm is running the unload a document algorithm for the active document of the browsing context being navigated, then abort these steps without affecting the unload a document algorithm or the aforementioned history traversal task.
If the prompt to unload a document algorithm is being run for the active document of the browsing context being navigated, then abort these steps without affecting the prompt to unload a document algorithm.
Let gone async be false.

The handle redirects step later in this algorithm can in certain cases jump back to the step labeled fragment identifiers. Since, between those two steps, this algorithm goes from operating synchronously in the context of the calling task to operating asynchronously independent of the event loop, some of the intervening steps need to be able to handle both being synchronous and being asynchronous. The gone async flag is thus used to make these steps aware of which mode they are operating in.
Fragment identifiers: Apply the URL parser algorithm to the absolute URL of the new resource and the address of the active document of the browsing context being navigated. If all the components of the resulting parsed URLs, ignoring any fragment components, are identical, and the new resource is to be fetched using HTTP GET or equivalent, and the parsed URL of the new resource has a fragment component that is not null (even if it is empty), then navigate to that fragment identifier and abort these steps.
If gone async is false, cancel any preexisting but not yet mature attempt to navigate the browsing context, including canceling any instances of the fetch algorithm started by those attempts. If one of those attempts has already created a new Document object, abort that Document also. (Navigation attempts that have matured already have session history entries, and are therefore handled during the update the session history with the new page algorithm, later.)
If the new resource is to be handled using a mechanism that does not affect the browsing context, e.g. ignoring the navigation request altogether because the specified scheme is not one of the supported protocols, then abort these steps and proceed with that mechanism instead.
If gone async is false, prompt to unload the Document object. If the user refused to allow the document to be unloaded, then abort these steps.

If this instance of the navigation algorithm gets canceled while this step is running, the prompt to unload a document algorithm must nonetheless be run to completion.
If gone async is false, abort the active document of the browsing context.
If the new resource is to be handled by displaying some sort of inline content, e.g. an error message because the specified scheme is not one of the supported protocols, or an inline prompt to allow the user to select a registered handler for the given scheme, then display the inline content and abort these steps.

In the case of a registered handler being used, the algorithm will be reinvoked with a new URL to handle the request.
If the browsing context being navigated is a nested browsing context, then put it in the delaying load events mode.

The user agent must take this nested browsing context out of the delaying load events mode when this navigation algorithm later matures, or when it terminates (whether due to having run all the steps, or being canceled, or being aborted), whichever happens first.
This is the step that attempts to obtain the resource, if necessary. Jump to the first appropriate substep:
If the resource has already been obtained (e.g. because it is being used to populate an object element's new child browsing context)

Skip this step. The data is already available.

If the new resource is a URL whose scheme is javascript
Queue a task to run these "javascript: URL" steps, associated with the active document of the browsing context being navigated:
1. If the origin of the source browsing context is not the same origin as the origin of the active document of the browsing context being navigated, then act as if the result of evaluating the script was the void value, and jump to the step labeled process results below.
2. Apply the URL parser to the URL being navigated.
3. Let parsed URL be the result of the URL parser.
4. Let script source be the empty string.
5. Append parsed URL's scheme data component to script source.
6. If parsed URL's query component is not null, then first append a "?" (U+003F) character to script source, and then append parsed URL's query component to script source.
7. If parsed URL's fragment component is not null, then first append a "#" (U+0023) character to script source, and then append parsed URL's fragment component to script source.
8. Replace script source with the result of applying the percent decode algorithm to script source.
9. Replace script source with the result of applying the UTF-8 decode algorithm to script source.
10. Let address be the address of the active document of the browsing context being navigated.
11. Create a script, using script source as the script source, address as the script source URL, JavaScript as the scripting language, and the script settings object of the Window object of the active document of the browsing context being navigated.
  
  Let result be the return value of the code entry-point of this script. If an exception was thrown, let result be void instead. (The result will be void also if scripting is disabled.)
12. Process results: If the result of executing the script is void (there is no return value), then the result of obtaining the resource for the URL is equivalent to an HTTP resource with an HTTP 204 No Content response.
  
  Otherwise, the result of obtaining the resource for the URL is equivalent to an HTTP resource with a 200 OK response whose Content-Type metadata is text/html and whose response body is the return value converted to a string value.
  
  When it comes time to set the document's address in the navigation algorithm, use address as the override URL.
The task source for this task is the DOM manipulation task source.

So for example a javascript: URL in an href attribute of an a element would only be evaluated when the link was followed, while such a URL in the src attribute of an iframe element would be evaluated in the context of the iframe's own nested browsing context when the iframe is being set up; once evaluated, its return value (if it was not void) would replace that browsing context's document, thus also changing the Window object of that browsing context.
If the new resource is to be fetched using HTTP GET or equivalent, and there are relevant application caches that are identified by a URL with the same origin as the URL in question, and that have this URL as one of their entries, excluding entries marked as foreign, and whose mode is fast, and the user agent is not in a mode where it will avoid using application caches

Fetch the resource from the most appropriate application cache of those that match.

For example, imagine an HTML page with an associated application cache displaying an image and a form, where the image is also used by several other application caches. If the user right-clicks on the image and chooses "View Image", then the user agent could decide to show the image from any of those caches, but it is likely that the most useful cache for the user would be the one that was used for the aforementioned HTML page. On the other hand, if the user submits the form, and the form does a POST submission, then the user agent will not use an application cache at all; the submission will be made to the network.

Otherwise

Fetch the new resource, with the manual redirect flag set.
If the steps above invoked the fetch algorith, the following requirements also apply:

If the resource is being fetched using a method other than one equivalent to HTTP's GET, or, if the navigation algorithm was invoked as a result of the form submission algorithm, then the fetching algorithm must be invoked from the origin of the active document of the source browsing context, if any.

Otherwise, if the browsing context being navigated is a child browsing context, then the fetching algorithm must be invoked from the browsing context scope origin of the browsing context container of the browsing context being navigated, if it has one.
If gone async is false, return to whatever algorithm invoked the navigation steps and continue running these steps asynchronously.
Let gone async be true.
Handle redirects: If fetching the resource results in a redirect, and either the URL of the target of the redirect has the same origin as the original resource, or the resource is being obtained using the POST method or a safe method (in HTTP terms), return to the step labeled fragment identifiers with the new resource, except that if the URL of the target of the redirect does not have a fragment identifier and the URL of the resource that led to the redirect does, then the fragment identifier of the resource that led to the redirect must be propagated to the URL of the target of the redirect.

So for instance, if the original URL was "http://example.com/#!sample" and "http://example.com/" is found to redirect to "https://example.com/", the URL of the new resource will be "https://example.com/#!sample".

Otherwise, if fetching the resource results in a redirect but the URL of the target of the redirect does not have the same origin as the original resource and the resource is being obtained using a method that is neither the POST method nor a safe method (in HTTP terms), then abort these steps. The user agent may indicate to the user that the navigation has been aborted for security reasons.
Wait for one or more bytes to be available or for the user agent to establish that the resource in question is empty. During this time, the user agent may allow the user to cancel this navigation attempt or start other navigation attempts.
Fallback in prefer-online mode: If the resource was not fetched from an application cache, and was to be fetched using HTTP GET or equivalent, and there are relevant application caches that are identified by a URL with the same origin as the URL in question, and that have this URL as one of their entries, excluding entries marked as foreign, and whose mode is prefer-online, and the user didn't cancel the navigation attempt during the earlier step, and the navigation attempt failed (e.g. the server returned a 4xx or 5xx status code or equivalent, or there was a DNS error), then:

Let candidate be the resource identified by the URL in question from the most appropriate application cache of those that match.

If candidate is not marked as foreign, then the user agent must discard the failed load and instead continue along these steps using candidate as the resource. The user agent may indicate to the user that the original page load failed, and that the page used was a previously cached resource.

This does not affect the address of the resource from which Request-URIs are obtained, as used to set the document's referrer in the create a Document object steps below; they still use the value as computed by the original fetch algorithm.
Fallback for fallback entries: If the resource was not fetched from an application cache, and was to be fetched using HTTP GET or equivalent, and its URL matches the fallback namespace of one or more relevant application caches, and the most appropriate application cache of those that match does not have an entry in its online whitelist that has the same origin as the resource's URL and that is a prefix match for the resource's URL, and the user didn't cancel the navigation attempt during the previous step, and the navigation attempt failed (e.g. the server returned a 4xx or 5xx status code or equivalent, or there was a DNS error), then:

Let candidate be the fallback resource specified for the fallback namespace in question. If multiple application caches match, the user agent must use the fallback of the most appropriate application cache of those that match.

If candidate is not marked as foreign, then the user agent must discard the failed load and instead continue along these steps using candidate as the resource. The document's address, if appropriate, will still be the originally requested URL, not the fallback URL, but the user agent may indicate to the user that the original page load failed, that the page used was a fallback resource, and what the URL of the fallback resource actually is.

This does not affect the address of the resource from which Request-URIs are obtained, as used to set the document's referrer in the create a Document object steps below; they still use the value as computed by the original fetch algorithm.
Resource handling: If the resource's out-of-band metadata (e.g. HTTP headers), not counting any type information (such as the Content-Type HTTP header), requires some sort of processing that will not affect the browsing context, then perform that processing and abort these steps.
Such processing might be triggered by, amongst other things, the following:
- HTTP status codes (e.g. 204 No Content or 205 Reset Content)
- Network errors (e.g. the network interface being unavailable)
- Cryptographic protocol failures (e.g. an incorrect TLS certificate)
Responses with HTTP Content-Disposition headers specifying the attachment disposition type must be handled as a download.

HTTP 401 responses that do not include a challenge recognized by the user agent must be processed as if they had no challenge, e.g. rendering the entity body as if the response had been 200 OK.

User agents may show the entity body of an HTTP 401 response even when the response does include a recognized challenge, with the option to login being included in a non-modal fashion, to enable the information provided by the server to be used by the user before authenticating. Similarly, user agents should allow the user to authenticate (in a non-modal fashion) against authentication challenges included in other responses such as HTTP 200 OK responses, effectively allowing resources to present HTTP login forms without requiring their use.
Let type be the sniffed type of the resource.
If the user agent has been configured to process resources of the given type using some mechanism other than rendering the content in a browsing context, then skip this step. Otherwise, if the type is one of the following types, jump to the appropriate entry in the following list, and process the resource as described there:

"text/html"

Follow the steps given in the HTML document section, and then, once they have completed, abort this navigate algorithm.

"application/xml"

"text/xml"

"image/svg+xml"

"application/xhtml+xml"

Any other type ending in "+xml" that is not an explicitly supported XML type

Follow the steps given in the XML document section. If that section determines that the content is not to be displayed as a generic XML document, then proceed to the next step in this overall set of steps. Otherwise, once the steps given in the XML document section have completed, abort this navigate algorithm.

"text/plain"

Follow the steps given in the plain text file section, and then, once they have completed, abort this navigate algorithm.

"multipart/x-mixed-replace"

Follow the steps given in the multipart/x-mixed-replace section, and then, once they have completed, abort this navigate algorithm.

A supported image, video, or audio type

Follow the steps given in the media section, and then, once they have completed, abort this navigate algorithm.

A type that will use an external application to render the content in the browsing context

Follow the steps given in the plugin section, and then, once they have completed, abort this navigate algorithm.

An explicitly supported XML type is one for which the user agent is configured to use an external application to render the content (either a plugin rendering directly in the browsing context, or a separate application), or one for which the user agent has dedicated processing rules (e.g. a Web browser with a built-in Atom feed viewer would be said to explicitly support the application/atom+xml MIME type), or one for which the user agent has a dedicated handler (e.g. one registered using registerContentHandler()).

Setting the document's address: If there is no override URL, then any Document created by these steps must have its address set to the URL that was originally to be fetched, ignoring any other data that was used to obtain the resource (e.g. the entity body in the case of a POST submission is not part of the document's address, nor is the URL of the fallback resource in the case of the original load having failed and that URL having been found to match a fallback namespace). However, if there is an override URL, then any Document created by these steps must have its address set to that URL instead.

An override URL is set when dereferencing a javascript: URL and when performing an overridden reload.

Creating a new Document object: when a Document is created as part of the above steps, the user agent must additionally run the following algorithm as part of creating the new object:
1. Create a new Window object, and associate it with the Document, with one exception: if the browsing context's only entry in its session history is the about:blank Document that was added when the browsing context was created, and navigation is occurring with replacement enabled, and that Document has the same origin as the new Document, then use the Window object of that Document instead, and change the document attribute of the Window object to point to the new Document.
2. Set the document's referrer to the address of the resource from which Request-URIs are obtained as determined when the fetch algorithm obtained the resource, if that algorithm was used and determined such a value; otherwise, set it to the empty string.
3. Implement the sandboxing for the Document.
4. If the active sandboxing flag set of the Document's browsing context or any of its ancestor browsing contexts (if any) have the sandboxed fullscreen browsing context flag set, then skip this step.
  
  If the Document's browsing context has a browsing context container and either it is not an iframe element, or its Document does not have the fullscreen enabled flag set, then also skip this step.
  
  Otherwise, set the Document's fullscreen enabled flag.
Non-document content: If, given type, the new resource is to be handled by displaying some sort of inline content, e.g. a native rendering of the content, an error message because the specified type is not supported, or an inline prompt to allow the user to select a registered handler for the given type, then display the inline content, and then abort these steps.

In the case of a registered handler being used, the algorithm will be reinvoked with a new URL to handle the request.
Otherwise, the document's type is such that the resource will not affect the browsing context, e.g. because the resource is to be handed to an external application or because it is an unknown type that will be processed as a download. Process the resource appropriately.

Some of the sections below, to which the above algorithm defers in certain cases, require the user agent to update the session history with the new page. When a user agent is required to do this, it must queue a task (associated with the Document object of the current entry, not the new one) to run the following steps:

Unload the Document object of the current entry, with the recycle parameter set to false.

If this instance of the navigation algorithm is canceled while this step is running the unload a document algorithm, then the unload a document algorithm must be allowed to run to completion, but this instance of the navigation algorithm must not run beyond this step. (In particular, for instance, the cancelation of this algorithm does not abort any event dispatch or script execution occurring as part of unloading the document or its descendants.)
If the navigation was initiated for entry update of an entry
1. Replace the Document of the entry being updated, and any other entries that referenced the same document as that entry, with the new Document.
2. Traverse the history to the new entry.
This can only happen if the entry being updated is not the current entry, and can never happen with replacement enabled. (It happens when the user tried to traverse to a session history entry that no longer had a Document object.)
Otherwise
1. Remove all the entries in the browsing context's session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.
  
  This doesn't necessarily have to affect the user agent's user interface.
2. Append a new entry at the end of the History object representing the new resource and its Document object and related state.
3. Traverse the history to the new entry. If the navigation was initiated with replacement enabled, then the traversal must itself be initiated with replacement enabled.
The navigation algorithm has now matured.
Fragment identifier loop: Spin the event loop for a user-agent-defined amount of time, as desired by the user agent implementor. (This is intended to allow the user agent to optimize the user experience in the face of performance concerns.)
If the Document object has no parser, or its parser has stopped parsing, or the user agent has reason to believe the user is no longer interested in scrolling to the fragment identifier, then abort these steps.
Scroll to the fragment identifier given in the document's address. If this fails to find an indicated part of the document, then return to the fragment identifier loop step.

The task source for this task is the networking task source.

5.6.2 Page load processing model for HTML files

When an HTML document is to be loaded in a browsing context, the user agent must queue a task to create a Document object, mark it as being an HTML document, set its content type to "text/html", create an HTML parser, and associate it with the document. Each task that the networking task source places on the task queue while the fetching algorithm runs must then fill the parser's input byte stream with the fetched bytes and cause the HTML parser to perform the appropriate processing of the input stream.

The input byte stream converts bytes into characters for use in the tokenizer. This process relies, in part, on character encoding information found in the real Content-Type metadata of the resource; the "sniffed type" is not used for this purpose.

When no more bytes are available, the user agent must queue a task for the parser to process the implied EOF character, which eventually causes a load event to be fired.

After creating the Document object, but before any script execution, certainly before the parser stops, the user agent must update the session history with the new page.

Application cache selection happens in the HTML parser.

The task source for the two tasks mentioned in this section must be the networking task source.

5.6.3 Page load processing model for XML files

When faced with displaying an XML file inline, user agents must first create a Document object, following the requirements of the XML and Namespaces in XML recommendations, RFC 7303, DOM, and other relevant specifications. [XML] [XMLNS] [RFC7303] [DOM]

The actual HTTP headers and other metadata, not the headers as mutated or implied by the algorithms given in this specification, are the ones that must be used when determining the character encoding according to the rules given in the above specifications. Once the character encoding is established, the document's character encoding must be set to that character encoding.

If the root element, as parsed according to the XML specifications cited above, is found to be an html element with an attribute manifest whose value is not the empty string, then, as soon as the element is inserted into the document, the user agent must resolve the value of that attribute relative to that element, and if that is successful, must apply the URL serializer algorithm to the resulting parsed URL with the exclude fragment flag set to obtain manifest URL, and then run the application cache selection algorithm with manifest URL as the manifest URL, passing in the newly-created Document. Otherwise, if the attribute is absent, its value is the empty string, or resolving its value fails, then as soon as the root element is inserted into the document, the user agent must run the application cache selection algorithm with no manifest, and passing in the Document.

Because the processing of the manifest attribute happens only once the root element is parsed, any URLs referenced by processing instructions before the root element (such as <?xml-stylesheet?> and <?xbl?> PIs) will be fetched from the network and cannot be cached.

User agents may examine the namespace of the root Element node of this Document object to perform namespace-based dispatch to alternative processing tools, e.g. determining that the content is actually a syndication feed and passing it to a feed handler. If such processing is to take place, abort the steps in this section, and jump to the next step (labeled non-document content) in the navigate steps above.

Otherwise, then, with the newly created Document, the user agent must update the session history with the new page. User agents may do this before the complete document has been parsed (thus achieving incremental rendering), and must do this before any scripts are to be executed.

Error messages from the parse process (e.g. XML namespace well-formedness errors) may be reported inline by mutating the Document.

5.6.4 Page load processing model for text files

When a plain text document is to be loaded in a browsing context, the user agent must queue a task to create a Document object, mark it as being an HTML document, set its content type to "text/plain", create an HTML parser, associate it with the document, act as if the tokenizer had emitted a start tag token with the tag name "pre" followed by a single "LF" (U+000A) character, and switch the HTML parser's tokenizer to the PLAINTEXT state. Each task that the networking task source places on the task queue while the fetching algorithm runs must then fill the parser's input byte stream with the fetched bytes and cause the HTML parser to perform the appropriate processing of the input stream.

The rules for how to convert the bytes of the plain text document into actual characters, and the rules for actually rendering the text to the user, are defined in RFC 2046, RFC 3676, and subsequent versions thereof. [RFC2046] [RFC3676]

The document's character encoding must be set to the character encoding used to decode the document.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, and passing in the newly-created Document.

When no more bytes are available, the user agent must queue a task for the parser to process the implied EOF character, which eventually causes a load event to be fired.

After creating the Document object, but potentially before the page has finished parsing, the user agent must update the session history with the new page.

User agents may add content to the head element of the Document, e.g. linking to a style sheet or an XBL binding, providing script, giving the document a title, etc.

In particular, if the user agent supports the Format=Flowed feature of RFC 3676 then the user agent would need to apply extra styling to cause the text to wrap correctly and to handle the quoting feature. This could be performed using, e.g., an XBL binding or a CSS extension.

The task source for the two tasks mentioned in this section must be the networking task source.

5.6.5 Page load processing model for `multipart/x-mixed-replace` resources

When a resource with the type multipart/x-mixed-replace is to be loaded in a browsing context, the user agent must parse the resource using the rules for multipart types. [RFC2046]

For each body part obtained from the resource, the user agent must run a new instance of the navigate algorithm, starting from the resource handling step, using the new body part as the resource being navigated, with replacement enabled if a previous body part from the same resource resulted in a Document object being created, and otherwise using the same setup as the navigate attempt that caused this section to be invoked in the first place.

For the purposes of algorithms processing these body parts as if they were complete stand-alone resources, the user agent must act as if there were no more bytes for those resources whenever the boundary following the body part is reached.

Thus, load events (and for that matter unload events) do fire for each body part loaded.

5.6.6 Page load processing model for media

When an image, video, or audio resource is to be loaded in a browsing context, the user agent should create a Document object, mark it as being an HTML document, set its content type to the sniffed MIME type of the resource (type in the navigate algorithm), append an html element to the Document, append a head element and a body element to the html element, append an element host element for the media, as described below, to the body element, and set the appropriate attribute of the element host element, as described below, to the address of the image, video, or audio resource.

The element host element to create for the media is the element given in the table below in the second cell of the row whose first cell describes the media. The appropriate attribute to set is the one given by the third cell in that same row.

Type of media	Element for the media	Appropriate attribute
Image	`img`	`src`
Video	`video`	`src`
Audio	`audio`	`src`

Then, the user agent must act as if it had stopped parsing.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, and passing in the newly-created Document.

After creating the Document object, but potentially before the page has finished fully loading, the user agent must update the session history with the new page.

User agents may add content to the head element of the Document, or attributes to the element host element, e.g. to link to a style sheet or an XBL binding, to provide a script, to give the document a title, to make the media autoplay, etc.

5.6.7 Page load processing model for content that uses plugins

When a resource that requires an external resource to be rendered is to be loaded in a browsing context, the user agent should create a Document object, mark it as being an HTML document and mark it as being a plugin document, set its content type to the sniffed MIME type of the resource (type in the navigate algorithm), append an html element to the Document, append a head element and a body element to the html element, append an embed to the body element, and set the src attribute of the embed element to the address of the resource.

The term plugin document is used by the Content Security Policy specification as part of the mechanism that ensures iframes can't be used to evade plugin-types directives. [CSP]

Then, the user agent must act as if it had stopped parsing.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, and passing in the newly-created Document.

After creating the Document object, but potentially before the page has finished fully loading, the user agent must update the session history with the new page.

User agents may add content to the head element of the Document, or attributes to the embed element, e.g. to link to a style sheet or an XBL binding, or to give the document a title.

If the Document's active sandboxing flag set has its sandboxed plugins browsing context flag set, the synthesized embed element will fail to render the content if the relevant plugin cannot be secured.

5.6.8 Page load processing model for inline content that doesn't have a DOM

When the user agent is to display a user agent page inline in a browsing context, the user agent should create a Document object, mark it as being an HTML document, set its content type to "text/html", and then either associate that Document with a custom rendering that is not rendered using the normal Document rendering rules, or mutate that Document until it represents the content the user agent wants to render.

Once the page has been set up, the user agent must act as if it had stopped parsing.

Upon creation of the Document object, the user agent must run the application cache selection algorithm with no manifest, passing in the newly-created Document.

After creating the Document object, but potentially before the page has been completely set up, the user agent must update the session history with the new page.

5.6.9 Navigating to a fragment identifier

When a user agent is supposed to navigate to a fragment identifier, then the user agent must run the following steps:

Remove all the entries in the browsing context's session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.

This doesn't necessarily have to affect the user agent's user interface.
Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context's document family.
Append a new entry at the end of the History object representing the new resource and its Document object and related state. Its URL must be set to the address to which the user agent was navigating. The title must be left unset.
Traverse the history to the new entry, with the asynchronous events flag set. This will scroll to the fragment identifier given in what is now the document's address.

If the scrolling fails because the relevant ID has not yet been parsed, then the original navigation algorithm will take care of the scrolling instead, as the last few steps of its update the session history with the new page algorithm.

When the user agent is required to scroll to the fragment identifier and the indicated part of the document, if any, is being rendered, the user agent must either change the scrolling position of the document using the following algorithm, or perform some other action such that the indicated part of the document is brought to the user's attention. If there is no indicated part, or if the indicated part is not being rendered, then the user agent must do nothing. The aforementioned algorithm is as follows:

Let target be the indicated part of the document, as defined below.
If target is the top of the document, then scroll to the beginning of the document for the Document, and abort these steps. [CSSOMVIEW]
Use the scroll an element into view algorithm to scroll target into view, with the align to top flag set. [CSSOMVIEW]
If target is a focusable element, run the focusing steps for that element.

The indicated part of the document is the one that the fragment identifier, if any, identifies. The semantics of the fragment identifier in terms of mapping it to a specific DOM Node is defined by the specification that defines the MIME type used by the Document (for example, the processing of fragment identifiers for XML MIME types is the responsibility of RFC7303). [RFC7303]

For HTML documents (and HTML MIME types), the following processing model must be followed to determine what the indicated part of the document is.

Apply the URL parser algorithm to the URL, and let fragid be the fragment component of the resulting parsed URL.
If fragid is the empty string, then the indicated part of the document is the top of the document; stop the algorithm here.
Let fragid bytes be the result of percent-decoding fragid.
Let decoded fragid be the result of applying the UTF-8 decoder algorithm to fragid bytes. If the UTF-8 decoder emits a decoder error, abort the decoder and instead jump to the step labeled no decoded fragid.
If there is an element in the DOM that has an ID exactly equal to decoded fragid, then the first such element in tree order is the indicated part of the document; stop the algorithm here.
No decoded fragid: If there is an a element in the DOM that has a name attribute whose value is exactly equal to fragid (not decoded fragid), then the first such element in tree order is the indicated part of the document; stop the algorithm here.
If fragid is an ASCII case-insensitive match for the string top, then the indicated part of the document is the top of the document; stop the algorithm here.
Otherwise, there is no indicated part of the document.

For the purposes of the interaction of HTML with Selectors' :target pseudo-class, the target element is the indicated part of the document, if that is an element; otherwise there is no target element. [SELECTORS]

The task source for the task mentioned in this section must be the DOM manipulation task source.

5.6.10 History traversal

When a user agent is required to traverse the history to a specified entry, optionally with replacement enabled, and optionally with the asynchronous events flag set, the user agent must act as follows.

This algorithm is not just invoked when explicitly going back or forwards in the session history — it is also invoked in other situations, for example when navigating a browsing context, as part of updating the session history with the new page.

If there is no longer a Document object for the entry in question, navigate the browsing context to the resource for that entry to perform an entry update of that entry, and abort these steps. The "navigate" algorithm reinvokes this "traverse" algorithm to complete the traversal, at which point there is a Document object and so this step gets skipped. The navigation must be done using the same source browsing context as was used the first time this entry was created. (This can never happen with replacement enabled.)

If the resource was obtained usign a non-idempotent action, for example a POST form submission, or if the resource is no longer available, for example because the computer is now offline and the page wasn't cached, navigating to it again might not be possible. In this case, the navigation will result in a different page than previously; for example, it might be an error message explaining the problem or offering to resubmit the form.
If the current entry's title was not set by the pushState() or replaceState() methods, then set its title to the value returned by the document.title IDL attribute.
If appropriate, update the current entry in the browsing context's Document object's History object to reflect any state that the user agent wishes to persist. The entry is then said to be an entry with persisted user state.
If the specified entry has a different Document object than the current entry, then run the following substeps:
1. Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context's document family.
2. If the origin of the Document of the specified entry is not the same as the origin of the Document of the current entry, then run the following sub-sub-steps:
  1. The current browsing context name must be stored with all the entries in the history that are associated with Document objects with the same origin as the active document and that are contiguous with the current entry.
  2. If the browsing context is a top-level browsing context, but not an auxiliary browsing context, then the browsing context's browsing context name must be unset.
3. Make the specified entry's Document object the active document of the browsing context.
4. If the specified entry has a browsing context name stored with it, then run the following sub-sub-steps:
  1. Set the browsing context's browsing context name to the name stored with the specified entry.
  2. Clear any browsing context names stored with all entries in the history that are associated with Document objects with the same origin as the new active document and that are contiguous with the specified entry.
5. If the specified entry's Document has any form controls whose autofill field name is "off", invoke the reset algorithm of each of those elements.
6. If the current document readiness of the specified entry's Document is "complete", queue a task to run the following sub-sub-steps:
  1. If the Document's page showing flag is true, then abort this task (i.e. don't fire the event below).
  2. Set the Document's page showing flag to true.
  3. Run any session history document visibility change steps for Document that are defined by other applicable specifications.
    
    This is specifically intended for use by the Page Visibility specification. [PAGEVIS]
  4. Fire a trusted event with the name pageshow at the Window object of that Document, but with its target set to the Document object (and the currentTarget set to the Window object), using the PageTransitionEvent interface, with the persisted attribute initialized to true. This event must not bubble, must not be cancelable, and has no default action.
Set the document's address to the URL of the specified entry.
If the specified entry has a URL whose fragment identifier differs from that of the current entry's when compared in a case-sensitive manner, and the two share the same Document object, then let hash changed be true, and let old URL be the URL of the current entry and new URL be the URL of the specified entry. Otherwise, let hash changed be false.
If the traversal was initiated with replacement enabled, remove the entry immediately before the specified entry in the session history.
If the specified entry is not an entry with persisted user state, but its URL has a fragment identifier, scroll to the fragment identifier.
If the entry is an entry with persisted user state, the user agent may update aspects of the document and its rendering, for instance the scroll position or values of form fields, that it had previously recorded.

This can even include updating the dir attribute of textarea elements or input elements whose type attribute is in either the Text state or the Search state, if the persisted state includes the directionality of user input in such controls.
If the entry is a state object entry, let state be a structured clone of that state object. Otherwise, let state be null.
Set history.state to state.
Let state changed be true if the Document of the specified entry has a latest entry, and that entry is not the specified entry; otherwise let it be false.
Let the latest entry of the Document of the specified entry be the specified entry.
If the asynchronous events flag is not set, then run the following steps synchronously. Otherwise, the asynchronous events flag is set; queue a task to run the following substeps.
1. If state changed is true, fire a trusted event with the name popstate at the Window object of the Document, using the PopStateEvent interface, with the state attribute initialized to the value of state. This event must bubble but not be cancelable and has no default action.
2. If hash changed is true, then fire a trusted event with the name hashchange at the browsing context's Window object, using the HashChangeEvent interface, with the oldURL attribute initialized to old URL and the newURL attribute initialized to new URL. This event must bubble but not be cancelable and has no default action.
The current entry is now the specified entry.

The task source for the tasks mentioned above is the DOM manipulation task source.

5.6.10.1 Event definitions

The popstate event is fired in certain cases when navigating to a session history entry.

[Constructor(DOMString type, optional PopStateEventInit eventInitDict)]
interface PopStateEvent : Event {
  readonly attribute any state;
};

dictionary PopStateEventInit : EventInit {
  any state;
};

event . state: Returns a copy of the information that was provided to pushState() or replaceState().

The state attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to null. It represents the context information for the event, or null, if the state represented is the initial state of the Document.

The hashchange event is fired when navigating to a session history entry whose URL differs from that of the previous one only in the fragment identifier.

[Constructor(DOMString type, optional HashChangeEventInit eventInitDict)]
interface HashChangeEvent : Event {
  readonly attribute DOMString oldURL;
  readonly attribute DOMString newURL;
};

dictionary HashChangeEventInit : EventInit {
  DOMString oldURL;
  DOMString newURL;
};

event . oldURL: Returns the URL of the session history entry that was previously current.
event . newURL: Returns the URL of the session history entry that is now current.

The oldURL attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to null. It represents context information for the event, specifically the URL of the session history entry that was traversed from.

The newURL attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to null. It represents context information for the event, specifically the URL of the session history entry that was traversed to.

The pageshow event is fired when traversing to a session history entry. The pagehide event is fired when traversing from a session history entry. The specification uses the page showing flag to ensure that scripts receive these events in a consistent manner (e.g. that they never receive two pagehide events in a row without an intervening pageshow, or vice versa).

[Constructor(DOMString type, optional PageTransitionEventInit eventInitDict)]
interface PageTransitionEvent : Event {
  readonly attribute boolean persisted;
};

dictionary PageTransitionEventInit : EventInit {
  boolean persisted;
};

event . persisted

For the pageshow event, returns false if the page is newly being loaded (and the load event will fire). Otherwise, returns true.

For the pagehide event, returns false if the page is going away for the last time. Otherwise, returns true, meaning that (if nothing conspires to make the page unsalvageable) the page might be reused if the user navigates back to this page.

Things that can cause the page to be unsalvageable include:

document.open()
Listening for beforeunload events
Listening for unload events
Having iframes that are not salvageable
Active WebSocket objects
Aborting a Document

The persisted attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to false. It represents the context information for the event.

5.6.11 Unloading documents

A Document has a salvageable state, which must initially be true, a fired unload flag, which must initially be false, and a page showing flag, which must initially be false.

Event loops have a termination nesting level counter, which must initially be zero.

When a user agent is to prompt to unload a document, it must run the following steps.

Increase the event loop's termination nesting level by one.
Increase the Document's ignore-opens-during-unload counter by one.
Let event be a new trusted BeforeUnloadEvent event object with the name beforeunload, which does not bubble but is cancelable.
Dispatch: Dispatch event at the Document's Window object.
Decrease the event loop's termination nesting level by one.
Release the storage mutex.
If any event listeners were triggered by the earlier dispatch step, then set the Document's salvageable state to false.
If the returnValue attribute of the event object is not the empty string, or if the event was canceled, then the user agent should ask the user to confirm that they wish to unload the document.

The prompt shown by the user agent may include the string of the returnValue attribute, or some leading subset thereof. (A user agent may want to truncate the string to 1024 characters for display, for instance.)

The user agent must pause while waiting for the user's response.

If the user did not confirm the page navigation, then the user agent refused to allow the document to be unloaded.
If this algorithm was invoked by another instance of the "prompt to unload a document" algorithm (i.e. through the steps below that invoke this algorithm for all descendant browsing contexts), then jump to the step labeled end.
Let descendants be the list of the descendant browsing contexts of the Document.
If descendants is not an empty list, then for each browsing context b in descendants run the following substeps:
1. Prompt to unload the active document of the browsing context b. If the user refused to allow the document to be unloaded, then the user implicitly also refused to allow this document to be unloaded; jump to the step labeled end.
2. If the salvageable state of the active document of the browsing context b is false, then set the salvageable state of this document to false also.
End: Decrease the Document's ignore-opens-during-unload counter by one.

When a user agent is to unload a document, it must run the following steps. These steps are passed an argument, recycle, which is either true or false, indicating whether the Document object is going to be re-used. (This is set by the document.open() method.)

Increase the event loop's termination nesting level by one.
Increase the Document's ignore-opens-during-unload counter by one.
If the Document's page showing flag is false, then jump to the step labeled unload event below (i.e. skip firing the pagehide event and don't rerun the unloading document visibility change steps).
Set the Document's page showing flag to false.
Fire a trusted event with the name pagehide at the Window object of the Document, but with its target set to the Document object (and the currentTarget set to the Window object), using the PageTransitionEvent interface, with the persisted attribute initialized to true if the Document object's salvageable state is true, and false otherwise. This event must not bubble, must not be cancelable, and has no default action.
Run any unloading document visibility change steps for Document that are defined by other applicable specifications.

This is specifically intended for use by the Page Visibility specification. [PAGEVIS]
Unload event: If the Document's fired unload flag is false, fire a simple event named unload at the Document's Window object.
Decrease the event loop's termination nesting level by one.
Release the storage mutex.
If any event listeners were triggered by the earlier unload event step, then set the Document object's salvageable state to false and set the Document's fired unload flag to true.
Run any unloading document cleanup steps for Document that are defined by this specification and other applicable specifications.
If this algorithm was invoked by another instance of the "unload a document" algorithm (i.e. by the steps below that invoke this algorithm for all descendant browsing contexts), then jump to the step labeled end.
Let descendants be the list of the descendant browsing contexts of the Document.
If descendants is not an empty list, then for each browsing context b in descendants run the following substeps:
1. Unload the active document of the browsing context b with the recycle parameter set to false.
2. If the salvageable state of the active document of the browsing context b is false, then set the salvageable state of this document to false also.
If both the Document's salvageable state and recycle are false, then the Document's browsing context must discard the Document.
End: Decrease the Document's ignore-opens-during-unload counter by one.

This specification defines the following unloading document cleanup steps. Other specifications can define more.

Make disappear any WebSocket objects that were created by the WebSocket() constructor from the Document's Window object.

If this affected any WebSocket objects, then set Document's salvageable state to false.
If the Document's salvageable state is false, forcibly close any EventSource objects that whose constructor was invoked from the Document's Window object.
If the Document's salvageable state is false, empty the Document's Window's list of active timers.

5.6.11.1 Event definition

interface BeforeUnloadEvent : Event {
           attribute DOMString returnValue;
};

event . returnValue [ = value ]

Returns the current return value of the event (the message to show the user).

Can be set, to update the message.

There are no BeforeUnloadEvent-specific initialization methods.

The returnValue attribute represents the message to show the user. When the event is created, the attribute must be set to the empty string. On getting, it must return the last value it was set to. On setting, the attribute must be set to the new value.

5.6.12 Aborting a document load

If a Document is aborted, the user agent must run the following steps:

Abort the active documents of every child browsing context. If this results in any of those Document objects having their salvageable state set to false, then set this Document's salvageable state to false also.
Cancel any instances of the fetch algorithm in the context of this Document, discarding any tasks queued for them, and discarding any further data received from the network for them. If this resulted in any instances of the fetch algorithm being canceled or any queued tasks or any network data getting discarded, then set the Document's salvageable state to false.
If the Document has an active parser, then abort that parser and set the Document's salvageable state to false.

User agents may allow users to explicitly invoke the abort a document algorithm for a Document. If the user does so, then, if that Document is an active document, the user agent should queue a task to fire a simple event named abort at that Document's Window object before invoking the abort algorithm.

5.7 Offline Web applications

5.7.1 Introduction

This section is non-normative.

In order to enable users to continue interacting with Web applications and documents even when their network connection is unavailable — for instance, because they are traveling outside of their ISP's coverage area — authors can provide a manifest which lists the files that are needed for the Web application to work offline and which causes the user's browser to keep a copy of the files for use offline.

To illustrate this, consider a simple clock applet consisting of an HTML page "clock.html", a CSS style sheet "clock.css", and a JavaScript script "clock.js".

Before adding the manifest, these three files might look like this:

EXAMPLE offline/clock/clock1.html

EXAMPLE offline/clock/clock1.css

EXAMPLE offline/clock/clock1.js

If the user tries to open the "clock.html" page while offline, though, the user agent (unless it happens to have it still in the local cache) will fail with an error.

The author can instead provide a manifest of the three files, say "clock.appcache":

EXAMPLE offline/clock/clock2.appcache

With a small change to the HTML file, the manifest (served as text/cache-manifest) is linked to the application:

EXAMPLE offline/clock/clock2.html

Now, if the user goes to the page, the browser will cache the files and make them available even when the user is offline.

Authors are encouraged to include the main page in the manifest also, but in practice the page that referenced the manifest is automatically cached even if it isn't explicitly mentioned.

With the exception of "no-store" directive, HTTP cache headers and restrictions on caching pages served over TLS (encrypted, using https:) are overridden by manifests. Thus, pages will not expire from an application cache before the user agent has updated it, and even applications served over TLS can be made to work offline.

5.7.1.1 Supporting offline caching for legacy applications

This section is non-normative.

The application cache feature works best if the application logic is separate from the application and user data, with the logic (markup, scripts, style sheets, images, etc) listed in the manifest and stored in the application cache, with a finite number of static HTML pages for the application, and with the application and user data stored in Web Storage or a client-side Indexed Database, updated dynamically using Web Sockets, XMLHttpRequest, server-sent events, or some other similar mechanism.

This model results in a fast experience for the user: the application immediately loads, and fresh data is obtained as fast as the network will allow it (possibly while stale data shows).

Legacy applications, however, tend to be designed so that the user data and the logic are mixed together in the HTML, with each operation resulting in a new HTML page from the server.

For example, consider a news application. The typical architecture of such an application, when not using the application cache feature, is that the user fetches the main page, and the server returns a dynamically-generated page with the current headlines and the user interface logic mixed together.

A news application designed for the application cache feature, however, would instead have the main page just consist of the logic, and would then have the main page fetch the data separately from the server, e.g. using XMLHttpRequest.

The mixed-content model does not work well with the application cache feature: since the content is cached, it would result in the user always seeing the stale data from the previous time the cache was updated.

While there is no way to make the legacy model work as fast as the separated model, it can at least be retrofitted for offline use using the prefer-online application cache mode. To do so, list all the static resources used by the HTML page you want to have work offline in an application cache manifest, use the manifest attribute to select that manifest from the HTML file, and then add the following line at the bottom of the manifest:

SETTINGS:
prefer-online
NETWORK:
*

This causes the application cache to only be used for master entries when the user is offline, and causes the application cache to be used as an atomic HTTP cache (essentially pinning resources listed in the manifest), while allowing all resources not listed in the manifest to be accessed normally when the user is online.

5.7.1.2 Event summary

This section is non-normative.

When the user visits a page that declares a manifest, the browser will try to update the cache. It does this by fetching a copy of the manifest and, if the manifest has changed since the user agent last saw it, redownloading all the resources it mentions and caching them anew.

As this is going on, a number of events get fired on the ApplicationCache object to keep the script updated as to the state of the cache update, so that the user can be notified appropriately. The events are as follows:

Event name	Interface	Fired when...	Next events
`checking`	`Event`	The user agent is checking for an update, or attempting to download the manifest for the first time. This is always the first event in the sequence.	`noupdate`, `downloading`, `obsolete`, `error`
`noupdate`	`Event`	The manifest hadn't changed.	Last event in sequence.
`downloading`	`Event`	The user agent has found an update and is fetching it, or is downloading the resources listed by the manifest for the first time.	`progress`, `error`, `cached`, `updateready`
`progress`	`ProgressEvent`	The user agent is downloading resources listed by the manifest. The event object's `total` attribute returns the total number of files to be downloaded. The event object's `loaded` attribute returns the number of files processed so far.	`progress`, `error`, `cached`, `updateready`
`cached`	`Event`	The resources listed in the manifest have been downloaded, and the application is now cached.	Last event in sequence.
`updateready`	`Event`	The resources listed in the manifest have been newly redownloaded, and the script can use `swapCache()` to switch to the new cache.	Last event in sequence.
`obsolete`	`Event`	The manifest was found to have become a 404 or 410 page, so the application cache is being deleted.	Last event in sequence.
`error`	`Event`	The manifest was a 404 or 410 page, so the attempt to cache the application has been aborted.	Last event in sequence.
		The manifest hadn't changed, but the page referencing the manifest failed to download properly.
		A fatal error occurred while fetching the resources listed in the manifest.
		The manifest changed while the update was being run.	The user agent will try fetching the files again momentarily.

These events are cancelable; their default action is for the user agent to show download progress information. If the page shows its own update UI, canceling the events will prevent the user agent from showing redundant progress information.

5.7.2 Application caches

An application cache is a set of cached resources consisting of:

One or more resources (including their out-of-band metadata, such as HTTP headers, if any), identified by URLs, each falling into one (or more) of the following categories:

Master entries
These are documents that were added to the cache because a browsing context was navigated to that document and the document indicated that this was its cache, using the manifest attribute.

The manifest
This is the resource corresponding to the URL that was given in a master entry's html element's manifest attribute. The manifest is fetched and processed during the application cache download process. All the master entries have the same origin as the manifest.

Explicit entries
These are the resources that were listed in the cache's manifest in an explicit section.

Fallback entries
These are the resources that were listed in the cache's manifest in a fallback section.

Explicit entries and Fallback entries can be marked as foreign, which means that they have a manifest attribute but that it doesn't point at this cache's manifest.

A URL in the list can be flagged with multiple different types, and thus an entry can end up being categorized as multiple entries. For example, an entry can be a manifest entry and an explicit entry at the same time, if the manifest is listed within the manifest.
Zero or more fallback namespaces, each of which is mapped to a fallback entry.

These are URLs used as prefix match patterns for resources that are to be fetched from the network if possible, or to be replaced by the corresponding fallback entry if not. Each namespace URL has the same origin as the manifest.
Zero or more URLs that form the online whitelist namespaces.

These are used as prefix match patterns, and declare URLs for which the user agent will ignore the application cache, instead fetching them normally (i.e. from the network or local HTTP cache as appropriate).
An online whitelist wildcard flag, which is either open or blocking.

The open state indicates that any URL not listed as cached is to be implicitly treated as being in the online whitelist namespaces; the blocking state indicates that URLs not listed explicitly in the manifest are to be treated as unavailable.
A cache mode flag, which is either in the fast state or the prefer-online state.

Each application cache has a completeness flag, which is either complete or incomplete.

An application cache group is a group of application caches, identified by the absolute URL of a resource manifest which is used to populate the caches in the group.

An application cache is newer than another if it was created after the other (in other words, application caches in an application cache group have a chronological order).

Only the newest application cache in an application cache group can have its completeness flag set to incomplete; the others are always all complete.

Each application cache group has an update status, which is one of the following: idle, checking, downloading.

A relevant application cache is an application cache that is the newest in its group to be complete.

Each application cache group has a list of pending master entries. Each entry in this list consists of a resource and a corresponding Document object. It is used during the application cache download process to ensure that new master entries are cached even if the application cache download process was already running for their application cache group when they were loaded.

An application cache group can be marked as obsolete, meaning that it must be ignored when looking at what application cache groups exist.

A cache host is a Document or a SharedWorkerGlobalScope object. A cache host can be associated with an application cache. [WEBWORKERS]

A Document initially is not associated with an application cache, but can become associated with one early during the page load process, when steps in the parser and in the navigation sections cause cache selection to occur.

A SharedWorkerGlobalScope can be associated with an application cache when it is created. [WEBWORKERS]

Each cache host has an associated ApplicationCache object.

Multiple application caches in different application cache groups can contain the same resource, e.g. if the manifests all reference that resource. If the user agent is to select an application cache from a list of relevant application caches that contain a resource, the user agent must use the application cache that the user most likely wants to see the resource from, taking into account the following:

which application cache was most recently updated,
which application cache was being used to display the resource from which the user decided to look at the new resource, and
which application cache the user prefers.

A URL matches a fallback namespace if there exists a relevant application cache whose manifest's URL has the same origin as the URL in question, and that has a fallback namespace that is a prefix match for the URL being examined. If multiple fallback namespaces match the same URL, the longest one is the one that matches. A URL looking for a fallback namespace can match more than one application cache at a time, but only matches one namespace in each cache.

If a manifest http://example.com/app1/manifest declares that http://example.com/resources/images is a fallback namespace, and the user navigates to HTTP://EXAMPLE.COM:80/resources/images/cat.png, then the user agent will decide that the application cache identified by http://example.com/app1/manifest contains a namespace with a match for that URL.

5.7.3 The cache manifest syntax

5.7.3.1 Some sample manifests

This section is non-normative.

This example manifest requires two images and a style sheet to be cached and whitelists a CGI script.

CACHE MANIFEST
# the above line is required

# this is a comment
# there can be as many of these anywhere in the file
# they are all ignored
  # comments can have spaces before them
  # but must be alone on the line

# blank lines are ignored too

# these are files that need to be cached they can either be listed
# first, or a "CACHE:" header could be put before them, as is done
# lower down.
images/sound-icon.png
images/background.png
# note that each file has to be put on its own line

# here is a file for the online whitelist -- it isn't cached, and
# references to this file will bypass the cache, always hitting the
# network (or trying to, if the user is offline).
NETWORK:
comm.cgi

# here is another set of files to cache, this time just the CSS file.
CACHE:
style/default.css

It could equally well be written as follows:

CACHE MANIFEST
NETWORK:
comm.cgi
CACHE:
style/default.css
images/sound-icon.png
images/background.png

Offline application cache manifests can use absolute paths or even absolute URLs:

CACHE MANIFEST

/main/home
/main/app.js
/settings/home
/settings/app.js
http://img.example.com/logo.png
http://img.example.com/check.png
http://img.example.com/cross.png

The following manifest defines a catch-all error page that is displayed for any page on the site while the user is offline. It also specifies that the online whitelist wildcard flag is open, meaning that accesses to resources on other sites will not be blocked. (Resources on the same site are already not blocked because of the catch-all fallback namespace.)

So long as all pages on the site reference this manifest, they will get cached locally as they are fetched, so that subsequent hits to the same page will load the page immediately from the cache. Until the manifest is changed, those pages will not be fetched from the server again. When the manifest changes, then all the files will be redownloaded.

Subresources, such as style sheets, images, etc, would only be cached using the regular HTTP caching semantics, however.

CACHE MANIFEST
FALLBACK:
/ /offline.html
NETWORK:
*

5.7.3.2 Writing cache manifests

Manifests must be served using the text/cache-manifest MIME type. All resources served using the text/cache-manifest MIME type must follow the syntax of application cache manifests, as described in this section.

An application cache manifest is a text file, whose text is encoded using UTF-8. Data in application cache manifests is line-based. Newlines must be represented by "LF" (U+000A) characters, "CR" (U+000D) characters, or "CR" (U+000D) "LF" (U+000A) pairs. [RFC3629]

This is a willful violation of RFC 2046, which requires all text/* types to only allow CRLF line breaks. This requirement, however, is outdated; the use of CR, LF, and CRLF line breaks is commonly supported and indeed sometimes CRLF is not supported by text editors. [RFC2046]

The first line of an application cache manifest must consist of the string "CACHE", a single U+0020 SPACE character, the string "MANIFEST", and either a U+0020 SPACE character, a "tab" (U+0009) character, a "LF" (U+000A) character, or a "CR" (U+000D) character. The first line may optionally be preceded by a "BOM" (U+FEFF) character. If any other text is found on the first line, it is ignored.

Subsequent lines, if any, must all be one of the following:

A blank line

Blank lines must consist of zero or more U+0020 SPACE and "tab" (U+0009) characters only.

A comment

Comment lines must consist of zero or more U+0020 SPACE and "tab" (U+0009) characters, followed by a single "#" (U+0023) character, followed by zero or more characters other than "LF" (U+000A) and "CR" (U+000D) characters.

Comments must be on a line on their own. If they were to be included on a line with a URL, the "#" would be mistaken for part of a fragment identifier.

A section header

Section headers change the current section. There are four possible section headers:

CACHE:: Switches to the explicit section.
FALLBACK:: Switches to the fallback section.
NETWORK:: Switches to the online whitelist section.
SETTINGS:: Switches to the settings section.

Section header lines must consist of zero or more U+0020 SPACE and "tab" (U+0009) characters, followed by one of the names above (including the ":)" (U+003A) character followed by zero or more U+0020 SPACE and "tab" (U+0009) characters.

Ironically, by default, the current section is the explicit section.

Data for the current section

The format that data lines must take depends on the current section.

When the current section is the explicit section, data lines must consist of zero or more U+0020 SPACE and "tab" (U+0009) characters, a valid URL identifying a resource other than the manifest itself, and then zero or more U+0020 SPACE and "tab" (U+0009) characters.

When the current section is the fallback section, data lines must consist of zero or more U+0020 SPACE and "tab" (U+0009) characters, a valid URL identifying a resource other than the manifest itself, one or more U+0020 SPACE and "tab" (U+0009) characters, another valid URL identifying a resource other than the manifest itself, and then zero or more U+0020 SPACE and "tab" (U+0009) characters.

When the current section is the online whitelist section, data lines must consist of zero or more U+0020 SPACE and "tab" (U+0009) characters, either a single "*" (U+002A) character or a valid URL identifying a resource other than the manifest itself, and then zero or more U+0020 SPACE and "tab" (U+0009) characters.

When the current section is the settings section, data lines must consist of zero or more U+0020 SPACE and "tab" (U+0009) characters, a setting, and then zero or more U+0020 SPACE and "tab" (U+0009) characters.

Currently only one setting is defined:

The cache mode setting: This consists of the string "prefer-online". It sets the cache mode to prefer-online. (The cache mode defaults to fast.)

Within a settings section, each setting must occur no more than once.

Manifests may contain sections more than once. Sections may be empty.

URLs that are to be fallback pages associated with fallback namespaces, and those namespaces themselves, must be given in fallback sections, with the namespace being the first URL of the data line, and the corresponding fallback page being the second URL. All the other pages to be cached must be listed in explicit sections.

Fallback namespaces and fallback entries must have the same origin as the manifest itself.

A fallback namespace must not be listed more than once.

Namespaces that the user agent is to put into the online whitelist must all be specified in online whitelist sections. (This is needed for any URL that the page is intending to use to communicate back to the server.) To specify that all URLs are automatically whitelisted in this way, a "*" (U+002A) character may be specified as one of the URLs.

Authors should not include namespaces in the online whitelist for which another namespace in the online whitelist is a prefix match.

Relative URLs must be given relative to the manifest's own URL. All URLs in the manifest must have the same scheme as the manifest itself (either explicitly or implicitly, through the use of relative URLs). [URL]

URLs in manifests must not have fragment identifiers (i.e. the U+0023 NUMBER SIGN character isn't allowed in URLs in manifests).

Fallback namespaces and namespaces in the online whitelist are matched by prefix match.

5.7.3.3 Parsing cache manifests

When a user agent is to parse a manifest, it means that the user agent must run the following steps:

UTF-8 decode the byte stream corresponding with the manifest to be parsed.

The UTF-8 decode algorithm strips a leading BOM, if any.
Let base URL be the absolute URL representing the manifest.
Apply the URL parser steps to the base URL, so that the components from its parsed URL can be used by the subseqent steps of this algorithm.
Let explicit URLs be an initially empty list of absolute URLs for explicit entries.
Let fallback URLs be an initially empty mapping of fallback namespaces to absolute URLs for fallback entries.
Let online whitelist namespaces be an initially empty list of absolute URLs for an online whitelist.
Let online whitelist wildcard flag be blocking.
Let cache mode flag be fast.
Let input be the decoded text of the manifest's byte stream.
Let position be a pointer into input, initially pointing at the first character.
If the characters starting from position are "CACHE", followed by a U+0020 SPACE character, followed by "MANIFEST", then advance position to the next character after those. Otherwise, this isn't a cache manifest; abort this algorithm with a failure while checking for the magic signature.
If the character at position is neither a U+0020 SPACE character, a "tab" (U+0009) character, "LF" (U+000A) character, nor a "CR" (U+000D) character, then this isn't a cache manifest; abort this algorithm with a failure while checking for the magic signature.
This is a cache manifest. The algorithm cannot fail beyond this point (though bogus lines can get ignored).
Collect a sequence of characters that are not "LF" (U+000A) or "CR" (U+000D) characters, and ignore those characters. (Extra text on the first line, after the signature, is ignored.)
Let mode be "explicit".
Start of line: If position is past the end of input, then jump to the last step. Otherwise, collect a sequence of characters that are "LF" (U+000A), "CR" (U+000D), U+0020 SPACE, or "tab" (U+0009) characters.
Now, collect a sequence of characters that are not "LF" (U+000A) or "CR" (U+000D) characters, and let the result be line.
Drop any trailing U+0020 SPACE and "tab" (U+0009) characters at the end of line.
If line is the empty string, then jump back to the step labeled start of line.
If the first character in line is a "#" (U+0023) character, then jump back to the step labeled start of line.
If line equals "CACHE:" (the word "CACHE" followed by a ":)" (U+003A) character, then set mode to "explicit" and jump back to the step labeled start of line.
If line equals "FALLBACK:" (the word "FALLBACK" followed by a ":)" (U+003A) character, then set mode to "fallback" and jump back to the step labeled start of line.
If line equals "NETWORK:" (the word "NETWORK" followed by a ":)" (U+003A) character, then set mode to "online whitelist" and jump back to the step labeled start of line.
If line equals "SETTINGS:" (the word "SETTINGS" followed by a ":)" (U+003A) character, then set mode to "settings" and jump back to the step labeled start of line.
If line ends with a ":" (U+003A) character, then set mode to "unknown" and jump back to the step labeled start of line.
This is either a data line or it is syntactically incorrect.
Let position be a pointer into line, initially pointing at the start of the string.
Let tokens be a list of strings, initially empty.
While position doesn't point past the end of line:
1. Let current token be an empty string.
2. While position doesn't point past the end of line and the character at position is neither a U+0020 SPACE nor a "tab" (U+0009) character, add the character at position to current token and advance position to the next character in input.
3. Add current token to the tokens list.
4. While position doesn't point past the end of line and the character at position is either a U+0020 SPACE or a "tab" (U+0009) character, advance position to the next character in input.
Process tokens as follows:

If mode is "explicit"

Resolve the first item in tokens, relative to base URL, with the URL character encoding set to UTF-8; ignore the rest.

If this fails, then jump back to the step labeled start of line.

If the resulting parsed URL has a different scheme component than base URL (the manifest's URL), then jump back to the step labeled start of line.

Let new URL be the result of applying the URL serializer algorithm to the resulting parsed URL, with the exclude fragment flag set.

Add new URL to the explicit URLs.

If mode is "fallback"

Let part one be the first token in tokens, and let part two be the second token in tokens.

Resolve part one and part two, relative to base URL, with the URL character encoding set to UTF-8.

If either fails, then jump back to the step labeled start of line.

If the absolute URL corresponding to either part one or part two does not have the same origin as the manifest's URL, then jump back to the step labeled start of line.

Let part one be the result of applying the URL serializer algorithm to the first resulting parsed URL, with the exclude fragment flag set.

Let part two be the result of applying the URL serializer algorithm to the second resulting parsed URL, with the exclude fragment flag set.

If part one is already in the fallback URLs mapping as a fallback namespace, then jump back to the step labeled start of line.

Otherwise, add part one to the fallback URLs mapping as a fallback namespace, mapped to part two as the fallback entry.

If mode is "online whitelist"

If the first item in tokens is a "*" (U+002A) character, then set online whitelist wildcard flag to open and jump back to the step labeled start of line.

Otherwise, resolve the first item in tokens, relative to base URL, with the URL character encoding set to UTF-8; ignore the rest.

If this fails, then jump back to the step labeled start of line.

If the resulting parsed URL has a different scheme component than base URL (the manifest's URL), then jump back to the step labeled start of line.

Let new URL be the result of applying the URL serializer algorithm to the resulting parsed URL, with the exclude fragment flag set.

Add new URL to the online whitelist namespaces.

If mode is "settings"

If tokens contains a single token, and that token is a case-sensitive match for the string "prefer-online", then set cache mode flag to prefer-online and jump back to the step labeled start of line.

Otherwise, the line is an unsupported setting: do nothing; the line is ignored.

If mode is "unknown"

Do nothing. The line is ignored.
Jump back to the step labeled start of line. (That step jumps to the next, and last, step when the end of the file is reached.)
Return the explicit URLs list, the fallback URLs mapping, the online whitelist namespaces, the online whitelist wildcard flag, and the cache mode flag.

The resource that declares the manifest (with the manifest attribute) will always get taken from the cache, whether it is listed in the cache or not, even if it is listed in an online whitelist namespace.

If a resource is listed in the explicit section or as a fallback entry in the fallback section, the resource will always be taken from the cache, regardless of any other matching entries in the fallback namespaces or online whitelist namespaces.

When a fallback namespace and an online whitelist namespace overlap, the online whitelist namespace has priority.

The online whitelist wildcard flag is applied last, only for URLs that match neither the online whitelist namespace nor the fallback namespace and that are not listed in the explicit section.

5.7.4 Downloading or updating an application cache

When the user agent is required (by other parts of this specification) to start the application cache download process for an absolute URL purported to identify a manifest, or for an application cache group, potentially given a particular cache host, and potentially given a master resource, the user agent must run the steps below. These steps are always run asynchronously, in parallel with the event loop tasks.

Some of these steps have requirements that only apply if the user agent shows caching progress. Support for this is optional. Caching progress UI could consist of a progress bar or message panel in the user agent's interface, or an overlay, or something else. Certain events fired during the application cache download process allow the script to override the display of such an interface. (Such events are delayed until after the load event has fired.) The goal of this is to allow Web applications to provide more seamless update mechanisms, hiding from the user the mechanics of the application cache mechanism. User agents may display user interfaces independent of this, but are encouraged to not show prominent update progress notifications for applications that cancel the relevant events.

The application cache download process steps are as follows:

Optionally, wait until the permission to start the application cache download process has been obtained from the user and until the user agent is confident that the network is available. This could include doing nothing until the user explicitly opts-in to caching the site, or could involve prompting the user for permission. The algorithm might never get past this point. (This step is particularly intended to be used by user agents running on severely space-constrained devices or in highly privacy-sensitive environments).
Atomically, so as to avoid race conditions, perform the following substeps:
1. Pick the appropriate substeps:
  
  If these steps were invoked with an absolute URL purported to identify a manifest
  
  Let manifest URL be that absolute URL.
  
  If there is no application cache group identified by manifest URL, then create a new application cache group identified by manifest URL. Initially, it has no application caches. One will be created later in this algorithm.
  
  If these steps were invoked with an application cache group
  
  Let manifest URL be the absolute URL of the manifest used to identify the application cache group to be updated.
  
  If that application cache group is obsolete, then abort this instance of the application cache download process. This can happen if another instance of this algorithm found the manifest to be 404 or 410 while this algorithm was waiting in the first step above.
2. Let cache group be the application cache group identified by manifest URL.
3. If these steps were invoked with a master resource, then add the resource, along with the resource's Document, to cache group's list of pending master entries.
4. If these steps were invoked with a cache host, and the status of cache group is checking or downloading, then queue a post-load task to fire a simple event named checking that is cancelable at the ApplicationCache singleton of that cache host. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent is checking to see if it can download the application.
5. If these steps were invoked with a cache host, and the status of cache group is downloading, then also queue a post-load task to fire a simple event named downloading that is cancelable at the ApplicationCache singleton of that cache host. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user the application is being downloaded.
6. If the status of the cache group is either checking or downloading, then abort this instance of the application cache download process, as an update is already in progress.
7. Set the status of cache group to checking.
8. For each cache host associated with an application cache in cache group, queue a post-load task to fire a simple event that is cancelable named checking at the ApplicationCache singleton of the cache host. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent is checking for the availability of updates.
The remainder of the steps run asynchronously.

If cache group already has an application cache in it, then this is an upgrade attempt. Otherwise, this is a cache attempt.
If this is a cache attempt, then this algorithm was invoked with a cache host; queue a post-load task to fire a simple event named checking that is cancelable at the ApplicationCache singleton of that cache host. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent is checking for the availability of updates.
Fetching the manifest: Fetch the resource from manifest URL with the synchronous flag set, and let manifest be that resource. HTTP caching semantics should be honored for this request.

Parse manifest according to the rules for parsing manifests, obtaining a list of explicit entries, fallback entries and the fallback namespaces that map to them, entries for the online whitelist, and values for the online whitelist wildcard flag and the cache mode flag.

The MIME type of the resource is ignored — it is assumed to be text/cache-manifest. In the future, if new manifest formats are supported, the different types will probably be distinguished on the basis of the file signatures (for the current format, that is the "CACHE MANIFEST" string at the top of the file).
If fetching the manifest fails due to a 404 or 410 response or equivalent, then run these substeps:
1. Mark cache group as obsolete. This cache group no longer exists for any purpose other than the processing of Document objects already associated with an application cache in the cache group.
2. Let task list be an empty list of tasks.
3. For each cache host associated with an application cache in cache group, create a task to fire a simple event named obsolete that is cancelable at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the application is no longer available for offline use.
4. For each entry in cache group's list of pending master entries, create a task to fire a simple event that is cancelable named error (not obsolete!) at the ApplicationCache singleton of the Document for this entry, if there still is one, and append it to task list. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.
5. If cache group has an application cache whose completeness flag is incomplete, then discard that application cache.
6. If appropriate, remove any user interface indicating that an update for this cache is in progress.
7. Let the status of cache group be idle.
8. For each task in task list, queue that task as a post-load task.
9. Abort the application cache download process.
Otherwise, if fetching the manifest fails in some other way (e.g. the server returns another 4xx or 5xx response or equivalent, or there is a DNS error, or the connection times out, or the user cancels the download, or the parser for manifests fails when checking the magic signature), or if the server returned a redirect, then run the cache failure steps. [HTTP]
If this is an upgrade attempt and the newly downloaded manifest is byte-for-byte identical to the manifest found in the newest application cache in cache group, or the server reported it as "304 Not Modified" or equivalent, then run these substeps:
1. Let cache be the newest application cache in cache group.
2. Let task list be an empty list of tasks.
3. For each entry in cache group's list of pending master entries, wait for the resource for this entry to have either completely downloaded or failed.
  
  If the download failed (e.g. the server returns a 4xx or 5xx response or equivalent, or there is a DNS error, the connection times out, or the user cancels the download), or if the resource is labeled with the "no-store" cache directive, then create a task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the Document for this entry, if there still is one, and append it to task list. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.
  
  Otherwise, associate the Document for this entry with cache; store the resource for this entry in cache, if it isn't already there, and categorize its entry as a master entry. If applying the URL parser algorithm to the resource's URL results in a parsed URL that has a non-null fragment component, the URL used for the entry in cache must instead be the absolute URL obtained from applying the URL serializer algorithm to the parsed URL with the exclude fragment flag set (application caches never include fragment identifiers).
4. For each cache host associated with an application cache in cache group, create a task to fire a simple event that is cancelable named noupdate at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the application is up to date.
5. Empty cache group's list of pending master entries.
6. If appropriate, remove any user interface indicating that an update for this cache is in progress.
7. Let the status of cache group be idle.
8. For each task in task list, queue that task as a post-load task.
9. Abort the application cache download process.
Let new cache be a newly created application cache in cache group. Set its completeness flag to incomplete.
For each entry in cache group's list of pending master entries, associate the Document for this entry with new cache.
Set the status of cache group to downloading.
For each cache host associated with an application cache in cache group, queue a post-load task to fire a simple event that is cancelable named downloading at the ApplicationCache singleton of the cache host. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that a new version is being downloaded.
Let file list be an empty list of URLs with flags.
Add all the URLs in the list of explicit entries obtained by parsing manifest to file list, each flagged with "explicit entry".
Add all the URLs in the list of fallback entries obtained by parsing manifest to file list, each flagged with "fallback entry".
If this is an upgrade attempt, then add all the URLs of master entries in the newest application cache in cache group whose completeness flag is complete to file list, each flagged with "master entry".
If any URL is in file list more than once, then merge the entries into one entry for that URL, that entry having all the flags that the original entries had.
For each URL in file list, run the following steps. These steps may be run in parallel for two or more of the URLs at a time. If, while running these steps, the ApplicationCache object's abort() method sends a signal to this instance of the application cache download process algorithm, then run the cache failure steps instead.
1. If the resource URL being processed was flagged as neither an "explicit entry" nor or a "fallback entry", then the user agent may skip this URL.
  
  This is intended to allow user agents to expire resources not listed in the manifest from the cache. Generally, implementors are urged to use an approach that expires lesser-used resources first.
2. For each cache host associated with an application cache in cache group, queue a post-load task to fire a trusted event with the name progress, which does not bubble, which is cancelable, and which uses the ProgressEvent interface, at the ApplicationCache singleton of the cache host. The lengthComputable attribute must be set to true, the total attribute must be set to the number of files in file list, and the loaded attribute must be set to the number of files in file list that have been either downloaded or skipped so far. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that a file is being downloaded in preparation for updating the application. [PROGRESS-EVENTS]
3. Fetch the resource, from the origin of the URL manifest URL, with the synchronous flag set and the manual redirect flag set. If this is an upgrade attempt, then use the newest application cache in cache group as an HTTP cache, and honor HTTP caching semantics (such as expiration, ETags, and so forth) with respect to that cache. User agents may also have other caches in place that are also honored.
  
  If the resource in question is already being downloaded for other reasons then the existing download process can sometimes be used for the purposes of this step, as defined by the fetching algorithm.
  
  An example of a resource that might already be being downloaded is a large image on a Web page that is being seen for the first time. The image would get downloaded to satisfy the img element on the page, as well as being listed in the cache manifest. According to the rules for fetching that image only need be downloaded once, and it can be used both for the cache and for the rendered Web page.
4. If the previous step fails (e.g. the server returns a 4xx or 5xx response or equivalent, or there is a DNS error, or the connection times out, or the user cancels the download), or if the server returned a redirect, or if the resource is labeled with the "no-store" cache directive, then run the first appropriate step from the following list: [HTTP]
  
  If the URL being processed was flagged as an "explicit entry" or a "fallback entry"
  
  If these steps are being run in parallel for any other URLs in file list, then abort these steps for those other URLs. Run the cache failure steps.
  
  Redirects are fatal because they are either indicative of a network problem (e.g. a captive portal); or would allow resources to be added to the cache under URLs that differ from any URL that the networking model will allow access to, leaving orphan entries; or would allow resources to be stored under URLs different than their true URLs. All of these situations are bad.
  
  If the error was a 404 or 410 HTTP response or equivalent
  
  If the resource was labeled with the "no-store" cache directive
  
  Skip this resource. It is dropped from the cache.
  
  Otherwise
  
  Copy the resource and its metadata from the newest application cache in cache group whose completeness flag is complete, and act as if that was the fetched resource, ignoring the resource obtained from the network.
  
  User agents may warn the user of these errors as an aid to development.
  
  These rules make errors for resources listed in the manifest fatal, while making it possible for other resources to be removed from caches when they are removed from the server, without errors, and making non-manifest resources survive server-side errors.
  
  Except for the "no-store" directive, HTTP caching rules that would cause a file to be expired or otherwise not cached are ignored for the purposes of the application cache download process.
5. Otherwise, the fetching succeeded. Store the resource in the new cache.
  
  If the user agent is not able to store the resource (e.g. because of quota restrictions), the user agent may prompt the user or try to resolve the problem in some other manner (e.g. automatically pruning content in other caches). If the problem cannot be resolved, the user agent must run the cache failure steps.
6. If the URL being processed was flagged as an "explicit entry" in file list, then categorize the entry as an explicit entry.
7. If the URL being processed was flagged as a "fallback entry" in file list, then categorize the entry as a fallback entry.
8. If the URL being processed was flagged as an "master entry" in file list, then categorize the entry as a master entry.
9. As an optimization, if the resource is an HTML or XML file whose root element is an html element with a manifest attribute whose value doesn't match the manifest URL of the application cache being processed, then the user agent should mark the entry as being foreign.
For each cache host associated with an application cache in cache group, queue a post-load task to fire a trusted event with the name progress, which does not bubble, which is cancelable, and which uses the ProgressEvent interface, at the ApplicationCache singleton of the cache host. The lengthComputable attribute must be set to true, the total and the loaded attributes must be set to the number of files in file list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that all the files have been downloaded. [PROGRESS-EVENTS]
Store the list of fallback namespaces, and the URLs of the fallback entries that they map to, in new cache.
Store the URLs that form the new online whitelist in new cache.
Store the value of the new online whitelist wildcard flag in new cache.
Store the value of the new cache mode flag in new cache.
For each entry in cache group's list of pending master entries, wait for the resource for this entry to have either completely downloaded or failed.

If the download failed (e.g. the server returns a 4xx or 5xx response or equivalent, or there is a DNS error, the connection times out, or the user cancels the download), or if the resource is labeled with the "no-store" cache directive, then run these substeps:
1. Unassociate the Document for this entry from new cache.
2. Queue a post-load task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the Document for this entry, if there still is one. The default action of this event must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.
3. If this is a cache attempt and this entry is the last entry in cache group's list of pending master entries, then run these further substeps:
  1. Discard cache group and its only application cache, new cache.
  2. If appropriate, remove any user interface indicating that an update for this cache is in progress.
  3. Abort the application cache download process.
4. Otherwise, remove this entry from cache group's list of pending master entries.
Otherwise, store the resource for this entry in new cache, if it isn't already there, and categorize its entry as a master entry.
Fetch the resource from manifest URL again, with the synchronous flag set, and let second manifest be that resource. HTTP caching semantics should again be honored for this request.

Since caching can be honored, authors are encouraged to avoid setting the cache headers on the manifest in such a way that the user agent would simply not contact the network for this second request; otherwise, the user agent would not notice if the cache had changed during the cache update process.
If the previous step failed for any reason, or if the fetching attempt involved a redirect, or if second manifest and manifest are not byte-for-byte identical, then schedule a rerun of the entire algorithm with the same parameters after a short delay, and run the cache failure steps.
Otherwise, store manifest in new cache, if it's not there already, and categorize its entry as the manifest.
Set the completeness flag of new cache to complete.
Let task list be an empty list of tasks.
If this is a cache attempt, then for each cache host associated with an application cache in cache group, create a task to fire a simple event that is cancelable named cached at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the application has been cached and that they can now use it offline.

Otherwise, it is an upgrade attempt. For each cache host associated with an application cache in cache group, create a task to fire a simple event that is cancelable named updateready at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that a new version is available and that they can activate it by reloading the page.
If appropriate, remove any user interface indicating that an update for this cache is in progress.
Set the update status of cache group to idle.
For each task in task list, queue that task as a post-load task.

The cache failure steps are as follows:

Let task list be an empty list of tasks.
For each entry in cache group's list of pending master entries, run the following further substeps. These steps may be run in parallel for two or more entries at a time.
1. Wait for the resource for this entry to have either completely downloaded or failed.
2. Unassociate the Document for this entry from its application cache, if it has one.
3. Create a task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the Document for this entry, if there still is one, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.
For each cache host still associated with an application cache in cache group, create a task to fire a simple event that is cancelable named error at the ApplicationCache singleton of the cache host, and append it to task list. The default action of these events must be, if the user agent shows caching progress, the display of some sort of user interface indicating to the user that the user agent failed to save the application for offline use.
Empty cache group's list of pending master entries.
If cache group has an application cache whose completeness flag is incomplete, then discard that application cache.
If appropriate, remove any user interface indicating that an update for this cache is in progress.
Let the status of cache group be idle.
If this was a cache attempt, discard cache group altogether.
For each task in task list, queue that task as a post-load task.
Abort the application cache download process.

Attempts to fetch resources as part of the application cache download process may be done with cache-defeating semantics, to avoid problems with stale or inconsistent intermediary caches.

User agents may invoke the application cache download process, in the background, for any application cache group, at any time (with no cache host). This allows user agents to keep caches primed and to update caches even before the user visits a site.

Each Document has a list of pending application cache download process tasks that is used to delay events fired by the algorithm above until the document's load event has fired. When the Document is created, the list must be empty.

When the steps above say to queue a post-load task task, where task is a task that dispatches an event on a target ApplicationCache object target, the user agent must run the appropriate steps from the following list:

If target's Document is ready for post-load tasks: Queue the task task.
Otherwise: Add task to target's Document's list of pending application cache download process tasks.

The task source for these tasks is the networking task source.

5.7.5 The application cache selection algorithm

When the application cache selection algorithm algorithm is invoked with a Document document and optionally a manifest URL manifest URL, the user agent must run the first applicable set of steps from the following list:

If there is a manifest URL, and document was loaded from an application cache, and the URL of the manifest of that cache's application cache group is not the same as manifest URL

Mark the entry for the resource from which document was taken in the application cache from which it was loaded as foreign.

Restart the current navigation from the top of the navigation algorithm, undoing any changes that were made as part of the initial load (changes can be avoided by ensuring that the step to update the session history with the new page is only ever completed after this application cache selection algorithm is run, though this is not required).

The navigation will not result in the same resource being loaded, because "foreign" entries are never picked during navigation.

User agents may notify the user of the inconsistency between the cache manifest and the document's own metadata, to aid in application development.

If document was loaded from an application cache, and that application cache still exists (it is not now obsolete)

Associate document with the application cache from which it was loaded. Invoke, in the background, the application cache download process for that application cache's application cache group, with document as the cache host.

If document was loaded using HTTP GET or equivalent, and, there is a manifest URL, and manifest URL has the same origin as document

Invoke, in the background, the application cache download process for manifest URL, with document as the cache host and with the resource from which document was parsed as the master resource.

If there are relevant application caches that are identified by a URL with the same origin as the URL of document, and that have this URL as one of their entries, excluding entries marked as foreign, then the user agent should use the most appropriate application cache of those that match as an HTTP cache for any subresource loads. User agents may also have other caches in place that are also honored.

Otherwise

The Document is not associated with any application cache.

If there was a manifest URL, the user agent may report to the user that it was ignored, to aid in application development.

5.7.6 Changes to the networking model

When a cache host is associated with an application cache whose completeness flag is complete, any and all loads for resources related to that cache host other than those for child browsing contexts must go through the following steps instead of immediately invoking the mechanisms appropriate to that resource's scheme:

If the resource is not to be fetched using the HTTP GET mechanism or equivalent, or if applying the URL parser algorithm to both its URL and the application cache's manifest's URL results in two parsed URLs with different scheme components, then fetch the resource normally and abort these steps.
If the resource's URL is a master entry, the manifest, an explicit entry, or a fallback entry in the application cache, then get the resource from the cache (instead of fetching it), and abort these steps.
If there is an entry in the application cache's online whitelist that has the same origin as the resource's URL and that is a prefix match for the resource's URL, then fetch the resource normally and abort these steps.
If the resource's URL has the same origin as the manifest's URL, and there is a fallback namespace f in the application cache that is a prefix match for the resource's URL, then:

Fetch the resource normally. If this results in a redirect to a resource with another origin (indicative of a captive portal), or a 4xx or 5xx status code or equivalent, or if there were network errors (but not if the user canceled the download), then instead get, from the cache, the resource of the fallback entry corresponding to the fallback namespace f. Abort these steps.
If the application cache's online whitelist wildcard flag is open, then fetch the resource normally and abort these steps.
Fail the resource load as if there had been a generic network error.

The above algorithm ensures that so long as the online whitelist wildcard flag is blocking, resources that are not present in the manifest will always fail to load (at least, after the application cache has been primed the first time), making the testing of offline applications simpler.

5.7.7 Expiring application caches

As a general rule, user agents should not expire application caches, except on request from the user, or after having been left unused for an extended period of time.

Application caches and cookies have similar implications with respect to privacy (e.g. if the site can identify the user when providing the cache, it can store data in the cache that can be used for cookie resurrection). Implementors are therefore encouraged to expose application caches in a manner related to HTTP cookies, allowing caches to be expunged together with cookies and other origin-specific data.

For example, a user agent could have a "delete site-specific data" feature that clears all cookies, application caches, local storage, databases, etc, from an origin all at once.

5.7.8 Disk space

User agents should consider applying constraints on disk usage of application caches, and care should be taken to ensure that the restrictions cannot be easily worked around using subdomains.

User agents should allow users to see how much space each domain is using, and may offer the user the ability to delete specific application caches.

For predictability, quotas should be based on the uncompressed size of data stored.

How quotas are presented to the user is not defined by this specification. User agents are encouraged to provide features such as allowing a user to indicate that certain sites are trusted to use more than the default quota, e.g. by asynchronously presenting a user interface while a cache is being updated, or by having an explicit whitelist in the user agent's configuration interface.

5.7.9 Application cache API

interface ApplicationCache : EventTarget {

  // update status
  const unsigned short UNCACHED = 0;
  const unsigned short IDLE = 1;
  const unsigned short CHECKING = 2;
  const unsigned short DOWNLOADING = 3;
  const unsigned short UPDATEREADY = 4;
  const unsigned short OBSOLETE = 5;
  readonly attribute unsigned short status;

  // updates
  void update();
  void abort();
  void swapCache();

  // events
           attribute EventHandler onchecking;
           attribute EventHandler onerror;
           attribute EventHandler onnoupdate;
           attribute EventHandler ondownloading;
           attribute EventHandler onprogress;
           attribute EventHandler onupdateready;
           attribute EventHandler oncached;
           attribute EventHandler onobsolete;
};

cache = window . applicationCache

(In a window.) Returns the ApplicationCache object that applies to the active document of that Window.

cache = self . applicationCache

(In a shared worker.) Returns the ApplicationCache object that applies to the current shared worker.

cache . status

Returns the current status of the application cache, as given by the constants defined below.

cache . update()

Invokes the application cache download process.

Throws an InvalidStateError exception if there is no application cache to update.

Calling this method is not usually necessary, as user agents will generally take care of updating application caches automatically.

The method can be useful in situations such as long-lived applications. For example, a Web mail application might stay open in a browser tab for weeks at a time. Such an application could want to test for updates each day.

cache . abort()

Cancels the application cache download process.

This method is intended to be used by Web application showing their own caching progress UI, in case the user wants to stop the update (e.g. because bandwidth is limited).

cache . swapCache()

Switches to the most recent application cache, if there is a newer one. If there isn't, throws an InvalidStateError exception.

This does not cause previously-loaded resources to be reloaded; for example, images do not suddenly get reloaded and style sheets and scripts do not get reparsed or reevaluated. The only change is that subsequent requests for cached resources will obtain the newer copies.

The updateready event will fire before this method can be called. Once it fires, the Web application can, at its leisure, call this method to switch the underlying cache to the one with the more recent updates. To make proper use of this, applications have to be able to bring the new features into play; for example, reloading scripts to enable new features.

An easier alternative to swapCache() is just to reload the entire page at a time suitable for the user, using location.reload().

There is a one-to-one mapping from cache hosts to ApplicationCache objects. The applicationCache attribute on Window objects must return the ApplicationCache object associated with the Window object's active document. The applicationCache attribute on SharedWorkerGlobalScope objects must return the ApplicationCache object associated with the worker.

A Window or SharedWorkerGlobalScope object has an associated ApplicationCache object even if that cache host has no actual application cache.

The status attribute, on getting, must return the current state of the application cache that the ApplicationCache object's cache host is associated with, if any. This must be the appropriate value from the following list:

UNCACHED (numeric value 0): The ApplicationCache object's cache host is not associated with an application cache at this time.
IDLE (numeric value 1): The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is idle, and that application cache is the newest cache in its application cache group, and the application cache group is not marked as obsolete.
CHECKING (numeric value 2): The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is checking.
DOWNLOADING (numeric value 3): The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is downloading.
UPDATEREADY (numeric value 4): The ApplicationCache object's cache host is associated with an application cache whose application cache group's update status is idle, and whose application cache group is not marked as obsolete, but that application cache is not the newest cache in its group.
OBSOLETE (numeric value 5): The ApplicationCache object's cache host is associated with an application cache whose application cache group is marked as obsolete.

If the update() method is invoked, the user agent must invoke the application cache download process, in the background, for the application cache group of the application cache with which the ApplicationCache object's cache host is associated, but without giving that cache host to the algorithm. If there is no such application cache, or if its application cache group is marked as obsolete, then the method must throw an InvalidStateError exception instead.

If the abort() method is invoked, the user agent must send a signal to the current application cache download process for the application cache group of the application cache with which the ApplicationCache object's cache host is associated, if any. If there is no such application cache, or it does not have a current application cache download process, then do nothing.

If the swapCache() method is invoked, the user agent must run the following steps:

Check that ApplicationCache object's cache host is associated with an application cache. If it is not, then throw an InvalidStateError exception and abort these steps.
Let cache be the application cache with which the ApplicationCache object's cache host is associated. (By definition, this is the same as the one that was found in the previous step.)
If cache's application cache group is marked as obsolete, then unassociate the ApplicationCache object's cache host from cache and abort these steps. (Resources will now load from the network instead of the cache.)
Check that there is an application cache in the same application cache group as cache whose completeness flag is complete and that is newer than cache. If there is not, then throw an InvalidStateError exception and abort these steps.
Let new cache be the newest application cache in the same application cache group as cache whose completeness flag is complete.
Unassociate the ApplicationCache object's cache host from cache and instead associate it with new cache.

Event handler	Event handler event type
`onchecking`	`checking`
`onerror`	`error`
`onnoupdate`	`noupdate`
`ondownloading`	`downloading`
`onprogress`	`progress`
`onupdateready`	`updateready`
`oncached`	`cached`
`onobsolete`	`obsolete`

5.7.10 Browser state

[NoInterfaceObject]
interface NavigatorOnLine {
  readonly attribute boolean onLine;
};

window . navigator . onLine

Returns false if the user agent is definitely offline (disconnected from the network). Returns true if the user agent might be online.

The events online and offline are fired when the value of this attribute changes.

The navigator.onLine attribute must return false if the user agent will not contact the network when the user follows links or when a script requests a remote page (or knows that such an attempt would fail), and must return true otherwise.

When the value that would be returned by the navigator.onLine attribute of a Window or WorkerGlobalScope changes from true to false, the user agent must queue a task to fire a simple event named offline at the Window or WorkerGlobalScope object.

On the other hand, when the value that would be returned by the navigator.onLine attribute of a Window or WorkerGlobalScope changes from false to true, the user agent must queue a task to fire a simple event named online at the Window or WorkerGlobalScope object.

The task source for these tasks is the networking task source.

This attribute is inherently unreliable. A computer can be connected to a network without having Internet access.

In this example, an indicator is updated as the browser goes online and offline.

<!DOCTYPE HTML>
<html>
 <head>
  <title>Online status</title>
  <script>
   function updateIndicator() {
     document.getElementById('indicator').textContent = navigator.onLine ? 'online' : 'offline';
   }
  </script>
 </head>
 <body onload="updateIndicator()" ononline="updateIndicator()" onoffline="updateIndicator()">
  <p>The network is: <span id="indicator">(state unknown)</span>
 </body>
</html>

6 Web application APIs

6.1 Scripting

6.1.1 Introduction

Various mechanisms can cause author-provided executable code to run in the context of a document. These mechanisms include, but are probably not limited to:

Processing of script elements.
Navigating to javascript: URLs.
Event handlers, whether registered through the DOM using addEventListener(), by explicit event handler content attributes, by event handler IDL attributes, or otherwise.
Processing of technologies like XBL or SVG that have their own scripting features.

6.1.2 Enabling and disabling scripting

Scripting is enabled in a browsing context when all of the following conditions are true:

The user agent supports scripting.
The user has not disabled scripting for this browsing context at this time. (User agents may provide users with the option to disable scripting globally, or in a finer-grained manner, e.g. on a per-origin basis.)
The browsing context's active document's active sandboxing flag set does not have its sandboxed scripts browsing context flag set.

Scripting is disabled in a browsing context when any of the above conditions are false (i.e. when scripting is not enabled).

Scripting is enabled for a node if the Document object of the node (the node itself, if it is itself a Document object) has an associated browsing context, and scripting is enabled in that browsing context.

Scripting is disabled for a node if there is no such browsing context, or if scripting is disabled in that browsing context.

6.1.3 Processing model

6.1.3.1 Definitions

This specification describes three kinds of JavaScript global environments: the document environment, the dedicated worker environment, and the shared worker environment. The dedicated worker environment and the shared worker environment are both types of worker environments.

Except where otherwise specified, a JavaScript global environment is a document environment.

A script has:

A code entry-point

A code entry-point represents a block of executable code that the script exposes to other scripts and to the user agent. Typically, the code corresponding to the code entry-point is executed immediately after the script is parsed, but for event handlers, it is called each time the handler is invoked.

In JavaScript script blocks, this corresponds to the execution context of the global code.

Optionally, a muted errors flag

A flag which, if set, means that error information will not be provided for errors in this script (used to mute errors for cross-origin scripts, since that can leak private information).

A settings object

A script settings object, various settings that are shared with other scripts in the same context.

A script settings object specifies algorithms for obtaining the following:

A script execution environment for each language supported by the user agent

The characteristics of the script execution environment depend on the language, and are not defined by this specification.

In JavaScript, the script execution environment consists of the interpreter, the stack of execution contexts, the global code and function code and the Function objects resulting, and so forth.

A global object

An object that provides the APIs that can be called by the code in scripts that use this settings object.

This is typically a Window object or a WorkerGlobalScope object. When a global object is an empty object, it can't do anything that interacts with the environment.

If the global object is a Window object, then, in JavaScript, the ThisBinding of the global execution context for this script must be the Window object's WindowProxy object, rather than the global object. [ECMA262]

This is a willful violation of the JavaScript specification current at the time of writing (ECMAScript edition 5, as defined in section 10.4.1.1 Initial Global Execution Context, step 3). The JavaScript specification requires that the this keyword in the global scope return the global object, but this is not compatible with the security design prevalent in implementations as specified herein. [ECMA262]

A responsible browsing context

A browsing context that is assigned responsibility for actions taken by the scripts that use this script settings object.

When a script creates and navigates a new top-level browsing context, the opener attribute of the new browsing context's Window object will be set to the responsible browsing context's WindowProxy object.

A responsible document

A Document that is assigned responsibility for actions taken by the scripts that use this script settings object.

For example, the address of the responsible document is used to set the address of any Documents created using createDocument().

A responsible event loop

An event loop that is used when it would not be immediately clear what event loop to use.

An API referrer source

Either a Document (specifically, the responsible document), or a URL, which is used by some APIs to determine what value to use for the Referer (sic) header in calls to the fetching algorithm.

An API URL character encoding

A character encoding used to encode URLs by APIs called by scripts that use this script settings object.

An API base URL

An absolute URL used by APIs called by scripts that use this script settings object to resolve relative URLs.

An origin and an effective script origin

An instrument used in security checks.

The relevant settings object for a global object o is the script settings object whose global object is o. (There is always a 1:1 mapping of global objects to script settings objects.)

The relevant settings object for a script s is the settings object of s.

6.1.3.2 Script settings for browsing contexts

Whenever a new Window object is created, it must also create a script settings object whose algorithms are defined as follows:

The script execution environments

When the script settings object is created, for each language supported by the user agent, create an appropriate execution environment as defined by the relevant specification.

When a script execution environment is needed, return the appropriate one from those created when the script settings object was created.

The global object

Return the Window object itself.

The responsible browsing context

Return the browsing context with which the Window object is associated.

The responsible document

Return the Document with which the Window is currently associated.

The responsible event loop

Return the event loop that is associated with the unit of related similar-origin browsing contexts to which the Window object's browsing context belongs.

The API referrer source

Return the Document with which the Window is currently associated.

The API URL character encoding

Return the current character encoding of the Document with which the Window is currently associated.

The API base URL

Return the current base URL of the Document with which the Window is currently associated.

The origin

Return the origin of the Document with which the Window is currently associated.

The effective script origin

Return the effective script origin of the Document with which the Window is currently associated.

6.1.3.3 Calling scripts

Each unit of related similar-origin browsing contexts has a stack of script settings objects, which must be initially empty. When a new script settings object is pushed onto this stack, the specified script settings object is to be added to the stack; when the script settings object on this stack that was most recently pushed onto it is to be popped from the stack, it must be removed. Entries on this stack can be labeled as candidate entry settings objects.

When a user agent is to jump to a code entry-point for a script s, the user agent must run the following steps:

Let context be the settings object of s.
Prepare to run a callback with context as the script settings object. If this returns "do not run" then abort these steps.
Make the appropriate script execution environment specified by context execute the s's code entry-point.
Clean up after running a callback.

The steps to prepare to run a callback with a script settings object o are as follows. They return either "run" or "do not run".

If the global object specified by o is a Window object whose Document object is not fully active, then return "do not run" and abort these steps.
If scripting is disabled for the responsible browsing context specified by o, then return "do not run" and abort these steps.
Push o onto the stack of script settings objects, and label it as a candidate entry settings object.
Return "run".

The steps to clean up after running a callback are as follows:

Pop the current incumbent settings object from the stack of script settings objects.
If the stack of script settings objects is now empty, run the global script clean-up jobs. (These cannot run scripts.)
If the stack of script settings objects is now empty, perform a microtask checkpoint. (If this runs scripts, these algorithms will be invoked reentrantly.)

These algorithms are not invoked by one script directly calling another, but they can be invoked reentrantly in an indirect manner, e.g. if a script dispatches an event which has event listeners registered.

When a JavaScript SourceElements production is to be evaluated, the settings object of the script corresponding to that SourceElements must be pushed onto the stack of script settings objects before the evaluation begins, and popped when the evaluation ends (regardless of whether it's an abrupt completion or not).

The entry settings object is the most-recently added script settings object in the stack of script settings objects that is labeled as a candidate entry settings object. If the stack is empty, or has no entries labeled as such, then there is no entry settings object. It is used to obtain, amongst other things, the API base URL to resolve relative URLs used in scripts running in that unit of related similar-origin browsing contexts.

The incumbent settings object is the script settings object in the stack of script settings objects that was most-recently added (i.e. the last one on the stack). If the stack is empty, then there is no incumbent settings object. It is used in some security checks.

The WebIDL specification also uses these algorithms. [WEBIDL]

Each unit of related similar-origin browsing contexts has a performing a microtask checkpoint flag, which must initially be false. It is used to prevent reentrant invocation of the algorithm to invoke MutationObserver objects. For the purposes of MutationObserver objects, each unit of related similar-origin browsing contexts is a distinct scripting environment.

Each unit of related similar-origin browsing contexts has a global script clean-up jobs list, which must initially be empty. A global script clean-up job cannot run scripts, and cannot be sensitive to the order in which other clean-up jobs are executed. The File API uses this to release blob: URLs. [FILEAPI]

When the user agent is to run the global script clean-up jobs, the user agent must perform each of the jobs in the global script clean-up jobs list and then empty the list.

6.1.3.4 Creating scripts

When the specification says that a script is to be created, given some script source, a script source URL, its scripting language, a script settings object, and optionally a muted errors flag, the user agent must run the following steps:

Let script be a new script that this algorithm will subsequently initialize.
If scripting is disabled for browsing context passed to this algorithm, then abort these steps, as if the script source described a program that did nothing but return void.
Obtain the appropriate script execution environment for the given scripting language from the script settings object provided.
Parse/compile/initialize the source of the script using the script execution environment, as appropriate for the scripting language, and thus obtain script's code entry-point.
Let script's settings object be the script settings object provided.
If the muted errors flag was set, then set script's muted errors flag.
If all the steps above succeeded (in particular, if the script was compiled successfully), Jump to script's code entry-point.

Otherwise, report the error for script, with the problematic position (line number and column number), using the global object specified by the script settings object as the target. If the error is still not handled after this, then the error may be reported to the user.

6.1.3.5 Killing scripts

User agents may impose resource limitations on scripts, for example CPU quotas, memory limits, total execution time limits, or bandwidth limitations. When a script exceeds a limit, the user agent may either throw a QuotaExceededError exception, abort the script without an exception, prompt the user, or throttle script execution.

For example, the following script never terminates. A user agent could, after waiting for a few seconds, prompt the user to either terminate the script or let it continue.

<script>
 while (true) { /* loop */ }
</script>

User agents are encouraged to allow users to disable scripting whenever the user is prompted either by a script (e.g. using the window.alert() API) or because of a script's actions (e.g. because it has exceeded a time limit).

If scripting is disabled while a script is executing, the script should be terminated immediately.

User agents may allow users to specifically disable scripts just for the purposes of closing a browsing context.

For example, the prompt mentioned in the example above could also offer the user with a mechanism to just close the page entirely, without running any unload event handlers.

6.1.3.6 Runtime script errors

When the user agent is required to report an error for a particular script script with a particular position line:col, using a particular target target, it must run these steps, after which the error is either handled or not handled:

If target is in error reporting mode, then abort these steps; the error is not handled.
Let target be in error reporting mode.
Let message be a user-agent-defined string describing the error in a helpful manner.
Let error object be the object that represents the error: in the case of an uncaught exception, that would be the object that was thrown; in the case of a JavaScript error that would be an Error object. If there is no corresponding object, then the null value must be used instead.
Let location be an absolute URL that corresponds to the resource from which script was obtained.

The resource containing the script will typically be the file from which the Document was parsed, e.g. for inline script elements or event handler content attributes; or the JavaScript file that the script was in, for external scripts. Even for dynamically-generated scripts, user agents are strongly encouraged to attempt to keep track of the original source of a script. For example, if an external script uses the document.write() API to insert an inline script element during parsing, the URL of the resource containing the script would ideally be reported as being the external script, and the line number might ideally be reported as the line with the document.write() call or where the string passed to that call was first constructed. Naturally, implementing this can be somewhat non-trivial.

User agents are similarly encouraged to keep careful track of the original line numbers, even in the face of document.write() calls mutating the document as it is parsed, or event handler content attributes spanning multiple lines.
If script has muted errors, then set message to "Script error.", set location to the empty string, set line and col to 0, and set error object to null.
Let event be a new trusted ErrorEvent object that does not bubble but is cancelable, and which has the event name error.
Initialize event's message attribute to message.
Initialize event's filename attribute to location.
Initialize event's lineno attribute to line.
Initialize event's colno attribute to col.
Initialize event's error attribute to error object.
Dispatch event at target.
Let target no longer be in error reporting mode.
If event was canceled, then the error is handled. Otherwise, the error is not handled.

6.1.3.6.1 Runtime script errors in documents

Whenever an uncaught runtime script error occurs in one of the scripts associated with a Document, the user agent must report the error for the relevant script, with the problematic position (line number and column number) in the resource containing the script, using the global object specified by the script's settings object as the target. If the error is still not handled after this, then the error may be reported to the user.

6.1.3.6.2 The `ErrorEvent` interface

[Constructor(DOMString type, optional ErrorEventInit eventInitDict)]
interface ErrorEvent : Event {
  readonly attribute DOMString message;
  readonly attribute DOMString filename;
  readonly attribute unsigned long lineno;
  readonly attribute unsigned long colno;
  readonly attribute any error;
};

dictionary ErrorEventInit : EventInit {
  DOMString message;
  DOMString filename;
  unsigned long lineno;
  unsigned long colno;
  any error;
};

The message attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to the empty string. It represents the error message.

The filename attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to the empty string. It represents the absolute URL of the script in which the error originally occurred.

The lineno attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to zero. It represents the line number where the error occurred in the script.

The colno attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to zero. It represents the column number where the error occurred in the script.

The error attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to null. Where appropriate, it is set to the object representing the error (e.g. the exception object in the case of an uncaught DOM exception).

6.1.4 Event loops

6.1.4.1 Definitions

To coordinate events, user interaction, scripts, rendering, networking, and so forth, user agents must use event loops as described in this section.

There must be at least one event loop per user agent, and at most one event loop per unit of related similar-origin browsing contexts.

When there is more than one event loop for a unit of related browsing contexts, complications arise when a browsing context in that group is navigated such that it switches from one unit of related similar-origin browsing contexts to another. This specification does not currently describe how to handle these complications.

An event loop always has at least one browsing context. If an event loop's browsing contexts all go away, then the event loop goes away as well. A browsing context always has an event loop coordinating its activities.

An event loop has one or more task queues. A task queue is an ordered list of tasks, which are algorithms that are responsible for such work as:

Events

Asynchronously dispatching an Event object at a particular EventTarget object is often done by a dedicated task.

Not all events are dispatched using the task queue, many are dispatched synchronously during other tasks.

Parsing

The HTML parser tokenizing one or more bytes, and then processing any resulting tokens, is typically a task.

Callbacks

Calling a callback asynchronously is often done by a dedicated task.

Using a resource

When an algorithm fetches a resource, if the fetching occurs asynchronously then the processing of the resource once some or all of the resource is available is performed by a task.

Reacting to DOM manipulation

Some elements have tasks that trigger in response to DOM manipulation, e.g. when that element is inserted into the document.

Each task is associated with a Document; if the task was queued in the context of an element, then it is the element's Document; if the task was queued in the context of a browsing context, then it is the browsing context's active document at the time the task was queued; if the task was queued by or for a script then the document is the responsible document specified by the script's settings object.

A task is intended for a specific event loop: the event loop that is handling tasks for the task's associated Document.

When a user agent is to queue a task, it must add the given task to one of the task queues of the relevant event loop.

Each task is defined as coming from a specific task source. All the tasks from one particular task source and destined to a particular event loop (e.g. the callbacks generated by timers of a Document, the events fired for mouse movements over that Document, the tasks queued for the parser of that Document) must always be added to the same task queue, but tasks from different task sources may be placed in different task queues.

For example, a user agent could have one task queue for mouse and key events (the user interaction task source), and another for everything else. The user agent could then give keyboard and mouse events preference over other tasks three quarters of the time, keeping the interface responsive but not starving other task queues, and never processing events from any one task source out of order.

A user agent may have one storage mutex. This mutex is used to control access to shared state like cookies. At any one point, the storage mutex is either free, or owned by a particular event loop or instance of the fetching algorithm.

If a user agent does not implement a storage mutex, it is exempt from implementing the requirements that require it to acquire or release it.

User agent implementors have to make a choice between two evils. On the one hand, not implementing the storage mutex means that there is a risk of data corruption: a site could, for instance, try to read a cookie, increment its value, then write it back out, using the new value of the cookie as a unique identifier for the session; if the site does this twice in two different browser windows at the same time, it might end up using the same "unique" identifier for both sessions, with potentially disastrous effects. On the other hand, implementing the storage mutex has potentially serious performance implications: whenever a site uses Web Storage or cookies, all other sites that try to use Web Storage or cookies are blocked until the first site finishes.

Whenever a script calls into a plugin, and whenever a plugin calls into a script, the user agent must release the storage mutex.

6.1.4.2 Processing model

An event loop must continually run through the following steps for as long as it exists:

Run the oldest task on one of the event loop's task queues, if any, ignoring tasks whose associated Documents are not fully active. The user agent may pick any task queue.
If the storage mutex is now owned by the event loop, release it so that it is once again free.
If a task was run in the first step above, remove that task from its task queue.
If this event loop is not a worker's event loop, run these substeps:
1. Perform a microtask checkpoint.
2. Provide a stable state.
3. If necessary, update the rendering or user interface of any Document or browsing context to reflect the current state.
Otherwise, if this event loop is running for a WorkerGlobalScope, but there are no events in the event loop's task queues and the WorkerGlobalScope object's closing flag is true, then destroy the event loop, aborting these steps.
Return to the first step of the event loop.

When a user agent is to perform a microtask checkpoint, if the performing a microtask checkpoint flag is false, then the user agent must run the following steps:

Let the performing a microtask checkpoint flag be true.
Perform a custom elements checkpoint. [CUSTOM]
Sort the tables with pending sorts.
Invoke MutationObserver objects for the unit of related similar-origin browsing contexts to which the responsible browsing context specified by the script's settings object belongs, using the task wrapper algorithm as the steps to invoke each callback.

This will typically invoke scripted callbacks, which eventually calls the clean up after running a callback steps, which call this perform a microtask checkpoint algorithm again, which is why we use the performing a microtask checkpoint flag to avoid reentrancy.
Let the performing a microtask checkpoint flag be false.

When the user agent is to provide a stable state, if any asynchronously-running algorithms are awaiting a stable state, then the user agent must run their synchronous section and then resume running their asynchronous algorithm (if appropriate).

A synchronous section never mutates the DOM, runs any script, or has any side-effects detectable from another synchronous section, and thus synchronous sections can be run in any order, and cannot spin the event loop.

Steps in synchronous sections are marked with ⌛.

The task wrapper algorithm, which is implicitly invoked in the context of an event loop and is used to invoke a given callback in a specific way, is as follows:

Invoke callback as specified.

The above will change in due course; see bug 20821.

When an algorithm says to spin the event loop until a condition goal is met, the user agent must run the following steps:

Let task source be the task source of the currently running task.
Let old stack of script settings objects be a copy of the stack of script settings objects.
Empty the stack of script settings objects.
Run the global script clean-up jobs.
Perform a microtask checkpoint.
Stop the currently running task, allowing the event loop to resume, but continue these steps asynchronously.

This causes the event loop to move on to the second step of its processing model (defined above).
Wait until the condition goal is met.
Queue a task to continue running these steps, using the task source task source. Wait until this task runs before continuing these steps.
Replace the stack of script settings objects with the old stack of script settings objects.
Return to the caller.

Some of the algorithms in this specification, for historical reasons, require the user agent to pause while running a task until a condition goal is met. This means running the following steps:

If any asynchronously-running algorithms are awaiting a stable state, then run their synchronous section and then resume running their asynchronous algorithm. (See the event loop processing model definition above for details.)
If necessary, update the rendering or user interface of any Document or browsing context to reflect the current state.
Wait until the condition goal is met. While a user agent has a paused task, the corresponding event loop must not run further tasks, and any script in the currently running task must block. User agents should remain responsive to user input while paused, however, albeit in a reduced capacity since the event loop will not be doing anything.

When a user agent is to obtain the storage mutex as part of running a task, it must run through the following steps:

If the storage mutex is already owned by this task's event loop, then abort these steps.
Otherwise, pause until the storage mutex can be taken by the event loop.
Take ownership of the storage mutex.

6.1.4.3 Generic task sources

The following task sources are used by a number of mostly unrelated features in this and other specifications.

The DOM manipulation task source

This task source is used for features that react to DOM manipulations, such as things that happen asynchronously when an element is inserted into the document.

The user interaction task source

This task source is used for features that react to user interaction, for example keyboard or mouse input.

Asynchronous events sent in response to user input (e.g. click events) must be fired using tasks queued with the user interaction task source. [DOMEVENTS]

The networking task source

This task source is used for features that trigger in response to network activity.

The history traversal task source

This task source is used to queue calls to history.back() and similar APIs.

6.1.5 Events

6.1.5.1 Event handlers

Many objects can have event handlers specified. These act as non-capture event listeners for the object on which they are specified. [DOM]

An event handler has a name, which always starts with "on" and is followed by the name of the event for which it is intended.

An event handler can either have the value null, or be set to a callback object, or be set to an internal raw uncompiled handler. The EventHandler callback function type describes how this is exposed to scripts. Initially, event handlers must be set to null.

Event handlers are exposed in one of two ways.

The first way, common to all event handlers, is as an event handler IDL attribute.

The second way is as an event handler content attribute. Event handlers on HTML elements and some of the event handlers on Window objects are exposed in this way.

An event handler IDL attribute is an IDL attribute for a specific event handler. The name of the IDL attribute is the same as the name of the event handler.

Event handler IDL attributes, on setting, must set the corresponding event handler to their new value, and on getting, must return the result of getting the current value of the event handler in question (this can throw an exception, in which case the getting propagates it to the caller, it does not catch it).

If an event handler IDL attribute exposes an event handler of an object that doesn't exist, it must always return null on getting and must do nothing on setting.

This can happen in particular for event handler IDL attribute on body elements that do not have corresponding Window objects.

Certain event handler IDL attributes have additional requirements, in particular the onmessage attribute of MessagePort objects.

An event handler content attribute is a content attribute for a specific event handler. The name of the content attribute is the same as the name of the event handler.

Event handler content attributes, when specified, must contain valid JavaScript code which, when parsed, would match the FunctionBody production after automatic semicolon insertion. [ECMA262]

When an event handler content attribute is set, the user agent must set the corresponding event handler to an internal raw uncompiled handler consisting of the attribute's new value and the script location where the attribute was set to this value

When an event handler content attribute is removed, the user agent must set the corresponding event handler to null.

When an event handler H of an element or object T implementing the EventTarget interface is first set to a non-null value, the user agent must append an event listener to the list of event listeners associated with T with type set to the event handler event type corresponding to H, capture set to false, and listener set to the event handler processing algorithm defined below. [DOM]

The listener is emphatically not the event handler itself. Every event handler ends up registering the same listener, the algorithm defined below, which takes care of invoking the right callback, and processing the callback's return value.

This only happens the first time the event handler's value is set. Since listeners are called in the order they were registered, the order of event listeners for a particular event type will always be first the event listeners registered with addEventListener() before the first time the event handler was set to a non-null value, then the callback to which it is currently set, if any, and finally the event listeners registered with addEventListener() after the first time the event handler was set to a non-null value.

This example demonstrates the order in which event listeners are invoked. If the button in this example is clicked by the user, the page will show four alerts, with the text "ONE", "TWO", "THREE", and "FOUR" respectively.

<button id="test">Start Demo</button>
<script>
 var button = document.getElementById('test');
 button.addEventListener('click', function () { alert('ONE') }, false);
 button.setAttribute('onclick', "alert('NOT CALLED')"); // event handler listener is registered here
 button.addEventListener('click', function () { alert('THREE') }, false);
 button.onclick = function () { alert('TWO'); };
 button.addEventListener('click', function () { alert('FOUR') }, false);
</script>

The interfaces implemented by the event object do not influence whether an event handler is triggered or not.

The event handler processing algorithm for an event handler H and an Event object E is as follows:

Let callback be the result of getting the current value of the event handler H.
If callback is null, then abort these steps.
Process the Event object E as follows:

If E is an ErrorEvent object and the event handler IDL attribute's type is OnErrorEventHandler

Invoke callback with five arguments, the first one having the value of E's message attribute, the second having the value of E's filename attribute, the third having the value of E's lineno attribute, the fourth having the value of E's colno attribute, the fifth having the value of E's error attribute, and with the callback this value set to E's currentTarget. Let return value be the callback's return value. [WEBIDL]

Otherwise

Invoke callback with one argument, the value of which is the Event object E, with the callback this value set to E's currentTarget. Let return value be the callback's return value. [WEBIDL]

In this step, invoke means to run the jump to a code entry-point algorithm.
Process return value as follows:

If the event type is mouseover

If the event type is error and E is an ErrorEvent object

If return value is a WebIDL boolean true value, then cancel the event.

If the event type is beforeunload

The event handler IDL attribute's type is OnBeforeUnloadEventHandler, and the return value will therefore have been coerced into either the value null or a DOMString.

If the return value is null, then cancel the event.

Otherwise, If the Event object E is a BeforeUnloadEvent object, and the Event object E's returnValue attribute's value is the empty string, then set the returnValue attribute's value to return value.

Otherwise

If return value is a WebIDL boolean false value, then cancel the event.

The EventHandler callback function type represents a callback used for event handlers. It is represented in Web IDL as follows:

[TreatNonCallableAsNull]
callback EventHandlerNonNull = any (Event event);
typedef EventHandlerNonNull? EventHandler;

In JavaScript, any Function object implements this interface.

For example, the following document fragment:

<body onload="alert(this)" onclick="alert(this)">

...leads to an alert saying "[object Window]" when the document is loaded, and an alert saying "[object HTMLBodyElement]" whenever the user clicks something in the page.

The return value of the function affects whether the event is canceled or not: as described above, if the return value is false, the event is canceled (except for mouseover events, where the return value has to be true to cancel the event). With beforeunload events, the value is instead used to determine the message to show the user.

For historical reasons, the onerror handler has different arguments:

[TreatNonCallableAsNull]
callback OnErrorEventHandlerNonNull = any ((Event or DOMString) event, optional DOMString source, optional unsigned long lineno, optional unsigned long column, optional any error);
typedef OnErrorEventHandlerNonNull? OnErrorEventHandler;

Similarly, the onbeforeunload handler has a different return value:

[TreatNonCallableAsNull]
callback OnBeforeUnloadEventHandlerNonNull = DOMString? (Event event);
typedef OnBeforeUnloadEventHandlerNonNull? OnBeforeUnloadEventHandler;

An internal raw uncompiled handler is a tuple with the following information:

An uncompiled script body
A location where the script body originated, in case an error needs to be reported

When the user agent is to get the current value of the event handler H, it must run these steps:

If H's value is an internal raw uncompiled handler, run these substeps:
1. If H is an element's event handler, then let element be the element, and document be the element's Document.
  
  Otherwise, H is a Window object's event handler: let element be null, and let document be the Document most recently associated with that Window object.
2. If document is not in a browsing context, or if scripting is enabled for document's browsing context, then return null and abort the algorithm for getting the current value of the event handler.
3. Let body be the uncompiled script body in the internal raw uncompiled handler.
4. Let location be the location where the script body originated, as given by the internal raw uncompiled handler.
5. If element is not null and element has a form owner, let form owner be that form owner. Otherwise, let form owner be null.
6. Let script settings be the script settings object created for the Window object with which document is currently associated.
7. Obtain the script execution environment for JavaScript from script settings.
8. If body is not parsable as FunctionBody or if parsing detects an early error, then follow these substeps:
  1. Set H's value to null.
  2. Report the error for the appropriate script and with the appropriate position (line number and column number) given by location, using the global object specified by script settings as the target. If the error is still not handled after this, then the error may be reported to the user.
  3. Jump to the step labeled end below.
  FunctionBody is defined in ECMAScript edition 5 section 13 Function Definition. Early error is defined in ECMAScript edition 5 section 16 Errors. [ECMA262]
9. If body begins with a Directive Prologue that contains a Use Strict Directive then let strict be true, otherwise let strict be false.
  
  The terms "Directive Prologue" and "Use Strict Directive" are defined in ECMAScript edition 5 section 14.1 Directive Prologues and the Use Strict Directive. [ECMA262]
10. Using the script execution environment obtained above, create a function object (as defined in ECMAScript edition 5 section 13.2 Creating Function Objects), with:
  Parameter list FormalParameterList
  
  If H is an onerror event handler of a Window object
  
  Let the function have five arguments, named event, source, lineno, colno, and error.
  
  Otherwise
  
  Let the function have a single argument called event.
  
  Function body FunctionBody
  
  The result of parsing body above.
  
  Lexical Environment Scope
  1. Let Scope be the result of NewObjectEnvironment(document, the global environment).
  2. If form owner is not null, let Scope be the result of NewObjectEnvironment(>form owner, Scope).
  3. If element is not null, let Scope be the result of NewObjectEnvironment(element, Scope).
  NewObjectEnvironment() is defined in ECMAScript edition 5 section 10.2.2.3 NewObjectEnvironment (O, E). [ECMA262]
  Boolean flag Strict
  
  The value of strict.
  Let function be this new function.
11. Let script be a new script.
12. Let script's code entry-point be function.
13. Let script's settings object be script settings.
14. Set H to function.
End: Return H's value.

6.1.5.2 Event handlers on elements, `Document` objects, and `Window` objects

The following are the event handlers (and their corresponding event handler event types) that must be supported by all HTML elements, as both event handler content attributes and event handler IDL attributes; and that must be supported by all Document and Window objects, as event handler IDL attributes:

Event handler	Event handler event type
`onabort`	`abort`
`oncancel`	`cancel`
`oncanplay`	`canplay`
`oncanplaythrough`	`canplaythrough`
`onchange`	`change`
`onclick`	`click`
`oncuechange`	`cuechange`
`ondblclick`	`dblclick`
`ondurationchange`	`durationchange`
`onemptied`	`emptied`
`onended`	`ended`
`oninput`	`input`
`oninvalid`	`invalid`
`onkeydown`	`keydown`
`onkeypress`	`keypress`
`onkeyup`	`keyup`
`onloadeddata`	`loadeddata`
`onloadedmetadata`	`loadedmetadata`
`onloadstart`	`loadstart`
`onmousedown`	`mousedown`
`onmouseenter`	`mouseenter`
`onmouseleave`	`mouseleave`
`onmousemove`	`mousemove`
`onmouseout`	`mouseout`
`onmouseover`	`mouseover`
`onmouseup`	`mouseup`
`onmousewheel`	`mousewheel`
`onpause`	`pause`
`onplay`	`play`
`onplaying`	`playing`
`onprogress`	`progress`
`onratechange`	`ratechange`
`onreset`	`reset`
`onseeked`	`seeked`
`onseeking`	`seeking`
`onselect`	`select`
`onshow`	`show`
`onstalled`	`stalled`
`onsubmit`	`submit`
`onsuspend`	`suspend`
`ontimeupdate`	`timeupdate`
`ontoggle`	`toggle`
`onvolumechange`	`volumechange`
`onwaiting`	`waiting`

The following are the event handlers (and their corresponding event handler event types) that must be supported by all HTML elements other than body and frameset elements, as both event handler content attributes and event handler IDL attributes; that must be supported by all Document objects, as event handler IDL attributes; and that must be supported by all Window objects, as event handler IDL attributes on the Window objects themselves, and with corresponding event handler content attributes and event handler IDL attributes exposed on all body and frameset elements that are owned by that Window object's Documents:

Event handler	Event handler event type
`onblur`	`blur`
`onerror`	`error`
`onfocus`	`focus`
`onload`	`load`
`onresize`	`resize`
`onscroll`	`scroll`

The following are the event handlers (and their corresponding event handler event types) that must be supported by Window objects, as event handler IDL attributes on the Window objects themselves, and with corresponding event handler content attributes and event handler IDL attributes exposed on all body and frameset elements that are owned by that Window object's Documents:

Event handler	Event handler event type
`onafterprint`	`afterprint`
`onbeforeprint`	`beforeprint`
`onbeforeunload`	`beforeunload`
`onhashchange`	`hashchange`
`onmessage`	`message`
`onoffline`	`offline`
`ononline`	`online`
`onpagehide`	`pagehide`
`onpageshow`	`pageshow`
`onpopstate`	`popstate`
`onstorage`	`storage`
`onunload`	`unload`

The following are the event handlers (and their corresponding event handler event types) that must be supported on Document objects as event handler IDL attributes:

Event handler	Event handler event type
`onreadystatechange`	`readystatechange`

6.1.5.2.1 IDL definitions

[NoInterfaceObject]
interface GlobalEventHandlers {
           attribute EventHandler onabort;
           attribute EventHandler onblur;
           attribute EventHandler oncancel;
           attribute EventHandler oncanplay;
           attribute EventHandler oncanplaythrough;
           attribute EventHandler onchange;
           attribute EventHandler onclick;
           attribute EventHandler oncuechange;
           attribute EventHandler ondblclick;
           attribute EventHandler ondurationchange;
           attribute EventHandler onemptied;
           attribute EventHandler onended;
           attribute OnErrorEventHandler onerror;
           attribute EventHandler onfocus;
           attribute EventHandler oninput;
           attribute EventHandler oninvalid;
           attribute EventHandler onkeydown;
           attribute EventHandler onkeypress;
           attribute EventHandler onkeyup;
           attribute EventHandler onload;
           attribute EventHandler onloadeddata;
           attribute EventHandler onloadedmetadata;
           attribute EventHandler onloadstart;
           attribute EventHandler onmousedown;
  [LenientThis] attribute EventHandler onmouseenter;
  [LenientThis] attribute EventHandler onmouseleave;
           attribute EventHandler onmousemove;
           attribute EventHandler onmouseout;
           attribute EventHandler onmouseover;
           attribute EventHandler onmouseup;
           attribute EventHandler onmousewheel;
           attribute EventHandler onpause;
           attribute EventHandler onplay;
           attribute EventHandler onplaying;
           attribute EventHandler onprogress;
           attribute EventHandler onratechange;
           attribute EventHandler onreset;
           attribute EventHandler onresize;
           attribute EventHandler onscroll;
           attribute EventHandler onseeked;
           attribute EventHandler onseeking;
           attribute EventHandler onselect;
           attribute EventHandler onshow;
           attribute EventHandler onstalled;
           attribute EventHandler onsubmit;
           attribute EventHandler onsuspend;
           attribute EventHandler ontimeupdate;
           attribute EventHandler ontoggle;
           attribute EventHandler onvolumechange;
           attribute EventHandler onwaiting;
};

[NoInterfaceObject]
interface WindowEventHandlers {
           attribute EventHandler onafterprint;
           attribute EventHandler onbeforeprint;
           attribute OnBeforeUnloadEventHandler onbeforeunload;
           attribute EventHandler onhashchange;
           attribute EventHandler onmessage;
           attribute EventHandler onoffline;
           attribute EventHandler ononline;
           attribute EventHandler onpagehide;
           attribute EventHandler onpageshow;
           attribute EventHandler onpopstate;
           attribute EventHandler onstorage;
           attribute EventHandler onunload;
};

6.1.5.3 Event firing

Certain operations and methods are defined as firing events on elements. For example, the click() method on the HTMLElement interface is defined as firing a click event on the element. [DOMEVENTS]

Firing a simple event named e means that a trusted event with the name e, which does not bubble (except where otherwise stated) and is not cancelable (except where otherwise stated), and which uses the Event interface, must be created and dispatched at the given target.

Firing a synthetic mouse event named e means that an event with the name e, which is trusted (except where otherwise stated), does not bubble (except where otherwise stated), is not cancelable (except where otherwise stated), and which uses the MouseEvent interface, must be created and dispatched at the given target. The event object must have its screenX, screenY, clientX, clientY, and button attributes initialized to 0, its ctrlKey, shiftKey, altKey, and metaKey attributes initialized according to the current state of the key input device, if any (false for any keys that are not available), its detail attribute initialized to 1, and its relatedTarget attribute initialized to null (except where otherwise stated). The getModifierState() method on the object must return values appropriately describing the state of the key input device at the time the event is created.

Firing a click event means firing a synthetic mouse event named click, which bubbles and is cancelable.

The default action of these events is to do nothing except where otherwise stated.

6.1.5.4 Events and the `Window` object

When an event is dispatched at a DOM node in a Document in a browsing context, if the event is not a load event, the user agent must act as if, for the purposes of event dispatching, the Window object is the parent of the Document object. [DOM]

6.2 Base64 utility methods

The atob() and btoa() methods allow authors to transform content to and from the base64 encoding.

[NoInterfaceObject]
interface WindowBase64 {
  DOMString btoa(DOMString btoa);
  DOMString atob(DOMString atob);
};
Window implements WindowBase64;

In these APIs, for mnemonic purposes, the "b" can be considered to stand for "binary", and the "a" for "ASCII". In practice, though, for primarily historical reasons, both the input and output of these functions are Unicode strings.

result = window . btoa( data )

Takes the input data, in the form of a Unicode string containing only characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, and converts it to its base64 representation, which it returns.

Throws an InvalidCharacterError exception if the input string contains any out-of-range characters.

result = window . atob( data )

Takes the input data, in the form of a Unicode string containing base64-encoded binary data, decodes it, and returns a string consisting of characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, corresponding to that binary data.

Throws an InvalidCharacterError exception if the input string is not valid base64 data.

The WindowBase64 interface adds to the Window interface and the WorkerGlobalScope interface (part of Web workers).

The btoa() method must throw an InvalidCharacterError exception if the method's first argument contains any character whose code point is greater than U+00FF. Otherwise, the user agent must convert that argument to a sequence of octets whose nth octet is the eight-bit representation of the code point of the nth character of the argument, and then must apply the base64 algorithm to that sequence of octets, and return the result. [RFC4648]

The atob() method must run the following steps to parse the string passed in the method's first argument:

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Remove all space characters from input.
If the length of input divides by 4 leaving no remainder, then: if input ends with one or two "=" (U+003D) characters, remove them from input.
If the length of input divides by 4 leaving a remainder of 1, throw an InvalidCharacterError exception and abort these steps.
If input contains a character that is not in the following list of characters and character ranges, throw an InvalidCharacterError exception and abort these steps:
- "+" (U+002B)
- "/" (U+002F)
- Alphanumeric ASCII characters
Let output be a string, initially empty.
Let buffer be a buffer that can have bits appended to it, initially empty.

While position does not point past the end of input, run these substeps:

Find the character pointed to by position in the first column of the following table. Let n be the number given in the second cell of the same row.

Character	Number
A	0
B	1
C	2
D	3
E	4
F	5
G	6
H	7
I	8
J	9
K	10
L	11
M	12
N	13
O	14
P	15
Q	16
R	17
S	18
T	19
U	20
V	21
W	22
X	23
Y	24
Z	25
a	26
b	27
c	28
d	29
e	30
f	31
g	32
h	33
i	34
j	35
k	36
l	37
m	38
n	39
o	40
p	41
q	42
r	43
s	44
t	45
u	46
v	47
w	48
x	49
y	50
z	51
0	52
1	53
2	54
3	55
4	56
5	57
6	58
7	59
8	60
9	61
+	62
/	63

Append to buffer the six bits corresponding to number, most significant bit first.
If buffer has accumulated 24 bits, interpret them as three 8-bit big-endian numbers. Append the three characters with code points equal to those numbers to output, in the same order, and then empty buffer.
Advance position by one character.

If buffer is not empty, it contains either 12 or 18 bits. If it contains 12 bits, discard the last four and interpret the remaining eight as an 8-bit big-endian number. If it contains 18 bits, discard the last two and interpret the remaining 16 as two 8-bit big-endian numbers. Append the one or two characters with code points equal to those one or two numbers to output, in the same order.

The discarded bits mean that, for instance, atob("YQ") and atob("YR") both return "a".
Return output.

6.3 Dynamic markup insertion

APIs for dynamically inserting markup into the document interact with the parser, and thus their behavior varies depending on whether they are used with HTML documents (and the HTML parser) or XHTML in XML documents (and the XML parser).

6.3.1 Opening the input stream

The open() method comes in several variants with different numbers of arguments.

document = document . open( [ type [, replace ] ] )

Causes the Document to be replaced in-place, as if it was a new Document object, but reusing the previous object, which is then returned.

If the type argument is omitted or has the value "text/html", then the resulting Document has an HTML parser associated with it, which can be given data to parse using document.write(). Otherwise, all content passed to document.write() will be parsed as plain text.

If the replace argument is present and has the value "replace", the existing entries in the session history for the Document object are removed.

The method has no effect if the Document is still being parsed.

Throws an InvalidStateError exception if the Document is an XML document.

window = document . open( url, name, features [, replace ] )

Works like the window.open() method.

Document objects have an ignore-opens-during-unload counter, which is used to prevent scripts from invoking the document.open() method (directly or indirectly) while the document is being unloaded. Initially, the counter must be set to zero.

When called with two arguments, the document.open() method must act as follows:

If the Document object is not flagged as an HTML document, throw an InvalidStateError exception and abort these steps.
If the Document object is not an active document, then abort these steps.
Let type be the value of the first argument.
If the second argument is an ASCII case-insensitive match for the value "replace", then let replace be true.

Otherwise, if the browsing context's session history contains only one Document, and that was the about:blank Document created when the browsing context was created, and that Document has never had the unload a document algorithm invoked on it (e.g. by a previous call to document.open()), then let replace be true.

Otherwise, let replace be false.
If the Document has an active parser whose script nesting level is greater than zero, then the method does nothing. Abort these steps and return the Document object on which the method was invoked.

This basically causes document.open() to be ignored when it's called in an inline script found during parsing, while still letting it have an effect when called asynchronously.
Similarly, if the Document's ignore-opens-during-unload counter is greater than zero, then the method does nothing. Abort these steps and return the Document object on which the method was invoked.

This basically causes document.open() to be ignored when it's called from a beforeunload pagehide, or unload event handler while the Document is being unloaded.
Release the storage mutex.
Set the Document's salvageable state to false.
Prompt to unload the Document object. If the user refused to allow the document to be unloaded, then abort these steps and return the Document object on which the method was invoked.
Unload the Document object, with the recycle parameter set to true.
Abort the Document.
Unregister all event listeners registered on the Document node and its descendants.
Remove any tasks associated with the Document in any task source.
Remove all child nodes of the document, without firing any mutation events.
Replace the Document's singleton objects with new instances of those objects. (This includes in particular the Window, Location, History, ApplicationCache, and Navigator, objects, the various BarProp objects, the two Storage objects, the various HTMLCollection objects, and objects defined by other specifications, like Selection and the document's UndoManager. It also includes all the Web IDL prototypes in the JavaScript binding, including the Document object's prototype.)

The new Window object has a new script settings object.
Change the document's character encoding to UTF-8.
If the Document is ready for post-load tasks, then set the Document object's reload override flag and set the Document's reload override buffer to the empty string.
Set the Document's salvageable state back to true.
Change the document's address to the address of the responsible document specified by the entry settings object.
If the Document's iframe load in progress flag is set, set the Document's mute iframe load flag.
Create a new HTML parser and associate it with the document. This is a script-created parser (meaning that it can be closed by the document.open() and document.close() methods, and that the tokenizer will wait for an explicit call to document.close() before emitting an end-of-file token). The encoding confidence is irrelevant.
Set the current document readiness of the document to "loading".
If type is an ASCII case-insensitive match for the string "replace", then, for historical reasons, set it to the string "text/html".

Otherwise:

If the type string contains a ";" (U+003B) character, remove the first such character and all characters from it up to the end of the string.

Strip leading and trailing whitespace from type.
If type is not now an ASCII case-insensitive match for the string "text/html", then act as if the tokenizer had emitted a start tag token with the tag name "pre" followed by a single "LF" (U+000A) character, then switch the HTML parser's tokenizer to the PLAINTEXT state.
Remove all the entries in the browsing context's session history after the current entry. If the current entry is the last entry in the session history, then no entries are removed.

This doesn't necessarily have to affect the user agent's user interface.
Remove any tasks queued by the history traversal task source that are associated with any Document objects in the top-level browsing context's document family.
Remove any earlier entries that share the same Document.
If replace is false, then add a new entry, just before the last entry, and associate with the new entry the text that was parsed by the previous parser associated with the Document object, as well as the state of the document at the start of these steps. This allows the user to step backwards in the session history to see the page before it was blown away by the document.open() call. This new entry does not have a Document object, so a new one will be created if the session history is traversed to that entry.
Finally, set the insertion point to point at just before the end of the input stream (which at this point will be empty).
Return the Document on which the method was invoked.

The document.open() method does not affect whether a Document is ready for post-load tasks or completely loaded.

When called with four arguments, the open() method on the Document object must call the open() method on the Window object of the Document object, with the same arguments as the original call to the open() method, and return whatever that method returned. If the Document object has no Window object, then the method must throw an InvalidAccessError exception.

6.3.2 Closing the input stream

document . close()

Closes the input stream that was opened by the document.open() method.

Throws an InvalidStateError exception if the Document is an XML document.

The close() method must run the following steps:

If the Document object is not flagged as an HTML document, throw an InvalidStateError exception and abort these steps.
If there is no script-created parser associated with the document, then abort these steps.
Insert an explicit "EOF" character at the end of the parser's input stream.
If there is a pending parsing-blocking script, then abort these steps.
Run the tokenizer, processing resulting tokens as they are emitted, and stopping when the tokenizer reaches the explicit "EOF" character or spins the event loop.

6.3.3 `document.write()`

document . write(text...)

In general, adds the given string(s) to the Document's input stream.

This method has very idiosyncratic behavior. In some cases, this method can affect the state of the HTML parser while the parser is running, resulting in a DOM that does not correspond to the source of the document (e.g. if the string written is the string "<plaintext>" or "<!--"). In other cases, the call can clear the current page first, as if document.open() had been called. In yet more cases, the method is simply ignored, or throws an exception. To make matters worse, the exact behavior of this method can in some cases be dependent on network latency, which can lead to failures that are very hard to debug. For all these reasons, use of this method is strongly discouraged.

This method throws an InvalidStateError exception when invoked on XML documents.

Document objects have an ignore-destructive-writes counter, which is used in conjunction with the processing of script elements to prevent external scripts from being able to use document.write() to blow away the document by implicitly calling document.open(). Initially, the counter must be set to zero.

The document.write(...) method must act as follows:

If the method was invoked on an XML document, throw an InvalidStateError exception and abort these steps.
If the Document object is not an active document, then abort these steps.
If the insertion point is undefined and either the Document's ignore-opens-during-unload counter is greater than zero or the Document's ignore-destructive-writes counter is greater than zero, abort these steps.
If the insertion point is undefined, call the open() method on the document object (with no arguments). If the user refused to allow the document to be unloaded, then abort these steps. Otherwise, the insertion point will point at just before the end of the (empty) input stream.
Insert the string consisting of the concatenation of all the arguments to the method into the input stream just before the insertion point.
If the Document object's reload override flag is set, then append the string consisting of the concatenation of all the arguments to the method to the Document's reload override buffer.
If there is no pending parsing-blocking script, have the HTML parser process the characters that were inserted, one at a time, processing resulting tokens as they are emitted, and stopping when the tokenizer reaches the insertion point or when the processing of the tokenizer is aborted by the tree construction stage (this can happen if a script end tag token is emitted by the tokenizer).
If the document.write() method was called from script executing inline (i.e. executing because the parser parsed a set of script tags), then this is a reentrant invocation of the parser.
Finally, return from the method.

6.3.4 `document.writeln()`

document . writeln(text...)

Adds the given string(s) to the Document's input stream, followed by a newline character. If necessary, calls the open() method implicitly first.

This method throws an InvalidStateError exception when invoked on XML documents.

The document.writeln(...) method, when invoked, must act as if the document.write() method had been invoked with the same argument(s), plus an extra argument consisting of a string containing a single line feed character (U+000A).

6.4 Timers

The setTimeout() and setInterval() methods allow authors to schedule timer-based callbacks.

[NoInterfaceObject]
interface WindowTimers {
  long setTimeout(Function handler, optional long timeout, any... arguments);
  long setTimeout(DOMString handler, optional long timeout, any... arguments);
  void clearTimeout(long handle);
  long setInterval(Function handler, optional long timeout, any... arguments);
  long setInterval(DOMString handler, optional long timeout, any... arguments);
  void clearInterval(long handle);
};
Window implements WindowTimers;

handle = window . setTimeout( handler [, timeout [, arguments... ] ] ): Schedules a timeout to run handler after timeout milliseconds. Any arguments are passed straight through to the handler.
handle = window . setTimeout( code [, timeout ] ): Schedules a timeout to compile and run code after timeout milliseconds.
window . clearTimeout( handle ): Cancels the timeout set with setTimeout() identified by handle.
handle = window . setInterval( handler [, timeout [, arguments... ] ] ): Schedules a timeout to run handler every timeout milliseconds. Any arguments are passed straight through to the handler.
handle = window . setInterval( code [, timeout ] ): Schedules a timeout to compile and run code every timeout milliseconds.
window . clearInterval( handle ): Cancels the timeout set with setInterval() identified by handle.

Timers can be nested; after five such nested timers, however, the interval is forced to be at least four milliseconds.

This API does not guarantee that timers will run exactly on schedule. Delays due to CPU load, other tasks, etc, are to be expected.

The WindowTimers interface adds to the Window interface and the WorkerGlobalScope interface (part of Web workers).

Each object that implements the WindowTimers interface has a list of active timers. Each entry in this lists is identified by a number, which must be unique within the list for the lifetime of the object that implements the WindowTimers interface.

The setTimeout() method must return the value returned by the timer initialization steps, passing them the method's arguments, the object on which the method for which the algorithm is running is implemented (a Window or WorkerGlobalScope object) as the method context, and the repeat flag set to false.

The setInterval() method must return the value returned by the timer initialization steps, passing them the method's arguments, the object on which the method for which the algorithm is running is implemented (a Window or WorkerGlobalScope object) as the method context, and the repeat flag set to true.

The clearTimeout() and clearInterval() methods must clear the entry identified as handle from the list of active timers of the WindowTimers object on which the method was invoked, where handle is the argument passed to the method, if any. (If handle does not identify an entry in the list of active timers of the WindowTimers object on which the method was invoked, the method does nothing.)

The timer initialization steps, which are invoked with some method arguments, a method context, a repeat flag which can be true or false, and optionally (and only if the repeat flag is true) a previous handle, are as follows:

Let method context proxy be method context if that is a WorkerGlobalScope object, or else the WindowProxy that corresponds to method context.
If previous handle was provided, let handle be previous handle; otherwise, let handle be a user-agent-defined integer that is greater than zero that will identify the timeout to be set by this call in the list of active timers.
If previous handle was not provided, add an entry to the list of active timers for handle.
Let task be a task that runs the following substeps:
1. If the entry for handle in the list of active timers has been cleared, then abort this task's substeps.
2. Run the appropriate set of steps from the following list:
  If the first method argument is a Function
  
  Call the Function. Use the third and subsequent method arguments (if any) as the arguments for invoking the Function. Use method context proxy as the thisArg for invoking the Function. [ECMA262]
  
  Otherwise
  1. Let script source be the first method argument.
  2. Let script language be JavaScript.
  3. Let settings object be method context's script settings object.
  4. Create a script using script source as the script source, the URL where script source can be found, scripting language as the scripting language, and settings object as the script settings object.
3. If the repeat flag is true, then call timer initialization steps again, passing them the same method arguments, the same method context, with the repeat flag still set to true, and with the previous handle set to handler.
Let timeout be the second method argument, or zero if the argument was omitted.
If the currently running task is a task that was created by this algorithm, then let nesting level be the task's timer nesting level. Otherwise, let nesting level be zero.
If nesting level is greater than 5, and timeout is less than 4, then increase timeout to 4.
Increment nesting level by one.
Let task's timer nesting level be nesting level.
Return handle, and then continue running this algorithm asynchronously.
If method context is a Window object, wait until the Document associated with method context has been fully active for a further timeout milliseconds (not necessarily consecutively).

Otherwise, method context is a WorkerGlobalScope object; wait until timeout milliseconds have passed with the worker not suspended (not necessarily consecutively).
Wait until any invocations of this algorithm that had the same method context, that started before this one, and whose timeout is equal to or less than this one's, have completed.

Argument conversion as defined by Web IDL (for example, invoking toString() methods on objects passed as the first argument) happens in the algorithms defined in Web IDL, before this algorithm is invoked.
So for example, the following rather silly code will result in the log containing "ONE TWO ":
```
var log = '';
function logger(s) { log += s + ' '; }

setTimeout({ toString: function () {
  setTimeout("logger('ONE')", 100);
  return "logger('TWO')";
} }, 100);
```
Optionally, wait a further user-agent defined length of time.

This is intended to allow user agents to pad timeouts as needed to optimise the power usage of the device. For example, some processors have a low-power mode where the granularity of timers is reduced; on such platforms, user agents can slow timers down to fit this schedule instead of requiring the processor to use the more accurate mode with its associated higher power usage.
Queue the task task.

Once the task has been processed, if the repeat flag is false, it is safe to remove the entry for handle from the list of active timers (there is no way for the entry's existence to be detected past this point, so it does not technically matter one way or the other).

The task source for these tasks is the timer task source.

To run tasks of several milliseconds back to back without any delay, while still yielding back to the browser to avoid starving the user interface (and to avoid the browser killing the script for hogging the CPU), simply queue the next timer before performing work:

function doExpensiveWork() {
  var done = false;
  // ...
  // this part of the function takes up to five milliseconds
  // set done to true if we're done
  // ...
  return done;
}

function rescheduleWork() {
  var handle = setTimeout(rescheduleWork, 0); // preschedule next iteration
  if (doExpensiveWork())
    clearTimeout(handle); // clear the timeout if we don't need it
}

function scheduleWork() {
  setTimeout(rescheduleWork, 0);
}

scheduleWork(); // queues a task to do lots of work

6.5 User prompts

6.5.1 Simple dialogs

window . alert(message)

Displays a modal alert with the given message, and waits for the user to dismiss it.

A call to the navigator.yieldForStorageUpdates() method is implied when this method is invoked.

result = window . confirm(message)

Displays a modal OK/Cancel prompt with the given message, waits for the user to dismiss it, and returns true if the user clicks OK and false if the user clicks Cancel.

A call to the navigator.yieldForStorageUpdates() method is implied when this method is invoked.

result = window . prompt(message [, default] )

Displays a modal text field prompt with the given message, waits for the user to dismiss it, and returns the value that the user entered. If the user cancels the prompt, then returns null instead. If the second argument is present, then the given value is used as a default.

A call to the navigator.yieldForStorageUpdates() method is implied when this method is invoked.

The alert(message) method, when invoked, must run the following steps:

If the event loop's termination nesting level is non-zero, optionally abort these steps.
Release the storage mutex.
Optionally, abort these steps. (For example, the user agent might give the user the option to ignore all alerts, and would thus abort at this step whenever the method was invoked.)
Show the given message to the user.
Optionally, pause while waiting for the user to acknowledge the message.

The confirm(message) method, when invoked, must run the following steps:

If the event loop's termination nesting level is non-zero, optionally abort these steps, returning false.
Release the storage mutex.
Optionally, return false and abort these steps. (For example, the user agent might give the user the option to ignore all prompts, and would thus abort at this step whenever the method was invoked.)
Show the given message to the user, and ask the user to respond with a positive or negative response.
Pause until the user responds either positively or negatively.
If the user responded positively, return true; otherwise, the user responded negatively: return false.

The prompt(message, default) method, when invoked, must run the following steps:

If the event loop's termination nesting level is non-zero, optionally abort these steps, returning null.
Release the storage mutex.
Optionally, return null and abort these steps. (For example, the user agent might give the user the option to ignore all prompts, and would thus abort at this step whenever the method was invoked.)
Show the given message to the user, and ask the user to either respond with a string value or abort. The response must be defaulted to the value given by default.
Pause while waiting for the user's response.
If the user aborts, then return null; otherwise, return the string that the user responded with.

6.5.2 Printing

window . print()

Prompts the user to print the page.

A call to the navigator.yieldForStorageUpdates() method is implied when this method is invoked.

When the print() method is invoked, if the Document is ready for post-load tasks, then the user agent must synchronously run the printing steps. Otherwise, the user agent must only set the print when loaded flag on the Document.

User agents should also run the printing steps whenever the user asks for the opportunity to obtain a physical form (e.g. printed copy), or the representation of a physical form (e.g. PDF copy), of a document.

The printing steps are as follows:

The user agent may display a message to the user or abort these steps (or both).

For instance, a kiosk browser could silently ignore any invocations of the print() method.

For instance, a browser on a mobile device could detect that there are no printers in the vicinity and display a message saying so before continuing to offer a "save to PDF" option.
The user agent must fire a simple event named beforeprint at the Window object of the Document that is being printed, as well as any nested browsing contexts in it.

The beforeprint event can be used to annotate the printed copy, for instance adding the time at which the document was printed.
The user agent must release the storage mutex.
The user agent should offer the user the opportunity to obtain a physical form (or the representation of a physical form) of the document. The user agent may wait for the user to either accept or decline before returning; if so, the user agent must pause while the method is waiting. Even if the user agent doesn't wait at this point, the user agent must use the state of the relevant documents as they are at this point in the algorithm if and when it eventually creates the alternate form.
The user agent must fire a simple event named afterprint at the Window object of the Document that is being printed, as well as any nested browsing contexts in it.

The afterprint event can be used to revert annotations added in the earlier event, as well as showing post-printing UI. For instance, if a page is walking the user through the steps of applying for a home loan, the script could automatically advance to the next step after having printed a form or other.

6.6 System state and capabilities

6.6.1 The `Navigator` object

The navigator attribute of the Window interface must return an instance of the Navigator interface, which represents the identity and state of the user agent (the client), and allows Web pages to register themselves as potential protocol and content handlers:

interface Navigator {
  // objects implementing this interface also implement the interfaces given below
};
Navigator implements NavigatorID;
Navigator implements NavigatorLanguage;
Navigator implements NavigatorOnLine;
Navigator implements NavigatorContentUtils;
Navigator implements NavigatorStorageUtils;
Navigator implements NavigatorPlugins;

These interfaces are defined separately so that other specifications can re-use parts of the Navigator interface.

6.6.1.1 Client identification

[NoInterfaceObject]
interface NavigatorID {
  readonly attribute DOMString appCodeName; // constant "Mozilla"
  readonly attribute DOMString appName;
  readonly attribute DOMString appVersion;
  readonly attribute DOMString platform;
  readonly attribute DOMString product; // constant "Gecko"
  boolean taintEnabled(); // constant false
  readonly attribute DOMString userAgent;
};

In certain cases, despite the best efforts of the entire industry, Web browsers have bugs and limitations that Web authors are forced to work around.

This section defines a collection of attributes that can be used to determine, from script, the kind of user agent in use, in order to work around these issues.

Client detection should always be limited to detecting known current versions; future versions and unknown versions should always be assumed to be fully compliant.

window . navigator . appCodeName: Returns the string "Mozilla".
window . navigator . appName: Returns the name of the browser.
window . navigator . appVersion: Returns the version of the browser.
window . navigator . platform: Returns the name of the platform.
window . navigator . product: Returns the string "Gecko".
window . navigator . taintEnabled(): Returns false.
window . navigator . userAgent: Returns the complete User-Agent header.

appCodeName: Must return the string "Mozilla".
appName: Must return either the string "Netscape" or the full name of the browser, e.g. "Mellblom Browsernator".
appVersion: Must return either the string "4.0" or a string representing the version of the browser in detail, e.g. "1.0 (VMS; en-US) Mellblomenator/9000".
platform: Must return either the empty string or a string representing the platform on which the browser is executing, e.g. "MacIntel", "Win32", "FreeBSD i386", "WebTV OS".
product: Must return the string "Gecko".
taintEnabled(): Must return false.
userAgent: Must return the string used for the value of the "User-Agent" header in HTTP requests, or the empty string if no such header is ever sent.

Any information in this API that varies from user to user can be used to profile the user. In fact, if enough such information is available, a user can actually be uniquely identified. For this reason, user agent implementors are strongly urged to include as little information in this API as possible.

6.6.1.2 Language preferences

[NoInterfaceObject]
interface NavigatorLanguage {
  readonly attribute DOMString? language;
};

window . navigator . language: Returns a language tag representing the user's preferred language.

language: Must return a valid BCP 47 language tag representing either a plausible language or the user's preferred language. [BCP47]

To determine a plausible language, the user agent should bear in mind the following:

Any information in this API that varies from user to user can be used to profile or identify the user.
If the user is not using a service that obfuscates the user's point of origin (e.g. the Tor anonymity network), then the value that is least likely to distinguish the user from other users with similar origins (e.g. from the same IP address block) is the language used by the majority of such users. [TOR]
If the user is using an anonymizing service, then the value "en-US" is suggested; if all users of the service use that same value, that reduces the possibility of distinguishing the users from each other.

6.6.1.3 Custom scheme and content handlers

[NoInterfaceObject]
interface NavigatorContentUtils {
  // content handler registration
  void registerProtocolHandler(DOMString scheme, DOMString url, DOMString title);
  void registerContentHandler(DOMString mimeType, DOMString url, DOMString title);
  void unregisterProtocolHandler(DOMString scheme, DOMString url);
  void unregisterContentHandler(DOMString mimeType, DOMString url);
};

The registerProtocolHandler() method allows Web sites to register themselves as possible handlers for particular schemes. For example, an online telephone messaging service could register itself as a handler of the sms: scheme, so that if the user clicks on such a link, he is given the opportunity to use that Web site. Analogously, the registerContentHandler() method allows Web sites to register themselves as possible handlers for content in a particular MIME type. For example, the same online telephone messaging service could register itself as a handler for text/vcard files, so that if the user has no native application capable of handling vCards, his Web browser can instead suggest he use that site to view contact information stored on vCards that he opens. [RFC5724] [RFC6350]

window . navigator . registerProtocolHandler(scheme, url, title)

window . navigator . registerContentHandler(mimeType, url, title)

Registers a handler for the given scheme or content type, at the given URL, with the given title.

The string "%s" in the URL is used as a placeholder for where to put the URL of the content to be handled.

Throws a SecurityError exception if the user agent blocks the registration (this might happen if trying to register as a handler for "http", for instance).

Throws a SyntaxError exception if the "%s" string is missing in the URL.

User agents may, within the constraints described in this section, do whatever they like when the methods are called. A UA could, for instance, prompt the user and offer the user the opportunity to add the site to a shortlist of handlers, or make the handlers his default, or cancel the request. UAs could provide such a UI through modal UI or through a non-modal transient notification interface. UAs could also simply silently collect the information, providing it only when relevant to the user.

User agents should keep track of which sites have registered handlers (even if the user has declined such registrations) so that the user is not repeatedly prompted with the same request.

The arguments to the methods have the following meanings and corresponding implementation requirements. The requirements that involve throwing exceptions must be processed in the order given below, stopping at the first exception thrown. (So the exceptions for the first argument take precedence over the exceptions for the second argument.)

scheme (registerProtocolHandler() only)

A scheme, such as mailto or web+auth. The scheme must be compared in an ASCII case-insensitive manner by user agents for the purposes of comparing with the scheme part of URLs that they consider against the list of registered handlers.

The scheme value, if it contains a colon (as in "mailto:"), will never match anything, since schemes don't contain colons.

If the registerProtocolHandler() method is invoked with a scheme that is neither a whitelisted scheme nor a scheme whose value starts with the substring "web+" and otherwise contains only lowercase ASCII letters, and whose length is at least five characters (including the "web+" prefix), the user agent must throw a SecurityError exception.

The following schemes are the whitelisted schemes:

bitcoin
geo
im
irc
ircs
magnet
mailto
mms
news
nntp
sip
sms
smsto
ssh
tel
urn
webcal
wtai
xmpp

This list can be changed. If there are schemes that should be added, please send feedback.

This list excludes any schemes that could reasonably be expected to be supported inline, e.g. in an iframe, such as http or (more theoretically) gopher. If those were supported, they could potentially be used in man-in-the-middle attacks, by replacing pages that have frames with such content with content under the control of the protocol handler. If the user agent has native support for the schemes, this could further be used for cookie-theft attacks.

mimeType (registerContentHandler() only)

A MIME type, such as model/vnd.flatland.3dml or application/vnd.google-earth.kml+xml. The MIME type must be compared in an ASCII case-insensitive manner by user agents for the purposes of comparing with MIME types of documents that they consider against the list of registered handlers.

User agents must compare the given values only to the MIME type/subtype parts of content types, not to the complete type including parameters. Thus, if mimeType values passed to this method include characters such as commas or whitespace, or include MIME parameters, then the handler being registered will never be used.

The type is compared to the MIME type used by the user agent after the sniffing algorithms have been applied.

If the registerContentHandler() method is invoked with a MIME type that is in the type blacklist or that the user agent has deemed a privileged type, the user agent must throw a SecurityError exception.

The following MIME types are in the type blacklist:

application/x-www-form-urlencoded
application/xhtml+xml
application/xml
image/gif
image/jpeg
image/png
image/svg+xml
multipart/x-mixed-replace
text/cache-manifest
text/css
text/html
text/ping
text/plain
text/xml
All types that the user agent supports displaying natively in a browsing context during navigation, except for application/rss+xml and application/atom+xml

This list can be changed. If there are MIME types that should be added, please send feedback.

url

A string used to build the URL of the page that will handle the requests.

User agents must throw a SyntaxError exception if the url argument passed to one of these methods does not contain the exact literal string "%s".

User agents must throw a SyntaxError exception if resolving the url argument relative to the API base URL specified by the entry settings object is not successful.

The resulting absolute URL would by definition not be a valid URL as it would include the string "%s" which is not a valid component in a URL.

User agents must throw a SecurityError exception if the resulting absolute URL has an origin that differs from the origin specified by the entry settings object.

This is forcibly the case if the %s placeholder is in the scheme, host, or port parts of the URL.

The resulting absolute URL is the proto-URL. It identifies the handler for the purposes of the methods described below.

When the user agent uses this handler, it must replace the first occurrence of the exact literal string "%s" in the url argument with an escaped version of the absolute URL of the content in question (as defined below), then resolve the resulting URL, relative to the API base URL specified by the entry settings object at the time the registerContentHandler() or registerProtocolHandler() methods were invoked, and then navigate an appropriate browsing context to the resulting URL using the GET method (or equivalent for non-HTTP URLs).

To get the escaped version of the absolute URL of the content in question, the user agent must replace every character in that absolute URL that is not a character in the URL default encode set with the result of UTF-8 percent encoding that character.

If the user had visited a site at http://example.com/ that made the following call:

navigator.registerContentHandler('application/x-soup', 'soup?url=%s', 'SoupWeb™')

...and then, much later, while visiting http://www.example.net/, clicked on a link such as:

<a href="chickenkïwi.soup">Download our Chicken Kïwi soup!</a>

...then, assuming this chickenkïwi.soup file was served with the MIME type application/x-soup, the UA might navigate to the following URL:

http://example.com/soup?url=http://www.example.net/chickenk%C3%AFwi.soup

This site could then fetch the chickenkïwi.soup file and do whatever it is that it does with soup (synthesize it and ship it to the user, or whatever).

title

A descriptive title of the handler, which the UA might use to remind the user what the site in question is.

This section does not define how the pages registered by these methods are used, beyond the requirements on how to process the url value (see above). To some extent, the processing model for navigating across documents defines some cases where these methods are relevant, but in general UAs may use this information wherever they would otherwise consider handing content to native plugins or helper applications.

UAs must not use registered content handlers to handle content that was returned as part of a non-GET transaction (or rather, as part of any non-idempotent transaction), as the remote site would not be able to fetch the same data.

In addition to the registration methods, there are also methods for determining if particular handlers have been registered, and for unregistering handlers.

state = window . navigator . unregisterProtocolHandler(scheme, url)
state = window . navigator . unregisterContentHandler(mimeType, url): Unregisters the handler given by the arguments.

The handler state strings are the following strings. Each string describes several situations, as given by the following list.

new: The described handler has never been registered for the given scheme or type.; The described handler was once registered for the given scheme or type, but the site has since unregistered it. If the handler were to be reregistered, the user would be notified accordingly.; The described handler was once registered for the given scheme or type, but the site has since unregistered it, but the user has indicated that the site is to be blocked from registering the type again, so the user agent would ignore further registration attempts.
registered: An attempt was made to register the described handler for the given scheme or type, but the user has not yet been notified, and the user agent would ignore further registration attempts. (Maybe the user agent batches registration requests to display them when the user requests to be notified about them, and the user has not yet requested that the user agent notify it of the previous registration attempt.); The described handler is registered for the given scheme or type (maybe, or maybe not, as the default handler).; The described handler is permanently blocked from being (re)registered. (Maybe the user marked the registration attempt as spam, or blocked the site for other reasons.)
declined: An attempt was made to register the described handler for the given scheme or type, but the user has not yet been notified; however, the user might be notified if another registration attempt were to be made. (Maybe the last registration attempt was made while the page was in the background and the user closed the page without looking at it, and the user agent requires confirmation for this registration attempt.); An attempt was made to register the described handler for the given scheme or type, but the user has not yet responded.; An attempt was made to register the described handler for the given scheme or type, but the user declined the offer. The user has not indicated that the handler is to be permanently blocked, however, so another attempt to register the described handler might result in the user being prompted again.; The described handler was once registered for the given scheme or type, but the user has since removed it. The user has not indicated that the handler is to be permanently blocked, however, so another attempt to register the described handler might result in the user being prompted again.

The unregisterProtocolHandler() method must unregister the handler described by the two arguments to the method, where the first argument gives the scheme and the second gives the string used to build the URL of the page that will handle the requests.

The first argument must be compared to the schemes for which custom protocol handlers are registered in an ASCII case-insensitive manner to find the relevant handlers.

The second argument must be preprocessed as described below, and if that is successful, must then be matched against the proto-URLs of the relevant handlers to find the described handler.

The unregisterContentHandler() method must unregister the handler described by the two arguments to the method, where the first argument gives the MIME type and the second gives the string used to build the URL of the page that will handle the requests.

The first argument must be compared to the MIME types for which custom content handlers are registered in an ASCII case-insensitive manner to find the relevant handlers.

The second argument must be preprocessed as described below, and if that is successful, must then be matched against the proto-URLs of the relevant handlers to find the described handler.

The second argument of the four methods described above must be preprocessed as follows:

If the string does not contain the substring "%s", abort these steps. There's no matching handler.
Resolve the string relative to the API base URL specified by the entry settings object.
If this fails, then throw a SyntaxError exception, aborting the method.
If the resulting absolute URL's origin is not the same origin as the origin specified by the entry settings object, throw a SecurityError exception, aborting the method.
Return the resulting absolute URL as the result of preprocessing the argument.

6.6.1.3.1 Security and privacy

These mechanisms can introduce a number of concerns, in particular privacy concerns.

Hijacking all Web usage. User agents should not allow schemes that are key to its normal operation, such as http or https, to be rerouted through third-party sites. This would allow a user's activities to be trivially tracked, and would allow user information, even in secure connections, to be collected.

Hijacking defaults. User agents are strongly urged to not automatically change any defaults, as this could lead the user to send data to remote hosts that the user is not expecting. New handlers registering themselves should never automatically cause those sites to be used.

Registration spamming. User agents should consider the possibility that a site will attempt to register a large number of handlers, possibly from multiple domains (e.g. by redirecting through a series of pages each on a different domain, and each registering a handler for video/mpeg — analogous practices abusing other Web browser features have been used by pornography Web sites for many years). User agents should gracefully handle such hostile attempts, protecting the user.

Misleading titles. User agents should not rely wholly on the title argument to the methods when presenting the registered handlers to the user, since sites could easily lie. For example, a site hostile.example.net could claim that it was registering the "Cuddly Bear Happy Content Handler". User agents should therefore use the handler's domain in any UI along with any title.

Hostile handler metadata. User agents should protect against typical attacks against strings embedded in their interface, for example ensuring that markup or escape characters in such strings are not executed, that null bytes are properly handled, that over-long strings do not cause crashes or buffer overruns, and so forth.

Leaking Intranet URLs. The mechanism described in this section can result in secret Intranet URLs being leaked, in the following manner:

The user registers a third-party content handler as the default handler for a content type.
The user then browses his corporate Intranet site and accesses a document that uses that content type.
The user agent contacts the third party and hands the third party the URL to the Intranet content.

No actual confidential file data is leaked in this manner, but the URLs themselves could contain confidential information. For example, the URL could be http://www.corp.example.com/upcoming-aquisitions/the-sample-company.egf, which might tell the third party that Example Corporation is intending to merge with The Sample Company. Implementors might wish to consider allowing administrators to disable this feature for certain subdomains, content types, or schemes.

Leaking secure URLs. User agents should not send HTTPS URLs to third-party sites registered as content handlers without the user's informed consent, for the same reason that user agents sometimes avoid sending Referer (sic) HTTP headers from secure sites to third-party sites.

Leaking credentials. User agents must never send username or password information in the URLs that are escaped and included sent to the handler sites. User agents may even avoid attempting to pass to Web-based handlers the URLs of resources that are known to require authentication to access, as such sites would be unable to access the resources in question without prompting the user for credentials themselves (a practice that would require the user to know whether to trust the third-party handler, a decision many users are unable to make or even understand).

Interface interference. User agents should be prepared to handle intentionally long arguments to the methods. For example, if the user interface exposed consists of an "accept" button and a "deny" button, with the "accept" binding containing the name of the handler, it's important that a long name not cause the "deny" button to be pushed off the screen.

Fingerprinting users. Since a site can detect if it has attempted to register a particular handler or not, whether or not the user responds, the mechanism can be used to store data. User agents are therefore strongly urged to treat registrations in the same manner as cookies: clearing cookies for a site should also clear all registrations for that site, and disabling cookies for a site should also disable registrations.

6.6.1.3.2 Sample user interface

This section is non-normative.

A simple implementation of this feature for a desktop Web browser might work as follows.

The registerContentHandler() method could display a modal dialog box:

In this dialog box, "Kittens at work" is the title of the page that invoked the method, "http://kittens.example.org/" is the URL of that page, "application/x-meowmeow" is the string that was passed to the registerContentHandler() method as its first argument (mimeType), "http://kittens.example.org/?show=%s" was the second argument (url), and "Kittens-at-work displayer" was the third argument (title).

If the user clicks the Cancel button, then nothing further happens. If the user clicks the "Trust" button, then the handler is remembered.

When the user then attempts to fetch a URL that uses the "application/x-meowmeow" MIME type, then it might display a dialog as follows:

In this dialog, the third option is the one that was primed by the site registering itself earlier.

If the user does select that option, then the browser, in accordance with the requirements described in the previous two sections, will redirect the user to "http://kittens.example.org/?show=data%3Aapplication/x-meowmeow;base64,S2l0dGVucyBhcmUgdGhlIGN1dGVzdCE%253D".

The registerProtocolHandler() method would work equivalently, but for schemes instead of unknown content types.

6.6.1.4 Manually releasing the storage mutex

[NoInterfaceObject]
interface NavigatorStorageUtils {
  readonly attribute boolean cookieEnabled;
  void yieldForStorageUpdates();
};

window . navigator . cookieEnabled

Returns false if setting a cookie will be ignored, and true otherwise.

window . navigator . yieldForStorageUpdates()

If a script uses the document.cookie API, or the localStorage API, the browser will block other scripts from accessing cookies or storage until the first script finishes. [WEBSTORAGE]

Calling the navigator.yieldForStorageUpdates() method tells the user agent to unblock any other scripts that may be blocked, even though the script hasn't returned.

Values of cookies and items in the Storage objects of localStorage attributes can change after calling this method, whence its name. [WEBSTORAGE]

The cookieEnabled attribute must return true if the user agent attempts to handle cookies according to the cookie specification, and false if it ignores cookie change requests. [COOKIES]

The yieldForStorageUpdates() method, when invoked, must, if the storage mutex is owned by the event loop of the task that resulted in the method being called, release the storage mutex so that it is once again free. Otherwise, it must do nothing.

6.6.1.5 Plugins

[NoInterfaceObject]
interface NavigatorPlugins {
  readonly attribute PluginArray plugins;
  readonly attribute MimeTypeArray mimeTypes;
  readonly attribute boolean javaEnabled;
};

interface PluginArray {
  void refresh(optional boolean reload = false);
  readonly attribute unsigned long length;
  getter Plugin? item(unsigned long index);
  getter Plugin? namedItem(DOMString name);
};

interface MimeTypeArray {
  readonly attribute unsigned long length;
  getter MimeType? item(unsigned long index);
  getter MimeType? namedItem(DOMString name);
};

interface Plugin {
  readonly attribute DOMString name;
  readonly attribute DOMString description;
  readonly attribute DOMString filename;
  readonly attribute unsigned long length;
  getter MimeType? item(unsigned long index);
  getter MimeType? namedItem(DOMString name);
};

interface MimeType {
  readonly attribute DOMString type;
  readonly attribute DOMString description;
  readonly attribute DOMString suffixes; // comma-separated
  readonly attribute Plugin enabledPlugin;
};

window . navigator . plugins . refresh( [ refresh ] ): Updates the lists of supported plugins and MIME types for this page, and reloads the page if the lists have changed.
window . navigator . plugins . length: Returns the number of plugins, represented by Plugin objects, that the user agent reports.
plugin = window . navigator . plugins . item(index)
window . navigator . plugins[index]: Returns the specified Plugin object.
plugin = window . navigator . plugins . item(name)
window . navigator . plugins[name]: Returns the Plugin object for the plugin with the given name.
window . navigator . mimeTypes . length: Returns the number of MIME types, represented by MimeType objects, supported by the plugins that the user agent reports.
mimeType = window . navigator . mimeTypes . item(index)
window . navigator . mimeTypes[index]: Returns the specified MimeType object.
mimeType = window . navigator . mimeTypes . item(name)
window . navigator . mimeTypes[name]: Returns the MimeType object for the given MIME type.
plugin . name: Returns the plugin's name.
plugin . description: Returns the plugin's description.
plugin . filename: Returns the plugin library's filename, if applicable on the current platform.
plugin . length: Returns the number of MIME types, represented by MimeType objects, supported by the plugin.
mimeType = plugin . item(index)
plugin[index]: Returns the specified MimeType object.
mimeType = plugin . item(name)
plugin[name]: Returns the MimeType object for the given MIME type.
mimeType . type: Returns the MIME type.
mimeType . description: Returns the MIME type's description.
mimeType . suffixes: Returns the MIME type's typical file extensions, in a comma-separated list.
mimeType . enabledPlugin: Returns the Plugin object that implements this MIME type.
window . navigator . javaEnabled: Returns true if there's a plugin that supports the MIME type "application/x-java-vm".

The navigator.plugins attribute must return a PluginArray object. The same object must be returned each time.

The navigator.mimeTypes attribute must return a MimeTypeArray object. The same object must be returned each time.

A PluginArray object represents none, some, or all of the plugins supported by the user agent, each of which is represented by a Plugin object. Each of these Plugin objects may be hidden plugins. A hidden plugin can't be enumerated, but can still be inspected by using its name.

The fewer plugins are represented by the PluginArray object, and of those, the more that are hidden, the more the user's privacy will be protected. Each exposed plugin increases the number of bits that can be derived for fingerprinting. Hiding a plugin helps, but unless it is an extremely rare plugin, it is likely that a site attempting to derive the list of plugins can still determine whether the plugin is supported or not by probing for it by name (the names of popular plugins are widely known). Therefore not exposing a plugin at all is preferred. Unfortunately, many legacy sites use this feature to determine, for example, which plugin to use to play video. Not exposing any plugins at all might therefore not be entirely plausible.

The PluginArray objects created by a user agent must not be live. The set of plugins represented by the objects must not change once an object is created, except when it is updated by the refresh() method.

Each plugin represented by a PluginArray can support a number of MIME types. For each such plugin, the user agent must pick one or more of these MIME types to be those that are explicitly supported.

The explicitly supported MIME types of a plugin are those that are exposed through the Plugin and MimeTypeArray interfaces. As with plugins themselves, any variation between users regarding what is exposed allows sites to fingerprint users. User agents are therefore encouraged to expose the same MIME types for all users of a plugin, regardless of the actual types supported... at least, within the constraints imposed by compatibility with legacy content.

The supported property indices of a PluginArray object are the numbers from zero to the number of non-hidden plugins represented by the object, if any.

The length attribute must return the number of non-hidden plugins represented by the object.

The item() method of a PluginArray object must return null if the argument is not one of the object's supported property indices, and otherwise must return the result of running the following steps, using the method's argument as index:

Let list be the Plugin objects representing the non-hidden plugins represented by the PluginArray object.
Sort list alphabetically by the name of each Plugin.
Return the indexth entry in list.

It is important for privacy that the order of plugins not leak additional information, e.g. the order in which plugins were installed.

The supported property names of a PluginArray object are the values of the name attributes of all the Plugin objects represented by the PluginArray object. The properties exposed in this way must not be enumerable.

The namedItem() method of a PluginArray object must return null if the argument is not one of the object's supported property names, and otherwise must return the Plugin object, of those represented by the PluginArray object, that has a name equal to the method's argument.

The refresh() method of the PluginArray object of a Navigator object, when invoked, must check to see if any plugins have been installed or reconfigured since the user agent created the PluginArray object. If so, and the method's argument is true, then the user agent must act as if the location.reload() method was called instead. Otherwise, the user agent must update the PluginArray object and MimeTypeArray object created for attributes of that Navigator object, and the Plugin and MimeType objects created for those PluginArray and MimeTypeArray objects, using the same Plugin objects for cases where the name is the same, and the same MimeType objects for cases where the type is the same, and creating new objects for cases where there were no matching objects immediately prior to the refresh() call. Old Plugin and MimeType objects must continue to return the same values that they had prior to the update, though naturally now the data is stale and may appear inconsistent (for example, an old MimeType entry might list as its enabledPlugin a Plugin object that no longer lists that MimeType as a supported MimeType).

A MimeTypeArray object represents the MIME types explicitly supported by plugins supported by the user agent, each of which is represented by a MimeType object.

The MimeTypeArray objects created by a user agent must not be live. The set of MIME types represented by the objects must not change once an object is created, except when it is updated by the PluginArray object's refresh() method.

The supported property indices of a MimeTypeArray object are the numbers from zero to the number of MIME types explicitly supported by non-hidden plugins represented by the corresponding PluginArray object, if any.

The length attribute must return the number of MIME types explicitly supported by non-hidden plugins represented by the corresponding PluginArray object, if any.

The item() method of a MimeTypeArray object must return null if the argument is not one of the object's supported property indices, and otherwise must return the result of running the following steps, using the method's argument as index:

Let list be the MimeType objects representing the MIME types explicitly supported by non-hidden plugins represented by the corresponding PluginArray object, if any.
Sort list alphabetically by the type of each MimeType.
Return the indexth entry in list.

It is important for privacy that the order of MIME types not leak additional information, e.g. the order in which plugins were installed.

The supported property names of a MimeTypeArray object are the values of the type attributes of all the MimeType objects represented by the MimeTypeArray object. The properties exposed in this way must not be enumerable.

The namedItem() method of a MimeTypeArray object must return null if the argument is not one of the object's supported property names, and otherwise must return the MimeType object that has a type equal to the method's argument.

A Plugin object represents a plugin. It has several attributes to provide details about the plugin, and can be enumerated to obtain the list of MIME types that it explicitly supports.

The Plugin objects created by a user agent must not be live. The set of MIME types represented by the objects, and the values of the objects' attributes, must not change once an object is created, except when updated by the PluginArray object's refresh() method.

The reported MIME types for a Plugin object are the MIME types explicitly supported by the corresponding plugin when this object was last created or updated by PluginArray.refresh(), whichever happened most recently.

The supported property indices of a Plugin object are the numbers from zero to the number of reported MIME types.

The length attribute must return the number of reported MIME types.

The item() method of a Plugin object must return null if the argument is not one of the object's supported property indices, and otherwise must return the result of running the following steps, using the method's argument as index:

Let list be the MimeType objects representing the reported MIME types.
Sort list alphabetically by the type of each MimeType.
Return the indexth entry in list.

It is important for privacy that the order of MIME types not leak additional information, e.g. the order in which plugins were installed.

The supported property names of a Plugin object are the values of the type attributes of the MimeType objects representing the reported MIME types. The properties exposed in this way must not be enumerable.

The namedItem() method of a Plugin object must return null if the argument is not one of the object's supported property names, and otherwise must return the MimeType object that has a type equal to the method's argument.

The name attribute must return the plugin's name.

The description and filename attributes must return user-agent-defined (or, in all likelihood, plugin-defined) strings. In each case, the same string must be returned each time, except that the strings returned may change when the PluginArray.refresh() method updates the object.

If the values returned by the description or filename attributes vary between versions of a plugin, they can be used both as a fingerprinting vector and, even more importantly, as a trivial way to determine what security vulnerabilities a plugin (and thus a browser) may have. It is thus highly recommended that the description attribute just return the same value as the name attribute, and that the filename attribute return the empty string.

A MimeType object represents a MIME type that is, or was, explicitly supported by a plugin.

The MimeType objects created by a user agent must not be live. The values of the objects' attributes must not change once an object is created, except when updated by the PluginArray object's refresh() method.

The type attribute must return the valid MIME type with no parameters describing the MIME type.

The description and suffixes attributes must return user-agent-defined (or, in all likelihood, plugin-defined) strings. In each case, the same string must be returned each time, except that the strings returned may change when the PluginArray.refresh() method updates the object.

If the values returned by the description or suffxies attributes vary between versions of a plugin, they can be used both as a fingerprinting vector and, even more importantly, as a trivial way to determine what security vulnerabilities a plugin (and thus a browser) may have. It is thus highly recommended that the description attribute just return the same value as the type attribute, and that the suffixes attribute return the empty string.

Commas in the suffixes attribute are interpreted as separating subsequent filename extensions, as in "htm,html".

The enabledPlugin attribute must return the Plugin object that represents the plugin that explicitly supported the MIME type that this MimeType object represents when this object was last created or updated by PluginArray.refresh(), whichever happened most recently.

The navigator.javaEnabled attribute must return true if the user agent supports a plugin that supports the MIME type "application/x-java-vm".

6.6.2 The `External` interface

The external attribute of the Window interface must return an instance of the External interface. The same object must be returned each time.

interface External {
  void AddSearchProvider(DOMString engineURL);
  unsigned long IsSearchProviderInstalled(DOMString engineURL);
};

window . external . AddSearchProvider( url )

Adds the search engine described by the OpenSearch description document at url. [OPENSEARCH]

The OpenSearch description document has to be on the same server as the script that calls this method.

installed = window . external . IsSearchProviderInstalled( url )

Returns a value based on comparing url to the URLs of the results pages of the installed search engines.

0: None of the installed search engines match url.
1: One or more installed search engines match url, but none are the user's default search engine.
2: The user's default search engine matches url.

The url is compared to the URLs of the results pages of the installed search engines using a prefix match. Only results pages on the same domain as the script that calls this method are checked.

Another way of exposing search engines using OpenSearch description documents is using a link element with the search link type.

The AddSearchProvider() method, when invoked, must run the following steps:

Optionally, abort these steps. User agents may implement the method as a stub method that never does anything, or may arbitrarily ignore invocations with particular arguments for security, privacy, or usability reasons.
Resolve the value of the method's first argument relative to the API base URL specified by the entry settings object.
If this fails, abort these steps.
Process the resulting absolute URL as the URL to an OpenSearch description document. [OPENSEARCH]

The IsSearchProviderInstalled() method, when invoked, must run the following steps:

Optionally, return 0 and abort these steps. User agents may implement the method as a stub method that never returns a non-zero value, or may arbitrarily ignore invocations with particular arguments for security, privacy, or usability reasons.
If the origin specified by the entry settings object is an opaque identifier (i.e. it has no host component), then return 0 and abort these steps.
Let host1 be the host component of the origin specified by the entry settings object.
Resolve the scriptURL argument relative to the API base URL specified by the entry settings object.
If this fails, return 0 and abort these steps.
Let host2 be the host component of the resulting parsed URL.
If the longest suffix in the Public Suffix List that matches the end of host1 is different than the longest suffix in the Public Suffix List that matches the end of host2, then return 0 and abort these steps. [PSL]

If the next domain component of host1 and host2 after their common suffix are not the same, then return 0 and abort these steps.
Let search engines be the list of search engines known by the user agent and made available to the user by the user agent for which the resulting absolute URL is a prefix match of the search engine's URL, if any. For search engines registered using OpenSearch description documents, the URL of the search engine corresponds to the URL given in a Url element whose rel attribute is "results" (the default). [OPENSEARCH]
If search engines is empty, return 0 and abort these steps.
If the user's default search engine (as determined by the user agent) is one of the search engines in search engines, then return 2 and abort these steps.
Return 1.

7 User interaction

7.1 The `hidden` attribute

All HTML elements may have the hidden content attribute set. The hidden attribute is a boolean attribute. When specified on an element, it indicates that the element is not yet, or is no longer, directly relevant to the page's current state, or that it is being used to declare content to be reused by other parts of the page as opposed to being directly accessed by the user. User agents should not render elements that have the hidden attribute specified. This requirement may be implemented indirectly through the style layer. For example, an HTML+CSS user agent could implement these requirements using the rules suggested in the Rendering section.

Because this attribute is typically implemented using CSS, it's also possible to override it using CSS. For instance, a rule that applies 'display: block' to all elements will cancel the effects of the hidden attribute. Authors therefore have to take care when writing their style sheets to make sure that the attribute is still styled as expected.

In the following skeletal example, the attribute is used to hide the Web game's main screen until the user logs in:

  <h1>The Example Game</h1>
  <section id="login">
   <h2>Login</h2>
   <form>
    ...
    <!-- calls login() once the user's credentials have been checked -->
   </form>
   <script>
    function login() {
      // switch screens
      document.getElementById('login').hidden = true;
      document.getElementById('game').hidden = false;
    }
   </script>
  </section>
  <section id="game" hidden>
   ...
  </section>

The hidden attribute must not be used to hide content that could legitimately be shown in another presentation. For example, it is incorrect to use hidden to hide panels in a tabbed dialog, because the tabbed interface is merely a kind of overflow presentation — one could equally well just show all the form controls in one big page with a scrollbar. It is similarly incorrect to use this attribute to hide content just from one presentation — if something is marked hidden, it is hidden from all presentations, including, for instance, printers.

Elements that are not themselves hidden must not hyperlink to elements that are hidden. The for attributes of label and output elements that are not themselves hidden must similarly not refer to elements that are hidden. In both cases, such references would cause user confusion.

Elements and scripts may, however, refer to elements that are hidden in other contexts.

For example, it would be incorrect to use the href attribute to link to a section marked with the hidden attribute. If the content is not applicable or relevant, then there is no reason to link to it.

It would be fine, however, to use the ARIA aria-describedby attribute to refer to descriptions that are themselves hidden. While hiding the descriptions implies that they are not useful alone, they could be written in such a way that they are useful in the specific context of being referenced from the images that they describe.

Similarly, a canvas element with the hidden attribute could be used by a scripted graphics engine as an off-screen buffer, and a form control could refer to a hidden form element using its form attribute.

Accessibility APIs are encouraged to provide a way to expose structured content while marking it as hidden in the default view. Such content should not be perceivable to users in the normal document flow in any modality, whether using Assistive Technology (AT) or mainstream User Agents.

When such features are available, User Agents may use them to expose the full semantics of hidden elements to AT when appropriate, if such content is referenced indirectly by an ID reference or valid hash-name reference. This allows ATs to access the structure of these hidden elements upon user request, while keeping the content hidden in all presentations of the normal document flow. Authors who wish to prevent user-initiated viewing of a hidden element should not reference the element with such a mechanism.

Because some User Agents have flattened hidden content when exposing such content to AT, authors should not reference hidden content which would lose essential meaning when flattened.

Elements in a section hidden by the hidden attribute are still active, e.g. scripts and form controls in such sections still execute and submit respectively. Only their presentation to the user changes.

The hidden IDL attribute must reflect the content attribute of the same name.

7.2 Inert subtrees

A node (in particular elements and text nodes) can be marked as inert. When a node is inert, then the user agent must act as if the node was absent for the purposes of targeting user interaction events, may ignore the node for the purposes of text search user interfaces (commonly known as "find in page"), and may prevent the user from selecting text in that node. User agents should allow the user to override the restrictions on search and text selection, however.

For example, consider a page that consists of just a single inert paragraph positioned in the middle of a body. If a user moves their pointing device from the body over to the inert paragraph and clicks on the paragraph, no mouseover event would be fired, and the mousemove and click events would be fired on the body element rather than the paragraph.

When a node is inert, it also can't be focusable.

An entire Document can be marked as blocked by a modal dialog subject. While a Document is so marked, every node that is in the Document, with the exception of the subject element and its descendants, must be marked inert. (The elements excepted by this paragraph can additionally be marked inert through other means; being part of a modal dialog does not "protect" a node from being marked inert.)

Only one element at a time can mark a Document as being blocked by a modal dialog.

7.3 Activation

element . click(): Acts as if the element was clicked.

The click() method must run synthetic click activation steps on the element.

7.4 Focus

When an element is focused, key events received by the document must be targeted at that element. There may be no element focused; when no element is focused, key events received by the document must be targeted at the body element, if there is one, or else at the Document's root element, if there is one. If there is no root element, key events must not be fired.

User agents may track focus for each browsing context or Document individually, or may support only one focused element per top-level browsing context — user agents should follow platform conventions in this regard.

Which elements within a top-level browsing context currently have focus must be independent of whether or not the top-level browsing context itself has the system focus.

When a child browsing context is focused, its browsing context container must also have focus.

When an element is focused, the element matches the CSS :focus pseudo-class.

7.4.1 Sequential focus navigation and the `tabindex` attribute

The tabindex content attribute allows authors to control whether an element is supposed to be focusable, whether it is supposed to be reachable using sequential focus navigation, and what is to be the relative order of the element for the purposes of sequential focus navigation. The name "tab index" comes from the common use of the "tab" key to navigate through the focusable elements. The term "tabbing" refers to moving forward through the focusable elements that can be reached using sequential focus navigation.

The tabindex attribute, if specified, must have a value that is a valid integer.

Each element can have a tabindex focus flag set, as defined below. This flag is a factor that contributes towards determining whether an element is focusable, as described in the next section.

If the attribute is specified, it must be parsed using the rules for parsing integers. The attribute's values have the following meanings:

If the attribute is omitted or parsing the value returns an error

The user agent should follow platform conventions to determine if the element's tabindex focus flag is set and, if so, whether the element can be reached using sequential focus navigation, and if so, what its relative order should be.

Modulo platform conventions, it is suggested that for the following elements, the tabindex focus flag be set:

a elements that have an href attribute
link elements that have an href attribute
button elements
input elements whose type attribute are not in the Hidden state
select elements
textarea elements
Editing hosts
Browsing context containers

One valid reason to ignore the platform conventions and always allow an element to be focused (by setting its tabindex focus flag) would be if the user's only mechanism for activating an element is through a keyboard action that triggers the focused element.

If the value is a negative integer

The user agent must set the element's tabindex focus flag, but should not allow the element to be reached using sequential focus navigation.

One valid reason to ignore the requirement that sequential focus navigation not allow the author to lead to the element would be if the user's only mechanism for moving the focus is sequential focus navigation. For instance, a keyboard-only user would be unable to click on a text field with a negative tabindex, so that user's user agent would be well justified in allowing the user to tab to the control regardless.

If the value is a zero

The user agent must set the element's tabindex focus flag, should allow the element to be reached using sequential focus navigation, and should follow platform conventions to determine the element's relative order.

If the value is greater than zero

The user agent must set the element's tabindex focus flag, should allow the element to be reached using sequential focus navigation, and should place the element in the sequential focus navigation order so that it is:

before any focusable element whose tabindex attribute has been omitted or whose value, when parsed, returns an error,
before any focusable element whose tabindex attribute has a value equal to or less than zero,
after any element whose tabindex attribute has a value greater than zero but less than the value of the tabindex attribute on the element,
after any element whose tabindex attribute has a value equal to the value of the tabindex attribute on the element but that is earlier in the document in tree order than the element,
before any element whose tabindex attribute has a value equal to the value of the tabindex attribute on the element but that is later in the document in tree order than the element, and
before any element whose tabindex attribute has a value greater than the value of the tabindex attribute on the element.

An element that has its tabindex focus flag set but does not otherwise have an activation behavior defined has an activation behavior that does nothing.

This means that an element that is only focusable because of its tabindex attribute will fire a click event in response to a non-mouse activation (e.g. hitting the "enter" key while the element is focused).

The tabIndex IDL attribute must reflect the value of the tabindex content attribute. Its default value is 0 for elements that are focusable and −1 for elements that are not focusable.

7.4.2 Focus management

An element is focusable if all of the following conditions are met:

The element's tabindex focus flag is set.
The element is either being rendered or is a descendant of a canvas element that represents embedded content.
The element is not inert.
The element is not disabled.

In addition, each shape that is generated for an area element, any user-agent-provided interface components of media elements (e.g. a play button), and distinct user interface components of form controls (e.g. "up" and "down" buttons on an <input type=number> spin control), should be focusable, unless platform conventions dictate otherwise or unless their corresponding element is disabled. (A single area element can correspond to multiple shapes, since image maps can be reused with multiple images on a page.)

Notwithstanding the above, user agents may make any element or part of an element focusable, especially to aid with accessibility or to better match platform conventions.

The focusing steps for an element are as follows:

If the element is not in a Document, or if the element's Document has no browsing context, or if the element's Document's browsing context has no top-level browsing context, or if the element is not focusable, or if the element is already focused, then abort these steps.
If focusing the element will remove the focus from another element, then run the unfocusing steps for that element.
Make the element the currently focused element in its top-level browsing context.

Some elements, most notably area, can correspond to more than one distinct focusable area. If a particular area was indicated when the element was focused, then that is the area that must get focus; otherwise, e.g. when using the focus() method, the first such region in tree order is the one that must be focused.
The user agent may apply relevant platform-specific conventions for focusing widgets.

For example, some platforms select the contents of a text field when that field is focused.
Fire a simple event named focus at the element.

User agents must synchronously run the focusing steps for an element whenever the user moves the focus to a focusable element.

The unfocusing steps for an element are as follows:

If the element is an input element, and the change event applies to the element, and the element does not have a defined activation behavior, and the user has changed the element's value or its list of selected files while the control was focused without committing that change, then fire a simple event that bubbles named change at the element.
Unfocus the element.
Fire a simple event named blur at the element.

When an element that is focused stops being a focusable element, or stops being focused without another element being explicitly focused in its stead, the user agent should synchronously run the unfocusing steps for the affected element only.

For example, this might happen because the element is removed from its Document, or has a hidden attribute added. It would also happen to an input element when the element gets disabled.

7.4.3 Document-level focus APIs

document . activeElement: Returns the currently focused element.
document . hasFocus(): Returns true if the document has focus; otherwise, returns false.
window . focus(): Focuses the window. Use of this method is discouraged. Allow the user to control window focus instead.
window . blur(): Unfocuses the window. Use of this method is discouraged. Allow the user to control window focus instead.

The activeElement attribute on Document objects must return the element in the document that is focused. If no element in the Document is focused, this must return the body element.

When a child browsing context is focused, its browsing context container is also focused, by definition. For example, if the user moves the focus to a text field in an iframe, the iframe is the element with focus in the parent browsing context.

The hasFocus() method on Document objects must return true if the Document's browsing context is focused, and all its ancestor browsing contexts are also focused, and the top-level browsing context has the system focus. If the Document has no browsing context or if its browsing context has no top-level browsing context, then the method will always return false.

The focus() method on the Window object, when invoked, provides a hint to the user agent that the script believes the user might be interested in the contents of the browsing context of the Window object on which the method was invoked.

User agents are encouraged to have this focus() method trigger some kind of notification.

The blur() method on the Window object, when invoked, provides a hint to the user agent that the script believes the user probably is not currently interested in the contents of the browsing context of the Window object on which the method was invoked, but that the contents might become interesting again in the future.

User agents are encouraged to ignore calls to this blur() method entirely.

Historically the focus() and blur() methods actually affected the system focus, but hostile sites widely abuse this behavior to the user's detriment.

7.4.4 Element-level focus APIs

element . focus()

Focuses the element.

element . blur()

Unfocuses the element. Use of this method is discouraged. Focus another element instead.

Do not use this method to hide the focus ring. Do not use any other method that hides the focus ring from keyboard users, in particular do not use a CSS rule to override the 'outline' property. Removal of the focus ring leads to serious accessibility issues for users who navigate and interact with interactive content using the keyboard.

The focus() method, when invoked, must run the following algorithm:

If the element is marked as locked for focus, then abort these steps.
Mark the element as locked for focus.
Run the focusing steps for the element.
Unmark the element as locked for focus.

The blur() method, when invoked, should run the unfocusing steps for the element on which the method was called instead. User agents may selectively or uniformly ignore calls to this method for usability reasons.

For example, if the blur() method is unwisely being used to remove the focus ring for aesthetics reasons, the page would become unusable by keyboard users. Ignoring calls to this method would thus allow keyboard users to interact with the page.

7.5 Assigning keyboard shortcuts

7.5.1 Introduction

This section is non-normative.

Each element that can be activated or focused can be assigned a single key combination to activate it, using the accesskey attribute.

The exact shortcut is determined by the user agent, based on information about the user's keyboard, what keyboard shortcuts already exist on the platform, and what other shortcuts have been specified on the page, using the information provided in the accesskey attribute as a guide.

In order to ensure that a relevant keyboard shortcut is available on a wide variety of input devices, the author can provide a number of alternatives in the accesskey attribute.

Each alternative consists of a single character, such as a letter or digit.

User agents can provide users with a list of the keyboard shortcuts, but authors are encouraged to do so also. The accessKeyLabel IDL attribute returns a string representing the actual key combination assigned by the user agent.

In this example, an author has provided a button that can be invoked using a shortcut key. To support full keyboards, the author has provided "C" as a possible key. To support devices equipped only with numeric keypads, the author has provided "1" as another possibly key.

<input type=button value=Collect onclick="collect()"
       accesskey="C 1" id=c>

To tell the user what the shortcut key is, the author has this script here opted to explicitly add the key combination to the button's label:

function addShortcutKeyLabel(button) {
  if (button.accessKeyLabel != '')
    button.value += ' (' + button.accessKeyLabel + ')';
}
addShortcutKeyLabel(document.getElementById('c'));

Browsers on different platforms will show different labels, even for the same key combination, based on the convention prevalent on that platform. For example, if the key combination is the Control key, the Shift key, and the letter C, a Windows browser might display "Ctrl+Shift+C", whereas a Mac browser might display "^⇧C", while an Emacs browser might just display "C-C". Similarly, if the key combination is the Alt key and the Escape key, Windows might use "Alt+Esc", Mac might use "⌥⎋", and an Emacs browser might use "M-ESC" or "ESC ESC".

In general, therefore, it is unwise to attempt to parse the value returned from the accessKeyLabel IDL attribute.

7.5.2 The `accesskey` attribute

All HTML elements may have the accesskey content attribute set. The accesskey attribute's value is used by the user agent as a guide for creating a keyboard shortcut that activates or focuses the element.

If specified, the value must be an ordered set of unique space-separated tokens that are case-sensitive, each of which must be exactly one Unicode code point in length.

In the following example, a variety of links are given with access keys so that keyboard users familiar with the site can more quickly navigate to the relevant pages:

<nav>
 <p>
  <a title="Consortium Activities" accesskey="A" href="/Consortium/activities">Activities</a> |
  <a title="Technical Reports and Recommendations" accesskey="T" href="/TR/">Technical Reports</a> |
  <a title="Alphabetical Site Index" accesskey="S" href="/Consortium/siteindex">Site Index</a> |
  <a title="About This Site" accesskey="B" href="/Consortium/">About Consortium</a> |
  <a title="Contact Consortium" accesskey="C" href="/Consortium/contact">Contact</a>
 </p>
</nav>

In the following example, the search field is given two possible access keys, "s" and "0" (in that order). A user agent on a device with a full keyboard might pick Ctrl+Alt+S as the shortcut key, while a user agent on a small device with just a numeric keypad might pick just the plain unadorned key 0:

<form action="/search">
 <label>Search: <input type="search" name="q" accesskey="s 0"></label>
 <input type="submit">
</form>

In the following example, a button has possible access keys described. A script then tries to update the button's label to advertise the key combination the user agent selected.

<input type=submit accesskey="N @ 1" value="Compose">
...
<script>
 function labelButton(button) {
   if (button.accessKeyLabel)
     button.value += ' (' + button.accessKeyLabel + ')';
 }
 var inputs = document.getElementsByTagName('input');
 for (var i = 0; i < inputs.length; i += 1) {
   if (inputs[i].type == "submit")
     labelButton(inputs[i]);
 }
</script>

On one user agent, the button's label might become "Compose (⌘N)". On another, it might become "Compose (Alt+⇧+1)". If the user agent doesn't assign a key, it will be just "Compose". The exact string depends on what the assigned access key is, and on how the user agent represents that key combination.

7.5.3 Processing model

An element's assigned access key is a key combination derived from the element's accesskey content attribute. Initially, an element must not have an assigned access key.

Whenever an element's accesskey attribute is set, changed, or removed, the user agent must update the element's assigned access key by running the following steps:

If the element has no accesskey attribute, then skip to the fallback step below.
Otherwise, split the attribute's value on spaces, and let keys be the resulting tokens.
For each value in keys in turn, in the order the tokens appeared in the attribute's value, run the following substeps:
1. If the value is not a string exactly one Unicode code point in length, then skip the remainder of these steps for this value.
2. If the value does not correspond to a key on the system's keyboard, then skip the remainder of these steps for this value.
3. If the user agent can find a mix of zero or more modifier keys that, combined with the key that corresponds to the value given in the attribute, can be used as the access key, then the user agent may assign that combination of keys as the element's assigned access key and abort these steps.
Fallback: Optionally, the user agent may assign a key combination of its choosing as the element's assigned access key and then abort these steps.
If this step is reached, the element has no assigned access key.

Once a user agent has selected and assigned an access key for an element, the user agent should not change the element's assigned access key unless the accesskey content attribute is changed or the element is moved to another Document.

User agents might expose elements that have an accesskey attribute in other ways as well, e.g. in a menu displayed in response to a specific key combination.

The accessKey IDL attribute must reflect the accesskey content attribute.

The accessKeyLabel IDL attribute must return a string that represents the element's assigned access key, if any. If the element does not have one, then the IDL attribute must return the empty string.

7.6 Editing

7.6.1 Making document regions editable: The `contenteditable` content attribute

The contenteditable attribute is an enumerated attribute whose keywords are the empty string, true, and false. The empty string and the true keyword map to the true state. The false keyword maps to the false state. In addition, there is a third state, the inherit state, which is the missing value default (and the invalid value default).

The true state indicates that the element is editable. The inherit state indicates that the element is editable if its parent is. The false state indicates that the element is not editable.

element . contentEditable [ = value ]

Returns "true", "false", or "inherit", based on the state of the contenteditable attribute.

Can be set, to change that state.

Throws a SyntaxError exception if the new value isn't one of those strings.

element . isContentEditable

Returns true if the element is editable; otherwise, returns false.

The contentEditable IDL attribute, on getting, must return the string "true" if the content attribute is set to the true state, "false" if the content attribute is set to the false state, and "inherit" otherwise. On setting, if the new value is an ASCII case-insensitive match for the string "inherit" then the content attribute must be removed, if the new value is an ASCII case-insensitive match for the string "true" then the content attribute must be set to the string "true", if the new value is an ASCII case-insensitive match for the string "false" then the content attribute must be set to the string "false", and otherwise the attribute setter must throw a SyntaxError exception.

The isContentEditable IDL attribute, on getting, must return true if the element is either an editing host or editable, and false otherwise.

7.6.2 Making entire documents editable: The `designMode` IDL attribute

Documents have a designMode, which can be either enabled or disabled.

document . designMode [ = value ]

Returns "on" if the document is editable, and "off" if it isn't.

Can be set, to change the document's current state. This focuses the document and resets the selection in that document.

The designMode IDL attribute on the Document object takes two values, "on" and "off". On setting, the new value must be compared in an ASCII case-insensitive manner to these two values; if it matches the "on" value, then designMode must be enabled, and if it matches the "off" value, then designMode must be disabled. Other values must be ignored.

On getting, if designMode is enabled, the IDL attribute must return the value "on"; otherwise it is disabled, and the attribute must return the value "off".

The last state set must persist until the document is destroyed or the state is changed. Initially, documents must have their designMode disabled.

When the designMode changes from being disabled to being enabled, the user agent must synchronously reset the document's active range's start and end boundary points to be at the start of the Document and then run the focusing steps for the root element of the Document, if any.

7.6.3 Best practices for in-page editors

Authors are encouraged to set the 'white-space' property on editing hosts and on markup that was originally created through these editing mechanisms to the value 'pre-wrap'. Default HTML whitespace handling is not well suited to WYSIWYG editing, and line wrapping will not work correctly in some corner cases if 'white-space' is left at its default value.

As an example of problems that occur if the default 'normal' value is used instead, consider the case of the user typing "yellow␣␣ball", with two spaces (here represented by "␣") between the words. With the editing rules in place for the default value of 'white-space' ('normal'), the resulting markup will either consist of "yellow  ball" or "yellow  ball"; i.e., there will be a non-breaking space between the two words in addition to the regular space. This is necessary because the 'normal' value for 'white-space' requires adjacent regular spaces to be collapsed together.

In the former case, "yellow⍽" might wrap to the next line ("⍽" being used here to represent a non-breaking space) even though "yellow" alone might fit at the end of the line; in the latter case, "⍽ball", if wrapped to the start of the line, would have visible indentation from the non-breaking space.

When 'white-space' is set to 'pre-wrap', however, the editing rules will instead simply put two regular spaces between the words, and should the two words be split at the end of a line, the spaces would be neatly removed from the rendering.

7.6.4 Editing APIs

The definition of the terms active range, editing host, and editable, the user interface requirements of elements that are editing hosts or editable, the execCommand(), queryCommandEnabled(), queryCommandIndeterm(), queryCommandState(), queryCommandSupported(), and queryCommandValue() methods, text selections, and the delete the selection algorithm are defined in the HTML Editing APIs specification. The interaction of editing and the undo/redo features in user agents is defined by the UndoManager and DOM Transaction specification. [EDITING] [UNDO]

7.6.5 Spelling and grammar checking

User agents can support the checking of spelling and grammar of editable text, either in form controls (such as the value of textarea elements), or in elements in an editing host (e.g. using contenteditable).

For each element, user agents must establish a default behavior, either through defaults or through preferences expressed by the user. There are three possible default behaviors for each element:

true-by-default: The element will be checked for spelling and grammar if its contents are editable.
false-by-default: The element will never be checked for spelling and grammar.
inherit-by-default: The element's default behavior is the same as its parent element's. Elements that have no parent element cannot have this as their default behavior.

The spellcheck attribute is an enumerated attribute whose keywords are the empty string, true and false. The empty string and the true keyword map to the true state. The false keyword maps to the false state. In addition, there is a third state, the default state, which is the missing value default (and the invalid value default).

The true state indicates that the element is to have its spelling and grammar checked. The default state indicates that the element is to act according to a default behavior, possibly based on the parent element's own spellcheck state, as defined below. The false state indicates that the element is not to be checked.

element . spellcheck [ = value ]

Returns true if the element is to have its spelling and grammar checked; otherwise, returns false.

Can be set, to override the default and set the spellcheck content attribute.

The spellcheck IDL attribute, on getting, must return true if the element's spellcheck content attribute is in the true state, or if the element's spellcheck content attribute is in the default state and the element's default behavior is true-by-default, or if the element's spellcheck content attribute is in the default state and the element's default behavior is inherit-by-default and the element's parent element's spellcheck IDL attribute would return true; otherwise, if none of those conditions applies, then the attribute must instead return false.

The spellcheck IDL attribute is not affected by user preferences that override the spellcheck content attribute, and therefore might not reflect the actual spellchecking state.

On setting, if the new value is true, then the element's spellcheck content attribute must be set to the literal string "true", otherwise it must be set to the literal string "false".

User agents must only consider the following pieces of text as checkable for the purposes of this feature:

The value of input elements whose type attributes are in the Text, Search, URL, or E-mail states and that are mutable (i.e. that do not have the readonly attribute specified and that are not disabled).
The value of textarea elements that do not have a readonly attribute and that are not disabled.
Text in Text nodes that are children of editing hosts or editable elements.
Text in attributes of editable elements.

For text that is part of a Text node, the element with which the text is associated is the element that is the immediate parent of the first character of the word, sentence, or other piece of text. For text in attributes, it is the attribute's element. For the values of input and textarea elements, it is the element itself.

To determine if a word, sentence, or other piece of text in an applicable element (as defined above) is to have spelling- and grammar-checking enabled, the UA must use the following algorithm:

If the user has disabled the checking for this text, then the checking is disabled.
Otherwise, if the user has forced the checking for this text to always be enabled, then the checking is enabled.
Otherwise, if the element with which the text is associated has a spellcheck content attribute, then: if that attribute is in the true state, then checking is enabled; otherwise, if that attribute is in the false state, then checking is disabled.
Otherwise, if there is an ancestor element with a spellcheck content attribute that is not in the default state, then: if the nearest such ancestor's spellcheck content attribute is in the true state, then checking is enabled; otherwise, checking is disabled.
Otherwise, if the element's default behavior is true-by-default, then checking is enabled.
Otherwise, if the element's default behavior is false-by-default, then checking is disabled.
Otherwise, if the element's parent element has its checking enabled, then checking is enabled.
Otherwise, checking is disabled.

If the checking is enabled for a word/sentence/text, the user agent should indicate spelling and grammar errors in that text. User agents should take into account the other semantics given in the document when suggesting spelling and grammar corrections. User agents may use the language of the element to determine what spelling and grammar rules to use, or may use the user's preferred language settings. UAs should use input element attributes such as pattern to ensure that the resulting value is valid, where possible.

If checking is disabled, the user agent should not indicate spelling or grammar errors for that text.

Even when checking is enabled, user agents may opt to not report spelling or grammar errors in text that the user agent deems the user has no interest in having checked (e.g. text that was already present when the page was loaded, or that the user did not type, or text in controls that the user has not focused, or in parts of e-mail addresses that the user agent is not confident were misspelt).

The element with ID "a" in the following example would be the one used to determine if the word "Hello" is checked for spelling errors. In this example, it would not be.

<div contenteditable="true">
 <span spellcheck="false" id="a">Hell</span><em>o!</em>
</div>

The element with ID "b" in the following example would have checking enabled (the leading space character in the attribute's value on the input element causes the attribute to be ignored, so the ancestor's value is used instead, regardless of the default).

<p spellcheck="true">
 <label>Name: <input spellcheck=" false" id="b"></label>
</p>

This specification does not define the user interface for spelling and grammar checkers. A user agent could offer on-demand checking, could perform continuous checking while the checking is enabled, or could use other interfaces.

8 The HTML syntax

This section only describes the rules for resources labeled with an HTML MIME type. Rules for XML resources are discussed in the section below entitled "The XHTML syntax".

8.1 Writing HTML documents

This section only applies to documents, authoring tools, and markup generators. In particular, it does not apply to conformance checkers; conformance checkers must use the requirements given in the next section ("parsing HTML documents").

Documents must consist of the following parts, in the given order:

Optionally, a single "BOM" (U+FEFF) character.
Any number of comments and space characters.
A DOCTYPE.
Any number of comments and space characters.
The root element, in the form of an html element.
Any number of comments and space characters.

The various types of content mentioned above are described in the next few sections.

In addition, there are some restrictions on how character encoding declarations are to be serialized, as discussed in the section on that topic.

Space characters before the root html element, and space characters at the start of the html element and before the head element, will be dropped when the document is parsed; space characters after the root html element will be parsed as if they were at the end of the body element. Thus, space characters around the root element do not round-trip.

It is suggested that newlines be inserted after the DOCTYPE, after any comments that are before the root element, after the html element's start tag (if it is not omitted), and after any comments that are inside the html element but before the head element.

Many strings in the HTML syntax (e.g. the names of elements and their attributes) are case-insensitive, but only for uppercase ASCII letters and lowercase ASCII letters. For convenience, in this section this is just referred to as "case-insensitive".

8.1.1 The DOCTYPE

A DOCTYPE is a required preamble.

DOCTYPEs are required for legacy reasons. When omitted, browsers tend to use a different rendering mode that is incompatible with some specifications. Including the DOCTYPE in a document ensures that the browser makes a best-effort attempt at following the relevant specifications.

A DOCTYPE must consist of the following components, in this order:

A string that is an ASCII case-insensitive match for the string "<!DOCTYPE".
One or more space characters.
A string that is an ASCII case-insensitive match for the string "html".
Optionally, a DOCTYPE legacy string or an obsolete permitted DOCTYPE string (defined below).
Zero or more space characters.
A ">" (U+003E) character.

In other words, <!DOCTYPE html>, case-insensitively.

For the purposes of HTML generators that cannot output HTML markup with the short DOCTYPE "<!DOCTYPE html>", a DOCTYPE legacy string may be inserted into the DOCTYPE (in the position defined above). This string must consist of:

One or more space characters.
A string that is an ASCII case-insensitive match for the string "SYSTEM".
One or more space characters.
A U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (the quote mark).
The literal string "about:legacy-compat".
A matching U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (i.e. the same character as in the earlier step labeled quote mark).

In other words, <!DOCTYPE html SYSTEM "about:legacy-compat"> or <!DOCTYPE html SYSTEM 'about:legacy-compat'>, case-insensitively except for the part in single or double quotes.

The DOCTYPE legacy string should not be used unless the document is generated from a system that cannot output the shorter string.

To help authors transition from HTML4 and XHTML1, an obsolete permitted DOCTYPE string can be inserted into the DOCTYPE (in the position defined above). This string must consist of:

One or more space characters.
A string that is an ASCII case-insensitive match for the string "PUBLIC".
One or more space characters.
A U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (the first quote mark).
The string from one of the cells in the first column of the table below. The row to which this cell belongs is the selected row.
A matching U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (i.e. the same character as in the earlier step labeled first quote mark).
If a system identifier is used,
1. One or more space characters.
2. A U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (the third quote mark).
3. The string from the cell in the second column of the selected row.
4. A matching U+0022 QUOTATION MARK or U+0027 APOSTROPHE character (i.e. the same character as in the earlier step labeled third quote mark).

Allowed values for public and system identifiers in an obsolete permitted DOCTYPE string.
Public identifier	System identifier	System identifier optional?
`-//W3C//DTD HTML 4.0//EN`	`http://www.w3.org/TR/REC-html40/strict.dtd`	Yes
`-//W3C//DTD HTML 4.01//EN`	`http://www.w3.org/TR/html4/strict.dtd`	Yes
`-//W3C//DTD XHTML 1.0 Strict//EN`	`http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd`	No
`-//W3C//DTD XHTML 1.1//EN`	`http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd`	No

A DOCTYPE containing an obsolete permitted DOCTYPE string is an obsolete permitted DOCTYPE. Authors should not use obsolete permitted DOCTYPEs, as they are unnecessarily long.

8.1.2 Elements

There are five different kinds of elements: void elements, raw text elements, escapable raw text elements, foreign elements, and normal elements.

Void elements: area, base, br, col, embed, hr, img, input, keygen, link, meta, param, source, track, wbr
Raw text elements: script, style
escapable raw text elements: textarea, title
Foreign elements: Elements from the MathML namespace and the SVG namespace.
Normal elements: All other allowed HTML elements are normal elements.

Tags are used to delimit the start and end of elements in the markup. Raw text, escapable raw text, and normal elements have a start tag to indicate where they begin, and an end tag to indicate where they end. The start and end tags of certain normal elements can be omitted, as described below in the section on optional tags. Those that cannot be omitted must not be omitted. Void elements only have a start tag; end tags must not be specified for void elements. Foreign elements must either have a start tag and an end tag, or a start tag that is marked as self-closing, in which case they must not have an end tag.

The contents of the element must be placed between just after the start tag (which might be implied, in certain cases) and just before the end tag (which again, might be implied in certain cases). The exact allowed contents of each individual element depend on the content model of that element, as described earlier in this specification. Elements must not contain content that their content model disallows. In addition to the restrictions placed on the contents by those content models, however, the five types of elements have additional syntactic requirements.

Void elements can't have any contents (since there's no end tag, no content can be put between the start tag and the end tag).

Raw text elements can have text, though it has restrictions described below.

Escapable raw text elements can have text and character references, but the text must not contain an ambiguous ampersand. There are also further restrictions described below.

Foreign elements whose start tag is marked as self-closing can't have any contents (since, again, as there's no end tag, no content can be put between the start tag and the end tag). Foreign elements whose start tag is not marked as self-closing can have text, character references, CDATA sections, other elements, and comments, but the text must not contain the character "<" (U+003C) or an ambiguous ampersand.

The HTML syntax does not support namespace declarations, even in foreign elements.

For instance, consider the following HTML fragment:

<p>
 <svg>
  <metadata>
   <!-- this is invalid -->
   <cdr:license xmlns:cdr="http://www.example.com/cdr/metadata" name="MIT"/>
  </metadata>
 </svg>
</p>

The innermost element, cdr:license, is actually in the SVG namespace, as the "xmlns:cdr" attribute has no effect (unlike in XML). In fact, as the comment in the fragment above says, the fragment is actually non-conforming. This is because the SVG specification does not define any elements called "cdr:license" in the SVG namespace.

Normal elements can have text, character references, other elements, and comments, but the text must not contain the character "<" (U+003C) or an ambiguous ampersand. Some normal elements also have yet more restrictions on what content they are allowed to hold, beyond the restrictions imposed by the content model and those described in this paragraph. Those restrictions are described below.

Tags contain a tag name, giving the element's name. HTML elements all have names that only use alphanumeric ASCII characters. In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive.

8.1.2.1 Start tags

Start tags must have the following format:

The first character of a start tag must be a "<" (U+003C) character.
The next few characters of a start tag must be the element's tag name.
If there are to be any attributes in the next step, there must first be one or more space characters.
Then, the start tag may have a number of attributes, the syntax for which is described below. Attributes must be separated from each other by one or more space characters.
After the attributes, or after the tag name if there are no attributes, there may be one or more space characters. (Some attributes are required to be followed by a space. See the attributes section below.)
Then, if the element is one of the void elements, or if the element is a foreign element, then there may be a single "/" (U+002F) character. This character has no effect on void elements, but on foreign elements it marks the start tag as self-closing.
Finally, start tags must be closed by a ">" (U+003E) character.

8.1.2.2 End tags

End tags must have the following format:

The first character of an end tag must be a "<" (U+003C) character.
The second character of an end tag must be a "/" (U+002F) character.
The next few characters of an end tag must be the element's tag name.
After the tag name, there may be one or more space characters.
Finally, end tags must be closed by a ">" (U+003E) character.

8.1.2.3 Attributes

Attributes for an element are expressed inside the element's start tag.

Attributes have a name and a value. Attribute names must consist of one or more characters other than the space characters, U+0000 NULL, U+0022 QUOTATION MARK ("), U+0027 APOSTROPHE ('), ">" (U+003E), "/" (U+002F), and "=" (U+003D) characters, the control characters, and any characters that are not defined by Unicode. In the HTML syntax, attribute names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that are an ASCII case-insensitive match for the attribute's name.

Attribute values are a mixture of text and character references, except with the additional restriction that the text cannot contain an ambiguous ampersand.

Attributes can be specified in four different ways:

Empty attribute syntax

Just the attribute name. The value is implicitly the empty string.

In the following example, the disabled attribute is given with the empty attribute syntax:

<input disabled>

If an attribute using the empty attribute syntax is to be followed by another attribute, then there must be a space character separating the two.

Unquoted attribute value syntax

The attribute name, followed by zero or more space characters, followed by a single U+003D EQUALS SIGN character, followed by zero or more space characters, followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal space characters, any U+0022 QUOTATION MARK characters ("), U+0027 APOSTROPHE characters ('), "=" (U+003D) characters, "<" (U+003C) characters, ">" (U+003E) characters, or U+0060 GRAVE ACCENT characters (`), and must not be the empty string.

In the following example, the value attribute is given with the unquoted attribute value syntax:

<input value=yes>

If an attribute using the unquoted attribute syntax is to be followed by another attribute or by the optional "/" (U+002F) character allowed in step 6 of the start tag syntax above, then there must be a space character separating the two.

Single-quoted attribute value syntax

The attribute name, followed by zero or more space characters, followed by a single U+003D EQUALS SIGN character, followed by zero or more space characters, followed by a single "'" (U+0027) character, followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal "'" (U+0027) characters, and finally followed by a second single "'" (U+0027) character.

In the following example, the type attribute is given with the single-quoted attribute value syntax:

<input type='checkbox'>

If an attribute using the single-quoted attribute syntax is to be followed by another attribute, then there must be a space character separating the two.

Double-quoted attribute value syntax

The attribute name, followed by zero or more space characters, followed by a single U+003D EQUALS SIGN character, followed by zero or more space characters, followed by a single """ (U+0022) character, followed by the attribute value, which, in addition to the requirements given above for attribute values, must not contain any literal U+0022 QUOTATION MARK characters ("), and finally followed by a second single """ (U+0022) character.

In the following example, the name attribute is given with the double-quoted attribute value syntax:

<input name="be evil">

If an attribute using the double-quoted attribute syntax is to be followed by another attribute, then there must be a space character separating the two.

There must never be two or more attributes on the same start tag whose names are an ASCII case-insensitive match for each other.

When a foreign element has one of the namespaced attributes given by the local name and namespace of the first and second cells of a row from the following table, it must be written using the name given by the third cell from the same row.

Local name	Namespace	Attribute name
`actuate`	XLink namespace	`xlink:actuate`
`arcrole`	XLink namespace	`xlink:arcrole`
`href`	XLink namespace	`xlink:href`
`role`	XLink namespace	`xlink:role`
`show`	XLink namespace	`xlink:show`
`title`	XLink namespace	`xlink:title`
`type`	XLink namespace	`xlink:type`
`base`	XML namespace	`xml:base`
`lang`	XML namespace	`xml:lang`
`space`	XML namespace	`xml:space`
`xmlns`	XMLNS namespace	`xmlns`
`xlink`	XMLNS namespace	`xmlns:xlink`

No other namespaced attribute can be expressed in the HTML syntax.

Whether the attributes in the table above are conforming or not is defined by other specifications (e.g. the SVG and MathML specifications); this section only describes the syntax rules if the attributes are serialized using the HTML syntax.

8.1.2.4 Optional tags

Certain tags can be omitted.

Omitting an element's start tag in the situations described below does not mean the element is not present; it is implied, but it is still there. For example, an HTML document always has a root html element, even if the string <html> doesn't appear anywhere in the markup.

An html element's start tag may be omitted if the first thing inside the html element is not a comment.

An html element's end tag may be omitted if the html element is not immediately followed by a comment.

A head element's start tag may be omitted if the element is empty, or if the first thing inside the head element is an element.

A head element's end tag may be omitted if the head element is not immediately followed by a space character or a comment.

A body element's end tag may be omitted if the body element is not immediately followed by a comment.

An li element's end tag may be omitted if the li element is immediately followed by another li element or if there is no more content in the parent element.

A dt element's end tag may be omitted if the dt element is immediately followed by another dt element or a dd element.

A dd element's end tag may be omitted if the dd element is immediately followed by another dd element or a dt element, or if there is no more content in the parent element.

An rb element's end tag may be omitted if the rb element is immediately followed by an rb, rt, rtc or rp element, or if there is no more content in the parent element.

An rt element's end tag may be omitted if the rt element is immediately followed by an rb, rt, rtc, or rp element, or if there is no more content in the parent element.

An rtc element's end tag may be omitted if the rtc element is immediately followed by an rb, rtc or rp element, or if there is no more content in the parent element.

An rp element's end tag may be omitted if the rp element is immediately followed by an rb, rt, rtc or rp element, or if there is no more content in the parent element.

An optgroup element's end tag may be omitted if the optgroup element is immediately followed by another optgroup element, or if there is no more content in the parent element.

A colgroup element's start tag may be omitted if the first thing inside the colgroup element is a col element, and if the element is not immediately preceded by another colgroup element whose end tag has been omitted. (It can't be omitted if the element is empty.)

A colgroup element's end tag may be omitted if the colgroup element is not immediately followed by a space character or a comment.

A thead element's end tag may be omitted if the thead element is immediately followed by a tbody or tfoot element.

A tbody element's end tag may be omitted if the tbody element is immediately followed by a tbody or tfoot element, or if there is no more content in the parent element.

A tfoot element's end tag may be omitted if the tfoot element is immediately followed by a tbody element, or if there is no more content in the parent element.

A tr element's end tag may be omitted if the tr element is immediately followed by another tr element, or if there is no more content in the parent element.

A td element's end tag may be omitted if the td element is immediately followed by a td or th element, or if there is no more content in the parent element.

A th element's end tag may be omitted if the th element is immediately followed by a td or th element, or if there is no more content in the parent element.

However, a start tag must never be omitted if it has any attributes.

8.1.2.5 Restrictions on content models

For historical reasons, certain elements have extra restrictions beyond even the restrictions given by their content model.

A table element must not contain tr elements, even though these elements are technically allowed inside table elements according to the content models described in this specification. (If a tr element is put inside a table in the markup, it will in fact imply a tbody start tag before it.)

A single newline may be placed immediately after the start tag of pre and textarea elements. If the element's contents are intended to start with a newline, two consecutive newlines thus need to be included by the author.

The following two pre blocks are equivalent:

<pre>Hello</pre>

<pre>
Hello</pre>

8.1.2.6 Restrictions on the contents of raw text and escapable raw text elements

The text in raw text and escapable raw text elements must not contain any occurrences of the string "</" (U+003C LESS-THAN SIGN, U+002F SOLIDUS) followed by characters that case-insensitively match the tag name of the element followed by one of "tab" (U+0009), "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), U+0020 SPACE, ">" (U+003E), or "/" (U+002F).

8.1.3 Text

Text is allowed inside elements, attribute values, and comments. Extra constraints are placed on what is and what is not allowed in text based on where the text is to be put, as described in the other sections.

8.1.3.1 Newlines

Newlines in HTML may be represented either as "CR" (U+000D) characters, "LF" (U+000A) characters, or pairs of "CR" (U+000D), "LF" (U+000A) characters in that order.

Where character references are allowed, a character reference of a "LF" (U+000A) character (but not a "CR" (U+000D) character) also represents a newline.

8.1.4 Character references

In certain cases described in other sections, text may be mixed with character references. These can be used to escape characters that couldn't otherwise legally be included in text.

Character references must start with a U+0026 AMPERSAND character (&). Following this, there are three possible kinds of character references:

Named character references: The ampersand must be followed by one of the names given in the named character references section, using the same case. The name must be one that is terminated by a ";" (U+003B) character.
Decimal numeric character reference: The ampersand must be followed by a "#" (U+0023) character, followed by one or more ASCII digits, representing a base-ten integer that corresponds to a Unicode code point that is allowed according to the definition below. The digits must then be followed by a ";" (U+003B) character.
Hexadecimal numeric character reference: The ampersand must be followed by a "#" (U+0023) character, which must be followed by either a "x" (U+0078) character or a "X" (U+0058) character, which must then be followed by one or more ASCII hex digits, representing a base-sixteen integer that corresponds to a Unicode code point that is allowed according to the definition below. The digits must then be followed by a ";" (U+003B) character.

The numeric character reference forms described above are allowed to reference any Unicode code point other than U+0000, U+000D, permanently undefined Unicode characters (noncharacters), surrogates (U+D800–U+DFFF), and control characters other than space characters.

An ambiguous ampersand is a U+0026 AMPERSAND character (&) that is followed by one or more alphanumeric ASCII characters, followed by a ";" (U+003B) character, where these characters do not match any of the names given in the named character references section.

8.1.5 CDATA sections

CDATA sections must consist of the following components, in this order:

The string "<![CDATA[".
Optionally, text, with the additional restriction that the text must not contain the string "]]>".
The string "]]>".

CDATA sections can only be used in foreign content (MathML or SVG). In this example, a CDATA section is used to escape the contents of an ms element:

<p>You can add a string to a number, but this stringifies the number:</p>
<math>
 <ms><![CDATA[x<y]]></ms>
 <mo>+</mo>
 <mn>3</mn>
 <mo>=</mo>
 <ms><![CDATA[x<y3]]></ms>
</math>

8.1.6 Comments

Comments must start with the four character sequence U+003C LESS-THAN SIGN, U+0021 EXCLAMATION MARK, U+002D HYPHEN-MINUS, U+002D HYPHEN-MINUS ().

8.2 Parsing HTML documents

This section only applies to user agents, data mining tools, and conformance checkers.

The rules for parsing XML documents into DOM trees are covered by the next section, entitled "The XHTML syntax".

User agents must use the parsing rules described in this section to generate the DOM trees from text/html resources. Together, these rules define what is referred to as the HTML parser.

While the HTML syntax described in this specification bears a close resemblance to SGML and XML, it is a separate language with its own parsing rules.

Some earlier versions of HTML (in particular from HTML2 to HTML4) were based on SGML and used SGML parsing rules. However, few (if any) web browsers ever implemented true SGML parsing for HTML documents; the only user agents to strictly handle HTML as an SGML application have historically been validators. The resulting confusion — with validators claiming documents to have one representation while widely deployed Web browsers interoperably implemented a different representation — has wasted decades of productivity. This version of HTML thus returns to a non-SGML basis.

Authors interested in using SGML tools in their authoring pipeline are encouraged to use XML tools and the XML serialization of HTML.

This specification defines the parsing rules for HTML documents, whether they are syntactically correct or not. Certain points in the parsing algorithm are said to be parse errors. The error handling for parse errors is well-defined (that's the processing rules described throughout this specification), but user agents, while parsing an HTML document, may abort the parser at the first parse error that they encounter for which they do not wish to apply the rules described in this specification.

Conformance checkers must report at least one parse error condition to the user if one or more parse error conditions exist in the document and must not report parse error conditions if none exist in the document. Conformance checkers may report more than one parse error condition if more than one parse error condition exists in the document.

Parse errors are only errors with the syntax of HTML. In addition to checking for parse errors, conformance checkers will also verify that the document obeys all the other conformance requirements described in this specification.

For the purposes of conformance checkers, if a resource is determined to be in the HTML syntax, then it is an HTML document.

As stated in the terminology section, references to element types that do not explicitly specify a namespace always refer to elements in the HTML namespace. For example, if the spec talks about "a div element", then that is an element with the local name "div", the namespace "http://www.w3.org/1999/xhtml", and the interface HTMLDivElement. Where possible, references to such elements are hyperlinked to their definition.

8.2.1 Overview of the parsing model

The input to the HTML parsing process consists of a stream of Unicode code points, which is passed through a tokenization stage followed by a tree construction stage. The output is a Document object.

Implementations that do not support scripting do not have to actually create a DOM Document object, but the DOM tree in such cases is still used as the model for the rest of the specification.

In the common case, the data handled by the tokenization stage comes from the network, but it can also come from script running in the user agent, e.g. using the document.write() API.

There is only one set of states for the tokenizer stage and the tree construction stage, but the tree construction stage is reentrant, meaning that while the tree construction stage is handling one token, the tokenizer might be resumed, causing further tokens to be emitted and processed before the first token's processing is complete.

In the following example, the tree construction stage will be called upon to handle a "p" start tag token while handling the "script" end tag token:

...
<script>
 document.write('<p>');
</script>
...

To handle these cases, parsers have a script nesting level, which must be initially set to zero, and a parser pause flag, which must be initially set to false.

8.2.2 The input byte stream

The stream of Unicode code points that comprises the input to the tokenization stage will be initially seen by the user agent as a stream of bytes (typically coming over the network or from the local file system). The bytes encode the actual characters according to a particular character encoding, which the user agent uses to decode the bytes into characters.

For XML documents, the algorithm user agents must use to determine the character encoding is given by the XML specification. This section does not apply to XML documents. [XML]

Usually, the encoding sniffing algorithm defined below is used to determine the character encoding.

Given a character encoding, the bytes in the input byte stream must be converted to Unicode code points for the tokenizer's input stream, as described by the rules for that encoding's decoder.

Bytes or sequences of bytes in the original byte stream that did not conform to the encoding specification (e.g. invalid UTF-8 byte sequences in a UTF-8 input byte stream) are errors that conformance checkers are expected to report.

Leading Byte Order Marks (BOMs) are not stripped by the decoder algorithms, they are stripped by the algorithm below.

The decoder algorithms describe how to handle invalid input; for security reasons, it is imperative that those rules be followed precisely. Differences in how invalid byte sequences are handled can result in, amongst other problems, script injection vulnerabilities ("XSS").

When the HTML parser is decoding an input byte stream, it uses a character encoding and a confidence. The confidence is either tentative, certain, or irrelevant. The encoding used, and whether the confidence in that encoding is tentative or certain, is used during the parsing to determine whether to change the encoding. If no encoding is necessary, e.g. because the parser is operating on a Unicode stream and doesn't have to use a character encoding at all, then the confidence is irrelevant.

Some algorithms feed the parser by directly adding characters to the input stream rather than adding bytes to the input byte stream.

8.2.2.1 Parsing with a known character encoding

When the HTML parser is to operate on an input byte stream that has a known definite encoding, then the character encoding is that encoding and the confidence is certain.

8.2.2.2 Determining the character encoding

In some cases, it might be impractical to unambiguously determine the encoding before parsing the document. Because of this, this specification provides for a two-pass mechanism with an optional pre-scan. Implementations are allowed, as described below, to apply a simplified parsing algorithm to whatever bytes they have available before beginning to parse the document. Then, the real parser is started, using a tentative encoding derived from this pre-parse and other out-of-band metadata. If, while the document is being loaded, the user agent discovers a character encoding declaration that conflicts with this information, then the parser can get reinvoked to perform a parse of the document with the real encoding.

User agents must use the following algorithm, called the encoding sniffing algorithm, to determine the character encoding to use when decoding a document in the first pass. This algorithm takes as input any out-of-band metadata available to the user agent (e.g. the Content-Type metadata of the document) and all the bytes available so far, and returns a character encoding and a confidence that is either tentative or certain.

If the user has explicitly instructed the user agent to override the document's character encoding with a specific encoding, optionally return that encoding with the confidence certain and abort these steps.

Typically, user agents remember such user requests across sessions, and in some cases apply them to documents in iframes as well.
The user agent may wait for more bytes of the resource to be available, either in this step or at any later step in this algorithm. For instance, a user agent might wait 500ms or 1024 bytes, whichever came first. In general preparsing the source to find the encoding improves performance, as it reduces the need to throw away the data structures used when parsing upon finding the encoding information. However, if the user agent delays too long to obtain data to determine the encoding, then the cost of the delay could outweigh any performance improvements from the preparse.

The authoring conformance requirements for character encoding declarations limit them to only appearing in the first 1024 bytes. User agents are therefore encouraged to use the prescan algorithm below (as invoked by these steps) on the first 1024 bytes, but not to stall beyond that.
For each of the rows in the following table, starting with the first one and going down, if there are as many or more bytes available than the number of bytes in the first column, and the first bytes of the file match the bytes given in the first column, then return the encoding given in the cell in the second column of that row, with the confidence certain, and abort these steps:

Bytes in Hexadecimal Encoding

FE FF Big-endian UTF-16
FF FE Little-endian UTF-16
EF BB BF UTF-8

This step looks for Unicode Byte Order Marks (BOMs).

That this step happens before the next one honoring the HTTP Content-Type header is a willful violation of the HTTP specification, motivated by a desire to be maximally compatible with legacy content. [HTTP]
If the transport layer specifies a character encoding, and it is supported, return that encoding with the confidence certain, and abort these steps.
Optionally prescan the byte stream to determine its encoding. The end condition is that the user agent decides that scanning further bytes would not be efficient. User agents are encouraged to only prescan the first 1024 bytes. User agents may decide that scanning any bytes is not efficient, in which case these substeps are entirely skipped.

The aforementioned algorithm either aborts unsuccessfully or returns a character encoding. If it returns a character encoding, then this algorithm must be aborted, returning the same encoding, with confidence tentative.
If the HTML parser for which this algorithm is being run is associated with a Document that is itself in a nested browsing context, run these substeps:
1. Let new document be the Document with which the HTML parser is associated.
2. Let parent document be the Document through which new document is nested (the active document of the parent browsing context of new document).
3. If parent document's origin is not the same origin as new document's origin, then abort these substeps.
4. If parent document's character encoding is not an ASCII-compatible character encoding, then abort these substeps.
5. Return parent document's character encoding, with the confidence tentative, and abort the encoding sniffing algorithm's steps.
Otherwise, if the user agent has information on the likely encoding for this page, e.g. based on the encoding of the page when it was last visited, then return that encoding, with the confidence tentative, and abort these steps.
The user agent may attempt to autodetect the character encoding from applying frequency analysis or other algorithms to the data stream. Such algorithms may use information about the resource other than the resource's contents, including the address of the resource. If autodetection succeeds in determining a character encoding, and that encoding is a supported encoding, then return that encoding, with the confidence tentative, and abort these steps. [UNIVCHARDET]

The UTF-8 encoding has a highly detectable bit pattern. Documents that contain bytes with values greater than 0x7F which match the UTF-8 pattern are very likely to be UTF-8, while documents with byte sequences that do not match it are very likely not. User-agents are therefore encouraged to search for this common encoding. [PPUTF8] [UTF8DET]

Bytes in Hexadecimal	Encoding
FE FF	Big-endian UTF-16
FF FE	Little-endian UTF-16
EF BB BF	UTF-8

Otherwise, return an implementation-defined or user-specified default character encoding, with the confidence tentative.

In controlled environments or in environments where the encoding of documents can be prescribed (for example, for user agents intended for dedicated use in new networks), the comprehensive UTF-8 encoding is suggested.

In other environments, the default encoding is typically dependent on the user's locale (an approximation of the languages, and thus often encodings, of the pages that the user is likely to frequent). The following table gives suggested defaults based on the user's locale, for compatibility with legacy content. Locales are identified by BCP 47 language tags. [BCP47] [ENCODING]

Locale language		Suggested default encoding
ar	Arabic	windows-1256
ba	Bashkir	windows-1251
be	Belarusian	windows-1251
bg	Bulgarian	windows-1251
cs	Czech	windows-1250
el	Greek	ISO-8859-7
et	Estonian	windows-1257
fa	Persian	windows-1256
he	Hebrew	windows-1255
hr	Croatian	windows-1250
hu	Hungarian	ISO-8859-2
ja	Japanese	Shift_JIS
kk	Kazakh	windows-1251
ko	Korean	euc-kr
ku	Kurdish	windows-1254
ky	Kyrgyz	windows-1251
lt	Lithuanian	windows-1257
lv	Latvian	windows-1257
mk	Macedonian	windows-1251
pl	Polish	ISO-8859-2
ru	Russian	windows-1251
sah	Yakut	windows-1251
sk	Slovak	windows-1250
sl	Slovenian	ISO-8859-2
sr	Serbian	windows-1251
tg	Tajik	windows-1251
th	Thai	windows-874
tr	Turkish	windows-1254
tt	Tatar	windows-1251
uk	Ukrainian	windows-1251
vi	Vietnamese	windows-1258
zh-CN	Chinese (People's Republic of China)	GB18030
zh-TW	Chinese (Taiwan)	Big5
All other locales		windows-1252

The contents of this table are derived from the intersection of Windows, Chrome, and Firefox defaults.

The document's character encoding must immediately be set to the value returned from this algorithm, at the same time as the user agent uses the returned value to select the decoder to use for the input byte stream.

When an algorithm requires a user agent to prescan a byte stream to determine its encoding, given some defined end condition, then it must run the following steps. These steps either abort unsuccessfully or return a character encoding. If at any point during these steps (including during instances of the get an attribute algorithm invoked by this one) the user agent either runs out of bytes (meaning the position pointer created in the first step below goes beyond the end of the byte stream obtained so far) or reaches its end condition, then abort the prescan a byte stream to determine its encoding algorithm unsuccessfully.

Let position be a pointer to a byte in the input byte stream, initially pointing at the first byte.
Loop: If position points to:
A sequence of bytes starting with: 0x3C 0x21 0x2D 0x2D (ASCII '' sequence) and comes after the 0x3C byte that was found. (The two 0x2D bytes can be the same as the those in the '<!--' sequence.)

A sequence of bytes starting with: 0x3C, 0x4D or 0x6D, 0x45 or 0x65, 0x54 or 0x74, 0x41 or 0x61, and one of 0x09, 0x0A, 0x0C, 0x0D, 0x20, 0x2F (case-insensitive ASCII '<meta' followed by a space or slash)
1. Advance the position pointer so that it points at the next 0x09, 0x0A, 0x0C, 0x0D, 0x20, or 0x2F byte (the one in sequence of characters matched above).
2. Let attribute list be an empty list of strings.
3. Let got pragma be false.
4. Let need pragma be null.
5. Let charset be the null value (which, for the purposes of this algorithm, is distinct from an unrecognised encoding or the empty string).
6. Attributes: Get an attribute and its value. If no attribute was sniffed, then jump to the processing step below.
7. If the attribute's name is already in attribute list, then return to the step labeled attributes.
8. Add the attribute's name to attribute list.
9. Run the appropriate step from the following list, if one applies:
  
  If the attribute's name is "http-equiv"
  
  If the attribute's value is "content-type", then set got pragma to true.
  
  If the attribute's name is "content"
  
  Apply the algorithm for extracting a character encoding from a meta element, giving the attribute's value as the string to parse. If a character encoding is returned, and if charset is still set to null, let charset be the encoding returned, and set need pragma to true.
  
  If the attribute's name is "charset"
  
  Let charset be the result of getting an encoding from the attribute's value, and set need pragma to false.
10. Return to the step labeled attributes.
11. Processing: If need pragma is null, then jump to the step below labeled next byte.
12. If need pragma is true but got pragma is false, then jump to the step below labeled next byte.
13. If charset is a UTF-16 encoding, change the value of charset to UTF-8.
14. If charset is not a supported character encoding, then jump to the step below labeled next byte.
15. Abort the prescan a byte stream to determine its encoding algorithm, returning the encoding given by charset.
A sequence of bytes starting with a 0x3C byte (ASCII <), optionally a 0x2F byte (ASCII /), and finally a byte in the range 0x41-0x5A or 0x61-0x7A (an ASCII letter)
1. Advance the position pointer so that it points at the next 0x09 (ASCII TAB), 0x0A (ASCII LF), 0x0C (ASCII FF), 0x0D (ASCII CR), 0x20 (ASCII space), or 0x3E (ASCII >) byte.
2. Repeatedly get an attribute until no further attributes can be found, then jump to the step below labeled next byte.
A sequence of bytes starting with: 0x3C 0x21 (ASCII '<!')

A sequence of bytes starting with: 0x3C 0x2F (ASCII '</')

A sequence of bytes starting with: 0x3C 0x3F (ASCII '<?')

Advance the position pointer so that it points at the first 0x3E byte (ASCII >) that comes after the 0x3C byte that was found.

Any other byte

Do nothing with that byte.
Next byte: Move position so it points at the next byte in the input byte stream, and return to the step above labeled loop.

When the prescan a byte stream to determine its encoding algorithm says to get an attribute, it means doing this:

If the byte at position is one of 0x09 (ASCII TAB), 0x0A (ASCII LF), 0x0C (ASCII FF), 0x0D (ASCII CR), 0x20 (ASCII space), or 0x2F (ASCII /) then advance position to the next byte and redo this step.
If the byte at position is 0x3E (ASCII >), then abort the get an attribute algorithm. There isn't one.
Otherwise, the byte at position is the start of the attribute name. Let attribute name and attribute value be the empty string.
Process the byte at position as follows:

If it is 0x3D (ASCII =), and the attribute name is longer than the empty string

Advance position to the next byte and jump to the step below labeled value.

If it is 0x09 (ASCII TAB), 0x0A (ASCII LF), 0x0C (ASCII FF), 0x0D (ASCII CR), or 0x20 (ASCII space)

Jump to the step below labeled spaces.

If it is 0x2F (ASCII /) or 0x3E (ASCII >)

Abort the get an attribute algorithm. The attribute's name is the value of attribute name, its value is the empty string.

If it is in the range 0x41 (ASCII A) to 0x5A (ASCII Z)

Append the Unicode character with code point b+0x20 to attribute name (where b is the value of the byte at position). (This converts the input to lowercase.)

Anything else

Append the Unicode character with the same code point as the value of the byte at position to attribute name. (It doesn't actually matter how bytes outside the ASCII range are handled here, since only ASCII characters can contribute to the detection of a character encoding.)
Advance position to the next byte and return to the previous step.
Spaces: If the byte at position is one of 0x09 (ASCII TAB), 0x0A (ASCII LF), 0x0C (ASCII FF), 0x0D (ASCII CR), or 0x20 (ASCII space) then advance position to the next byte, then, repeat this step.
If the byte at position is not 0x3D (ASCII =), abort the get an attribute algorithm. The attribute's name is the value of attribute name, its value is the empty string.
Advance position past the 0x3D (ASCII =) byte.
Value: If the byte at position is one of 0x09 (ASCII TAB), 0x0A (ASCII LF), 0x0C (ASCII FF), 0x0D (ASCII CR), or 0x20 (ASCII space) then advance position to the next byte, then, repeat this step.
Process the byte at position as follows:
If it is 0x22 (ASCII ") or 0x27 (ASCII ')
1. Let b be the value of the byte at position.
2. Quote loop: Advance position to the next byte.
3. If the value of the byte at position is the value of b, then advance position to the next byte and abort the "get an attribute" algorithm. The attribute's name is the value of attribute name, and its value is the value of attribute value.
4. Otherwise, if the value of the byte at position is in the range 0x41 (ASCII A) to 0x5A (ASCII Z), then append a Unicode character to attribute value whose code point is 0x20 more than the value of the byte at position.
5. Otherwise, append a Unicode character to attribute value whose code point is the same as the value of the byte at position.
6. Return to the step above labeled quote loop.
If it is 0x3E (ASCII >)

Abort the get an attribute algorithm. The attribute's name is the value of attribute name, its value is the empty string.

If it is in the range 0x41 (ASCII A) to 0x5A (ASCII Z)

Append the Unicode character with code point b+0x20 to attribute value (where b is the value of the byte at position). Advance position to the next byte.

Anything else

Append the Unicode character with the same code point as the value of the byte at position to attribute value. Advance position to the next byte.
Process the byte at position as follows:

If it is 0x09 (ASCII TAB), 0x0A (ASCII LF), 0x0C (ASCII FF), 0x0D (ASCII CR), 0x20 (ASCII space), or 0x3E (ASCII >)

Abort the get an attribute algorithm. The attribute's name is the value of attribute name and its value is the value of attribute value.

If it is in the range 0x41 (ASCII A) to 0x5A (ASCII Z)

Append the Unicode character with code point b+0x20 to attribute value (where b is the value of the byte at position).

Anything else

Append the Unicode character with the same code point as the value of the byte at position to attribute value.
Advance position to the next byte and return to the previous step.

For the sake of interoperability, user agents should not use a pre-scan algorithm that returns different results than the one described above. (But, if you do, please at least let us know, so that we can improve this algorithm and benefit everyone...)

8.2.2.3 Character encodings

User agents must support the encodings defined in the Encoding standard. User agents should not support other encodings.

User agents must not support the CESU-8, UTF-7, BOCU-1 and SCSU encodings. [CESU8] [UTF7] [BOCU1] [SCSU]

Support for encodings based on EBCDIC is especially discouraged. This encoding is rarely used for publicly-facing Web content. Support for UTF-32 is also especially discouraged. This encoding is rarely used, and frequently implemented incorrectly.

This specification does not make any attempt to support EBCDIC-based encodings and UTF-32 in its algorithms; support and use of these encodings can thus lead to unexpected behavior in implementations of this specification.

8.2.2.4 Changing the encoding while parsing

When the parser requires the user agent to change the encoding, it must run the following steps. This might happen if the encoding sniffing algorithm described above failed to find a character encoding, or if it found a character encoding that was not the actual encoding of the file.

If the encoding that is already being used to interpret the input stream is a UTF-16 encoding, then set the confidence to certain and abort these steps. The new encoding is ignored; if it was anything but the same encoding, then it would be clearly incorrect.
If the new encoding is a UTF-16 encoding, change it to UTF-8.
If the new encoding is identical or equivalent to the encoding that is already being used to interpret the input stream, then set the confidence to certain and abort these steps. This happens when the encoding information found in the file matches what the encoding sniffing algorithm determined to be the encoding, and in the second pass through the parser if the first pass found that the encoding sniffing algorithm described in the earlier section failed to find the right encoding.
If all the bytes up to the last byte converted by the current decoder have the same Unicode interpretations in both the current encoding and the new encoding, and if the user agent supports changing the converter on the fly, then the user agent may change to the new converter for the encoding on the fly. Set the document's character encoding and the encoding used to convert the input stream to the new encoding, set the confidence to certain, and abort these steps.
Otherwise, navigate to the document again, with replacement enabled, and using the same source browsing context, but this time skip the encoding sniffing algorithm and instead just set the encoding to the new encoding and the confidence to certain. Whenever possible, this should be done without actually contacting the network layer (the bytes should be re-parsed from memory), even if, e.g., the document is marked as not being cacheable. If this is not possible and contacting the network layer would involve repeating a request that uses a method other than HTTP GET (or equivalent for non-HTTP URLs), then instead set the confidence to certain and ignore the new encoding. The resource will be misinterpreted. User agents may notify the user of the situation, to aid in application development.

8.2.2.5 Preprocessing the input stream

The input stream consists of the characters pushed into it as the input byte stream is decoded or from the various APIs that directly manipulate the input stream.

One leading U+FEFF BYTE ORDER MARK character must be ignored if any are present in the input stream.

The requirement to strip a U+FEFF BYTE ORDER MARK character regardless of whether that character was used to determine the byte order is a willful violation of Unicode, motivated by a desire to increase the resilience of user agents in the face of naïve transcoders.

Any occurrences of any characters in the ranges U+0001 to U+0008, U+000E to U+001F, U+007F to U+009F, U+FDD0 to U+FDEF, and characters U+000B, U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, U+2FFFE, U+2FFFF, U+3FFFE, U+3FFFF, U+4FFFE, U+4FFFF, U+5FFFE, U+5FFFF, U+6FFFE, U+6FFFF, U+7FFFE, U+7FFFF, U+8FFFE, U+8FFFF, U+9FFFE, U+9FFFF, U+AFFFE, U+AFFFF, U+BFFFE, U+BFFFF, U+CFFFE, U+CFFFF, U+DFFFE, U+DFFFF, U+EFFFE, U+EFFFF, U+FFFFE, U+FFFFF, U+10FFFE, and U+10FFFF are parse errors. These are all control characters or permanently undefined Unicode characters (noncharacters).

Any character that is a not a Unicode character, i.e. any isolated surrogate, is a parse error. (These can only find their way into the input stream via script APIs such as document.write().)

"CR" (U+000D) characters and "LF" (U+000A) characters are treated specially. All CR characters must be converted to LF characters, and any LF characters that immediately follow a CR character must be ignored. Thus, newlines in HTML DOMs are represented by LF characters, and there are never any CR characters in the input to the tokenization stage.

The next input character is the first character in the input stream that has not yet been consumed or explicitly ignored by the requirements in this section. Initially, the next input character is the first character in the input. The current input character is the last character to have been consumed.

The insertion point is the position (just before a character or just before the end of the input stream) where content inserted using document.write() is actually inserted. The insertion point is relative to the position of the character immediately after it, it is not an absolute offset into the input stream. Initially, the insertion point is undefined.

The "EOF" character in the tables below is a conceptual character representing the end of the input stream. If the parser is a script-created parser, then the end of the input stream is reached when an explicit "EOF" character (inserted by the document.close() method) is consumed. Otherwise, the "EOF" character is not a real character in the stream, but rather the lack of any further characters.

The handling of U+0000 NULL characters varies based on where the characters are found. In general, they are ignored except where doing so could plausibly introduce an attack vector. This handling is, by necessity, spread across both the tokenization stage and the tree construction stage.

8.2.3 Parse state

8.2.3.1 The insertion mode

The insertion mode is a state variable that controls the primary operation of the tree construction stage.

Initially, the insertion mode is "initial". It can change to "before html", "before head", "in head", "in head noscript", "after head", "in body", "text", "in table", "in table text", "in caption", "in column group", "in table body", "in row", "in cell", "in select", "in select in table", "in template", "after body", "in frameset", "after frameset", "after after body", and "after after frameset" during the course of the parsing, as described in the tree construction stage. The insertion mode affects how tokens are processed and whether CDATA sections are supported.

Several of these modes, namely "in head", "in body", "in table", and "in select", are special, in that the other modes defer to them at various times. When the algorithm below says that the user agent is to do something "using the rules for the m insertion mode", where m is one of these modes, the user agent must use the rules described under the m insertion mode's section, but must leave the insertion mode unchanged unless the rules in m themselves switch the insertion mode to a new value.

When the insertion mode is switched to "text" or "in table text", the original insertion mode is also set. This is the insertion mode to which the tree construction stage will return.

Similarly, to parse nested template elements, a stack of template insertion modes is used. It is initially empty. The current template insertion mode is the insertion mode that was most recently added to the stack of template insertion modes. The algorithms in the sections below will push insertion modes onto this stack, meaning that the specified insertion mode is to be added to the stack, and pop insertion modes from the stack, which means that the most recently added insertion mode must be removed from the stack.

When the steps below require the UA to reset the insertion mode appropriately, it means the UA must follow these steps:

Let last be false.
Let node be the last node in the stack of open elements.
Loop: If node is the first node in the stack of open elements, then set last to true, and, if the parser was originally created as part of the HTML fragment parsing algorithm (fragment case) set node to the context element.
If node is a select element, run these substeps:
1. If last is true, jump to the step below labeled done.
2. Let ancestor be node.
3. Loop: If ancestor is the first node in the stack of open elements, jump to the step below labeled done.
4. Let ancestor be the node before ancestor in the stack of open elements.
5. If ancestor is a template node, jump to the step below labeled done.
6. If ancestor is a table node, switch the insertion mode to "in select in table" and abort these steps.
7. Jump back to the step labeled loop.
8. Done: Switch the insertion mode to "in select" and abort these steps.
If node is a td or th element and last is false, then switch the insertion mode to "in cell" and abort these steps.
If node is a tr element, then switch the insertion mode to "in row" and abort these steps.
If node is a tbody, thead, or tfoot element, then switch the insertion mode to "in table body" and abort these steps.
If node is a caption element, then switch the insertion mode to "in caption" and abort these steps.
If node is a colgroup element, then switch the insertion mode to "in column group" and abort these steps.
If node is a table element, then switch the insertion mode to "in table" and abort these steps.
If node is a template element, then switch the insertion mode to the current template insertion mode and abort these steps.
If node is a head element and last is true, then switch the insertion mode to "in body" ("in body"! not "in head"!) and abort these steps. (fragment case)
If node is a head element and last is false, then switch the insertion mode to "in head" and abort these steps.
If node is a body element, then switch the insertion mode to "in body" and abort these steps.
If node is a frameset element, then switch the insertion mode to "in frameset" and abort these steps. (fragment case)
If node is an html element, run these substeps:
1. If the head element pointer is null, switch the insertion mode to "before head" and abort these steps. (fragment case)
2. Otherwise, the head element pointer is not null, switch the insertion mode to "after head" and abort these steps.
If last is true, then switch the insertion mode to "in body" and abort these steps. (fragment case)
Let node now be the node before node in the stack of open elements.
Return to the step labeled loop.

8.2.3.2 The stack of open elements

Initially, the stack of open elements is empty. The stack grows downwards; the topmost node on the stack is the first one added to the stack, and the bottommost node of the stack is the most recently added node in the stack (notwithstanding when the stack is manipulated in a random access fashion as part of the handling for misnested tags).

The "before html" insertion mode creates the html root element node, which is then added to the stack.

In the fragment case, the stack of open elements is initialized to contain an html element that is created as part of that algorithm. (The fragment case skips the "before html" insertion mode.)

The html node, however it is created, is the topmost node of the stack. It only gets popped off the stack when the parser finishes.

The current node is the bottommost node in this stack of open elements.

The adjusted current node is the context element if the stack of open elements has only one element in it and the parser was created by the HTML fragment parsing algorithm; otherwise, the adjusted current node is the current node.

Elements in the stack of open elements fall into the following categories:

Special: The following elements have varying levels of special parsing rules: HTML's address, applet, area, article, aside, base, basefont, bgsound, blockquote, body, br, button, caption, center, col, colgroup, dd, details, dir, div, dl, dt, embed, fieldset, figcaption, figure, footer, form, frame, frameset, h1, h2, h3, h4, h5, h6, head, header, hgroup, hr, html, iframe, img, input, isindex, li, link, listing, main, marquee, meta, nav, noembed, noframes, noscript, object, ol, p, param, plaintext, pre, script, section, select, source, style, summary, table, tbody, td, template, textarea, tfoot, th, thead, title, tr, track, ul, wbr, and xmp; MathML's mi, mo, mn, ms, mtext, and annotation-xml; and SVG's foreignObject, desc, and title.
Formatting: The following HTML elements are those that end up in the list of active formatting elements: a, b, big, code, em, font, i, nobr, s, small, strike, strong, tt, and u.
Ordinary: All other elements found while parsing an HTML document.

The stack of open elements is said to have an element target node in a specific scope consisting of a list of element types list when the following algorithm terminates in a match state:

Initialize node to be the current node (the bottommost node of the stack).
If node is the target node, terminate in a match state.
Otherwise, if node is one of the element types in list, terminate in a failure state.
Otherwise, set node to the previous entry in the stack of open elements and return to step 2. (This will never fail, since the loop will always terminate in the previous step if the top of the stack — an html element — is reached.)

The stack of open elements is said to have a particular element in scope when it has that element in the specific scope consisting of the following element types:

applet in the HTML namespace
caption in the HTML namespace
html in the HTML namespace
table in the HTML namespace
td in the HTML namespace
th in the HTML namespace
marquee in the HTML namespace
object in the HTML namespace
template in the HTML namespace
mi in the MathML namespace
mo in the MathML namespace
mn in the MathML namespace
ms in the MathML namespace
mtext in the MathML namespace
annotation-xml in the MathML namespace
foreignObject in the SVG namespace
desc in the SVG namespace
title in the SVG namespace

The stack of open elements is said to have a particular element in list item scope when it has that element in the specific scope consisting of the following element types:

All the element types listed above for the has an element in scope algorithm.
ol in the HTML namespace
ul in the HTML namespace

The stack of open elements is said to have a particular element in button scope when it has that element in the specific scope consisting of the following element types:

All the element types listed above for the has an element in scope algorithm.
button in the HTML namespace

The stack of open elements is said to have a particular element in table scope when it has that element in the specific scope consisting of the following element types:

html in the HTML namespace
table in the HTML namespace
template in the HTML namespace

The stack of open elements is said to have a particular element in select scope when it has that element in the specific scope consisting of all element types except the following:

optgroup in the HTML namespace
option in the HTML namespace

Nothing happens if at any time any of the elements in the stack of open elements are moved to a new location in, or removed from, the Document tree. In particular, the stack is not changed in this situation. This can cause, amongst other strange effects, content to be appended to nodes that are no longer in the DOM.

In some cases (namely, when closing misnested formatting elements), the stack is manipulated in a random-access fashion.

8.2.3.3 The list of active formatting elements

Initially, the list of active formatting elements is empty. It is used to handle mis-nested formatting element tags.

The list contains elements in the formatting category, and scope markers. The scope markers are inserted when entering applet elements, buttons, object elements, marquees, table cells, and table captions, and are used to prevent formatting from "leaking" into applet elements, buttons, object elements, marquees, and tables.

The scope markers are unrelated to the concept of an element being in scope.

In addition, each element in the list of active formatting elements is associated with the token for which it was created, so that further elements can be created for that token if necessary.

When the steps below require the UA to push onto the list of active formatting elements an element element, the UA must perform the following steps:

If there are already three elements in the list of active formatting elements after the last list marker, if any, or anywhere in the list if there are no list markers, that have the same tag name, namespace, and attributes as element, then remove the earliest such element from the list of active formatting elements. For these purposes, the attributes must be compared as they were when the elements were created by the parser; two elements have the same attributes if all their parsed attributes can be paired such that the two attributes in each pair have identical names, namespaces, and values (the order of the attributes does not matter).

This is the Noah's Ark clause. But with three per family instead of two.
Add element to the list of active formatting elements.

When the steps below require the UA to reconstruct the active formatting elements, the UA must perform the following steps:

If there are no entries in the list of active formatting elements, then there is nothing to reconstruct; stop this algorithm.
If the last (most recently added) entry in the list of active formatting elements is a marker, or if it is an element that is in the stack of open elements, then there is nothing to reconstruct; stop this algorithm.
Let entry be the last (most recently added) element in the list of active formatting elements.
Rewind: If there are no entries before entry in the list of active formatting elements, then jump to the step labeled create.
Let entry be the entry one earlier than entry in the list of active formatting elements.
If entry is neither a marker nor an element that is also in the stack of open elements, go to the step labeled rewind.
Advance: Let entry be the element one later than entry in the list of active formatting elements.
Create: Insert an HTML element for the token for which the element entry was created, to obtain new element.
Replace the entry for entry in the list with an entry for new element.
If the entry for new element in the list of active formatting elements is not the last entry in the list, return to the step labeled advance.

This has the effect of reopening all the formatting elements that were opened in the current body, cell, or caption (whichever is youngest) that haven't been explicitly closed.

The way this specification is written, the list of active formatting elements always consists of elements in chronological order with the least recently added element first and the most recently added element last (except for while steps 8 to 11 of the above algorithm are being executed, of course).

When the steps below require the UA to clear the list of active formatting elements up to the last marker, the UA must perform the following steps:

Let entry be the last (most recently added) entry in the list of active formatting elements.
Remove entry from the list of active formatting elements.
If entry was a marker, then stop the algorithm at this point. The list has been cleared up to the last marker.
Go to step 1.

8.2.3.4 The element pointers

Initially, the head element pointer and the form element pointer are both null.

Once a head element has been parsed (whether implicitly or explicitly) the head element pointer gets set to point to this node.

The form element pointer points to the last form element that was opened and whose end tag has not yet been seen. It is used to make form controls associate with forms in the face of dramatically bad markup, for historical reasons. It is ignored inside template elements.

8.2.3.5 Other parsing state flags

The scripting flag is set to "enabled" if scripting was enabled for the Document with which the parser is associated when the parser was created, and "disabled" otherwise.

The scripting flag can be enabled even when the parser was originally created for the HTML fragment parsing algorithm, even though script elements don't execute in that case.

The frameset-ok flag is set to "ok" when the parser is created. It is set to "not ok" after certain tokens are seen.

8.2.4 Tokenization

Implementations must act as if they used the following state machine to tokenize HTML. The state machine must start in the data state. Most states consume a single character, which may have various side-effects, and either switches the state machine to a new state to reconsume the same character, or switches it to a new state to consume the next character, or stays in the same state to consume the next character. Some states have more complicated behavior and can consume several characters before switching to another state. In some cases, the tokenizer state is also changed by the tree construction stage.

The exact behavior of certain states depends on the insertion mode and the stack of open elements. Certain states also use a temporary buffer to track progress.

The output of the tokenization step is a series of zero or more of the following tokens: DOCTYPE, start tag, end tag, comment, character, end-of-file. DOCTYPE tokens have a name, a public identifier, a system identifier, and a force-quirks flag. When a DOCTYPE token is created, its name, public identifier, and system identifier must be marked as missing (which is a distinct state from the empty string), and the force-quirks flag must be set to off (its other state is on). Start and end tag tokens have a tag name, a self-closing flag, and a list of attributes, each of which has a name and a value. When a start or end tag token is created, its self-closing flag must be unset (its other state is that it be set), and its attributes list must be empty. Comment and character tokens have data.

When a token is emitted, it must immediately be handled by the tree construction stage. The tree construction stage can affect the state of the tokenization stage, and can insert additional characters into the stream. (For example, the script element can result in scripts executing and using the dynamic markup insertion APIs to insert characters into the stream being tokenized.)

Creating a token and emitting it are distinct actions. It is possible for a token to be created but implicitly abandoned (never emitted), e.g. if the file ends unexpectedly while processing the characters that are being parsed into a start tag token.

When a start tag token is emitted with its self-closing flag set, if the flag is not acknowledged when it is processed by the tree construction stage, that is a parse error.

When an end tag token is emitted with attributes, that is a parse error.

When an end tag token is emitted with its self-closing flag set, that is a parse error.

An appropriate end tag token is an end tag token whose tag name matches the tag name of the last start tag to have been emitted from this tokenizer, if any. If no start tag has been emitted from this tokenizer, then no end tag token is appropriate.

Before each step of the tokenizer, the user agent must first check the parser pause flag. If it is true, then the tokenizer must abort the processing of any nested invocations of the tokenizer, yielding control back to the caller.

The tokenizer state machine consists of the states defined in the following subsections.

8.2.4.1 Data state

Consume the next input character:

U+0026 AMPERSAND (&): Switch to the character reference in data state.
"<" (U+003C): Switch to the tag open state.
U+0000 NULL: Parse error. Emit the current input character as a character token.
EOF: Emit an end-of-file token.
Anything else: Emit the current input character as a character token.

8.2.4.2 Character reference in data state

Switch to the data state.

Attempt to consume a character reference, with no additional allowed character.

If nothing is returned, emit a U+0026 AMPERSAND character (&) token.

Otherwise, emit the character tokens that were returned.

8.2.4.3 RCDATA state

Consume the next input character:

U+0026 AMPERSAND (&): Switch to the character reference in RCDATA state.
"<" (U+003C): Switch to the RCDATA less-than sign state.
U+0000 NULL: Parse error. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Emit an end-of-file token.
Anything else: Emit the current input character as a character token.

8.2.4.4 Character reference in RCDATA state

Switch to the RCDATA state.

Attempt to consume a character reference, with no additional allowed character.

If nothing is returned, emit a U+0026 AMPERSAND character (&) token.

Otherwise, emit the character tokens that were returned.

8.2.4.5 RAWTEXT state

Consume the next input character:

"<" (U+003C): Switch to the RAWTEXT less-than sign state.
U+0000 NULL: Parse error. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Emit an end-of-file token.
Anything else: Emit the current input character as a character token.

8.2.4.6 Script data state

Consume the next input character:

"<" (U+003C): Switch to the script data less-than sign state.
U+0000 NULL: Parse error. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Emit an end-of-file token.
Anything else: Emit the current input character as a character token.

8.2.4.7 PLAINTEXT state

Consume the next input character:

U+0000 NULL: Parse error. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Emit an end-of-file token.
Anything else: Emit the current input character as a character token.

8.2.4.8 Tag open state

Consume the next input character:

"!" (U+0021): Switch to the markup declaration open state.
"/" (U+002F): Switch to the end tag open state.
Uppercase ASCII letter: Create a new start tag token, set its tag name to the lowercase version of the current input character (add 0x0020 to the character's code point), then switch to the tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Lowercase ASCII letter: Create a new start tag token, set its tag name to the current input character, then switch to the tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
"?" (U+003F): Parse error. Switch to the bogus comment state.
Anything else: Parse error. Switch to the data state. Emit a U+003C LESS-THAN SIGN character token. Reconsume the current input character.

8.2.4.9 End tag open state

Consume the next input character:

Uppercase ASCII letter: Create a new end tag token, set its tag name to the lowercase version of the current input character (add 0x0020 to the character's code point), then switch to the tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Lowercase ASCII letter: Create a new end tag token, set its tag name to the current input character, then switch to the tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
">" (U+003E): Parse error. Switch to the data state.
EOF: Parse error. Switch to the data state. Emit a U+003C LESS-THAN SIGN character token and a U+002F SOLIDUS character token. Reconsume the EOF character.
Anything else: Parse error. Switch to the bogus comment state.

8.2.4.10 Tag name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the before attribute name state.
"/" (U+002F): Switch to the self-closing start tag state.
">" (U+003E): Switch to the data state. Emit the current tag token.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current tag token's tag name.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current tag token's tag name.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Append the current input character to the current tag token's tag name.

8.2.4.11 RCDATA less-than sign state

Consume the next input character:

"/" (U+002F): Set the temporary buffer to the empty string. Switch to the RCDATA end tag open state.
Anything else: Switch to the RCDATA state. Emit a U+003C LESS-THAN SIGN character token. Reconsume the current input character.

8.2.4.12 RCDATA end tag open state

Consume the next input character:

Uppercase ASCII letter: Create a new end tag token, and set its tag name to the lowercase version of the current input character (add 0x0020 to the character's code point). Append the current input character to the temporary buffer. Finally, switch to the RCDATA end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Lowercase ASCII letter: Create a new end tag token, and set its tag name to the current input character. Append the current input character to the temporary buffer. Finally, switch to the RCDATA end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Anything else: Switch to the RCDATA state. Emit a U+003C LESS-THAN SIGN character token and a U+002F SOLIDUS character token. Reconsume the current input character.

8.2.4.13 RCDATA end tag name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: If the current end tag token is an appropriate end tag token, then switch to the before attribute name state. Otherwise, treat it as per the "anything else" entry below.
"/" (U+002F): If the current end tag token is an appropriate end tag token, then switch to the self-closing start tag state. Otherwise, treat it as per the "anything else" entry below.
">" (U+003E): If the current end tag token is an appropriate end tag token, then switch to the data state and emit the current tag token. Otherwise, treat it as per the "anything else" entry below.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current tag token's tag name. Append the current input character to the temporary buffer.
Lowercase ASCII letter: Append the current input character to the current tag token's tag name. Append the current input character to the temporary buffer.
Anything else: Switch to the RCDATA state. Emit a U+003C LESS-THAN SIGN character token, a U+002F SOLIDUS character token, and a character token for each of the characters in the temporary buffer (in the order they were added to the buffer). Reconsume the current input character.

8.2.4.14 RAWTEXT less-than sign state

Consume the next input character:

"/" (U+002F): Set the temporary buffer to the empty string. Switch to the RAWTEXT end tag open state.
Anything else: Switch to the RAWTEXT state. Emit a U+003C LESS-THAN SIGN character token. Reconsume the current input character.

8.2.4.15 RAWTEXT end tag open state

Consume the next input character:

Uppercase ASCII letter: Create a new end tag token, and set its tag name to the lowercase version of the current input character (add 0x0020 to the character's code point). Append the current input character to the temporary buffer. Finally, switch to the RAWTEXT end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Lowercase ASCII letter: Create a new end tag token, and set its tag name to the current input character. Append the current input character to the temporary buffer. Finally, switch to the RAWTEXT end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Anything else: Switch to the RAWTEXT state. Emit a U+003C LESS-THAN SIGN character token and a U+002F SOLIDUS character token. Reconsume the current input character.

8.2.4.16 RAWTEXT end tag name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: If the current end tag token is an appropriate end tag token, then switch to the before attribute name state. Otherwise, treat it as per the "anything else" entry below.
"/" (U+002F): If the current end tag token is an appropriate end tag token, then switch to the self-closing start tag state. Otherwise, treat it as per the "anything else" entry below.
">" (U+003E): If the current end tag token is an appropriate end tag token, then switch to the data state and emit the current tag token. Otherwise, treat it as per the "anything else" entry below.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current tag token's tag name. Append the current input character to the temporary buffer.
Lowercase ASCII letter: Append the current input character to the current tag token's tag name. Append the current input character to the temporary buffer.
Anything else: Switch to the RAWTEXT state. Emit a U+003C LESS-THAN SIGN character token, a U+002F SOLIDUS character token, and a character token for each of the characters in the temporary buffer (in the order they were added to the buffer). Reconsume the current input character.

8.2.4.17 Script data less-than sign state

Consume the next input character:

"/" (U+002F): Set the temporary buffer to the empty string. Switch to the script data end tag open state.
"!" (U+0021): Switch to the script data escape start state. Emit a U+003C LESS-THAN SIGN character token and a U+0021 EXCLAMATION MARK character token.
Anything else: Switch to the script data state. Emit a U+003C LESS-THAN SIGN character token. Reconsume the current input character.

8.2.4.18 Script data end tag open state

Consume the next input character:

Uppercase ASCII letter: Create a new end tag token, and set its tag name to the lowercase version of the current input character (add 0x0020 to the character's code point). Append the current input character to the temporary buffer. Finally, switch to the script data end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Lowercase ASCII letter: Create a new end tag token, and set its tag name to the current input character. Append the current input character to the temporary buffer. Finally, switch to the script data end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Anything else: Switch to the script data state. Emit a U+003C LESS-THAN SIGN character token and a U+002F SOLIDUS character token. Reconsume the current input character.

8.2.4.19 Script data end tag name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: If the current end tag token is an appropriate end tag token, then switch to the before attribute name state. Otherwise, treat it as per the "anything else" entry below.
"/" (U+002F): If the current end tag token is an appropriate end tag token, then switch to the self-closing start tag state. Otherwise, treat it as per the "anything else" entry below.
">" (U+003E): If the current end tag token is an appropriate end tag token, then switch to the data state and emit the current tag token. Otherwise, treat it as per the "anything else" entry below.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current tag token's tag name. Append the current input character to the temporary buffer.
Lowercase ASCII letter: Append the current input character to the current tag token's tag name. Append the current input character to the temporary buffer.
Anything else: Switch to the script data state. Emit a U+003C LESS-THAN SIGN character token, a U+002F SOLIDUS character token, and a character token for each of the characters in the temporary buffer (in the order they were added to the buffer). Reconsume the current input character.

8.2.4.20 Script data escape start state

Consume the next input character:

"-" (U+002D): Switch to the script data escape start dash state. Emit a U+002D HYPHEN-MINUS character token.
Anything else: Switch to the script data state. Reconsume the current input character.

8.2.4.21 Script data escape start dash state

Consume the next input character:

"-" (U+002D): Switch to the script data escaped dash dash state. Emit a U+002D HYPHEN-MINUS character token.
Anything else: Switch to the script data state. Reconsume the current input character.

8.2.4.22 Script data escaped state

Consume the next input character:

"-" (U+002D): Switch to the script data escaped dash state. Emit a U+002D HYPHEN-MINUS character token.
"<" (U+003C): Switch to the script data escaped less-than sign state.
U+0000 NULL: Parse error. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Switch to the data state. Parse error. Reconsume the EOF character.
Anything else: Emit the current input character as a character token.

8.2.4.23 Script data escaped dash state

Consume the next input character:

"-" (U+002D): Switch to the script data escaped dash dash state. Emit a U+002D HYPHEN-MINUS character token.
"<" (U+003C): Switch to the script data escaped less-than sign state.
U+0000 NULL: Parse error. Switch to the script data escaped state. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Switch to the script data escaped state. Emit the current input character as a character token.

8.2.4.24 Script data escaped dash dash state

Consume the next input character:

"-" (U+002D): Emit a U+002D HYPHEN-MINUS character token.
"<" (U+003C): Switch to the script data escaped less-than sign state.
">" (U+003E): Switch to the script data state. Emit a U+003E GREATER-THAN SIGN character token.
U+0000 NULL: Parse error. Switch to the script data escaped state. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Switch to the script data escaped state. Emit the current input character as a character token.

8.2.4.25 Script data escaped less-than sign state

Consume the next input character:

"/" (U+002F): Set the temporary buffer to the empty string. Switch to the script data escaped end tag open state.
Uppercase ASCII letter: Set the temporary buffer to the empty string. Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the temporary buffer. Switch to the script data double escape start state. Emit a U+003C LESS-THAN SIGN character token and the current input character as a character token.
Lowercase ASCII letter: Set the temporary buffer to the empty string. Append the current input character to the temporary buffer. Switch to the script data double escape start state. Emit a U+003C LESS-THAN SIGN character token and the current input character as a character token.
Anything else: Switch to the script data escaped state. Emit a U+003C LESS-THAN SIGN character token. Reconsume the current input character.

8.2.4.26 Script data escaped end tag open state

Consume the next input character:

Uppercase ASCII letter: Create a new end tag token, and set its tag name to the lowercase version of the current input character (add 0x0020 to the character's code point). Append the current input character to the temporary buffer. Finally, switch to the script data escaped end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Lowercase ASCII letter: Create a new end tag token, and set its tag name to the current input character. Append the current input character to the temporary buffer. Finally, switch to the script data escaped end tag name state. (Don't emit the token yet; further details will be filled in before it is emitted.)
Anything else: Switch to the script data escaped state. Emit a U+003C LESS-THAN SIGN character token and a U+002F SOLIDUS character token. Reconsume the current input character.

8.2.4.27 Script data escaped end tag name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: If the current end tag token is an appropriate end tag token, then switch to the before attribute name state. Otherwise, treat it as per the "anything else" entry below.
"/" (U+002F): If the current end tag token is an appropriate end tag token, then switch to the self-closing start tag state. Otherwise, treat it as per the "anything else" entry below.
">" (U+003E): If the current end tag token is an appropriate end tag token, then switch to the data state and emit the current tag token. Otherwise, treat it as per the "anything else" entry below.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current tag token's tag name. Append the current input character to the temporary buffer.
Lowercase ASCII letter: Append the current input character to the current tag token's tag name. Append the current input character to the temporary buffer.
Anything else: Switch to the script data escaped state. Emit a U+003C LESS-THAN SIGN character token, a U+002F SOLIDUS character token, and a character token for each of the characters in the temporary buffer (in the order they were added to the buffer). Reconsume the current input character.

8.2.4.28 Script data double escape start state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE
"/" (U+002F)
">" (U+003E): If the temporary buffer is the string "script", then switch to the script data double escaped state. Otherwise, switch to the script data escaped state. Emit the current input character as a character token.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the temporary buffer. Emit the current input character as a character token.
Lowercase ASCII letter: Append the current input character to the temporary buffer. Emit the current input character as a character token.
Anything else: Switch to the script data escaped state. Reconsume the current input character.

8.2.4.29 Script data double escaped state

Consume the next input character:

"-" (U+002D): Switch to the script data double escaped dash state. Emit a U+002D HYPHEN-MINUS character token.
"<" (U+003C): Switch to the script data double escaped less-than sign state. Emit a U+003C LESS-THAN SIGN character token.
U+0000 NULL: Parse error. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Emit the current input character as a character token.

8.2.4.30 Script data double escaped dash state

Consume the next input character:

"-" (U+002D): Switch to the script data double escaped dash dash state. Emit a U+002D HYPHEN-MINUS character token.
"<" (U+003C): Switch to the script data double escaped less-than sign state. Emit a U+003C LESS-THAN SIGN character token.
U+0000 NULL: Parse error. Switch to the script data double escaped state. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Switch to the script data double escaped state. Emit the current input character as a character token.

8.2.4.31 Script data double escaped dash dash state

Consume the next input character:

"-" (U+002D): Emit a U+002D HYPHEN-MINUS character token.
"<" (U+003C): Switch to the script data double escaped less-than sign state. Emit a U+003C LESS-THAN SIGN character token.
">" (U+003E): Switch to the script data state. Emit a U+003E GREATER-THAN SIGN character token.
U+0000 NULL: Parse error. Switch to the script data double escaped state. Emit a U+FFFD REPLACEMENT CHARACTER character token.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Switch to the script data double escaped state. Emit the current input character as a character token.

8.2.4.32 Script data double escaped less-than sign state

Consume the next input character:

"/" (U+002F): Set the temporary buffer to the empty string. Switch to the script data double escape end state. Emit a U+002F SOLIDUS character token.
Anything else: Switch to the script data double escaped state. Reconsume the current input character.

8.2.4.33 Script data double escape end state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE
"/" (U+002F)
">" (U+003E): If the temporary buffer is the string "script", then switch to the script data escaped state. Otherwise, switch to the script data double escaped state. Emit the current input character as a character token.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the temporary buffer. Emit the current input character as a character token.
Lowercase ASCII letter: Append the current input character to the temporary buffer. Emit the current input character as a character token.
Anything else: Switch to the script data double escaped state. Reconsume the current input character.

8.2.4.34 Before attribute name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
"/" (U+002F): Switch to the self-closing start tag state.
">" (U+003E): Switch to the data state. Emit the current tag token.
Uppercase ASCII letter: Start a new attribute in the current tag token. Set that attribute's name to the lowercase version of the current input character (add 0x0020 to the character's code point), and its value to the empty string. Switch to the attribute name state.
U+0000 NULL: Parse error. Start a new attribute in the current tag token. Set that attribute's name to a U+FFFD REPLACEMENT CHARACTER character, and its value to the empty string. Switch to the attribute name state.
U+0022 QUOTATION MARK (")
"'" (U+0027)
"<" (U+003C)
"=" (U+003D): Parse error. Treat it as per the "anything else" entry below.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Start a new attribute in the current tag token. Set that attribute's name to the current input character, and its value to the empty string. Switch to the attribute name state.

8.2.4.35 Attribute name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the after attribute name state.
"/" (U+002F): Switch to the self-closing start tag state.
"=" (U+003D): Switch to the before attribute value state.
">" (U+003E): Switch to the data state. Emit the current tag token.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current attribute's name.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current attribute's name.
U+0022 QUOTATION MARK (")
"'" (U+0027)
"<" (U+003C): Parse error. Treat it as per the "anything else" entry below.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Append the current input character to the current attribute's name.

When the user agent leaves the attribute name state (and before emitting the tag token, if appropriate), the complete attribute's name must be compared to the other attributes on the same token; if there is already an attribute on the token with the exact same name, then this is a parse error and the new attribute must be removed from the token.

If an attribute is so removed from a token, it, along with the value that gets associated with it, if any, are never subsequently used by the parser, and are therefore effectively discarded. Removing the attribute in this way does not change its status as the "current attribute" for the purposes of the tokenizer, however.

8.2.4.36 After attribute name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
"/" (U+002F): Switch to the self-closing start tag state.
"=" (U+003D): Switch to the before attribute value state.
">" (U+003E): Switch to the data state. Emit the current tag token.
Uppercase ASCII letter: Start a new attribute in the current tag token. Set that attribute's name to the lowercase version of the current input character (add 0x0020 to the character's code point), and its value to the empty string. Switch to the attribute name state.
U+0000 NULL: Parse error. Start a new attribute in the current tag token. Set that attribute's name to a U+FFFD REPLACEMENT CHARACTER character, and its value to the empty string. Switch to the attribute name state.
U+0022 QUOTATION MARK (")
"'" (U+0027)
"<" (U+003C): Parse error. Treat it as per the "anything else" entry below.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Start a new attribute in the current tag token. Set that attribute's name to the current input character, and its value to the empty string. Switch to the attribute name state.

8.2.4.37 Before attribute value state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
U+0022 QUOTATION MARK ("): Switch to the attribute value (double-quoted) state.
U+0026 AMPERSAND (&): Switch to the attribute value (unquoted) state. Reconsume the current input character.
"'" (U+0027): Switch to the attribute value (single-quoted) state.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current attribute's value. Switch to the attribute value (unquoted) state.
">" (U+003E): Parse error. Switch to the data state. Emit the current tag token.
"<" (U+003C)
"=" (U+003D)
"`" (U+0060): Parse error. Treat it as per the "anything else" entry below.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Append the current input character to the current attribute's value. Switch to the attribute value (unquoted) state.

8.2.4.38 Attribute value (double-quoted) state

Consume the next input character:

U+0022 QUOTATION MARK ("): Switch to the after attribute value (quoted) state.
U+0026 AMPERSAND (&): Switch to the character reference in attribute value state, with the additional allowed character being U+0022 QUOTATION MARK (").
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current attribute's value.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Append the current input character to the current attribute's value.

8.2.4.39 Attribute value (single-quoted) state

Consume the next input character:

"'" (U+0027): Switch to the after attribute value (quoted) state.
U+0026 AMPERSAND (&): Switch to the character reference in attribute value state, with the additional allowed character being "'" (U+0027).
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current attribute's value.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Append the current input character to the current attribute's value.

8.2.4.40 Attribute value (unquoted) state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the before attribute name state.
U+0026 AMPERSAND (&): Switch to the character reference in attribute value state, with the additional allowed character being ">" (U+003E).
">" (U+003E): Switch to the data state. Emit the current tag token.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current attribute's value.
U+0022 QUOTATION MARK (")
"'" (U+0027)
"<" (U+003C)
"=" (U+003D)
"`" (U+0060): Parse error. Treat it as per the "anything else" entry below.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Append the current input character to the current attribute's value.

8.2.4.41 Character reference in attribute value state

Attempt to consume a character reference.

If nothing is returned, append a U+0026 AMPERSAND character (&) to the current attribute's value.

Otherwise, append the returned character tokens to the current attribute's value.

Finally, switch back to the attribute value state that switched into this state.

8.2.4.42 After attribute value (quoted) state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the before attribute name state.
"/" (U+002F): Switch to the self-closing start tag state.
">" (U+003E): Switch to the data state. Emit the current tag token.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Parse error. Switch to the before attribute name state. Reconsume the character.

8.2.4.43 Self-closing start tag state

Consume the next input character:

">" (U+003E): Set the self-closing flag of the current tag token. Switch to the data state. Emit the current tag token.
EOF: Parse error. Switch to the data state. Reconsume the EOF character.
Anything else: Parse error. Switch to the before attribute name state. Reconsume the character.

8.2.4.44 Bogus comment state

Consume every character up to and including the first ">" (U+003E) character or the end of the file (EOF), whichever comes first. Emit a comment token whose data is the concatenation of all the characters starting from and including the character that caused the state machine to switch into the bogus comment state, up to and including the character immediately before the last consumed character (i.e. up to the character just before the U+003E or EOF character), but with any U+0000 NULL characters replaced by U+FFFD REPLACEMENT CHARACTER characters. (If the comment was started by the end of the file (EOF), the token is empty. Similarly, the token is empty if it was generated by the string "<!>".)

Switch to the data state.

If the end of the file was reached, reconsume the EOF character.

8.2.4.45 Markup declaration open state

If the next two characters are both "-" (U+002D) characters, consume those two characters, create a comment token whose data is the empty string, and switch to the comment start state.

Otherwise, if the next seven characters are an ASCII case-insensitive match for the word "DOCTYPE", then consume those characters and switch to the DOCTYPE state.

Otherwise, if there is an adjusted current node and it is not an element in the HTML namespace and the next seven characters are a case-sensitive match for the string "[CDATA[" (the five uppercase letters "CDATA" with a U+005B LEFT SQUARE BRACKET character before and after), then consume those characters and switch to the CDATA section state.

Otherwise, this is a parse error. Switch to the bogus comment state. The next character that is consumed, if any, is the first character that will be in the comment.

8.2.4.46 Comment start state

Consume the next input character:

"-" (U+002D): Switch to the comment start dash state.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the comment token's data. Switch to the comment state.
">" (U+003E): Parse error. Switch to the data state. Emit the comment token.
EOF: Parse error. Switch to the data state. Emit the comment token. Reconsume the EOF character.
Anything else: Append the current input character to the comment token's data. Switch to the comment state.

8.2.4.47 Comment start dash state

Consume the next input character:

"-" (U+002D): Switch to the comment end state
U+0000 NULL: Parse error. Append a "-" (U+002D) character and a U+FFFD REPLACEMENT CHARACTER character to the comment token's data. Switch to the comment state.
">" (U+003E): Parse error. Switch to the data state. Emit the comment token.
EOF: Parse error. Switch to the data state. Emit the comment token. Reconsume the EOF character.
Anything else: Append a "-" (U+002D) character and the current input character to the comment token's data. Switch to the comment state.

8.2.4.48 Comment state

Consume the next input character:

"-" (U+002D): Switch to the comment end dash state
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the comment token's data.
EOF: Parse error. Switch to the data state. Emit the comment token. Reconsume the EOF character.
Anything else: Append the current input character to the comment token's data.

8.2.4.49 Comment end dash state

Consume the next input character:

"-" (U+002D): Switch to the comment end state
U+0000 NULL: Parse error. Append a "-" (U+002D) character and a U+FFFD REPLACEMENT CHARACTER character to the comment token's data. Switch to the comment state.
EOF: Parse error. Switch to the data state. Emit the comment token. Reconsume the EOF character.
Anything else: Append a "-" (U+002D) character and the current input character to the comment token's data. Switch to the comment state.

8.2.4.50 Comment end state

Consume the next input character:

">" (U+003E): Switch to the data state. Emit the comment token.
U+0000 NULL: Parse error. Append two "-" (U+002D) characters and a U+FFFD REPLACEMENT CHARACTER character to the comment token's data. Switch to the comment state.
"!" (U+0021): Parse error. Switch to the comment end bang state.
"-" (U+002D): Parse error. Append a "-" (U+002D) character to the comment token's data.
EOF: Parse error. Switch to the data state. Emit the comment token. Reconsume the EOF character.
Anything else: Parse error. Append two "-" (U+002D) characters and the current input character to the comment token's data. Switch to the comment state.

8.2.4.51 Comment end bang state

Consume the next input character:

"-" (U+002D): Append two "-" (U+002D) characters and a "!" (U+0021) character to the comment token's data. Switch to the comment end dash state.
">" (U+003E): Switch to the data state. Emit the comment token.
U+0000 NULL: Parse error. Append two "-" (U+002D) characters, a "!" (U+0021) character, and a U+FFFD REPLACEMENT CHARACTER character to the comment token's data. Switch to the comment state.
EOF: Parse error. Switch to the data state. Emit the comment token. Reconsume the EOF character.
Anything else: Append two "-" (U+002D) characters, a "!" (U+0021) character, and the current input character to the comment token's data. Switch to the comment state.

8.2.4.52 DOCTYPE state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the before DOCTYPE name state.
EOF: Parse error. Switch to the data state. Create a new DOCTYPE token. Set its force-quirks flag to on. Emit the token. Reconsume the EOF character.
Anything else: Parse error. Switch to the before DOCTYPE name state. Reconsume the character.

8.2.4.53 Before DOCTYPE name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
Uppercase ASCII letter: Create a new DOCTYPE token. Set the token's name to the lowercase version of the current input character (add 0x0020 to the character's code point). Switch to the DOCTYPE name state.
U+0000 NULL: Parse error. Create a new DOCTYPE token. Set the token's name to a U+FFFD REPLACEMENT CHARACTER character. Switch to the DOCTYPE name state.
">" (U+003E): Parse error. Create a new DOCTYPE token. Set its force-quirks flag to on. Switch to the data state. Emit the token.
EOF: Parse error. Switch to the data state. Create a new DOCTYPE token. Set its force-quirks flag to on. Emit the token. Reconsume the EOF character.
Anything else: Create a new DOCTYPE token. Set the token's name to the current input character. Switch to the DOCTYPE name state.

8.2.4.54 DOCTYPE name state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the after DOCTYPE name state.
">" (U+003E): Switch to the data state. Emit the current DOCTYPE token.
Uppercase ASCII letter: Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current DOCTYPE token's name.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current DOCTYPE token's name.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Append the current input character to the current DOCTYPE token's name.

8.2.4.55 After DOCTYPE name state

Consume the next input character:

"tab" (U+0009)

"LF" (U+000A)

"FF" (U+000C)

U+0020 SPACE

Ignore the character.

">" (U+003E)

Switch to the data state. Emit the current DOCTYPE token.

EOF

Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.

Anything else

If the six characters starting from the current input character are an ASCII case-insensitive match for the word "PUBLIC", then consume those characters and switch to the after DOCTYPE public keyword state.

Otherwise, if the six characters starting from the current input character are an ASCII case-insensitive match for the word "SYSTEM", then consume those characters and switch to the after DOCTYPE system keyword state.

Otherwise, this is a parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the bogus DOCTYPE state.

8.2.4.56 After DOCTYPE public keyword state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the before DOCTYPE public identifier state.
U+0022 QUOTATION MARK ("): Parse error. Set the DOCTYPE token's public identifier to the empty string (not missing), then switch to the DOCTYPE public identifier (double-quoted) state.
"'" (U+0027): Parse error. Set the DOCTYPE token's public identifier to the empty string (not missing), then switch to the DOCTYPE public identifier (single-quoted) state.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the bogus DOCTYPE state.

8.2.4.57 Before DOCTYPE public identifier state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
U+0022 QUOTATION MARK ("): Set the DOCTYPE token's public identifier to the empty string (not missing), then switch to the DOCTYPE public identifier (double-quoted) state.
"'" (U+0027): Set the DOCTYPE token's public identifier to the empty string (not missing), then switch to the DOCTYPE public identifier (single-quoted) state.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the bogus DOCTYPE state.

8.2.4.58 DOCTYPE public identifier (double-quoted) state

Consume the next input character:

U+0022 QUOTATION MARK ("): Switch to the after DOCTYPE public identifier state.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current DOCTYPE token's public identifier.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Append the current input character to the current DOCTYPE token's public identifier.

8.2.4.59 DOCTYPE public identifier (single-quoted) state

Consume the next input character:

"'" (U+0027): Switch to the after DOCTYPE public identifier state.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current DOCTYPE token's public identifier.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Append the current input character to the current DOCTYPE token's public identifier.

8.2.4.60 After DOCTYPE public identifier state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the between DOCTYPE public and system identifiers state.
">" (U+003E): Switch to the data state. Emit the current DOCTYPE token.
U+0022 QUOTATION MARK ("): Parse error. Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (double-quoted) state.
"'" (U+0027): Parse error. Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (single-quoted) state.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the bogus DOCTYPE state.

8.2.4.61 Between DOCTYPE public and system identifiers state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
">" (U+003E): Switch to the data state. Emit the current DOCTYPE token.
U+0022 QUOTATION MARK ("): Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (double-quoted) state.
"'" (U+0027): Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (single-quoted) state.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the bogus DOCTYPE state.

8.2.4.62 After DOCTYPE system keyword state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Switch to the before DOCTYPE system identifier state.
U+0022 QUOTATION MARK ("): Parse error. Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (double-quoted) state.
"'" (U+0027): Parse error. Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (single-quoted) state.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the bogus DOCTYPE state.

8.2.4.63 Before DOCTYPE system identifier state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
U+0022 QUOTATION MARK ("): Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (double-quoted) state.
"'" (U+0027): Set the DOCTYPE token's system identifier to the empty string (not missing), then switch to the DOCTYPE system identifier (single-quoted) state.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the bogus DOCTYPE state.

8.2.4.64 DOCTYPE system identifier (double-quoted) state

Consume the next input character:

U+0022 QUOTATION MARK ("): Switch to the after DOCTYPE system identifier state.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current DOCTYPE token's system identifier.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Append the current input character to the current DOCTYPE token's system identifier.

8.2.4.65 DOCTYPE system identifier (single-quoted) state

Consume the next input character:

"'" (U+0027): Switch to the after DOCTYPE system identifier state.
U+0000 NULL: Parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current DOCTYPE token's system identifier.
">" (U+003E): Parse error. Set the DOCTYPE token's force-quirks flag to on. Switch to the data state. Emit that DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Append the current input character to the current DOCTYPE token's system identifier.

8.2.4.66 After DOCTYPE system identifier state

Consume the next input character:

"tab" (U+0009)
"LF" (U+000A)
"FF" (U+000C)
U+0020 SPACE: Ignore the character.
">" (U+003E): Switch to the data state. Emit the current DOCTYPE token.
EOF: Parse error. Switch to the data state. Set the DOCTYPE token's force-quirks flag to on. Emit that DOCTYPE token. Reconsume the EOF character.
Anything else: Parse error. Switch to the bogus DOCTYPE state. (This does not set the DOCTYPE token's force-quirks flag to on.)

8.2.4.67 Bogus DOCTYPE state

Consume the next input character:

">" (U+003E): Switch to the data state. Emit the DOCTYPE token.
EOF: Switch to the data state. Emit the DOCTYPE token. Reconsume the EOF character.
Anything else: Ignore the character.

8.2.4.68 CDATA section state

Switch to the data state.

Consume every character up to the next occurrence of the three character sequence U+005D RIGHT SQUARE BRACKET U+005D RIGHT SQUARE BRACKET U+003E GREATER-THAN SIGN (]]>), or the end of the file (EOF), whichever comes first. Emit a series of character tokens consisting of all the characters consumed except the matching three character sequence at the end (if one was found before the end of the file).

If the end of the file was reached, reconsume the EOF character.

8.2.4.69 Tokenizing character references

This section defines how to consume a character reference, optionally with an additional allowed character, which, if specified where the algorithm is invoked, adds a character to the list of characters that cause there to not be a character reference.

This definition is used when parsing character references in text and in attributes.

The behavior depends on the identity of the next character (the one immediately after the U+0026 AMPERSAND character), as follows:

"tab" (U+0009)

"LF" (U+000A)

"FF" (U+000C)

U+0020 SPACE

U+003C LESS-THAN SIGN

U+0026 AMPERSAND

EOF

The additional allowed character, if there is one

Not a character reference. No characters are consumed, and nothing is returned. (This is not an error, either.)

"#" (U+0023)

Consume the U+0023 NUMBER SIGN.

The behavior further depends on the character after the U+0023 NUMBER SIGN:

U+0078 LATIN SMALL LETTER X

U+0058 LATIN CAPITAL LETTER X

Consume the X.

Follow the steps below, but using ASCII hex digits.

When it comes to interpreting the number, interpret it as a hexadecimal number.

Anything else

Follow the steps below, but using ASCII digits.

When it comes to interpreting the number, interpret it as a decimal number.

Consume as many characters as match the range of characters given above (ASCII hex digits or ASCII digits).

If no characters match the range, then don't consume any characters (and unconsume the U+0023 NUMBER SIGN character and, if appropriate, the X character). This is a parse error; nothing is returned.

Otherwise, if the next character is a U+003B SEMICOLON, consume that too. If it isn't, there is a parse error.

If one or more characters match the range, then take them all and interpret the string of characters as a number (either hexadecimal or decimal as appropriate).

If that number is one of the numbers in the first column of the following table, then this is a parse error. Find the row with that number in the first column, and return a character token for the Unicode character given in the second column of that row.

Number	Unicode character
0x00	U+FFFD	REPLACEMENT CHARACTER
0x80	U+20AC	EURO SIGN (€)
0x82	U+201A	SINGLE LOW-9 QUOTATION MARK (‚)
0x83	U+0192	LATIN SMALL LETTER F WITH HOOK (ƒ)
0x84	U+201E	DOUBLE LOW-9 QUOTATION MARK („)
0x85	U+2026	HORIZONTAL ELLIPSIS (…)
0x86	U+2020	DAGGER (†)
0x87	U+2021	DOUBLE DAGGER (‡)
0x88	U+02C6	MODIFIER LETTER CIRCUMFLEX ACCENT (ˆ)
0x89	U+2030	PER MILLE SIGN (‰)
0x8A	U+0160	LATIN CAPITAL LETTER S WITH CARON (Š)
0x8B	U+2039	SINGLE LEFT-POINTING ANGLE QUOTATION MARK (‹)
0x8C	U+0152	LATIN CAPITAL LIGATURE OE (Œ)
0x8E	U+017D	LATIN CAPITAL LETTER Z WITH CARON (Ž)
0x91	U+2018	LEFT SINGLE QUOTATION MARK (‘)
0x92	U+2019	RIGHT SINGLE QUOTATION MARK (’)
0x93	U+201C	LEFT DOUBLE QUOTATION MARK (“)
0x94	U+201D	RIGHT DOUBLE QUOTATION MARK (”)
0x95	U+2022	BULLET (•)
0x96	U+2013	EN DASH (–)
0x97	U+2014	EM DASH (—)
0x98	U+02DC	SMALL TILDE (˜)
0x99	U+2122	TRADE MARK SIGN (™)
0x9A	U+0161	LATIN SMALL LETTER S WITH CARON (š)
0x9B	U+203A	SINGLE RIGHT-POINTING ANGLE QUOTATION MARK (›)
0x9C	U+0153	LATIN SMALL LIGATURE OE (œ)
0x9E	U+017E	LATIN SMALL LETTER Z WITH CARON (ž)
0x9F	U+0178	LATIN CAPITAL LETTER Y WITH DIAERESIS (Ÿ)

Otherwise, if the number is in the range 0xD800 to 0xDFFF or is greater than 0x10FFFF, then this is a parse error. Return a U+FFFD REPLACEMENT CHARACTER character token.

Otherwise, return a character token for the Unicode character whose code point is that number. Additionally, if the number is in the range 0x0001 to 0x0008, 0x000D to 0x001F, 0x007F to 0x009F, 0xFDD0 to 0xFDEF, or is one of 0x000B, 0xFFFE, 0xFFFF, 0x1FFFE, 0x1FFFF, 0x2FFFE, 0x2FFFF, 0x3FFFE, 0x3FFFF, 0x4FFFE, 0x4FFFF, 0x5FFFE, 0x5FFFF, 0x6FFFE, 0x6FFFF, 0x7FFFE, 0x7FFFF, 0x8FFFE, 0x8FFFF, 0x9FFFE, 0x9FFFF, 0xAFFFE, 0xAFFFF, 0xBFFFE, 0xBFFFF, 0xCFFFE, 0xCFFFF, 0xDFFFE, 0xDFFFF, 0xEFFFE, 0xEFFFF, 0xFFFFE, 0xFFFFF, 0x10FFFE, or 0x10FFFF, then this is a parse error.

Anything else

Consume the maximum number of characters possible, with the consumed characters matching one of the identifiers in the first column of the named character references table (in a case-sensitive manner).

If no match can be made, then no characters are consumed, and nothing is returned. In this case, if the characters after the U+0026 AMPERSAND character (&) consist of a sequence of one or more alphanumeric ASCII characters followed by a U+003B SEMICOLON character (;), then this is a parse error.

If the character reference is being consumed as part of an attribute, and the last character matched is not a ";" (U+003B) character, and the next character is either a "=" (U+003D) character or an alphanumeric ASCII character, then, for historical reasons, all the characters that were matched after the U+0026 AMPERSAND character (&) must be unconsumed, and nothing is returned. However, if this next character is in fact a "=" (U+003D) character, then this is a parse error, because some legacy user agents will misinterpret the markup in those cases.

Otherwise, a character reference is parsed. If the last character matched is not a ";" (U+003B) character, there is a parse error.

Return one or two character tokens for the character(s) corresponding to the character reference name (as given by the second column of the named character references table).

If the markup contains (not in an attribute) the string I'm &notit; I tell you, the character reference is parsed as "not", as in, I'm ¬it; I tell you (and this is a parse error). But if the markup was I'm ∉ I tell you, the character reference would be parsed as "notin;", resulting in I'm ∉ I tell you (and no parse error).

8.2.5 Tree construction

The input to the tree construction stage is a sequence of tokens from the tokenization stage. The tree construction stage is associated with a DOM Document object when a parser is created. The "output" of this stage consists of dynamically modifying or extending that document's DOM tree.

This specification does not define when an interactive user agent has to render the Document so that it is available to the user, or when it has to begin accepting user input.

As each token is emitted from the tokenizer, the user agent must follow the appropriate steps from the following list, known as the tree construction dispatcher:

If there is no adjusted current node
If the adjusted current node is an element in the HTML namespace
If the adjusted current node is a MathML text integration point and the token is a start tag whose tag name is neither "mglyph" nor "malignmark"
If the adjusted current node is a MathML text integration point and the token is a character token
If the adjusted current node is an annotation-xml element in the MathML namespace and the token is a start tag whose tag name is "svg"
If the adjusted current node is an HTML integration point and the token is a start tag
If the adjusted current node is an HTML integration point and the token is a character token
If the token is an end-of-file token: Process the token according to the rules given in the section corresponding to the current insertion mode in HTML content.
Otherwise: Process the token according to the rules given in the section for parsing tokens in foreign content.

The next token is the token that is about to be processed by the tree construction dispatcher (even if the token is subsequently just ignored).

A node is a MathML text integration point if it is one of the following elements:

An mi element in the MathML namespace
An mo element in the MathML namespace
An mn element in the MathML namespace
An ms element in the MathML namespace
An mtext element in the MathML namespace

A node is an HTML integration point if it is one of the following elements:

An annotation-xml element in the MathML namespace whose start tag token had an attribute with the name "encoding" whose value was an ASCII case-insensitive match for the string "text/html"
An annotation-xml element in the MathML namespace whose start tag token had an attribute with the name "encoding" whose value was an ASCII case-insensitive match for the string "application/xhtml+xml"
A foreignObject element in the SVG namespace
A desc element in the SVG namespace
A title element in the SVG namespace

Not all of the tag names mentioned below are conformant tag names in this specification; many are included to handle legacy content. They still form part of the algorithm that implementations are required to implement to claim conformance.

The algorithm described below places no limit on the depth of the DOM tree generated, or on the length of tag names, attribute names, attribute values, Text nodes, etc. While implementors are encouraged to avoid arbitrary limits, it is recognized that practical concerns will likely force user agents to impose nesting depth constraints.

8.2.5.1 Creating and inserting nodes

While the parser is processing a token, it can enable or disable foster parenting. This affects the following algorithm.

The appropriate place for inserting a node, optionally using a particular override target, is the position in an element returned by running the following steps:

If there was an override target specified, then let target be the override target.

Otherwise, let target be the current node.
Determine the adjusted insertion location using the first matching steps from the following list:
If foster parenting is enabled and target is a table, tbody, tfoot, thead, or tr element
Foster parenting happens when content is misnested in tables.

Run these substeps:
1. Let last template be the last template element in the stack of open elements, if any.
2. Let last table be the last table element in the stack of open elements, if any.
3. If there is a last template and either there is no last table, or there is one, but last template is lower (more recently added) than last table in the stack of open elements, then: let adjusted insertion location be inside last template's template contents, after its last child (if any), and abort these substeps.
4. If there is no last table, then let adjusted insertion location be inside the first element in the stack of open elements (the html element), after its last child (if any), and abort these substeps. (fragment case)
5. If last table has a parent element, then let adjusted insertion location be inside last table's parent element, immediately before last table, and abort these substeps.
6. Let previous element be the element immediately above last table in the stack of open elements.
7. Let adjusted insertion location be inside previous element, after its last child (if any).
These steps are involved in part because it's possible for elements, the table element in this case in particular, to have been moved by a script around in the DOM, or indeed removed from the DOM entirely, after the element was inserted by the parser.
Otherwise

Let adjusted insertion location be inside target, after its last child (if any).
If the adjusted insertion location is inside a template element, let it instead be inside the template element's template contents, after its last child (if any).
Return the adjusted insertion location.

When the steps below require the UA to create an element for a token in a particular given namespace and with a particular intended parent, the UA must run the following steps:

Create a node implementing the interface appropriate for the element type corresponding to the tag name of the token in given namespace (as given in the specification that defines that element, e.g. for an a element in the HTML namespace, this specification defines it to be the HTMLAnchorElement interface), with the tag name being the name of that element, with the node being in the given namespace, and with the attributes on the node being those given in the given token.

The interface appropriate for an element in the HTML namespace that is not defined in this specification (or other applicable specifications) is HTMLUnknownElement. Elements in other namespaces whose interface is not defined by that namespace's specification must use the interface Element.

The ownerDocument of the newly created element must be the same as that of the intended parent.
If the newly created element has an xmlns attribute in the XMLNS namespace whose value is not exactly the same as the element's namespace, that is a parse error. Similarly, if the newly created element has an xmlns:xlink attribute in the XMLNS namespace whose value is not the XLink Namespace, that is a parse error.
If the newly created element is a resettable element, invoke its reset algorithm. (This initializes the element's value and checkedness based on the element's attributes.)
If the element is a form-associated element, and the form element pointer is not null, and there is no template element on the stack of open elements, and the newly created element is either not reassociateable or doesn't have a form attribute, and the intended parent is in the same home subtree as the element pointed to by the form element pointer, associate the newly created element with the form element pointed to by the form element pointer, and suppress the running of the reset the form owner algorithm when the parser subsequently attempts to insert the element.
Return the newly created element.

When the steps below require the user agent to insert a foreign element for a token in a given namespace, the user agent must run these steps:

Let the adjusted insertion location be the appropriate place for inserting a node.
Create an element for the token in the given namespace, with the intended parent being the element in which the adjusted insertion location finds itself.
If it is possible to insert an element at the adjusted insertion location, then insert the newly created element at the adjusted insertion location.

If the adjusted insertion location cannot accept more elements, e.g. because it's a Document that already has an element child, then the newly created element is dropped on the floor.
Push the element onto the stack of open elements so that it is the new current node.
Return the newly created element.

When the steps below require the user agent to insert an HTML element for a token, the user agent must insert a foreign element for the token, in the HTML namespace.

When the steps below require the user agent to adjust MathML attributes for a token, then, if the token has an attribute named definitionurl, change its name to definitionURL (note the case difference).

When the steps below require the user agent to adjust SVG attributes for a token, then, for each attribute on the token whose attribute name is one of the ones in the first column of the following table, change the attribute's name to the name given in the corresponding cell in the second column. (This fixes the case of SVG attributes that are not all lowercase.)

Attribute name on token	Attribute name on element
`attributename`	`attributeName`
`attributetype`	`attributeType`
`basefrequency`	`baseFrequency`
`baseprofile`	`baseProfile`
`calcmode`	`calcMode`
`clippathunits`	`clipPathUnits`
`contentscripttype`	`contentScriptType`
`contentstyletype`	`contentStyleType`
`diffuseconstant`	`diffuseConstant`
`edgemode`	`edgeMode`
`externalresourcesrequired`	`externalResourcesRequired`
`filterres`	`filterRes`
`filterunits`	`filterUnits`
`glyphref`	`glyphRef`
`gradienttransform`	`gradientTransform`
`gradientunits`	`gradientUnits`
`kernelmatrix`	`kernelMatrix`
`kernelunitlength`	`kernelUnitLength`
`keypoints`	`keyPoints`
`keysplines`	`keySplines`
`keytimes`	`keyTimes`
`lengthadjust`	`lengthAdjust`
`limitingconeangle`	`limitingConeAngle`
`markerheight`	`markerHeight`
`markerunits`	`markerUnits`
`markerwidth`	`markerWidth`
`maskcontentunits`	`maskContentUnits`
`maskunits`	`maskUnits`
`numoctaves`	`numOctaves`
`pathlength`	`pathLength`
`patterncontentunits`	`patternContentUnits`
`patterntransform`	`patternTransform`
`patternunits`	`patternUnits`
`pointsatx`	`pointsAtX`
`pointsaty`	`pointsAtY`
`pointsatz`	`pointsAtZ`
`preservealpha`	`preserveAlpha`
`preserveaspectratio`	`preserveAspectRatio`
`primitiveunits`	`primitiveUnits`
`refx`	`refX`
`refy`	`refY`
`repeatcount`	`repeatCount`
`repeatdur`	`repeatDur`
`requiredextensions`	`requiredExtensions`
`requiredfeatures`	`requiredFeatures`
`specularconstant`	`specularConstant`
`specularexponent`	`specularExponent`
`spreadmethod`	`spreadMethod`
`startoffset`	`startOffset`
`stddeviation`	`stdDeviation`
`stitchtiles`	`stitchTiles`
`surfacescale`	`surfaceScale`
`systemlanguage`	`systemLanguage`
`tablevalues`	`tableValues`
`targetx`	`targetX`
`targety`	`targetY`
`textlength`	`textLength`
`viewbox`	`viewBox`
`viewtarget`	`viewTarget`
`xchannelselector`	`xChannelSelector`
`ychannelselector`	`yChannelSelector`
`zoomandpan`	`zoomAndPan`

When the steps below require the user agent to adjust foreign attributes for a token, then, if any of the attributes on the token match the strings given in the first column of the following table, let the attribute be a namespaced attribute, with the prefix being the string given in the corresponding cell in the second column, the local name being the string given in the corresponding cell in the third column, and the namespace being the namespace given in the corresponding cell in the fourth column. (This fixes the use of namespaced attributes, in particular lang attributes in the XML namespace.)

Attribute name	Prefix	Local name	Namespace
`xlink:actuate`	`xlink`	`actuate`	XLink namespace
`xlink:arcrole`	`xlink`	`arcrole`	XLink namespace
`xlink:href`	`xlink`	`href`	XLink namespace
`xlink:role`	`xlink`	`role`	XLink namespace
`xlink:show`	`xlink`	`show`	XLink namespace
`xlink:title`	`xlink`	`title`	XLink namespace
`xlink:type`	`xlink`	`type`	XLink namespace
`xml:base`	`xml`	`base`	XML namespace
`xml:lang`	`xml`	`lang`	XML namespace
`xml:space`	`xml`	`space`	XML namespace
`xmlns`	(none)	`xmlns`	XMLNS namespace
`xmlns:xlink`	`xmlns`	`xlink`	XMLNS namespace

When the steps below require the user agent to insert a character while processing a token, the user agent must run the following steps:

Let data be the characters passed to the algorithm, or, if no characters were explicitly specified, the character of the character token being processed.
Let the adjusted insertion location be the appropriate place for inserting a node.
If the adjusted insertion location is in a Document node, then abort these steps.
The DOM will not let Document nodes have Text node children, so they are dropped on the floor.
If there is a Text node immediately before the adjusted insertion location, then append data to that Text node's data.

Otherwise, create a new Text node whose data is data and whose ownerDocument is the same as that of the element in which the adjusted insertion location finds itself, and insert the newly created node at the adjusted insertion location.

Here are some sample inputs to the parser and the corresponding number of Text nodes that they result in, assuming a user agent that executes scripts.

Input	Number of `Text` nodes
A<script> var script = document.getElementsByTagName('script')[0]; document.body.removeChild(script); </script>B	One `Text` node in the document, containing "AB".
A<script> var text = document.createTextNode('B'); document.body.appendChild(text); </script>C	Three `Text` nodes; "A" before the script, the script's contents, and "BC" after the script (the parser appends to the `Text` node created by the script).
A<script> var text = document.getElementsByTagName('script')[0].firstChild; text.data = 'B'; document.body.appendChild(text); </script>C	Two adjacent `Text` nodes in the document, containing "A" and "BC".
A<table>B<tr>C</tr>D</table>	One `Text` node before the table, containing "ABCD". (This is caused by foster parenting.)
A<table><tr> B</tr> C</table>	One `Text` node before the table, containing "A B C" (A-space-B-space-C). (This is caused by foster parenting.)
A<table><tr> B</tr> </em>C</table>	One `Text` node before the table, containing "A BC" (A-space-B-C), and one `Text` node inside the table (as a child of a `tbody`) with a single space character. (Space characters separated from non-space characters by non-character tokens are not affected by foster parenting, even if those other tokens then get ignored.)

When the steps below require the user agent to insert a comment while processing a comment token, optionally with an explicitly insertion position position, the user agent must run the following steps:

Let data be the data given in the comment token being processed.
If position was specified, then let the adjusted insertion location be position. Otherwise, let adjusted insertion location be the appropriate place for inserting a node.
Create a Comment node whose data attribute is set to data and whose ownerDocument is the same as that of the node in which the adjusted insertion location finds itself.
Insert the newly created node at the adjusted insertion location.

DOM mutation events must not fire for changes caused by the UA parsing the document. This includes the parsing of any content inserted using document.write() and document.writeln() calls. [DOMEVENTS]

However, mutation observers do fire, as required by the DOM specification.

8.2.5.2 Parsing elements that contain only text

The generic raw text element parsing algorithm and the generic RCDATA element parsing algorithm consist of the following steps. These algorithms are always invoked in response to a start tag token.

Insert an HTML element for the token.
If the algorithm that was invoked is the generic raw text element parsing algorithm, switch the tokenizer to the RAWTEXT state; otherwise the algorithm invoked was the generic RCDATA element parsing algorithm, switch the tokenizer to the RCDATA state.
Let the original insertion mode be the current insertion mode.
Then, switch the insertion mode to "text".

8.2.5.3 Closing elements that have implied end tags

When the steps below require the UA to generate implied end tags, then, while the current node is a dd element, a dt element, an li element, an option element, an optgroup element, a p element, an rb element, an rp element, an rt element, or an rtc element, the UA must pop the current node off the stack of open elements.

If a step requires the UA to generate implied end tags but lists an element to exclude from the process, then the UA must perform the above steps as if that element was not in the above list.

8.2.5.4 The rules for parsing tokens in HTML content

8.2.5.4.1 The "initial" insertion mode

When the user agent is to apply the rules for the "initial" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

Ignore the token.

A comment token

Insert a comment as the last child of the Document object.

A DOCTYPE token

If the DOCTYPE token's name is not a case-sensitive match for the string "html", or the token's public identifier is not missing, or the token's system identifier is neither missing nor a case-sensitive match for the string "about:legacy-compat", and none of the sets of conditions in the following list are matched, then there is a parse error.

The DOCTYPE token's name is a case-sensitive match for the string "html", the token's public identifier is the case-sensitive string "-//W3C//DTD HTML 4.0//EN", and the token's system identifier is either missing or the case-sensitive string "http://www.w3.org/TR/REC-html40/strict.dtd".
The DOCTYPE token's name is a case-sensitive match for the string "html", the token's public identifier is the case-sensitive string "-//W3C//DTD HTML 4.01//EN", and the token's system identifier is either missing or the case-sensitive string "http://www.w3.org/TR/html4/strict.dtd".
The DOCTYPE token's name is a case-sensitive match for the string "html", the token's public identifier is the case-sensitive string "-//W3C//DTD XHTML 1.0 Strict//EN", and the token's system identifier is the case-sensitive string "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd".
The DOCTYPE token's name is a case-sensitive match for the string "html", the token's public identifier is the case-sensitive string "-//W3C//DTD XHTML 1.1//EN", and the token's system identifier is the case-sensitive string "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd".

Conformance checkers may, based on the values (including presence or lack thereof) of the DOCTYPE token's name, public identifier, or system identifier, switch to a conformance checking mode for another language (e.g. based on the DOCTYPE token a conformance checker could recognize that the document is an HTML4-era document, and defer to an HTML4 conformance checker.)

Append a DocumentType node to the Document node, with the name attribute set to the name given in the DOCTYPE token, or the empty string if the name was missing; the publicId attribute set to the public identifier given in the DOCTYPE token, or the empty string if the public identifier was missing; the systemId attribute set to the system identifier given in the DOCTYPE token, or the empty string if the system identifier was missing; and the other attributes specific to DocumentType objects set to null and empty lists as appropriate. Associate the DocumentType node with the Document object so that it is returned as the value of the doctype attribute of the Document object.

Then, if the document is not an iframe srcdoc document, and the DOCTYPE token matches one of the conditions in the following list, then set the Document to quirks mode:

The force-quirks flag is set to on.
The name is set to anything other than "html" (compared case-sensitively).
The public identifier starts with: "+//Silmaril//dtd html Pro v0r11 19970101//"
The public identifier starts with: "-//AdvaSoft Ltd//DTD HTML 3.0 asWedit + extensions//"
The public identifier starts with: "-//AS//DTD HTML 3.0 asWedit + extensions//"
The public identifier starts with: "-//IETF//DTD HTML 2.0 Level 1//"
The public identifier starts with: "-//IETF//DTD HTML 2.0 Level 2//"
The public identifier starts with: "-//IETF//DTD HTML 2.0 Strict Level 1//"
The public identifier starts with: "-//IETF//DTD HTML 2.0 Strict Level 2//"
The public identifier starts with: "-//IETF//DTD HTML 2.0 Strict//"
The public identifier starts with: "-//IETF//DTD HTML 2.0//"
The public identifier starts with: "-//IETF//DTD HTML 2.1E//"
The public identifier starts with: "-//IETF//DTD HTML 3.0//"
The public identifier starts with: "-//IETF//DTD HTML 3.2 Final//"
The public identifier starts with: "-//IETF//DTD HTML 3.2//"
The public identifier starts with: "-//IETF//DTD HTML 3//"
The public identifier starts with: "-//IETF//DTD HTML Level 0//"
The public identifier starts with: "-//IETF//DTD HTML Level 1//"
The public identifier starts with: "-//IETF//DTD HTML Level 2//"
The public identifier starts with: "-//IETF//DTD HTML Level 3//"
The public identifier starts with: "-//IETF//DTD HTML Strict Level 0//"
The public identifier starts with: "-//IETF//DTD HTML Strict Level 1//"
The public identifier starts with: "-//IETF//DTD HTML Strict Level 2//"
The public identifier starts with: "-//IETF//DTD HTML Strict Level 3//"
The public identifier starts with: "-//IETF//DTD HTML Strict//"
The public identifier starts with: "-//IETF//DTD HTML//"
The public identifier starts with: "-//Metrius//DTD Metrius Presentational//"
The public identifier starts with: "-//Microsoft//DTD Internet Explorer 2.0 HTML Strict//"
The public identifier starts with: "-//Microsoft//DTD Internet Explorer 2.0 HTML//"
The public identifier starts with: "-//Microsoft//DTD Internet Explorer 2.0 Tables//"
The public identifier starts with: "-//Microsoft//DTD Internet Explorer 3.0 HTML Strict//"
The public identifier starts with: "-//Microsoft//DTD Internet Explorer 3.0 HTML//"
The public identifier starts with: "-//Microsoft//DTD Internet Explorer 3.0 Tables//"
The public identifier starts with: "-//Netscape Comm. Corp.//DTD HTML//"
The public identifier starts with: "-//Netscape Comm. Corp.//DTD Strict HTML//"
The public identifier starts with: "-//O'Reilly and Associates//DTD HTML 2.0//"
The public identifier starts with: "-//O'Reilly and Associates//DTD HTML Extended 1.0//"
The public identifier starts with: "-//O'Reilly and Associates//DTD HTML Extended Relaxed 1.0//"
The public identifier starts with: "-//SoftQuad Software//DTD HoTMetaL PRO 6.0::19990601::extensions to HTML 4.0//"
The public identifier starts with: "-//SoftQuad//DTD HoTMetaL PRO 4.0::19971010::extensions to HTML 4.0//"
The public identifier starts with: "-//Spyglass//DTD HTML 2.0 Extended//"
The public identifier starts with: "-//SQ//DTD HTML 2.0 HoTMetaL + extensions//"
The public identifier starts with: "-//Sun Microsystems Corp.//DTD HotJava HTML//"
The public identifier starts with: "-//Sun Microsystems Corp.//DTD HotJava Strict HTML//"
The public identifier starts with: "-//W3C//DTD HTML 3 1995-03-24//"
The public identifier starts with: "-//W3C//DTD HTML 3.2 Draft//"
The public identifier starts with: "-//W3C//DTD HTML 3.2 Final//"
The public identifier starts with: "-//W3C//DTD HTML 3.2//"
The public identifier starts with: "-//W3C//DTD HTML 3.2S Draft//"
The public identifier starts with: "-//W3C//DTD HTML 4.0 Frameset//"
The public identifier starts with: "-//W3C//DTD HTML 4.0 Transitional//"
The public identifier starts with: "-//W3C//DTD HTML Experimental 19960712//"
The public identifier starts with: "-//W3C//DTD HTML Experimental 970421//"
The public identifier starts with: "-//W3C//DTD W3 HTML//"
The public identifier starts with: "-//W3O//DTD W3 HTML 3.0//"
The public identifier is set to: "-//W3O//DTD W3 HTML Strict 3.0//EN//"
The public identifier starts with: "-//WebTechs//DTD Mozilla HTML 2.0//"
The public identifier starts with: "-//WebTechs//DTD Mozilla HTML//"
The public identifier is set to: "-/W3C/DTD HTML 4.0 Transitional/EN"
The public identifier is set to: "HTML"
The system identifier is set to: "http://www.ibm.com/data/dtd/v11/ibmxhtml1-transitional.dtd"
The system identifier is missing and the public identifier starts with: "-//W3C//DTD HTML 4.01 Frameset//"
The system identifier is missing and the public identifier starts with: "-//W3C//DTD HTML 4.01 Transitional//"

Otherwise, if the document is not an iframe srcdoc document, and the DOCTYPE token matches one of the conditions in the following list, then set the Document to limited-quirks mode:

The public identifier starts with: "-//W3C//DTD XHTML 1.0 Frameset//"
The public identifier starts with: "-//W3C//DTD XHTML 1.0 Transitional//"
The system identifier is not missing and the public identifier starts with: "-//W3C//DTD HTML 4.01 Frameset//"
The system identifier is not missing and the public identifier starts with: "-//W3C//DTD HTML 4.01 Transitional//"

The system identifier and public identifier strings must be compared to the values given in the lists above in an ASCII case-insensitive manner. A system identifier whose value is the empty string is not considered missing for the purposes of the conditions above.

Then, switch the insertion mode to "before html".

Anything else

If the document is not an iframe srcdoc document, then this is a parse error; set the Document to quirks mode.

In any case, switch the insertion mode to "before html", then reprocess the token.

8.2.5.4.2 The "before html" insertion mode

When the user agent is to apply the rules for the "before html" insertion mode, the user agent must handle the token as follows:

A DOCTYPE token

Parse error. Ignore the token.

A comment token

Insert a comment as the last child of the Document object.

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

Ignore the token.

A start tag whose tag name is "html"

Create an element for the token in the HTML namespace, with the Document as the intended parent. Append it to the Document object. Put this element in the stack of open elements.

If the Document is being loaded as part of navigation of a browsing context, then: if the newly created element has a manifest attribute whose value is not the empty string, then resolve the value of that attribute to an absolute URL, relative to the newly created element, and if that is successful, run the application cache selection algorithm with the result of applying the URL serializer algorithm to the resulting parsed URL with the exclude fragment flag set; otherwise, if there is no such attribute, or its value is the empty string, or resolving its value fails, run the application cache selection algorithm with no manifest. The algorithm must be passed the Document object.

Switch the insertion mode to "before head".

An end tag whose tag name is one of: "head", "body", "html", "br"

Act as described in the "anything else" entry below.

Any other end tag

Parse error. Ignore the token.

Anything else

Create an html element whose ownerDocument is the Document object. Append it to the Document object. Put this element in the stack of open elements.

If the Document is being loaded as part of navigation of a browsing context, then: run the application cache selection algorithm with no manifest, passing it the Document object.

Switch the insertion mode to "before head", then reprocess the token.

The root element can end up being removed from the Document object, e.g. by scripts; nothing in particular happens in such cases, content continues being appended to the nodes as described in the next section.

8.2.5.4.3 The "before head" insertion mode

When the user agent is to apply the rules for the "before head" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

Ignore the token.

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

A start tag whose tag name is "head"

Insert an HTML element for the token.

Set the head element pointer to the newly created head element.

Switch the insertion mode to "in head".

An end tag whose tag name is one of: "head", "body", "html", "br"

Act as described in the "anything else" entry below.

Any other end tag

Parse error. Ignore the token.

Anything else

Insert an HTML element for a "head" start tag token with no attributes.

Set the head element pointer to the newly created head element.

Switch the insertion mode to "in head".

Reprocess the current token.

8.2.5.4.4 The "in head" insertion mode

When the user agent is to apply the rules for the "in head" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

A start tag whose tag name is one of: "base", "basefont", "bgsound", "link"

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

A start tag whose tag name is "meta"

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

If the element has a charset attribute, and getting an encoding from its value results in a supported ASCII-compatible character encoding or a UTF-16 encoding, and the confidence is currently tentative, then change the encoding to the resulting encoding.

Otherwise, if the element has an http-equiv attribute whose value is an ASCII case-insensitive match for the string "Content-Type", and the element has a content attribute, and applying the algorithm for extracting a character encoding from a meta element to that attribute's value returns a supported ASCII-compatible character encoding or a UTF-16 encoding, and the confidence is currently tentative, then change the encoding to the extracted encoding.

A start tag whose tag name is "title"

Follow the generic RCDATA element parsing algorithm.

A start tag whose tag name is "noscript", if the scripting flag is enabled

A start tag whose tag name is one of: "noframes", "style"

Follow the generic raw text element parsing algorithm.

A start tag whose tag name is "noscript", if the scripting flag is disabled

Insert an HTML element for the token.

Switch the insertion mode to "in head noscript".

A start tag whose tag name is "script"

Run these steps:

Let the adjusted insertion location be the appropriate place for inserting a node.
Create an element for the token in the HTML namespace, with the intended parent being the element in which the adjusted insertion location finds itself.
Mark the element as being "parser-inserted" and unset the element's "force-async" flag.

This ensures that, if the script is external, any document.write() calls in the script will execute in-line, instead of blowing the document away, as would happen in most other cases. It also prevents the script from executing until the end tag is seen.
If the parser was originally created for the HTML fragment parsing algorithm, then mark the script element as "already started". (fragment case)
Insert the newly created element at the adjusted insertion location.
Push the element onto the stack of open elements so that it is the new current node.
Switch the tokenizer to the script data state.
Let the original insertion mode be the current insertion mode.
Switch the insertion mode to "text".

An end tag whose tag name is "head"

Pop the current node (which will be the head element) off the stack of open elements.

Switch the insertion mode to "after head".

An end tag whose tag name is one of: "body", "html", "br"

Act as described in the "anything else" entry below.

A start tag whose tag name is "template"

Insert an HTML element for the token.

Insert a marker at the end of the list of active formatting elements.

Set the frameset-ok flag to "not ok".

Switch the insertion mode to "in template".

Push "in template" onto the stack of template insertion modes so that it is the new current template insertion mode.

An end tag whose tag name is "template"

If there is no template element on the stack of open elements, then this is a parse error; ignore the token.

Otherwise, run these steps:

Generate implied end tags.
If the current node is not a template element, then this is a parse error.
Pop elements from the stack of open elements until a template element has been popped from the stack.
Clear the list of active formatting elements up to the last marker.
Pop the current template insertion mode off the stack of template insertion modes.
Reset the insertion mode appropriately.

A start tag whose tag name is "head"

Any other end tag

Parse error. Ignore the token.

Anything else

Pop the current node (which will be the head element) off the stack of open elements.

Switch the insertion mode to "after head".

Reprocess the token.

8.2.5.4.5 The "in head noscript" insertion mode

When the user agent is to apply the rules for the "in head noscript" insertion mode, the user agent must handle the token as follows:

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

An end tag whose tag name is "noscript"

Pop the current node (which will be a noscript element) from the stack of open elements; the new current node will be a head element.

Switch the insertion mode to "in head".

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

A comment token

A start tag whose tag name is one of: "basefont", "bgsound", "link", "meta", "noframes", "style"

Process the token using the rules for the "in head" insertion mode.

An end tag whose tag name is "br"

Act as described in the "anything else" entry below.

A start tag whose tag name is one of: "head", "noscript"

Any other end tag

Parse error. Ignore the token.

Anything else

Pop the current node (which will be a noscript element) from the stack of open elements; the new current node will be a head element.

Switch the insertion mode to "in head".

Reprocess the token.

8.2.5.4.6 The "after head" insertion mode

When the user agent is to apply the rules for the "after head" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

A start tag whose tag name is "body"

Insert an HTML element for the token.

Set the frameset-ok flag to "not ok".

Switch the insertion mode to "in body".

A start tag whose tag name is "frameset"

Insert an HTML element for the token.

Switch the insertion mode to "in frameset".

A start tag whose tag name is one of: "base", "basefont", "bgsound", "link", "meta", "noframes", "script", "style", "template", "title"

Push the node pointed to by the head element pointer onto the stack of open elements.

Process the token using the rules for the "in head" insertion mode.

Remove the node pointed to by the head element pointer from the stack of open elements. (It might not be the current node at this point.)

The head element pointer cannot be null at this point.

An end tag whose tag name is "template"

Process the token using the rules for the "in head" insertion mode.

An end tag whose tag name is one of: "body", "html", "br"

Act as described in the "anything else" entry below.

A start tag whose tag name is "head"

Any other end tag

Parse error. Ignore the token.

Anything else

Insert an HTML element for a "body" start tag token with no attributes.

Switch the insertion mode to "in body".

Reprocess the current token.

8.2.5.4.7 The "in body" insertion mode

When the user agent is to apply the rules for the "in body" insertion mode, the user agent must handle the token as follows:

A character token that is U+0000 NULL

Parse error. Ignore the token.

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

Reconstruct the active formatting elements, if any.

Any other character token

Reconstruct the active formatting elements, if any.

Set the frameset-ok flag to "not ok".

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

If there is a template element on the stack of open elements, then ignore the token.

Otherwise, for each attribute on the token, check to see if the attribute is already present on the top element of the stack of open elements. If it is not, add the attribute and its corresponding value to that element.

A start tag whose tag name is one of: "base", "basefont", "bgsound", "link", "meta", "noframes", "script", "style", "template", "title"

An end tag whose tag name is "template"

Process the token using the rules for the "in head" insertion mode.

A start tag whose tag name is "body"

If the second element on the stack of open elements is not a body element, if the stack of open elements has only one node on it, or if there is a template element on the stack of open elements, then ignore the token. (fragment case)

Otherwise, set the frameset-ok flag to "not ok"; then, for each attribute on the token, check to see if the attribute is already present on the body element (the second element) on the stack of open elements, and if it is not, add the attribute and its corresponding value to that element.

A start tag whose tag name is "frameset"

If the stack of open elements has only one node on it, or if the second element on the stack of open elements is not a body element, then ignore the token. (fragment case)

If the frameset-ok flag is set to "not ok", ignore the token.

Otherwise, run the following steps:

Remove the second element on the stack of open elements from its parent node, if it has one.
Pop all the nodes from the bottom of the stack of open elements, from the current node up to, but not including, the root html element.
Insert an HTML element for the token.
Switch the insertion mode to "in frameset".

An end-of-file token

If there is a node in the stack of open elements that is not either a dd element, a dt element, an li element, a p element, a tbody element, a td element, a tfoot element, a th element, a thead element, a tr element, the body element, or the html element, then this is a parse error.

If the stack of template insertion modes is not empty, then process the token using the rules for the "in template" insertion mode.

Otherwise, stop parsing.

An end tag whose tag name is "body"

If the stack of open elements does not have a body element in scope, this is a parse error; ignore the token.

Otherwise, if there is a node in the stack of open elements that is not either a dd element, a dt element, an li element, an optgroup element, an option element, a p element, an rb element, an rp element, an rt element, an rtc element, a tbody element, a td element, a tfoot element, a th element, a thead element, a tr element, the body element, or the html element, then this is a parse error.

Switch the insertion mode to "after body".

An end tag whose tag name is "html"

If the stack of open elements does not have a body element in scope, this is a parse error; ignore the token.

Switch the insertion mode to "after body".

Reprocess the token.

A start tag whose tag name is one of: "address", "article", "aside", "blockquote", "center", "details", "dialog", "dir", "div", "dl", "fieldset", "figcaption", "figure", "footer", "header", "hgroup", "main", "nav", "ol", "p", "section", "summary", "ul"

If the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for the token.

A start tag whose tag name is one of: "h1", "h2", "h3", "h4", "h5", "h6"

If the stack of open elements has a p element in button scope, then close a p element.

If the current node is an HTML element whose tag name is one of "h1", "h2", "h3", "h4", "h5", or "h6", then this is a parse error; pop the current node off the stack of open elements.

Insert an HTML element for the token.

A start tag whose tag name is one of: "pre", "listing"

If the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for the token.

If the next token is a "LF" (U+000A) character token, then ignore that token and move on to the next one. (Newlines at the start of pre blocks are ignored as an authoring convenience.)

Set the frameset-ok flag to "not ok".

A start tag whose tag name is "form"

If the form element pointer is not null, and there is no template element on the stack of open elements, then this is a parse error; ignore the token.

Otherwise:

If the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for the token, and, if there is no template element on the stack of open elements, set the form element pointer to point to the element created.

A start tag whose tag name is "li"

Run these steps:

Set the frameset-ok flag to "not ok".
Initialize node to be the current node (the bottommost node of the stack).
Loop: If node is an li element, then run these substeps:
1. Generate implied end tags, except for li elements.
2. If the current node is not an li element, then this is a parse error.
3. Pop elements from the stack of open elements until an li element has been popped from the stack.
4. Jump to the step labeled done below.
If node is in the special category, but is not an address, div, or p element, then jump to the step labeled done below.
Otherwise, set node to the previous entry in the stack of open elements and return to the step labeled loop.
Done: If the stack of open elements has a p element in button scope, then close a p element.
Finally, insert an HTML element for the token.

A start tag whose tag name is one of: "dd", "dt"

Run these steps:

Set the frameset-ok flag to "not ok".
Initialize node to be the current node (the bottommost node of the stack).
Loop: If node is a dd element, then run these substeps:
1. Generate implied end tags, except for dd elements.
2. If the current node is not a dd element, then this is a parse error.
3. Pop elements from the stack of open elements until a dd element has been popped from the stack.
4. Jump to the step labeled done below.
If node is a dt element, then run these substeps:
1. Generate implied end tags, except for dt elements.
2. If the current node is not a dt element, then this is a parse error.
3. Pop elements from the stack of open elements until a dt element has been popped from the stack.
4. Jump to the step labeled done below.
If node is in the special category, but is not an address, div, or p element, then jump to the step labeled done below.
Otherwise, set node to the previous entry in the stack of open elements and return to the step labeled loop.
Done: If the stack of open elements has a p element in button scope, then close a p element.
Finally, insert an HTML element for the token.

A start tag whose tag name is "plaintext"

If the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for the token.

Switch the tokenizer to the PLAINTEXT state.

Once a start tag with the tag name "plaintext" has been seen, that will be the last token ever seen other than character tokens (and the end-of-file token), because there is no way to switch out of the PLAINTEXT state.

A start tag whose tag name is "button"

If the stack of open elements has a button element in scope, then run these substeps:
1. Parse error.
2. Generate implied end tags.
3. Pop elements from the stack of open elements until a button element has been popped from the stack.
Reconstruct the active formatting elements, if any.
Insert an HTML element for the token.
Set the frameset-ok flag to "not ok".

An end tag whose tag name is one of: "address", "article", "aside", "blockquote", "button", "center", "details", "dialog", "dir", "div", "dl", "fieldset", "figcaption", "figure", "footer", "header", "hgroup", "listing", "main", "nav", "ol", "pre", "section", "summary", "ul"

If the stack of open elements does not have an element in scope that is an HTML element and with the same tag name as that of the token, then this is a parse error; ignore the token.

Otherwise, run these steps:

Generate implied end tags.
If the current node is not an HTML element with the same tag name as that of the token, then this is a parse error.
Pop elements from the stack of open elements until an HTML element with the same tag name as the token has been popped from the stack.

An end tag whose tag name is "form"

If there is no template element on the stack of open elements, then run these substeps:

Let node be the element that the form element pointer is set to, or null if it is not set to an element.
Set the form element pointer to null. Otherwise, let node be null.
If node is null or if the stack of open elements does not have node in scope, then this is a parse error; abort these steps and ignore the token.
Generate implied end tags.
If the current node is not node, then this is a parse error.
Remove node from the stack of open elements.

If there is a template element on the stack of open elements, then run these substeps instead:

If the stack of open elements does not have a form element in scope, then this is a parse error; abort these steps and ignore the token.
Generate implied end tags.
If the current node is not a form element, then this is a parse error.
Pop elements from the stack of open elements until a form element has been popped from the stack.

An end tag whose tag name is "p"

If the stack of open elements does not have a p element in button scope, then this is a parse error; insert an HTML element for a "p" start tag token with no attributes.

Close a p element.

An end tag whose tag name is "li"

If the stack of open elements does not have an li element in list item scope, then this is a parse error; ignore the token.

Otherwise, run these steps:

Generate implied end tags, except for li elements.
If the current node is not an li element, then this is a parse error.
Pop elements from the stack of open elements until an li element has been popped from the stack.

An end tag whose tag name is one of: "dd", "dt"

If the stack of open elements does not have an element in scope that is an HTML element and with the same tag name as that of the token, then this is a parse error; ignore the token.

Otherwise, run these steps:

Generate implied end tags, except for HTML elements with the same tag name as the token.
If the current node is not an HTML element with the same tag name as that of the token, then this is a parse error.
Pop elements from the stack of open elements until an HTML element with the same tag name as the token has been popped from the stack.

An end tag whose tag name is one of: "h1", "h2", "h3", "h4", "h5", "h6"

If the stack of open elements does not have an element in scope that is an HTML element and whose tag name is one of "h1", "h2", "h3", "h4", "h5", or "h6", then this is a parse error; ignore the token.

Otherwise, run these steps:

Generate implied end tags.
If the current node is not an HTML element with the same tag name as that of the token, then this is a parse error.
Pop elements from the stack of open elements until an HTML element whose tag name is one of "h1", "h2", "h3", "h4", "h5", or "h6" has been popped from the stack.

An end tag whose tag name is "sarcasm"

Take a deep breath, then act as described in the "any other end tag" entry below.

A start tag whose tag name is "a"

If the list of active formatting elements contains an a element between the end of the list and the last marker on the list (or the start of the list if there is no marker on the list), then this is a parse error; run the adoption agency algorithm for the tag name "a", then remove that element from the list of active formatting elements and the stack of open elements if the adoption agency algorithm didn't already remove it (it might not have if the element is not in table scope).

In the non-conforming stream <a href="a">a<table><a href="b">b</table>x, the first a element would be closed upon seeing the second one, and the "x" character would be inside a link to "b", not to "a". This is despite the fact that the outer a element is not in table scope (meaning that a regular </a> end tag at the start of the table wouldn't close the outer a element). The result is that the two a elements are indirectly nested inside each other — non-conforming markup will often result in non-conforming DOMs when parsed.

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token. Push onto the list of active formatting elements that element.

A start tag whose tag name is one of: "b", "big", "code", "em", "font", "i", "s", "small", "strike", "strong", "tt", "u"

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token. Push onto the list of active formatting elements that element.

A start tag whose tag name is "nobr"

Reconstruct the active formatting elements, if any.

If the stack of open elements has a nobr element in scope, then this is a parse error; run the adoption agency algorithm for the tag name "nobr", then once again reconstruct the active formatting elements, if any.

Insert an HTML element for the token. Push onto the list of active formatting elements that element.

An end tag whose tag name is one of: "a", "b", "big", "code", "em", "font", "i", "nobr", "s", "small", "strike", "strong", "tt", "u"

Run the adoption agency algorithm for the token's tag name.

A start tag whose tag name is one of: "applet", "marquee", "object"

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token.

Insert a marker at the end of the list of active formatting elements.

Set the frameset-ok flag to "not ok".

An end tag token whose tag name is one of: "applet", "marquee", "object"

If the stack of open elements does not have an element in scope that is an HTML element and with the same tag name as that of the token, then this is a parse error; ignore the token.

Otherwise, run these steps:

Generate implied end tags.
If the current node is not an HTML element with the same tag name as that of the token, then this is a parse error.
Pop elements from the stack of open elements until an HTML element with the same tag name as the token has been popped from the stack.
Clear the list of active formatting elements up to the last marker.

A start tag whose tag name is "table"

If the Document is not set to quirks mode, and the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for the token.

Set the frameset-ok flag to "not ok".

Switch the insertion mode to "in table".

An end tag whose tag name is "br"

Parse error. Act as described in the next entry, as if this was a "br" start tag token, rather than an end tag token.

A start tag whose tag name is one of: "area", "br", "embed", "img", "keygen", "wbr"

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

Set the frameset-ok flag to "not ok".

A start tag whose tag name is "input"

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

If the token does not have an attribute with the name "type", or if it does, but that attribute's value is not an ASCII case-insensitive match for the string "hidden", then: set the frameset-ok flag to "not ok".

A start tag whose tag name is one of: "param", "source", "track"

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

A start tag whose tag name is "hr"

If the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

Set the frameset-ok flag to "not ok".

A start tag whose tag name is "image"

Parse error. Change the token's tag name to "img" and reprocess it. (Don't ask.)

A start tag whose tag name is "isindex"

If there is no template element on the stack of open elements and the form element pointer is not null, then ignore the token.

Otherwise:

Acknowledge the token's self-closing flag, if it is set.

Set the frameset-ok flag to "not ok".

If the stack of open elements has a p element in button scope, then close a p element.

Insert an HTML element for a "form" start tag token with no attributes, and, if there is no template element on the stack of open elements, set the form element pointer to point to the element created.

If the token has an attribute called "action", set the action attribute on the resulting form element to the value of the "action" attribute of the token.

Insert an HTML element for an "hr" start tag token with no attributes. Immediately pop the current node off the stack of open elements.

Reconstruct the active formatting elements, if any.

Insert an HTML element for a "label" start tag token with no attributes.

Insert characters (see below for what they should say).

Insert an HTML element for an "input" start tag token with all the attributes from the "isindex" token except "name", "action", and "prompt", and with an attribute named "name" with the value "isindex". (This creates an input element with the name attribute set to the magic balue "isindex".) Immediately pop the current node off the stack of open elements.

Insert more characters (see below for what they should say).

Pop the current node (which will be the label element created earlier) off the stack of open elements.

Insert an HTML element for an "hr" start tag token with no attributes. Immediately pop the current node off the stack of open elements.

Pop the current node (which will be the form element created earlier) off the stack of open elements, and, if there is no template element on the stack of open elements, set the form element pointer back to null.

Prompt: If the token has an attribute with the name "prompt", then the first stream of characters must be the same string as given in that attribute, and the second stream of characters must be empty. Otherwise, the two streams of character tokens together should, together with the input element, express the equivalent of "This is a searchable index. Enter search keywords: (input field)" in the user's preferred language.

A start tag whose tag name is "textarea"

Run these steps:

Insert an HTML element for the token.
If the next token is a "LF" (U+000A) character token, then ignore that token and move on to the next one. (Newlines at the start of textarea elements are ignored as an authoring convenience.)
Switch the tokenizer to the RCDATA state.
Let the original insertion mode be the current insertion mode.
Set the frameset-ok flag to "not ok".
Switch the insertion mode to "text".

A start tag whose tag name is "xmp"

If the stack of open elements has a p element in button scope, then close a p element.

Reconstruct the active formatting elements, if any.

Set the frameset-ok flag to "not ok".

Follow the generic raw text element parsing algorithm.

A start tag whose tag name is "iframe"

Set the frameset-ok flag to "not ok".

Follow the generic raw text element parsing algorithm.

A start tag whose tag name is "noembed"

A start tag whose tag name is "noscript", if the scripting flag is enabled

Follow the generic raw text element parsing algorithm.

A start tag whose tag name is "select"

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token.

Set the frameset-ok flag to "not ok".

If the insertion mode is one of "in table", "in caption", "in table body", "in row", or "in cell", then switch the insertion mode to "in select in table". Otherwise, switch the insertion mode to "in select".

A start tag whose tag name is one of: "optgroup", "option"

If the current node is an option element, then pop the current node off the stack of open elements.

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token.

A start tag whose tag name is one of: "rb", "rp", "rtc"

If the stack of open elements has a ruby element in scope, then generate implied end tags. If the current node is not then a ruby element, this is a parse error.

Insert an HTML element for the token.

A start tag whose tag name is "rt"

If the stack of open elements has a ruby element in scope, then generate implied end tags, except for rtc elements. If the current node is not then a ruby element or an rtc element, this is a parse error.

Insert an HTML element for the token.

A start tag whose tag name is "math"

Reconstruct the active formatting elements, if any.

Adjust MathML attributes for the token. (This fixes the case of MathML attributes that are not all lowercase.)

Adjust foreign attributes for the token. (This fixes the use of namespaced attributes, in particular XLink.)

Insert a foreign element for the token, in the MathML namespace.

If the token has its self-closing flag set, pop the current node off the stack of open elements and acknowledge the token's self-closing flag.

A start tag whose tag name is "svg"

Reconstruct the active formatting elements, if any.

Adjust SVG attributes for the token. (This fixes the case of SVG attributes that are not all lowercase.)

Adjust foreign attributes for the token. (This fixes the use of namespaced attributes, in particular XLink in SVG.)

Insert a foreign element for the token, in the SVG namespace.

If the token has its self-closing flag set, pop the current node off the stack of open elements and acknowledge the token's self-closing flag.

A start tag whose tag name is one of: "caption", "col", "colgroup", "frame", "head", "tbody", "td", "tfoot", "th", "thead", "tr"

Parse error. Ignore the token.

Any other start tag

Reconstruct the active formatting elements, if any.

Insert an HTML element for the token.

This element will be an ordinary element.

Any other end tag

Run these steps:

Initialize node to be the current node (the bottommost node of the stack).
Loop: If node is an HTML element with the same tag name as the token, then:
1. Generate implied end tags, except for HTML elements with the same tag name as the token.
2. If node is not the current node, then this is a parse error.
3. Pop all the nodes from the current node up to node, including node, then stop these steps.
Otherwise, if node is in the special category, then this is a parse error; ignore the token, and abort these steps.
Set node to the previous entry in the stack of open elements.
Return to the step labeled loop.

When the steps above say the user agent is to close a p element, it means that the user agent must run the following steps:

Generate implied end tags, except for p elements.
If the current node is not a p element, then this is a parse error.
Pop elements from the stack of open elements until a p element has been popped from the stack.

The adoption agency algorithm, which takes as its only argument a tag name subject for which the algorithm is being run, consists of the following steps:

If the current node is an HTML element whose tag name is subject, then run these substeps:
1. Let element be the current node.
2. Pop element off the stack of open elements.
3. If element is also in the list of active formatting elements, remove the element from the list.
4. Abort the adoption agency algorithm.
Let outer loop counter be zero.
Outer loop: If outer loop counter is greater than or equal to eight, then abort these steps.
Increment outer loop counter by one.
Let formatting element be the last element in the list of active formatting elements that:
- is between the end of the list and the last scope marker in the list, if any, or the start of the list otherwise, and
- has the tag name subject.
If there is no such element, then abort these steps and instead act as described in the "any other end tag" entry above.
If formatting element is not in the stack of open elements, then this is a parse error; remove the element from the list, and abort these steps.
If formatting element is in the stack of open elements, but the element is not in scope, then this is a parse error; abort these steps.
If formatting element is not the current node, this is a parse error. (But do not abort these steps.)
Let furthest block be the topmost node in the stack of open elements that is lower in the stack than formatting element, and is an element in the special category. There might not be one.
If there is no furthest block, then the UA must first pop all the nodes from the bottom of the stack of open elements, from the current node up to and including formatting element, then remove formatting element from the list of active formatting elements, and finally abort these steps.
Let common ancestor be the element immediately above formatting element in the stack of open elements.
Let a bookmark note the position of formatting element in the list of active formatting elements relative to the elements on either side of it in the list.
Let node and last node be furthest block. Follow these steps:
1. Let inner loop counter be zero.
2. Inner loop: Increment inner loop counter by one.
3. Let node be the element immediately above node in the stack of open elements, or if node is no longer in the stack of open elements (e.g. because it got removed by this algorithm), the element that was immediately above node in the stack of open elements before node was removed.
4. If node is formatting element, then go to the next step in the overall algorithm.
5. If inner loop counter is greater than three and node is in the list of active formatting elements, then remove node from the list of active formatting elements.
6. If node is not in the list of active formatting elements, then remove node from the stack of open elements and then go back to the step labeled inner loop.
7. Create an element for the token for which the element node was created, in the HTML namespace, with common ancestor as the intended parent; replace the entry for node in the list of active formatting elements with an entry for the new element, replace the entry for node in the stack of open elements with an entry for the new element, and let node be the new element.
8. If last node is furthest block, then move the aforementioned bookmark to be immediately after the new node in the list of active formatting elements.
9. Insert last node into node, first removing it from its previous parent node if any.
10. Let last node be node.
11. Return to the step labeled inner loop.
Insert whatever last node ended up being in the previous step at the appropriate place for inserting a node, but using common ancestor as the override target.
Create an element for the token for which formatting element was created, in the HTML namespace, with furthest block as the intended parent.
Take all of the child nodes of furthest block and append them to the element created in the last step.
Append that new element to furthest block.
Remove formatting element from the list of active formatting elements, and insert the new element into the list of active formatting elements at the position of the aforementioned bookmark.
Remove formatting element from the stack of open elements, and insert the new element into the stack of open elements immediately below the position of furthest block in that stack.
Jump back to the step labeled outer loop.

This algorithm's name, the "adoption agency algorithm", comes from the way it causes elements to change parents, and is in contrast with other possible algorithms for dealing with misnested content, which included the "incest algorithm", the "secret affair algorithm", and the "Heisenberg algorithm".

8.2.5.4.8 The "text" insertion mode

When the user agent is to apply the rules for the "text" insertion mode, the user agent must handle the token as follows:

A character token

This can never be a U+0000 NULL character; the tokenizer converts those to U+FFFD REPLACEMENT CHARACTER characters.

An end-of-file token

Perform a microtask checkpoint.

If the current node is a script element, mark the script element as "already started".

Pop the current node off the stack of open elements.

Switch the insertion mode to the original insertion mode and reprocess the token.

An end tag whose tag name is "script"

Provide a stable state.

Let script be the current node (which will be a script element).

Pop the current node off the stack of open elements.

Switch the insertion mode to the original insertion mode.

Let the old insertion point have the same value as the current insertion point. Let the insertion point be just before the next input character.

Increment the parser's script nesting level by one.

Prepare the script. This might cause some script to execute, which might cause new characters to be inserted into the tokenizer, and might cause the tokenizer to output more tokens, resulting in a reentrant invocation of the parser.

Decrement the parser's script nesting level by one. If the parser's script nesting level is zero, then set the parser pause flag to false.

Let the insertion point have the value of the old insertion point. (In other words, restore the insertion point to its previous value. This value might be the "undefined" value.)

At this stage, if there is a pending parsing-blocking script, then:

If the script nesting level is not zero:

Set the parser pause flag to true, and abort the processing of any nested invocations of the tokenizer, yielding control back to the caller. (Tokenization will resume when the caller returns to the "outer" tree construction stage.)

The tree construction stage of this particular parser is being called reentrantly, say from a call to document.write().

Otherwise:

Run these steps:

Let the script be the pending parsing-blocking script. There is no longer a pending parsing-blocking script.
Block the tokenizer for this instance of the HTML parser, such that the event loop will not run tasks that invoke the tokenizer.
If the parser's Document has a style sheet that is blocking scripts or the script's "ready to be parser-executed" flag is not set: spin the event loop until the parser's Document has no style sheet that is blocking scripts and the script's "ready to be parser-executed" flag is set.
If this parser has been aborted in the meantime, abort these steps.

This could happen if, e.g., while the spin the event loop algorithm is running, the browsing context gets closed, or the document.open() method gets invoked on the Document.
Unblock the tokenizer for this instance of the HTML parser, such that tasks that invoke the tokenizer can again be run.
Let the insertion point be just before the next input character.
Increment the parser's script nesting level by one (it should be zero before this step, so this sets it to one).
Execute the script.
Decrement the parser's script nesting level by one. If the parser's script nesting level is zero (which it always should be at this point), then set the parser pause flag to false.
Let the insertion point be undefined again.
If there is once again a pending parsing-blocking script, then repeat these steps from step 1.

Any other end tag

Pop the current node off the stack of open elements.

Switch the insertion mode to the original insertion mode.

8.2.5.4.9 The "in table" insertion mode

When the user agent is to apply the rules for the "in table" insertion mode, the user agent must handle the token as follows:

A character token, if the current node is table, tbody, tfoot, thead, or tr element

Let the pending table character tokens be an empty list of tokens.

Let the original insertion mode be the current insertion mode.

Switch the insertion mode to "in table text" and reprocess the token.

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "caption"

Clear the stack back to a table context. (See below.)

Insert a marker at the end of the list of active formatting elements.

Insert an HTML element for the token, then switch the insertion mode to "in caption".

A start tag whose tag name is "colgroup"

Clear the stack back to a table context. (See below.)

Insert an HTML element for the token, then switch the insertion mode to "in column group".

A start tag whose tag name is "col"

Clear the stack back to a table context. (See below.)

Insert an HTML element for a "colgroup" start tag token with no attributes, then switch the insertion mode to "in column group".

Reprocess the current token.

A start tag whose tag name is one of: "tbody", "tfoot", "thead"

Clear the stack back to a table context. (See below.)

Insert an HTML element for the token, then switch the insertion mode to "in table body".

A start tag whose tag name is one of: "td", "th", "tr"

Clear the stack back to a table context. (See below.)

Insert an HTML element for a "tbody" start tag token with no attributes, then switch the insertion mode to "in table body".

Reprocess the current token.

A start tag whose tag name is "table"

If the stack of open elements does not have a table element in table scope, ignore the token.

Otherwise:

Pop elements from this stack until a table element has been popped from the stack.

Reprocess the token.

An end tag whose tag name is "table"

If the stack of open elements does not have a table element in table scope, this is a parse error; ignore the token.

Otherwise:

Pop elements from this stack until a table element has been popped from the stack.

An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html", "tbody", "td", "tfoot", "th", "thead", "tr"

Parse error. Ignore the token.

A start tag whose tag name is one of: "style", "script", "template"

An end tag whose tag name is "template"

Process the token using the rules for the "in head" insertion mode.

A start tag whose tag name is "input"

If the token does not have an attribute with the name "type", or if it does, but that attribute's value is not an ASCII case-insensitive match for the string "hidden", then: act as described in the "anything else" entry below.

Otherwise:

Insert an HTML element for the token.

Pop that input element off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

A start tag whose tag name is "form"

Generate implied end tags.

If there is a template element on the stack of open elements, or if the form element pointer is not null, ignore the token.

Otherwise:

Insert an HTML element for the token, and set the form element pointer to point to the element created.

Pop that form element off the stack of open elements.

An end-of-file token

Process the token using the rules for the "in body" insertion mode.

Anything else

Parse error. Enable foster parenting, process the token using the rules for the "in body" insertion mode, and then disable foster parenting.

When the steps above require the UA to clear the stack back to a table context, it means that the UA must, while the current node is not a table, template, or html element, pop elements from the stack of open elements.

The current node being an html element after this process is a fragment case.

8.2.5.4.10 The "in table text" insertion mode

When the user agent is to apply the rules for the "in table text" insertion mode, the user agent must handle the token as follows:

A character token that is U+0000 NULL

Parse error. Ignore the token.

Any other character token

Append the character token to the pending table character tokens list.

Anything else

If any of the tokens in the pending table character tokens list are character tokens that are not space characters, then reprocess the character tokens in the pending table character tokens list using the rules given in the "anything else" entry in the "in table" insertion mode.

Otherwise, insert the characters given by the pending table character tokens list.

Switch the insertion mode to the original insertion mode and reprocess the token.

8.2.5.4.11 The "in caption" insertion mode

When the user agent is to apply the rules for the "in caption" insertion mode, the user agent must handle the token as follows:

An end tag whose tag name is "caption"

If the stack of open elements does not have a caption element in table scope, this is a parse error; ignore the token. (fragment case)

Otherwise:

Now, if the current node is not a caption element, then this is a parse error.

Pop elements from this stack until a caption element has been popped from the stack.

Clear the list of active formatting elements up to the last marker.

Switch the insertion mode to "in table".

A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "td", "tfoot", "th", "thead", "tr"

An end tag whose tag name is "table"

Clear the list of active formatting elements up to the last marker.

If the stack of open elements does not have a caption element in table scope, ignore the token. (fragment case)

Otherwise:

Pop elements from this stack until a caption element has been popped from the stack.

Switch the insertion mode to "in table".

Reprocess the token.

An end tag whose tag name is one of: "body", "col", "colgroup", "html", "tbody", "td", "tfoot", "th", "thead", "tr"

Parse error. Ignore the token.

Anything else

Process the token using the rules for the "in body" insertion mode.

8.2.5.4.12 The "in column group" insertion mode

When the user agent is to apply the rules for the "in column group" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

A start tag whose tag name is "col"

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

An end tag whose tag name is "colgroup"

If the current node is not a colgroup element, then this is a parse error; ignore the token.

Otherwise, pop the current node from the stack of open elements. Switch the insertion mode to "in table".

An end tag whose tag name is "col"

Parse error. Ignore the token.

A start tag whose tag name is "template"

An end tag whose tag name is "template"

Process the token using the rules for the "in head" insertion mode.

An end-of-file token

Process the token using the rules for the "in body" insertion mode.

Anything else

If the current node is not a colgroup element, then this is a parse error; ignore the token.

Otherwise, pop the current node from the stack of open elements.

Switch the insertion mode to "in table".

Reprocess the token.

8.2.5.4.13 The "in table body" insertion mode

When the user agent is to apply the rules for the "in table body" insertion mode, the user agent must handle the token as follows:

A start tag whose tag name is "tr"

Clear the stack back to a table body context. (See below.)

Insert an HTML element for the token, then switch the insertion mode to "in row".

A start tag whose tag name is one of: "th", "td"

Generate implied end tags.

Clear the stack back to a table body context. (See below.)

Insert an HTML element for a "tr" start tag token with no attributes, then switch the insertion mode to "in row".

Reprocess the current token.

An end tag whose tag name is one of: "tbody", "tfoot", "thead"

If the stack of open elements does not have an element in table scope that is an HTML element and with the same tag name as the token, this is a parse error; ignore the token.

Otherwise:

Clear the stack back to a table body context. (See below.)

Pop the current node from the stack of open elements. Switch the insertion mode to "in table".

A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "tfoot", "thead"

An end tag whose tag name is "table"

If the stack of open elements does not have a tbody, thead, or tfoot element in table scope, this is a parse error; ignore the token.

Otherwise:

Clear the stack back to a table body context. (See below.)

Pop the current node from the stack of open elements. Switch the insertion mode to "in table".

Reprocess the token.

An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html", "td", "th", "tr"

Parse error. Ignore the token.

Anything else

Process the token using the rules for the "in table" insertion mode.

When the steps above require the UA to clear the stack back to a table body context, it means that the UA must, while the current node is not a tbody, tfoot, thead, template, or html element, pop elements from the stack of open elements.

The current node being an html element after this process is a fragment case.

8.2.5.4.14 The "in row" insertion mode

When the user agent is to apply the rules for the "in row" insertion mode, the user agent must handle the token as follows:

A start tag whose tag name is one of: "th", "td"

Clear the stack back to a table row context. (See below.)

Insert an HTML element for the token, then switch the insertion mode to "in cell".

Insert a marker at the end of the list of active formatting elements.

An end tag whose tag name is "tr"

If the stack of open elements does not have a tr element in table scope, this is a parse error; ignore the token.

Otherwise:

Clear the stack back to a table row context. (See below.)

Pop the current node (which will be a tr element) from the stack of open elements. Switch the insertion mode to "in table body".

A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "tfoot", "thead", "tr"

An end tag whose tag name is "table"

If the stack of open elements does not have a tr element in table scope, this is a parse error; ignore the token.

Otherwise:

Clear the stack back to a table row context. (See below.)

Pop the current node (which will be a tr element) from the stack of open elements. Switch the insertion mode to "in table body".

Reprocess the token.

An end tag whose tag name is one of: "tbody", "tfoot", "thead"

If the stack of open elements does not have an element in table scope that is an HTML element and with the same tag name as the token, this is a parse error; ignore the token.

If the stack of open elements does not have a tr element in table scope, ignore the token.

Otherwise:

Clear the stack back to a table row context. (See below.)

Pop the current node (which will be a tr element) from the stack of open elements. Switch the insertion mode to "in table body".

Reprocess the token.

An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html", "td", "th"

Parse error. Ignore the token.

Anything else

Process the token using the rules for the "in table" insertion mode.

When the steps above require the UA to clear the stack back to a table row context, it means that the UA must, while the current node is not a tr, template, or html element, pop elements from the stack of open elements.

The current node being an html element after this process is a fragment case.

8.2.5.4.15 The "in cell" insertion mode

When the user agent is to apply the rules for the "in cell" insertion mode, the user agent must handle the token as follows:

An end tag whose tag name is one of: "td", "th"

If the stack of open elements does not have an element in table scope that is an HTML element and with the same tag name as that of the token, then this is a parse error; ignore the token.

Otherwise:

Now, if the current node is not an HTML element with the same tag name as the token, then this is a parse error.

Pop elements from the stack of open elements stack until an HTML element with the same tag name as the token has been popped from the stack.

Clear the list of active formatting elements up to the last marker.

Switch the insertion mode to "in row".

A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "td", "tfoot", "th", "thead", "tr"

If the stack of open elements does not have a td or th element in table scope, then this is a parse error; ignore the token. (fragment case)

Otherwise, close the cell (see below) and reprocess the token.

An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html"

Parse error. Ignore the token.

An end tag whose tag name is one of: "table", "tbody", "tfoot", "thead", "tr"

If the stack of open elements does not have an element in table scope that is an HTML element and with the same tag name as that of the token, then this is a parse error; ignore the token.

Otherwise, close the cell (see below) and reprocess the token.

Anything else

Process the token using the rules for the "in body" insertion mode.

Where the steps above say to close the cell, they mean to run the following algorithm:

Generate implied end tags.
If the current node is not now a td element or a th element, then this is a parse error.
Pop elements from the stack of open elements stack until a td element or a th element has been popped from the stack.
Clear the list of active formatting elements up to the last marker.
Switch the insertion mode to "in row".

The stack of open elements cannot have both a td and a th element in table scope at the same time, nor can it have neither when the close the cell algorithm is invoked.

8.2.5.4.16 The "in select" insertion mode

When the user agent is to apply the rules for the "in select" insertion mode, the user agent must handle the token as follows:

A character token that is U+0000 NULL

Parse error. Ignore the token.

Any other character token

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

A start tag whose tag name is "option"

If the current node is an option element, pop that node from the stack of open elements.

Insert an HTML element for the token.

A start tag whose tag name is "optgroup"

If the current node is an option element, pop that node from the stack of open elements.

If the current node is an optgroup element, pop that node from the stack of open elements.

Insert an HTML element for the token.

An end tag whose tag name is "optgroup"

First, if the current node is an option element, and the node immediately before it in the stack of open elements is an optgroup element, then pop the current node from the stack of open elements.

If the current node is an optgroup element, then pop that node from the stack of open elements. Otherwise, this is a parse error; ignore the token.

An end tag whose tag name is "option"

If the current node is an option element, then pop that node from the stack of open elements. Otherwise, this is a parse error; ignore the token.

An end tag whose tag name is "select"

If the stack of open elements does not have a select element in select scope, this is a parse error; ignore the token. (fragment case)

Otherwise:

Pop elements from the stack of open elements until a select element has been popped from the stack.

A start tag whose tag name is "select"

Pop elements from the stack of open elements until a select element has been popped from the stack.

It just gets treated like an end tag.

A start tag whose tag name is one of: "input", "keygen", "textarea"

If the stack of open elements does not have a select element in select scope, ignore the token. (fragment case)

Pop elements from the stack of open elements until a select element has been popped from the stack.

Reprocess the token.

A start tag whose tag name is one of: "script", "template"

An end tag whose tag name is "template"

Process the token using the rules for the "in head" insertion mode.

An end-of-file token

Process the token using the rules for the "in body" insertion mode.

Anything else

Parse error. Ignore the token.

8.2.5.4.17 The "in select in table" insertion mode

When the user agent is to apply the rules for the "in select in table" insertion mode, the user agent must handle the token as follows:

A start tag whose tag name is one of: "caption", "table", "tbody", "tfoot", "thead", "tr", "td", "th"

Pop elements from the stack of open elements until a select element has been popped from the stack.

Reprocess the token.

An end tag whose tag name is one of: "caption", "table", "tbody", "tfoot", "thead", "tr", "td", "th"

If the stack of open elements does not have an element in table scope that is an HTML element and with the same tag name as that of the token, then ignore the token.

Otherwise:

Pop elements from the stack of open elements until a select element has been popped from the stack.

Clear the list of active formatting elements up to the last marker.

Reprocess the token.

Anything else

Process the token using the rules for the "in select" insertion mode.

8.2.5.4.18 The "in template" insertion mode

When the user agent is to apply the rules for the "in template" insertion mode, the user agent must handle the token as follows:

A character token

A comment token

A DOCTYPE token

Process the token using the rules for the "in body" insertion mode.

A start tag whose tag name is one of: "base", "basefont", "bgsound", "link", "meta", "noframes", "script", "style", "template", "title"

An end tag whose tag name is "template"

Process the token using the rules for the "in head" insertion mode.

A start tag whose tag name is one of: "caption", "colgroup", "tbody", "tfoot", "thead"

Pop the current template insertion mode off the stack of template insertion modes.

Push "in table" onto the stack of template insertion modes so that it is the new current template insertion mode.

Switch the insertion mode to "in table", and reprocess the token.

A start tag whose tag name is "col"

Pop the current template insertion mode off the stack of template insertion modes.

Push "in column group" onto the stack of template insertion modes so that it is the new current template insertion mode.

Switch the insertion mode to "in column group", and reprocess the token.

A start tag whose tag name is "tr"

Pop the current template insertion mode off the stack of template insertion modes.

Push "in table body" onto the stack of template insertion modes so that it is the new current template insertion mode.

Switch the insertion mode to "in table body", and reprocess the token.

A start tag whose tag name is one of: "td", "th"

Pop the current template insertion mode off the stack of template insertion modes.

Push "in row" onto the stack of template insertion modes so that it is the new current template insertion mode.

Switch the insertion mode to "in row", and reprocess the token.

Any other start tag

Pop the current template insertion mode off the stack of template insertion modes.

Push "in body" onto the stack of template insertion modes so that it is the new current template insertion mode.

Switch the insertion mode to "in body", and reprocess the token.

Any other end tag

Parse error. Ignore the token.

An end-of-file token

If there is no template element on the stack of open elements, then stop parsing. (fragment case)

Otherwise, this is a parse error.

Pop elements from the stack of open elements until a template element has been popped from the stack.

Pop the current template insertion mode off the stack of template insertion modes.

Reprocess the token.

8.2.5.4.19 The "after body" insertion mode

When the user agent is to apply the rules for the "after body" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

Process the token using the rules for the "in body" insertion mode.

A comment token

Insert a comment as the last child of the first element in the stack of open elements (the html element).

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

An end tag whose tag name is "html"

If the parser was originally created as part of the HTML fragment parsing algorithm, this is a parse error; ignore the token. (fragment case)

Otherwise, switch the insertion mode to "after after body".

An end-of-file token

Stop parsing.

Anything else

Parse error. Switch the insertion mode to "in body" and reprocess the token.

8.2.5.4.20 The "in frameset" insertion mode

When the user agent is to apply the rules for the "in frameset" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

A comment token

Parse error. Insert a U+FFFD REPLACEMENT CHARACTER character.

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is "html"

Process the token using the rules for the "in body" insertion mode.

A start tag whose tag name is "frameset"

Insert an HTML element for the token.

An end tag whose tag name is "frameset"

If the current node is the root html element, then this is a parse error; ignore the token. (fragment case)

Otherwise, pop the current node from the stack of open elements.

If the parser was not originally created as part of the HTML fragment parsing algorithm (fragment case), and the current node is no longer a frameset element, then switch the insertion mode to "after frameset".

A start tag whose tag name is "frame"

Insert an HTML element for the token. Immediately pop the current node off the stack of open elements.

Acknowledge the token's self-closing flag, if it is set.

A start tag whose tag name is "noframes"

Process the token using the rules for the "in head" insertion mode.

An end-of-file token

If the current node is not the root html element, then this is a parse error.

The current node can only be the root html element in the fragment case.

Stop parsing.

Anything else

Parse error. Ignore the token.

8.2.5.4.21 The "after frameset" insertion mode

When the user agent is to apply the rules for the "after frameset" insertion mode, the user agent must handle the token as follows:

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE: Insert the character.
A comment token: Insert a comment.
A DOCTYPE token: Parse error. Ignore the token.
A start tag whose tag name is "html": Process the token using the rules for the "in body" insertion mode.
An end tag whose tag name is "html": Switch the insertion mode to "after after frameset".
A start tag whose tag name is "noframes": Process the token using the rules for the "in head" insertion mode.
An end-of-file token: Stop parsing.
Anything else: Parse error. Ignore the token.

8.2.5.4.22 The "after after body" insertion mode

When the user agent is to apply the rules for the "after after body" insertion mode, the user agent must handle the token as follows:

A comment token: Insert a comment as the last child of the Document object.
A DOCTYPE token
A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE
A start tag whose tag name is "html": Process the token using the rules for the "in body" insertion mode.
An end-of-file token: Stop parsing.
Anything else: Parse error. Switch the insertion mode to "in body" and reprocess the token.

8.2.5.4.23 The "after after frameset" insertion mode

When the user agent is to apply the rules for the "after after frameset" insertion mode, the user agent must handle the token as follows:

A comment token: Insert a comment as the last child of the Document object.
A DOCTYPE token
A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE
A start tag whose tag name is "html": Process the token using the rules for the "in body" insertion mode.
An end-of-file token: Stop parsing.
A start tag whose tag name is "noframes": Process the token using the rules for the "in head" insertion mode.
Anything else: Parse error. Ignore the token.

8.2.5.5 The rules for parsing tokens in foreign content

When the user agent is to apply the rules for parsing tokens in foreign content, the user agent must handle the token as follows:

A character token that is U+0000 NULL

A character token that is one of U+0009 CHARACTER TABULATION, "LF" (U+000A), "FF" (U+000C), "CR" (U+000D), or U+0020 SPACE

Any other character token

Set the frameset-ok flag to "not ok".

A comment token

A DOCTYPE token

Parse error. Ignore the token.

A start tag whose tag name is one of: "b", "big", "blockquote", "body", "br", "center", "code", "dd", "div", "dl", "dt", "em", "embed", "h1", "h2", "h3", "h4", "h5", "h6", "head", "hr", "i", "img", "li", "listing", "main", "meta", "nobr", "ol", "p", "pre", "ruby", "s", "small", "span", "strong", "strike", "sub", "sup", "table", "tt", "u", "ul", "var"

A start tag whose tag name is "font", if the token has any attributes named "color", "face", or "size"