4.7 Embedded content

4.7.1 Introduction

This section is non-normative.

To embed an image in HTML, when there is only a single image resource, use the img element and its src attribute.

<h2>From today's featured article</h2>
<img src="/uploads/100-marie-lloyd.jpg" alt="" width="100" height="150">
<p><b><a href="/wiki/Marie_Lloyd">Marie Lloyd</a></b> (1870–1922)
was an English <a href="/wiki/Music_hall">music hall</a> singer, ...

However, there are a number of situations for which the author might wish to use multiple image resources that the user agent can choose from:

The above situations are not mutually exclusive. For example, it is reasonable to combine different resources for different device-pixel-ratio with different resources for art direction.

While it is possible to solve these problems using scripting, doing so introduces some other problems:

With this in mind, this specification introduces a number of features to address the above problems in a declarative manner.

Device-pixel-ratio-based selection when the rendered size of the image is fixed

The src and srcset attributes on the img element can be used, using the x descriptor, to provide multiple images that only vary in their size (the smaller image is a scaled-down version of the bigger image).

The x descriptor is not appropriate when the rendered size of the image depends on the viewport width (viewport-based selection), but can be used together with art direction.

<h2>From today's featured article</h2>
<img src="/uploads/100-marie-lloyd.jpg"
     srcset="/uploads/150-marie-lloyd.jpg 1.5x, /uploads/200-marie-lloyd.jpg 2x"
     alt="" width="100" height="150">
<p><b><a href="/wiki/Marie_Lloyd">Marie Lloyd</a></b> (1870–1922)
was an English <a href="/wiki/Music_hall">music hall</a> singer, ...

The user agent can choose any of the given resources depending on the user's screen's pixel density, zoom level, and possibly other factors such as the user's network conditions.

For backwards compatibility with older user agents that don't yet understand the srcset attribute, one of the URLs is specified in the img element's src attribute. This will result in something useful (though perhaps lower-resolution than the user would like) being displayed even in older user agents. For new user agents, the src attribute participates in the resource selection, as if it was specified in srcset with a 1x descriptor.

The image's rendered size is given in the width and height attributes, which allows the user agent to allocate space for the image before it is downloaded.

Art direction-based selection

The picture element and the source element, together with the media attribute, can be used, to provide multiple images that vary the image content (for intance the smaller image might be a cropped version of the bigger image).

<picture>
  <source media="(min-width: 45em)" srcset="large.jpg">
  <source media="(min-width: 32em)" srcset="med.jpg">
  <img src="small.jpg" alt="The president giving an award.">
</picture>

The user agent will choose the first source element for which the media query in the media attribute matches, and then choose an appropriate URL from its srcset attribute.

The rendered size of the image varies depending on which resource is chosen. To specify dimensions that the user agent can use before having downloaded the image, CSS can be used.

img { width: 300px; height: 300px }
@media (min-width: 32em) { img { width: 500px; height:300px } }
@media (min-width: 45em) { img { width: 700px; height:400px } }

This example combines art direction- and device-pixel-ratio-based selection. A banner that takes half the viewport is provided in two versions, one for wide screens and one for narrow screens.

<h1>
 <picture>
  <source media="(max-width: 500px)" srcset="banner-phone.jpeg, banner-phone-HD.jpeg 2x">
  <img src="banner.jpeg" srcset="banner-HD.jpeg 2x" alt="The Breakfast Combo">
 </picture>
</h1>
Viewport-based selection

The srcset and sizes attributes can be used, using the w descriptor, to provide multiple images that only vary in their size (the smaller image is a scaled-down version of the bigger image).

In this example, a banner image takes up the entire viewport width (using appropriate CSS).

<h1><img sizes="100vw" srcset="wolf-400.jpg 400w, wolf-800.jpg 800w, wolf-1600.jpg 1600w"
     src="wolf-400.jpg" alt="The rad wolf"></h1>

The user agent will calculate the effective pixel density of each image from the specified w descriptors and the specified rendered size in the sizes attribute. It can then choose any of the given resources depending on the user's screen's pixel density, zoom level, and possibly other factors such as the user's network conditions.

If the user's screen is 320 CSS pixels wide, this is equivalent to specifying wolf-400.jpg 1.25x, wolf-800.jpg 2.5x, wolf-1600.jpg 5x. On the other hand, if the user's screen is 1200 CSS pixels wide, this is equivalent to specifying wolf-400.jpg 0.33x, wolf-800.jpg 0.67x, wolf-1600.jpg 1.33x. By using the w descriptors and the sizes attribute, the user agent can choose the correct image source to download regardless of how large the user's device is.

For backwards compatibility, one of the URLs is specified in the img element's src attribute. In new user agents, the src attribute is ignored when the srcset attribute uses w descriptors.

In this example, the sizes attribute could be omitted because the default value is 100vw.

In this example, the Web page has three layouts depending on the width of the viewport. The narrow layout has one column of images (the width of each image is about 100%), the middle layout has two columns of images (the width of each image is about 50%), and the widest layout has three columns of images, and some page margin (the width of each image is about 33%). It breaks between these layouts when the viewport is 30em wide and 50em wide, respectively.

<img sizes="(max-width: 30em) 100vw, (max-width: 50em) 50vw, calc(33vw - 100px)"
     srcset="swing-200.jpg 200w, swing-400.jpg 400w, swing-800.jpg 800w, swing-1600.jpg 1600w"
     src="swing-400.jpg" alt="Kettlebell Swing">

The sizes attribute sets up the layout breakpoints at 30em and 50em, and declares the image sizes between these breakpoints to be 100vw, 50vw, or calc(33vw - 100px). These sizes do not necessarily have to match up exactly with the actual image width as specified in the CSS.

The user agent will pick a width from the sizes attribute, using the first item with a <media-condition> (the part in parentheses) that evaluates to true, or using the last item (calc(33vw - 100px)) if they all evaluate to false.

For example, if the viewport width is 29em, then (max-width: 30em) evaluates to true and 100vw is used, so the image size, for the purpose of resource selection, is 29em. If the viewport width is instead 32em, then (max-width: 30em) evaluates to false, but (max-width: 50em) evaluates to true and 50vw is used, so the image size, for the purpose of resource selection, is 16em (half the viewport width). Notice that the slightly wider viewport results in a smaller image because of the different layout.

The user agent can then calculate the effective pixel density and choose an appropriate resource similarly to the previous example.

Image format-based selection

The type attribute on the source element can be used, to provide multiple images in different formats.

<h2>From today's featured article</h2>
<picture>
 <source srcset="/uploads/100-marie-lloyd.webp" type="image/webp">
 <source srcset="/uploads/100-marie-lloyd.jxr" type="image/vnd.ms-photo">
 <img src="/uploads/100-marie-lloyd.jpg" alt="" width="100" height="150">
</picture>
<p><b><a href="/wiki/Marie_Lloyd">Marie Lloyd</a></b> (1870–1922)
was an English <a href="/wiki/Music_hall">music hall</a> singer, ...

In this example, the user agent will choose the first source that has a type attribute with a supported MIME type. If the user agent supports WebP images, the first source element will be chosen. If not, but the user agent does support JPEG XR images, the second source element will be chosen. If neither of those formats are supported, the img element will be chosen.

4.7.2 Dependencies

Media Queries [MQ] Correct reference is http://dev.w3.org/csswg/mediaqueries-4/
<media-condition>
CSS Values and Units [CSSVALUES]
<length>
CSS Syntax [CSSSYNTAX]
Parse a comma-separated list of component values
component value
<whitespace-token>

4.7.3 The picture element

Categories:
Flow content.
Phrasing content.
Embedded content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
Zero or more source elements, followed by one img element, optionally intermixed with script-supporting elements.
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:
interface HTMLPictureElement : HTMLElement {};

The picture element is a container which provides multiples sources to its contained img element to allow authors to declaratively control or give hints to the user agent about which image resource to use, based on the screen pixel density, viewport size, image format, and other factors. It represents its children.

The picture element is somewhat different from the similar-looking video and audio elements. While all of them contain source elements, the source element's src attribute has no meaning when the element is nested within a picture element, and the resource selection algorithm is different. As well, the picture element itself does not display anything; it merely provides a context for its contained img element that enables it to choose from multiple URLs.

4.7.4 The source element when used with the picture element

Categories:
Same as for the source element.
Contexts in which this element can be used:
As a child of a picture element, before the img element.
Content model:
Same as for the source element.
Content attributes:
Global attributes
srcset - Images to use in different situations (e.g. high-resolution displays, small monitors, etc)
sizes - Image sizes between breakpoints
media - Applicable media
type - Type of embedded resource
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:
partial interface HTMLSourceElement {
           attribute DOMString srcset;
           attribute DOMString sizes;
           attribute DOMString media;
};

The authoring requirements in this section only apply if the source element has a parent that is a picture element.

The source element allows authors to specify multiple alternative source sets for img elements. It does not represent anything on its own.

The srcset attribute must be present, and must consist of one or more image candidate strings, each separated from the next by a "," (U+002C) character. If an image candidate string contains no descriptors and no space characters after the URL, the following image candidate string, if there is one, must begin with one or more space characters.

The sizes attribute may also be present. If present, the value must be a valid source size list.

The media attributes may also be present. If present, the value must contain a valid media query.

The type attribute may also be present. If present, the value must be a valid MIME type. It gives the type of the images in the source set, to allow the user agent to skip to the next source element if it does not support the given type.

If the type attribute is not specified, the user agent will not select a different source element if it finds that it does not support the image format after fetching it.

When a source element has a following sibling source element or img element with a srcset attribute specified, it must have at least one of the following:

The src attribute must not be present.

The IDL attributes srcset, sizes and media must reflect the respective content attributes of the same name.

4.7.5 The img element

Categories:
Flow content.
Phrasing content.
Embedded content.
Form-associated element.
If the element has a usemap attribute: Interactive content.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
Empty.
Content attributes:
Global attributes
alt - Replacement text for use when images are not available
src - Address of the resource
srcset - Images to use in different situations (e.g. high-resolution displays, small monitors, etc)
sizes - Image sizes between breakpoints
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
Tag omission in text/html:
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.
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
[NamedConstructor=Image(optional unsigned long width, optional unsigned long height)]
interface HTMLImageElement : HTMLElement {
           attribute DOMString alt;
           attribute DOMString src;
           attribute DOMString srcset;
           attribute DOMString sizes;
           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;
  readonly attribute DOMString currentSrc;
};

An img element represents an image.

The image given by the src and srcset attributes, and any previous sibling source elements' srcset attributes if the parent is a picture element, 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 (i.e. it is the img element's fallback content).

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 srcset attribute may also be present. If present, its value must consist of one or more image candidate strings, each separated from the next by a "," (U+002C) character. If an image candidate string contains no descriptors and no space characters after the URL, the following image candidate string, if there is one, must begin with one or more space characters.

An image candidate string consists of the following components, in order, with the further restrictions described below this list:

  1. Zero or more space characters.

  2. A valid non-empty URL that does not start or end with a "," (U+002C) character, referencing a non-interactive, optionally animated, image resource that is neither paged nor scripted.

  3. Zero or more space characters.

  4. Zero or one of the following:

  5. Zero or more space characters.

There must not be an image candidate string for an element that has the same width descriptor value as another image candidate string's width descriptor value for the same element.

There must not be an image candidate string for an element that has the same pixel density descriptor value as another image candidate string's pixel density descriptor value for the same element. For the purpose of this requirement, an image candidate string with no descriptors is equivalent to an image candidate string with a 1x descriptor.

If a source element has a sizes attribute present or an img element has a sizes attribute present, all image candidate strings for that element must have the width descriptor specified.

If an image candidate string for an source or img element has the width descriptor specified, all other image candidate strings for that element must also have the width descriptor specified.

The specified width in an image candidate string's width descriptor must match the intrinsic width in the resource given by the image candidate string's URL.

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]

If the srcset attribute is present, the sizes attribute may also be present. If present, its value must be a valid source size list.

A valid source size list is a string that matches the following grammar: [CSSVALUES] [MQ]

<source-size-list> = <source-size># [ , <source-size-value> ]? | <source-size-value>
<source-size> = <media-condition> <source-size-value>
<source-size-value> = <length>

A <source-size-value> must not be negative.

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 element has a current request and a pending request. The current request is initially set to a new image request. The pending request is initially set to null. The current request is usually referred to as the img element itself.

An image request has a state, current URL and image data.

An image request's state is one of the following:

Unavailable
The user agent hasn't obtained any image data, or has obtained some or all of the image data but hasn't yet decoded enough of the image to get the image dimensions.
Partially available
The user agent has obtained some of the image data and at least the image dimensions are available.
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).

An image request's current URL is initially the empty string.

An image request's image data is the decoded image data.

When an image request is either in the partially available state or in the completely available state, it is said to be available.

An image request 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, with the restart animation flag set if so stated, whenever that element is created, or has experienced relevant mutations, and whenever that element's adopting steps are run.

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 is in the unavailable state. When an img element has experienced relevant mutations, and whenever that element's adopting steps are run, if the user agent only obtains images on demand, the img element must return to the unavailable state.

The relevant mutations for an img element are as follows:

Each img element has a last selected source, which must initially be null.

Each image request has 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. Each image furthermore has an ignore higher-layer caching flag. 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, and must unset the ignore higher-layer caching flag for the copied entry. User agents may also remove images from such lists at any time (e.g. to save memory). User agents must remove entries in the list of available images as appropriate given higher-layer caching semantics for the resource (e.g. the HTTP Cache-Control response header) when the ignore higher-layer caching is unset.

The user agent can also store the image data in a separatly from the list of available images.

For example, if a resource has the HTTP response header Cache-Control: must-revalidate, the user agent would remove it from the list of available images but could keep the image data separately, and use that if the server responds with a 204 No Content status.

When the user agent is to update the image data of an img element, optionally with the restart animations flag set, it must run the following steps:

  1. If the element's Document is not the active document, abort these steps.

  2. If the user agent cannot support images, or its support for images has been disabled, then abort the image request for the current request and the pending request, set current request to the unavailable state, let pending request be null, and abort these steps.

  3. If the element does not have a srcset attribute specified and it does not have a parent or it has a parent but it is not a picture element, and it 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.

  4. Let the img element's last selected source be selected source.

  5. If selected source is not null, run these substeps:

    1. Resolve selected source, relative to the element, and let the result be absolute URL. If that is not successful, then abort these inner set of 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 ignore higher-layer caching flag for that entry, abort the image request for the current request and the pending request, let pending request be null, let current request be a new image request whose image data is that of the entry and whose state is set to the completely available state, update the presentation of the image appropriately, let the current request's current pixel density be selected pixel density, queue a task to restart the animation if restart animation is set, change current request's current URL to absolute URL, and then fire a simple event named load at the img element, and abort these steps.

  6. 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 ⌛.)

  7. ⌛ 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, srcset, and crossorigin attributes are all set in succession.

  8. ⌛ Let selected source and selected pixel density be the URL and pixel density that results from selecting an image source, respectively.

  9. ⌛ If selected source is null, run these substeps:

    1. ⌛ Set the current request to the broken state, abort the image request for the current request and the pending request, and let pending request be null.

    2. Queue a task to change the current request's current URL to the empty string, and then, if the element has a src attribute or a srcset attribute or a parent that is a picture element, fire a simple event named error at the img element.

    3. ⌛ Abort this algorithm.

  10. Queue a task to fire a progress event named loadstart at the img element.

  11. Resolve selected source, relative to the element, and let the result be absolute URL. If that is not successful, then abort the image request for the current request and the pending request, set the current request to the broken state, let pending request be null, queue a task to change the current request's current URL to absolute URL, fire a simple event named error at the img element and then fire a simple event named loadend at the img element, and abort these steps.

  12. ⌛ If the pending request is not null, and absolute URL is the same as the pending request's current URL, then abort these steps.

    ⌛ If absolute URL is the same as the current request's current URL, and current request is in the partially available state, then abort the image request for the pending request, queue a task to restart the animation if restart animation is set, and abort these steps.

    ⌛ If the pending request is not null, abort the image request for the pending request.

    ⌛ Let image request be a new image request whose current URL is absolute URL.

    ⌛ Let the pending request be image request.

    ⌛ Do a potentially CORS-enabled fetch of absolute URL, 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. Let this instance of the fetching algorithm be associated with image request.

    The resource obtained in this fashion, if any, is image request'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, if image request is the current request, must fire a progress event named progress at the img element.

  13. End the synchronous section, continuing the remaining steps asynchronously, but without missing any data from the fetch algorithm.

  14. 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 run the following steps:

    1. If image request is the pending request and the user agent is able to determine image request's image's width and height, abort the image request for the current request, upgrade the pending request to the current request and set the current request's state to partially available.

    2. Otherwise, if image request is the pending request and the user agent is able to determine that image request's image is corrupted in some fatal way such that the image dimensions cannot be obtained, abort the image request for the current request, upgrade the pending request to the current request and set the current request's state to broken.

    3. Otherwise, if image request is the current request, it is in the unavailable state, and the user agent is able to determine image request's image's width and height, set the current request's state to partially available.

    4. Otherwise, if image request is the current request, it is in the unavailable state, and the user agent is able to determine that image request's image is corrupted in some fatal way such that the image dimensions cannot be obtained, set the current request's state to broken.

    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 run the following steps:

    1. If the user agent is able to determine image request's image's width and height, and image request is pending request, abort the image request for the current request, upgrade the pending request to the current request, update the img element's presentation appropriately, and set image request's state to partially available.

    2. Otherwise, if the user agent is able to determine image request's image's width and height, and image request is current request, update the img element's presentation appropriately and set image request's state to partially available.

    3. Otherwise, if the user agent is able to determine that image request's image is corrupted in some fatal way such that the image dimensions cannot be obtained, and image request is pending request, abort the image request for the current request and the pending request, upgrade the pending request to the current request, set current request to the broken state, fire a simple event named error at the img element, fire a simple event named loadend at the img element, and abort these steps.

    4. Otherwise, if the user agent is able to determine that image request's image is corrupted in some fatal way such that the image dimensions cannot be obtained, and image request is current request, abort the image request for image request, fire a simple event named error at the img element, fire a simple event named loadend at the img element, and abort these steps.

    That task, and each subsequent task, that is queued by the networking task source while the image is being fetched, if image request is the current request, 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 these steps:

    1. Set image request to the completely available state.

    2. Add the image to the list of available images using the key key, with the ignore higher-layer caching flag set.

    3. Fire a progress event or simple event named load at the img element, depending on the resource in image request.

    4. Fire a progress event or simple event named loadend at the img element, depending on the resource in image request.

    Otherwise

    The image data is not in a supported file format; the user agent must set image request to the broken state, abort the image request for the current request and the pending request, upgrade the pending request to the current request, 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.

To abort the image request for an image request image request means to run the following steps:

  1. Forget image request's image data, if any.

  2. Abort any instance of the fetching algorithm for image request, discarding any pending tasks generated by that algorithm.

To upgrade the pending request to the current request for an img element means to run the following steps:

  1. Let the img element's current request be the pending request.

  2. Let the img element's pending request be null.

To fire a progress event or simple event named type at an element e, depending on resource r, means to fire a progress event named type at e if r is CORS-same-origin, and otherwise fire a simple event named type at e.

To restart the animation for an img element means that, if the image is an animated image, all animated images with the same absolute URL and the same image data in the img element's Document are expected to restart their animation.

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.


An img element is associated with a source set.

A source set is a set of zero or more image sources and a source size.

An image source is a URL, and optionally either a density descriptor, or a width descriptor.

A source size is a <source-size-value>. When a source size has a unit relative to the viewport, it must be interpreted relative to the img element's document's viewport. Other units must be interpreted the same as in Media Queries. [MQ]

When asked to select an image source for a given img element el, user agents must do the following:

  1. Update the source set for el.

  2. If el's source set is empty, return null as the URL and undefined as the pixel density and abort these steps.

  3. Otherwise, take el's source set and let it be source set.

  4. In a user agent-specific manner, choose one image source from source set. Let this be selected source.

  5. Return selected source and its associated pixel density.

When asked to update the source set for a given img element el, user agents must do the following:

  1. Set el's source set to an empty source set.

  2. If el has a parent node and that is a picture element, let elements be an array containing el's parent node's child elements, retaining relative order. Otherwise, let elements be array containing only el.

  3. Iterate through elements, doing the following for each item child:

    1. If child is el:

      1. If child has a srcset attribute, parse child's srcset attribute and let the returned source set be source set. Otherwise, let source set be an empty source set.

      2. Parse child's sizes attribute and let source set's source size be the returned value.

      3. If child has a src attribute whose value is not the empty string and source set does not contain an image source with a density descriptor value of 1, and no image source with a width descriptor, append child's src attribute value to source set.

      4. Let el's source set be source set.

      5. Abort this algorithm.

    2. If child is not a source element, continue to the next child. Otherwise, child is a source element.

    3. If child does not have a srcset attribute, continue to the next child.

    4. Parse child's srcset attribute and let the returned source set be source set.

    5. If source set has zero image sources, continue to the next child.

    6. If child has a media attribute, and its value is not a valid media query, or is a valid media query that evaluates to false, continue to the next child.

    7. Parse child's sizes attribute and let source set's source size be the returned value.

    8. If child has a type attribute, and its value is an unknown or unsupported MIME type, continue to the next child.

    9. Normalize the source densities of source set.

    10. Let el's source set be source set.

    11. Abort this algorithm.

Each img element independently considers its previous sibling source elements plus the img element itself for selecting an image source, ignoring any other (invalid) elements, including other img elements in the same picture element, or source elements that are following siblings of the relevant img element.

When asked to parse a srcset attribute from an element, parse the value of the element's srcset attribute as follows:

  1. Let input be the value passed to this algorithm.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Let raw candidates be an initially empty ordered list of URLs with associated unparsed descriptor list. The order of entries in the lists is the order in which entries are added to the lists.

  4. Splitting loop: Collect a sequence of characters that are space characters or U+002C COMMA characters.

  5. If position is past the end of input, then jump to the step labeled descriptor parser.

  6. Collect a sequence of characters that are not space characters, and let that be url.

  7. Let descriptors be a new empty list.

  8. If url ends with a "," (U+002C) character, follow these substeps:

    1. Remove all trailing U+002C COMMA characters from url.

    2. If url is empty, then jump to the step labeled splitting loop.

    Otherwise, follow these substeps:

    1. Let current token be the empty string.

    2. Let state be start.

    3. Let c be the character at position. Do the following depending on the value of state. For the purpose of this step, "EOF" is a special character representing that position is past the end of input.

      Start

      Do the following, depending on the value of c:

      Space character

      If current token is not empty, append current token to descriptors and let current token be the empty string. Set state to after token.

      "," (U+002C)

      Advance position to the next character in input. If current token is not empty, append current token to descriptors. Jump to the step labeled add candidate.

      "(" (U+0028)

      Append c to current token. Set state to in parens.

      EOF

      If current token is not empty, append current token to descriptors. Jump to the step labeled add candidate.

      Anything else

      Append c to current token.

      In parens

      Do the following, depending on the value of c:

      ")" (U+0029)

      Append c to current token. Set state to start.

      EOF

      Append current token to descriptors. Jump to the step labeled add candidate.

      Anything else

      Append c to current token.

      After token

      Do the following, depending on the value of c:

      Space character

      Stay in this state.

      EOF

      Jump to the step labeled add candidate.

      Anything else

      Set state to start. Set position to the previous character in input.

      Advance position to the next character in input. Repeat this step.

  9. Add candidate: Add url to raw candidates, associated with descriptors.

  10. Return to the step labeled splitting loop.

  11. Descriptor parser: Let candidates be an initially empty source set. The order of entries in the list is the order in which entries are added to the list.

  12. For each entry in raw candidates with URL url associated with the unparsed descriptor list descriptor list, run these substeps:

    1. Let error be no.

    2. Let width be absent.

    3. Let density be absent.

    4. Let future-compat-h be absent.

    5. For each token in descriptor list, run the appropriate set of steps from the following list:

      If the token consists of a valid non-negative integer followed by a U+0077 LATIN SMALL LETTER W character
      1. If width and density are not both absent, then let error be yes.

      2. Apply the rules for parsing non-negative integers to the token. If the result is zero, let error be yes. Otherwise, let width be the result.

      If the token consists of a valid floating-point number followed by a U+0078 LATIN SMALL LETTER X character
      1. If width, density and future-compat-h are not all absent, then let error be yes.

      2. Apply the rules for parsing floating-point number values to the token. If the result is less than zero, let error be yes. Otherwise, let density be the result.

        If density is zero, the intrinsic dimensions will be infinite. User agents are expected to have limits in how big images can be rendered, which is allowed by the hardware limitations clause.

      If the token consists of a valid non-negative integer followed by a U+0068 LATIN SMALL LETTER H character
      1. If future-compat-h and density are not both absent, then let error be yes.

      2. Apply the rules for parsing non-negative integers to the token. If the result is zero, let error be yes. Otherwise, let future-compat-h be the result.

    6. If error is still no, then add a new image source to candidates whose URL is url, associated with a width width if not absent and a pixel density density if not absent.

  13. Return candidates.

When asked to parse a sizes attribute from an element, parse a comma-separated list of component values from the value of the element's sizes attribute (or the empty string, if the attribute is absent), and let unparsed sizes list be the result. [CSSSYNTAX]

For each unparsed size in unparsed sizes list:

  1. Remove all consecutive <whitespace-token>s from the end of unparsed size. If unparsed size is now empty, continue to the next iteration of this algorithm.

  2. If the last component value in unparsed size is a valid non-negative <source-size-value>, let size be its value and remove the component value from unparsed size. Otherwise, continue to the next iteration of this algorithm.

  3. Remove all consecutive <whitespace-token>s from the end of unparsed size. If unparsed size is now empty, return size and exit this algorithm.

  4. Parse the remaining component values in unparsed size as a <media-condition>. If it does not parse correctly, or it does parse correctly but the <media-condition> evaluates to false, continue to the next iteration of this algorithm. [MQ]

  5. Return size and exit this algorithm.

If the above algorithm exhausts unparsed sizes list without returning a size value, return 100vw.

While a valid source size list only contains a bare <source-size-value> (without an accompanying <media-condition>) as the last entry in the <source-size-list>, the parsing algorithm technically allows such at any point in the list, and will accept it immediately as the size if the preceding entries in the list weren't used. This is to enable future extensions, and protect against simple author errors such as a final trailing comma.

An image source can have a density descriptor, a width descriptor, or no descriptor at all accompanying its URL. Normalizing a source set gives every image source a density descriptor.

When asked to normalize the source densities of a source set source set, the user agent must do the following:

  1. Let source size be source set's source size.

  2. For each image source in source set:

    1. If the image source has a density descriptor, continue to the next image source.

    2. Otherwise, if the image source has a width descriptor, replace the width descriptor with a density descriptor with a value of the width descriptor divided by the source size and a unit of x.

      If the source size is zero, the density would be infinity, which results in the intrinsic dimensions being zero by zero.

    3. Otherwise, give the image source a density descriptor of 1x.

The user agent may at any time run the following algorithm to update an img element's image in order to react to changes in the environment. (User agents are not required to ever run this algorithm; for example, if the user is not looking at the page any more, the user agent might want to wait until the user has returned to the page before determining which image to use, in case the environment changes again in the meantime.)

  1. 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 ⌛.)

  2. ⌛ If the img element does not have a srcset attribute specified and it either has no parent or it is not a picture element, is not in the completely available state, has image data whose resource type is multipart/x-mixed-replace, or if its update the image data algorithm is running, then abort this algorithm.

  3. ⌛ Let selected source and selected pixel density be the URL and pixel density that results from selecting an image source, respectively.

  4. ⌛ If selected source is null, then abort these steps.

  5. ⌛ If selected source and selected pixel density are the same as the element's last selected source and current pixel density, then abort these steps.

  6. Resolve selected source, relative to the element.

  7. ⌛ Let CORS mode be the state of the element's crossorigin content attribute.

  8. ⌛ If the resolve a URL algorithm is not successful, abort these steps.

  9. End the synchronous section, continuing the remaining steps asynchronously.

  10. Do a potentially CORS-enabled fetch of the resulting absolute URL, with the mode being CORS mode, the origin being the origin of the img element's Document, and the default origin behaviour set to taint.

    If this download fails in any way (other than the response code not being a 2xx code, as mentioned earlier), or if the image format is unsupported (as determined by applying the image sniffing rules, again as mentioned earlier), or if the resource type is multipart/x-mixed-replace, then abort these steps.

    Otherwise, wait for the fetch algorithm to queue its last task, and then continue with these steps. The data obtained in this way is used in the steps below.

  11. Queue a task to run the following substeps:

    1. If the img element has experienced relevant mutations since this algorithm started, then abort these steps.

    2. Let the img element's last selected source be selected source and the img element's current pixel density be selected pixel density.

    3. Let the img element's current request's current URL be the resulting absolute URL from the earlier step.

    4. Replace the img element's image data with the resource obtained by the earlier step of this algorithm. 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).

    5. Fire a simple event named load at the img element.


The task source for the tasks queued by algorithms in this section is the DOM manipulation task source.


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:

  1. 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.

  2. 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 usemap and ismap attributes can result in confusing behavior when used together with source elements with the media attribute specified in a picture element.

The img element supports dimension attributes.

The alt, src, srcset and sizes 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 . currentSrc

Returns the image's absolute URL.

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:

Otherwise, the attribute must return false.

The value of complete can thus change while a script is executing.

The currentSrc IDL attribute must return the img element's current request's current URL.

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.

4.7.5.1 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.

4.7.5.1.1 Examples of scenarios where users benefit from text alternatives for images
4.7.5.1.2 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.

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:

The example HTML code as displayed in a browser. 3 links each containing a rectangular image:  1 is blue with the text 'blue', 2 red with the text 'red' and 3 is green with the text 'green'.

  <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.

W3C logo used as link content.

  <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.

W3C logo used as link content.

  <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, as part of the text alternative, that the image is a logo. 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.

Print preview icon used as link content.

  <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.

Search icon used as button content.

  <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>
  
4.7.5.1.4 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 KingdomJapanAustralia
Spring5.3 (highest)2.42 (lowest)
Summer4.5 (highest)3.42 (lowest)
Autumn3.5 (highest)1.81.5 (lowest)
Winter1.5 (highest)1.21 (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.

For any of the examples in this section the details and summary elements could be used so that the text descriptions for the images are only displayed on demand:

Details element in the closed state.

Details element in the open state with list content displayed.

  <figure>
  <img src="flowchart.gif" alt="Flowchart: Dealing with a broken lamp.">
  <details>
  <summary>Dealing with a broken lamp</summary>
  <ol>
  lt;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>
  </details>
  </figure>
  

The details and summary elements are not currently well supported by browsers, until such times they are supported, if used, you will need to use scripting to provide the functionality. There are a number of scripted Polyfills and scripted custom controls available, in popular JavaScript UI widget libraries, which provide similar functionality.

4.7.5.1.5 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 pronunication.

Only euro 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.

Once 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...
4.7.5.1.6 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.

4.7.5.1.7 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 recommeneded 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>
  
4.7.5.1.8 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.  -->
  
4.7.5.1.9 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>
  
4.7.5.1.10 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 love you.

I <img src="heart.png" alt="love"> you.

My heart breaks.

My <img src="heart.png" alt="heart"> breaks.

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.

3 out of 5.

  <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>
  
4.7.5.1.12 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.

North KatoombaSouth 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> 

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 headduck'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>  
  
4.7.5.1.14 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> 
4.7.5.1.15 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 as 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>
  
4.7.5.1.16 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:

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.

The caption text in the example below is not a suitable text alternative and is not conforming to the Web Accessibility Guidelines 2.0. [WCAG]

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.

4.7.5.1.17 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.

4.7.5.1.18 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 house icon next to the word 'home'.

  <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!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.

Warning!Your session is about to expire.

  <p><img src="warning.png" width="15" height="15" alt="Warning!">
  Your session is about to expire</p>
  
4.7.5.1.19 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.

HTML 5.1 Nightly 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.

WebPlatform.org 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
4.7.5.1.20 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 -->
  
4.7.5.1.21 An image in a picture element

The picture element and any source elements it contains have no semantics for users, only the img element or its text alternative is displayed to users. Provide a text alternative for an img element without regard to it being within a picture element. Refer to Requirements for providing text to act as an alternative for images for more information on how to provide useful alt text for images.

Art directed images that rely on picture need to depict the same content (irrespective of size, pixel density, or any other discriminating factor). Therefore the appropriate text alternative for an image will always be the same irrespective of which source file ends up being chosen by the browser.

<h2>Is it a ghost?</h2>
<picture>
  <source media="(min-width: 32em)" srcset="large.jpg">
  <img src="small.jpg" alt="Reflection of a girls face in a train window.">
</picture>

The large and small versions (both versions are displayed for demonstration purposes) of the image portray the same scene: Reflection of a girls face in a train window, while the small version (displayed on smaller screens) is cropped, this does not effect the subject matter or the appropriateness of the alt text.

Reflection of a girls face in a train window. Reflection of a girls face in a train window.

4.7.5.1.22 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.

4.7.5.1.23 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:

4.7.6 The iframe element

Categories:
Flow content.
Phrasing content.
Embedded content.
Interactive content.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
Text that conforms to the requirements given in the prose.
Content attributes:
Global attributes
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
seamless - Whether to apply the document's styles to the nested content
allowfullscreen - Whether to allow the iframe's contents to use requestFullscreen()
width - Horizontal dimension
height - Vertical dimension
Tag omission in text/html:
Neither tag is omissible
Allowed ARIA role attribute values:
application, document, img or presentation.
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
interface HTMLIFrameElement : HTMLElement {
           attribute DOMString src;
           attribute DOMString srcdoc;
           attribute DOMString name;
  [PutForwards=value] readonly attribute DOMSettableTokenList sandbox;
           attribute boolean seamless;
           attribute boolean allowFullscreen;
           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. If the itemprop is specified on an iframe element, then the src attribute must also be specified.

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:

  1. Any number of comments and space characters.
  2. Optionally, a DOCTYPE.
  3. Any number of comments and space characters.
  4. The root element, in the form of an html element.
  5. 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]

Here a blog uses the srcdoc attribute in conjunction with the sandbox and seamless attributes described below to provide users of user agents that support this feature with an extra layer of protection from script injection in the blog post comments:

<article>
 <h1>I got my own magazine!</h1>
 <p>After much effort, I've finally found a publisher, and so now I
 have my own magazine! Isn't that awesome?! The first issue will come
 out in September, and we have articles about getting food, and about
 getting in boxes, it's going to be great!</p>
 <footer>
  <p>Written by <a href="/users/cap">cap</a>, 1 hour ago.
 </footer>
 <article>
  <footer> Thirteen minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
  <iframe seamless sandbox srcdoc="<p>did you get a cover picture yet?"></iframe>
 </article>
 <article>
  <footer> Nine minutes ago, <a href="/users/cap">cap</a> wrote: </footer>
  <iframe seamless sandbox srcdoc="<p>Yeah, you can see it <a href=&quot;/gallery?mode=cover&amp;amp;page=1&quot;>in my gallery</a>."></iframe>
 </article>
 <article>
  <footer> Five minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
  <iframe seamless sandbox srcdoc="<p>hey that's earl's table.
<p>you should get earl&amp;amp;me on the next cover."></iframe>
 </article>

Notice the way that quotes have to be escaped (otherwise the srcdoc attribute would end prematurely), and the way raw ampersands (e.g. in URLs or in prose) mentioned in the sandboxed content have to be doubly escaped — once so that the ampersand is preserved when originally parsing the srcdoc attribute, and once more to prevent the ampersand from being misinterpreted when parsing the sandboxed content.

Furthermore, notice that since the DOCTYPE is optional in iframe srcdoc documents, and the html, head, and body elements have optional start and end tags, and the title element is also optional in iframe srcdoc documents, the markup in a srcdoc attribute can be relatively succint despite representing an entire document, since only the contents of the body element need appear literally in the syntax. The other elements are still present, but only by implication.

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
  1. 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.

  2. If there exists an ancestor browsing context whose active document's address, ignoring fragment identifiers, is equal to url, then abort these steps.

  3. 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:

  1. Let child document be the active document of the iframe element's nested browsing context.

  2. If child document has its mute iframe load flag set, abort these steps.

  3. Set child document's iframe load in progress flag.

  4. Fire a simple event named load at the iframe element.

  5. 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, the iframe element's nested browsing context's iframe sandboxing flag set as the output, and, if the iframe has an allowfullscreen attribute, the allow fullscreen flag.

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 seamless attribute is a boolean attribute. When specified, it indicates that the iframe element's browsing context is to be rendered in a manner that makes it appear to be part of the containing document (seamlessly included in the parent document).

An HTML inclusion is effected using this attribute as in the following example. In this case, the inclusion is of a site-wide navigation bar. Any links in the iframe will, in new user agents, be automatically opened in the iframe's parent browsing context; for legacy user agents, the site could also include a base element with a target attribute with the value _parent. Similarly, in new user agents the styles of the parent page will be automatically applied to the contents of the frame, but to support legacy user agents authors might wish to include the styles explicitly.

<!DOCTYPE HTML>
<title>Mirror Mirror — MovieInfo™</title>
<header>
  <h1>Mirror Mirror</h1>
  <p>Part of the MovieInfo™ Database</p>
 <nav>
  <iframe seamless src="nav.inc"></iframe>
 </nav>
</header>
...

An iframe element is said to be in seamless mode when all of the following conditions are met:

When an iframe element is in seamless mode, the following requirements apply:

If the attribute is not specified, or if the origin conditions listed above are not met, then the user agent should render the nested browsing context in a manner that is clearly distinguishable as a separate browsing context, and the seamless browsing context flag must be set to false for that browsing context.

It is important that user agents recheck the above conditions whenever the active document of the nested browsing context of the iframe changes, such that the seamless browsing context flag gets unset if the nested browsing context is navigated to another origin.

The attribute can be set or removed dynamically, with the rendering updating in tandem.

The contenteditable attribute does not propagate into seamless iframes.


The allowfullscreen attribute is a boolean attribute. When specified, it indicates that Document objects in the iframe element's browsing context are to be allowed to use requestFullscreen() (if it's not blocked for other reasons, e.g. there is another ancestor iframe without this attribute set).

Here, an iframe is used to embed a player from a video site. The allowfullscreen attribute is needed to enable the player to show its video full-screen.

<article>
 <header>
  <p><img src="/usericons/1627591962735"> <b>Fred Flintstone</b></p>
  <p><a href="/posts/3095182851" rel=bookmark>12:44</a> — <a href="#acl-3095182851">Private Post</a></p>
 </header>
 <main>
  <p>Check out my new ride!</p>
  <iframe src="https://video.example.com/embed?id=92469812" allowfullscreen></iframe>
 </main>
</article>

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, sandbox, and seamless must reflect the respective content attributes of the same name.

The allowFullscreen IDL attribute must reflect the allowfullscreen content attribute.

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>

4.7.7 The embed element

Categories:
Flow content.
Phrasing content.
Embedded content.
Interactive content.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
Empty.
Content attributes:
Global attributes
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:
No end tag
Allowed ARIA role attribute values:
application, document or img or presentation.
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
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.

If the itemprop attribute is specified on an embed element, then the src attribute must also be specified.

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.

While any of the following conditions are occurring, any plugin instantiated for the element must be removed, and the embed element represents nothing:

An embed element is said to be potentially active when the following conditions are all met simultaneously:

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:

  1. If another task has since been queued to run the embed element setup steps for this element, then abort these steps.

  2. 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.

    5. Once the resource or plugin has completely loaded, queue a task to fire a simple event named load at the element.

    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.

    Once the plugin is completely loaded, queue a task to fire a simple event named load at the element.

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.

When an embed element represents a nested browsing context: if the embed element's nested browsing context's active document is not ready for post-load tasks, and when anything is delaying the load event of the embed element's browsing context's active document, and when the embed element's browsing context is in the delaying load events mode, the embed must delay the load event of its document.

The task source for the tasks mentioned in this section is the DOM manipulation task source.

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>

4.7.8 The object element

Categories:
Flow content.
Phrasing content.
Embedded content.
If the element has a usemap attribute: Interactive content.
Listed, submittable, and reassociateable form-associated element.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
Zero or more param elements, then, transparent.
Content attributes:
Global attributes
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.
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
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();
  boolean reportValidity();
  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.

If the itemprop attribute is specified on an object element, then the data attribute must also be specified.

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 user agent must queue a task to run the following steps to (re)determine what the object element represents. This task being queued or actively running must delay the load event of the element's document.

  1. 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.

  2. 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.

  3. 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.

  4. 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
        1. Let binary be false.

        2. 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.

        3. If the type specified in the resource's Content-Type metadata is "application/octet-stream", then set binary to true.

        4. 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.

        5. If there is a type attribute present on the object element, and its value is not application/octet-stream, then run the following steps:

          1. 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.

          2. Jump to the step below labeled handler.

        Otherwise, if the resource does not have associated Content-Type metadata
        1. 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.

        2. 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. Abort these steps. Once the resource is completely loaded, queue a task to fire a simple event named load at the element.

  5. 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. Otherwise abort these steps; once the plugin is completely loaded, queue a task to fire a simple event named load at the element.

  6. 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.

When an object element represents a nested browsing context: if the object element's nested browsing context's active document is not ready for post-load tasks, and when anything is delaying the load event of the object element's browsing context's active document, and when the object element's browsing context is in the delaying load events mode, the object must delay the load event of its document.

The task source for the tasks mentioned in this section is the DOM manipulation task source.

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(), reportValidity(), 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>

4.7.9 The param element

Categories:
None.
Contexts in which this element can be used:
As a child of an object element, before any flow content.
Content model:
Empty.
Content attributes:
Global attributes
name - Name of parameter
value - Value of parameter
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:
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>

4.7.10 The video element

Categories:
Flow content.
Phrasing content.
Embedded content.
If the element has a controls attribute: Interactive content.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
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.
Content attributes:
Global attributes
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.
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
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):

  1. 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.

  2. 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.

  3. Resolve the poster attribute's value relative to the element. If this fails, then there is no poster frame; abort these steps.

  4. Fetch the resulting absolute URL, from the element's Document's origin. This must delay the load event of the element's document.

  5. 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.

Frames of video must be obtained from the video track that was selected when the event loop last reached step 1.

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 synchronised with the current playback position, at the element's effective media volume. The user agent must play the audio from audio tracks that were enabled when the event loop last reached step 1.

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>

4.7.11 The audio element

Categories:
Flow content.
Phrasing content.
Embedded content.
If the element has a controls attribute: Interactive content.
If the element has a controls attribute: Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
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.
Content attributes:
Global attributes
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.
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
[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 synchronised with the current playback position, at the element's effective media volume. The user agent must play the audio from audio tracks that were enabled when the event loop last reached step 1.

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 be created with its preload attribute set to the literal value "auto". If the src argument is present, the object created must be created with its src content attribute set to the provided value (this will cause the user agent to 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.

4.7.12 The source element

Categories:
None.
Contexts in which this element can be used:
As a child of a media element, before any flow content or track elements.
Content model:
Empty.
Content attributes:
Global attributes
src - Address of the resource
type - Type of embedded resource
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:
interface HTMLSourceElement : HTMLElement {
           attribute DOMString src;
           attribute DOMString type;
};

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. [RFC4281]

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"'>

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 and type 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>

4.7.13 The track element

Categories:
None.
Contexts in which this element can be used:
As a child of a media element, before any flow content.
Content model:
Empty.
Content attributes:
Global attributes
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:
No end tag
Allowed ARIA role attribute values:
None
Allowed ARIA state and property attributes:
Global aria-* attributes
DOM interface:
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.)

4.7.14 Media elements

Media elements (audio and video, in this specification) implement the following interface:

enum CanPlayTypeResult { "" /* 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();
  CanPlayTypeResult 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;
  void fastSeek(double time);
  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) as given by the element's videoTracks attribute when the event loop last reached step 1, 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 when the event loop last reached step 1.

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.

4.7.14.1 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.
4.7.14.2 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.

If the itemprop attribute is specified on the media element, then the src attribute must also be specified.

The crossorigin content attribute on media elements is a CORS settings attribute.

If a media element is created with a src attribute, the user agent must synchronously invoke the media element's resource selection algorithm.

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.

4.7.14.3 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.) [RFC4281]

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 recognise 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.

4.7.14.4 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.

4.7.14.5 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.

  1. Abort any already-running instance of the resource selection algorithm for this element.

  2. 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.

    Basically, pending events and callbacks for the media element are discarded when the media element starts loading a new resource.

  3. 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.

  4. 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.

  5. Set the playbackRate attribute to the value of the defaultPlaybackRate attribute.

  6. Set the error attribute to null and the autoplaying flag to true.

  7. Invoke the media element's resource selection algorithm.

  8. 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 ⌛.

  1. Set the element's networkState attribute to the NETWORK_NO_SOURCE value.

  2. Set the element's show poster flag to true.

  3. Set the media element's delaying-the-load-event flag to true (this delays the load event).

  4. 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 ⌛.)

  5. ⌛ If the media element's blocked-on-parser flag is false, then populate the list of pending text tracks.

  6. ⌛ 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.

  7. ⌛ Set the media element's networkState to NETWORK_LOADING.

  8. Queue a task to fire a simple event named loadstart at the media element.

  9. 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:

      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. ⌛ Set the currentSrc attribute to absolute URL.

    7. End the synchronous section, continuing the remaining steps asynchronously.

    8. Run the resource fetch algorithm with absolute URL. If that algorithm returns without aborting this one, then the load failed.

    9. Failed with elements: Queue a task to fire a simple event named error at the candidate element.

    10. 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 ⌛.)

    11. Forget the media element's media-resource-specific tracks.

    12. Find next candidate: Let candidate be null.

    13. Search loop: If the node after pointer is the end of the list, then jump to the waiting step below.

    14. ⌛ If the node after pointer is a source element, let candidate be that element.

    15. ⌛ 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.

    16. ⌛ If candidate is null, jump back to the search loop step. Otherwise, jump back to the process candidate step.

    17. Waiting: Set the element's networkState attribute to the NETWORK_NO_SOURCE value.

    18. ⌛ Set the element's show poster flag to true.

    19. Queue a task to set the element's delaying-the-load-event flag to false. This stops delaying the load event.

    20. End the synchronous section, continuing the remaining steps asynchronously.

    21. Wait until the node after pointer is a node other than the end of the list. (This step might wait forever.)

    22. 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 ⌛.)

    23. ⌛ 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).

    24. ⌛ Set the networkState back to NETWORK_LOADING.

    25. ⌛ 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:

  1. 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.

  2. Remove all media-resource-specific text tracks from the media element's list of pending text tracks, if any.

  3. 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.

    3. Queue a task to 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.

  4. 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, 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 user agent must queue a task to set the element's delaying-the-load-event flag 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 each immediately queue a task to run the first appropriate steps from the following list. (A new task is used for this so that the work described below occurs relative to the media element event task source rather than the networking task source.)

    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. Let enable be unknown.

    4. If either the media resource or the address of the current media resource indicate a particular set of audio tracks to enable, or if the user agent has information that would facilitate the selection of specific audio tracks to improve the user's experience, then: if this audio track is one of the ones to enable, then set enable to true, otherwise, set enable to false.

      This could 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]

    5. If enable is still unknown, then, if the media element does not yet have an enabled audio track, then set enable to true, otherwise, set enable to false.

    6. If enable is true, then enable this audio track, otherwise, do not enable this audio track.

    7. 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 initialised 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. Let enable be unknown.

    4. If either the media resource or the address of the current media resource indicate a particular set of video tracks to enable, or if the user agent has information that would facilitate the selection of specific video tracks to improve the user's experience, then: if this video track is the first such video track, then set enable to true, otherwise, set enable to false.

      This could again be triggered by Media Fragments URI fragment identifier syntax.

    5. If enable is still unknown, then, if the media element does not yet have a selected video track, then set enable to true, otherwise, set enable to false.

    6. If enable is true, then select this track and unselect any previously selected video tracks, otherwise, do not select this video track. If other tracks are unselected, then a change event will be fired.

    7. 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 initialised 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 there is no enabled audio track, then enable an audio track. This will cause a change event to be fired.

    12. If there is no selected video track, then select a video track. This will cause a change event to be fired.

    13. 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.

  5. 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 minimise 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 normalised 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.

4.7.14.6 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. (This is a fingerprinting vector.)

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 initialised 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.

4.7.14.7 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:

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:

  1. 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.

  2. 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.

4.7.14.8 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 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:

  1. 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.

  2. As defined above, the ended IDL attribute starts returning true once the event loop returns to step 1.

  3. Queue a task to fire a simple event named timeupdate at the media element.

  4. 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.

  5. Queue a task to fire a simple event named ended at the media element.

  6. 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 word "reaches" here does not imply that the current playback position needs to have changed during normal playback; it could be via seeking, for instance.


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 normalised 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.

  1. If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's resource selection algorithm.

  2. 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.

  3. If the media element has a current media controller, then bring the media element up to speed with its new media controller.

  4. 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.

  5. Set the media element's autoplaying flag to false.

  6. 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:

  1. If the media element's networkState attribute has the value NETWORK_EMPTY, invoke the media element's resource selection algorithm.

  2. Run the internal pause steps for the media element.

The internal pause steps for a media element are as follows:

  1. Set the media element's autoplaying flag to false.

  2. 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.

  3. 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.

While the direction of playback is backwards, any corresponding audio must be muted. While 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:

  1. Let current cues be a list of cues, initialised 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.

  2. Let other cues be a list of cues, initialised to contain all the cues of hidden and showing text tracks of the media element that are not present in current cues.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. For each text track cue in missed cues, prepare an event named enter for the TextTrackCue object with the text track cue start time.

  11. 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.

  12. 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.

  13. 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.

  14. Queue each task in events, in list order.

  15. Sort affected tracks in the same order as the text tracks appear in the media element's list of text tracks, and remove duplicates.

  16. 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.

  17. 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.

  18. 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:

  1. 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 ⌛.)

  2. ⌛ If the media element is in a Document, abort these steps.

  3. ⌛ Run the internal pause steps for the media element.

4.7.14.9 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.

media . fastSeek( time )

Seeks to near the given time as fast as possible, trading precision for speed. (To seek to a precise time, use the currentTime attribute.)

This does nothing if the media resource has not been loaded.

The seeking attribute must initially have the value false.

The fastSeek() method must seek to the time given by the method's argument, with the approximate-for-speed flag set.

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 ⌛.

  1. Set the media element's show poster flag to false.

  2. If the media element's readyState is HAVE_NOTHING, abort these steps.

  3. 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.

  4. Set the seeking IDL attribute to true.

  5. 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.

  6. 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.

  7. If the new playback position is less than the earliest possible position, let it be that position instead.

  8. 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.

  9. 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.

  10. Queue a task to fire a simple event named seeking at the element.

  11. 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.

    This step sets the current playback position, and thus can immediately trigger other conditions, such as the rules regarding when playback "reaches the end of the media resource" (part of the logic that handles looping), even before the user agent is actually able to render the media data for that position (as determined in the next step).

    The currentTime attribute does not get updated asynchronously, as it returns the official playback position, not the current playback position.

  12. 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.

  13. Await a stable state. The synchronous section consists of all the remaining steps of this algorithm. (Steps in the synchronous section are marked with ⌛.)

  14. ⌛ Set the seeking IDL attribute to false.

  15. ⌛ Run the time marches on steps.

  16. Queue a task to fire a simple event named timeupdate at the element.

  17. Queue a task to fire a simple event named seeked at the element.


The seekable attribute must return a new static normalised 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.

4.7.14.10 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>
4.7.14.10.1 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 that was disabled is enabled, and whenever one that was enabled is 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
4.7.14.10.2 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>
4.7.14.11 Synchronising multiple media elements
4.7.14.11.1 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.

4.7.14.11.2 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:

  1. Let m be the media element in question.

  2. Let old controller be m's current media controller, if it currently has one, and null otherwise.

  3. Let new controller be null.

  4. Let m have no current media controller, if it currently has one.

  5. Remove the element's mediagroup content attribute, if any.

  6. If the new value is null, then jump to the update controllers step below.

  7. Let m's current media controller be the new value.

  8. Let new controller be m's current media controller.

  9. Bring the media element up to speed with its new media controller.

  10. Update controllers: If old controller and new controller are the same (whether both null or both the same controller) then abort these steps.

  11. If old controller is not null and still has one or more slaved media elements, then report the controller state for old controller.

  12. 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 normalised 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 normalised 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 normalised 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:

  1. If the new playback position is less than zero, then set it to zero.

  2. If the new playback position is greater than the media controller duration, then set it to the media controller duration.

  3. Set the media controller position to the new playback position.

  4. 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, 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:

  1. 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.

  2. If the MediaController's most recently reported readiness state is less than the new readiness state, then run these substeps:

    1. Let next state be the MediaController's most recently reported readiness state.

    2. Loop: Increment next state by one.

    3. 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.

    4. 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
  3. Let the MediaController's most recently reported readiness state be new readiness state.

  4. Initialise 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.
  5. 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.

  6. 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:

    1. 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.

    2. 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
  7. Let the MediaController's most recently reported playback state be new playback state.


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 MediaController interface:

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.

4.7.14.11.3 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:

  1. Let m be the media element in question.

  2. Let old controller be m's current media controller, if it currently has one, and null otherwise.

  3. Let new controller be null.

  4. Let m have no current media controller, if it currently has one.

  5. If m's mediagroup attribute is being removed, then jump to the update controllers step below.

  6. 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.

  7. Let m's current media controller be controller.

  8. Let new controller be m's current media controller.

  9. Bring the media element up to speed with its new media controller.

  10. Update controllers: If old controller and new controller are the same (whether both null or both the same controller) then abort these steps.

  11. If old controller is not null and still has one or more slaved media elements, then report the controller state for old controller.

  12. 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 scoped>
  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>
4.7.14.12 Timed text tracks
4.7.14.12.1 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:

  1. The text tracks corresponding to track element children of the media element, in tree order.
  2. Any text tracks added using the addTextTrack() method, in the order they were added, oldest first.
  3. 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:

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:

  1. If the media element's pending text track change notification flag is set, abort these steps.

  2. Set the media element's pending text track change notification flag.

  3. 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.

  4. 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]

4.7.14.12.2 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.

  1. 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.

  2. 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.

  3. Associate the text track list of cues with the rules for updating the text track rendering appropriate for the format in question.

  4. 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]
  5. 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.

  6. Set the new text track's readiness state to loaded.

  7. 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.

    For instance, if there are no other active subtitles, and this is a forced subtitle track (a subtitle track giving subtitles in the audio track's primary language, but only for audio that is actually in another language), then those subtitles might be activated here.

  8. Add the new text track to the media element's list of text tracks.

  9. 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 initialised to the text track's TextTrack object, at the media element's textTracks attribute's TextTrackList object.

4.7.14.12.3 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 initialised 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 initialised 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:

  1. If the element's blocked-on-parser flag is true, abort these steps.

  2. If the element's did-perform-automatic-track-selection flag is true, abort these steps.

  3. 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:

  1. Perform automatic text track selection for subtitles and captions.

  2. Perform automatic text track selection for descriptions.

  3. Perform automatic text track selection for chapters.

  4. 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

  5. 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:

  1. 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.

  2. If candidates is empty, then abort these steps.

  3. If any of the text tracks in candidates have a text track mode set to showing, abort these steps.

  4. 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:

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 ⌛.

  1. 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.

  2. If the text track's text track mode is not set to one of hidden or showing, abort these steps.

  3. If the text track's track element does not have a media element as a parent, abort these steps.

  4. Run the remainder of these steps asynchronously, allowing whatever caused these steps to run to continue.

  5. Top: Await a stable state. The synchronous section consists of the following steps. (The steps in the synchronous section are marked with ⌛.)

  6. ⌛ Set the text track readiness state to loading.

  7. ⌛ Let URL be the track URL of the track element.

  8. ⌛ 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.

  9. End the synchronous section, continuing the remaining steps asynchronously.

  10. 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), or if URL is the empty string, then 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. This task must use the DOM manipulation task source.

    If the fetching algorithm does not fail, but the type of the resource is not a supported text track format, or 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), then the task that is queued by the networking task source in which the aforementioned problem is found must change the text track readiness state to failed to load and fire a simple event named error at the track element.

    If the fetching algorithm does not fail, and the file was successfully processed, then the final task that is queued by the networking task source, after it has finished parsing the data, must change the text track readiness state to loaded, and fire a simple event named load at the track element.

    If, while the fetching algorithm is active, either:

    ...then the user agent must 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), and then queue a task that first changes the text track readiness state to failed to load and then fires a simple event named error at the track element. This task must use the DOM manipulation task source.

  11. Wait until the text track readiness state is no longer set to loading.

  12. 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.

  13. Jump to the step labeled top.

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.)

4.7.14.12.4 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.

For media-resource-specific text tracks of kind metadata, text track cues are exposed using the DataCue object unless there is a more appropriate TextTrackCue interface available. For example, if the media-resource-specific text track format is WebVTT, then VTTCue is more appropriate.

4.7.14.12.5 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:

  1. Create a new TextTrack object.

  2. 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.

  3. Add the new text track to the media element's list of text tracks.

  4. 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 initialised to the new text track's TextTrack object, at the media element's textTracks attribute's TextTrackList object.

  5. 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:

  1. 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.

  2. 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.

  3. If the given cue is in a text track list of cues, then remove cue from that text track list of cues.

  4. 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:

  1. 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.

  2. 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.

4.7.14.12.6 Text tracks exposing in-band metadata

Media resources often contain one or more media-resource-specific text tracks containing data that browsers don't render, but want to expose to script to allow being dealt with.

If the browser is unable to identify a TextTrackCue interface that is more appropriate to expose the data in the cues of a media-resource-specific text track, the DataCue object is used. [INBANDTRACKS]

[Constructor(double startTime, double endTime, ArrayBuffer data)]
   interface DataCue : TextTrackCue {
               attribute ArrayBuffer data;
};
cue = new DataCue( [ startTime, endTime, data ] )

Returns a new DataCue object, for use with the addCue() method.

The startTime argument sets the text track cue start time.

The endTime argument sets the text track cue end time.

The data argument is copied as the text track cue data.

cue . data [ = value ]

Returns the text track cue data in raw unparsed form.

Can be set.

The data attribute, on getting, must return the raw text track cue data of the text track cue that the TextTrackCue object represents. On setting, the text track cue data must be set to the new value.

The UA will use DataCue to expose only text track cue objects that belong to a text track that has a text track kind of metadata.

DataCue has a constructor to allow script to create DataCue objects in cases where generic metadata needs to be managed for a text track.

The rules for updating the text track rendering for a DataCue simply state that there is no rendering, even when the cues are in showing mode and the text track kind is one of subtitles or captions or descriptions or chapters.

4.7.14.12.7 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.

  1. Let list be a copy of the list of cues of the text track being processed.

  2. Remove from list any text track cue whose text track cue end time is before its text track cue start time.

  3. 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.

  4. 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.)

  5. Loop: If list is empty, jump to the step labeled end.

  6. Let current cue be the first cue in list, and then remove it from list.

  7. 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.

  8. 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.

  9. 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.

  10. 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]

  11. Append new chapter to current chapter's list of nested chapters, and let current chapter be new chapter's parent.

  12. Let current chapter be new chapter.

  13. Return to the step labeled loop.

  14. 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
4.7.14.12.8 Event handlers for objects of the text track APIs

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

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 TextTrack interface:

Event handler Event handler event type
oncuechange cuechange

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 TextTrackCue interface:

Event handler Event handler event type
onenter enter
onexit exit
4.7.14.12.9 Best practices for metadata text tracks

This section is non-normative.

Text tracks can be used for storing data relating to the media data, for interactive or augmented views.

For example, a page showing a sports broadcast could include information about the current score. Suppose a robotics competition was being streamed live. The image could be overlayed with the scores, as follows:

In order to make the score display render correctly whenever the user seeks to an arbitrary point in the video, the metadata text track cues need to be as long as is appropriate for the score. For example, in the frame above, there would be maybe one cue that lasts the length of the match that gives the match number, one cue that lasts until the blue alliance's score changes, and one cue that lasts until the red alliance's score changes. If the video is just a stream of the live event, the time in the bottom right would presumably be automatically derived from the current video time, rather than based on a cue. However, if the video was just the highlights, then that might be given in cues also.

The following shows what fragments of this could look like in a WebVTT file:

WEBVTT

...

05:10:00.000 --> 05:12:15.000
matchtype:qual
matchnumber:37

...

05:11:02.251 --> 05:11:17.198
red:78

05:11:03.672 --> 05:11:54.198
blue:66

05:11:17.198 --> 05:11:25.912
red:80

05:11:25.912 --> 05:11:26.522
red:83

05:11:26.522 --> 05:11:26.982
red:86

05:11:26.982 --> 05:11:27.499
red:89

...

The key here is to notice that the information is given in cues that span the length of time to which the relevant event applies. If, instead, the scores were given as zero-length (or very brief, nearly zero-length) cues when the score changes, for example saying "red+2" at 05:11:17.198, "red+3" at 05:11:25.912, etc, problems arise: primarily, seeking is much harder to implement, as the script has to walk the entire list of cues to make sure that no notifications have been missed; but also, if the cues are short it's possible the script will never see that they are active unless it listens to them specifically.

When using cues in this manner, authors are encouraged to use the cuechange event to update the current annotations. (In particular, using the timeupdate event would be less appropriate as it would require doing work even when the cues haven't changed, and, more importantly, would introduce a higher latency between when the metatata cues become active and when the display is updated, since timeupdate events are rate-limited.)

4.7.14.13 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

A media element has a playback volume, which is a fraction 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.

The volume IDL attribute must return the playback volume of any audio portions of the media element. On setting, if the new value is in the range 0.0 to 1.0 inclusive, the media element's playback volume 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.

A media element can also be muted. If anything is muting the element, then it is muted. (For example, when the direction of playback is backwards, the element is muted.)

The muted IDL attribute must return the value to which it was last set. When a media element is created, if the element has a muted content attribute specified, then the muted IDL attribute should be set to true; otherwise, the user agents may set the value to the user's preferred value (e.g. remembering the last set value across sessions, on a per-site basis or otherwise). While the muted IDL attribute is set to true, the media element must be muted.

Whenever either of the values that would be returned by the volume and muted IDL 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:

  1. 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.

  2. If the element's audio output is muted, the element's effective media volume is zero. Abort these steps.

  3. 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.

  4. Let volume be the playback volume of the audio portions of the media element, in range 0.0 (silent) to 1.0 (loudest).

  5. If the element has a current media controller, multiply volume by that MediaController object's media controller volume multiplier. (The media controller volume multiplier is in the range 0.0 to 1.0, so this can only reduce the value.)

  6. 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 content 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.

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>
4.7.14.14 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 normalised TimeRanges object, the ranges it represents must obey the following criteria:

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.

4.7.14.15 The TrackEvent interface
[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 initialised to. When the object is created, this attribute must be initialised to null. It represents the context information for the event.

4.7.14.16 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.
waiting Event The MediaController is now a blocked media controller.
ended Event All the slaved media elements have newly ended playback; the MediaController has reached the end of all the slaved media elements.
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 have 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.

The following event fires on TextTrack objects and track elements:

Event name Interface Fired when...
cuechange Event One or more cues in the track have become active or stopped being active.

The following events fire on TextTrackCue objects:

Event name Interface Fired when...
enter Event The cue has become active.
exit Event The cue has stopped being active.
4.7.14.17 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. [FETCH]

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.

4.7.14.18 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.

4.7.14.19 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 reaches step 1, 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.)

4.7.15 The map 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
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
Allowed ARIA state and property attributes:
Global aria-* attributes
DOM interface:
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="100,0,200,50" HREF="/toys/"> |
   <A HREF="/food/">Food</A>
   <AREA ALT="Food" COORDS="200,0,300,50" HREF="/food/"> |
   <A HREF="/books/">Books</A>
   <AREA ALT="Books" COORDS="300,0,400,50" HREF="/books/">
 </MAP>
</FOOTER>

4.7.16 The area element

Categories:
Flow content.
Phrasing content.
Contexts in which this element can be used:
Where phrasing content is expected, but only if there is a map element ancestor or a template element ancestor.
Content model:
Empty.
Content attributes:
Global attributes
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:
No end tag
Allowed ARIA role attribute values:
link role (default - do not set).
Allowed ARIA state and property attributes:
Global aria-* attributes
Any aria-* attributes applicable to the allowed roles.
DOM interface:
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
circ Non-conforming
Default state default
Polygon state poly
polygon Non-conforming
Rectangle state rect
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.

If the itemprop attribute is specified on an area element, then the href attribute must also be specified.

The activation behavior of area elements is to run the following steps:

  1. If the area element's Document is not fully active, then abort these steps.

  2. 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.

  3. 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]

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.

4.7.17 Image maps

4.7.17.1 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>
4.7.17.2 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:

  1. 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.

  2. If that returned null, then abort these steps. The image is not associated with an image map after all.

  3. 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.

  1. Remove all the area elements in areas that have no href attribute.

  2. 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.

  3. 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:

  1. Find the state that the element's shape attribute represents.

  2. 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.

  3. 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
  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.
  5. 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.

  6. 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.

  7. 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.

  8. 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 xi be the (2i)th entry in coords, and yi be the (2i+1)th entry in coords (the first entry in coords being the one with index 0).

    Let the coordinates be (xi, yi), 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.

4.7.18 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.

When the MathML annotation-xml element contains elements from the HTML namespace, such elements must all be flow content. [MATHML]

When the MathML token elements (mi, mo, mn, ms, and mtext) are descendants of HTML elements, they may contain phrasing content elements from the HTML namespace. [MATHML]

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>

4.7.19 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]

4.7.20 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:

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.