A vocabulary and associated APIs for HTML and XHTML
img
elementusemap
attribute: Interactive content.alt
- Replacement text for use when images are not availablesrc
- Address of the resourcecrossorigin
- How the element handles crossorigin requestsusemap
- Name of image map to use ismap
- Whether the image is a server-side image mapwidth
- Horizontal dimensionheight
- Vertical dimensionpresentation
role only, for an
img
element whose alt
attribute's value is empty (alt=""
), otherwise
any role value.aria-*
attributes
applicable to the allowed roles.[NamedConstructor=Image(optional unsigned long width, optional unsigned long height)] interface HTMLImageElement : HTMLElement { attribute DOMString alt; attribute DOMString src; attribute DOMString crossOrigin; attribute DOMString useMap; attribute boolean isMap; attribute unsigned long width; attribute unsigned long height; readonly attribute unsigned long naturalWidth; readonly attribute unsigned long naturalHeight; readonly attribute boolean complete; };
An img
element represents an image.
The image given by the src
attributes is the embedded content; the value of
the alt
attribute provides equivalent content for
those who cannot process images or who have image loading disabled.
The requirements on the alt
attribute's value are described
in the next section.
The src
attribute must be present, and must contain a
valid non-empty URL potentially surrounded by spaces referencing a non-interactive,
optionally animated, image resource that is neither paged nor scripted.
The requirements above imply that images can be static bitmaps (e.g. PNGs, GIFs, JPEGs), single-page vector documents (single-page PDFs, XML files with an SVG root element), animated bitmaps (APNGs, animated GIFs), animated vector graphics (XML files with an SVG root element that use declarative SMIL animation), and so forth. However, these definitions preclude SVG files with script, multipage PDF files, interactive MNG files, HTML documents, plain text documents, and so forth. [PNG] [GIF] [JPEG] [PDF] [XML] [APNG] [SVG] [MNG]
The img
element must not be used as a layout tool. In particular, img
elements should not be used to display transparent images, as such images rarely convey meaning and
rarely add anything useful to the document.
The crossorigin
attribute is a CORS
settings attribute. Its purpose is to allow images from third-party sites that allow
cross-origin access to be used with canvas
.
An img
is always in one of the following states:
When an img
element is either in the partially
available state or in the completely available state, it is
said to be available.
An img
element is initially unavailable.
When an img
element is available, it
provides a paint source whose width is the image's intrinsic width, whose height is
the image's intrinsic height, and whose appearance is the intrinsic appearance of the image.
In a browsing context where scripting is disabled, user agents may obtain images immediately or on demand. In a browsing context where scripting is enabled, user agents must obtain images immediately.
A user agent that obtains images immediately must synchronously update the image
data of an img
element whenever that element is created with a src
attribute.
A user agent that obtains images immediately must also synchronously
update the image data of an img
element whenever that element has its
src
or crossorigin
attribute set, changed, or removed.
A user agent that obtains images on demand must update the image data of an
img
element whenever it needs the image data (i.e. on demand), but only if the
img
element has a src
attribute, and only if the img
element is in the
unavailable state. When an img
element's src
or crossorigin
attribute set, changed, or removed, if the user
agent only obtains images on demand, the img
element must return to the unavailable state.
Each img
element has a last selected source, which must initially be
null, and a current pixel density, which must initially be undefined.
When an img
element has a current pixel density that is not 1.0, the
element's image data must be treated as if its resolution, in device pixels per CSS pixels, was
the current pixel density.
For example, if the current pixel density is 3.125, that means that there are 300 device pixels per CSS inch, and thus if the image data is 300x600, it has an intrinsic dimension of 96 CSS pixels by 192 CSS pixels.
Each Document
object must have a list of available images. Each image
in this list is identified by a tuple consisting of an absolute URL, a CORS
settings attribute mode, and, if the mode is not No
CORS, an origin. User agents may copy entries from one Document
object's list of available images to another at any time (e.g. when the
Document
is created, user agents can add to it all the images that are loaded in
other Document
s), but must not change the keys of entries copied in this way when
doing so. User agents may also remove images from such lists at any time (e.g. to save
memory).
When the user agent is to update the image data of an img
element, it
must run the following steps:
Return the img
element to the unavailable
state.
If an instance of the fetching algorithm is still running for this element, then abort that algorithm, discarding any pending tasks generated by that algorithm.
Forget the img
element's current image data, if any.
If the user agent cannot support images, or its support for images has been disabled, then abort these steps.
src
attribute specified and
its value is not the empty string, let selected source be the value of the
element's src
attribute, and selected pixel
density be 1.0. Otherwise, let selected source be null and selected pixel density be undefined.
Let the img
element's last selected source be selected source and the img
element's current pixel
density be selected pixel density.
If selected source is not null, run these substeps:
Resolve selected source, relative to the element. If that is not successful, abort these steps.
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.
If the list of available images contains an entry for key, then set the img
element to the completely
available state, update the presentation of the image appropriately, queue a
task to fire a simple event named load
at the
img
element, and abort these steps.
Asynchronously await a stable state, allowing the task that invoked this algorithm to continue. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ If another instance of this algorithm for this img
element was started
after this instance (even if it aborted and is no longer running), then abort these steps.
Only the last instance takes effect, to avoid multiple requests when, for
example, the src
and crossorigin
attributes are all set in
succession.
⌛ If selected source is null, then set the element to the broken state, queue a task to fire a simple
event named error
at the img
element, and
abort these steps.
⌛ Queue a task to fire a progress event named loadstart
at
the img
element.
⌛ Do a potentially CORS-enabled fetch of the absolute
URL that resulted from the earlier step, with the mode being the current state of the
element's crossorigin
content attribute, the origin being the origin of the img
element's
Document
, and the default origin behaviour set to taint.
The resource obtained in this fashion, if any, is the img
element's image data.
It can be either CORS-same-origin or CORS-cross-origin; this affects
the origin of the image itself (e.g. when used on a canvas
).
Fetching the image must delay the load event of the element's document until the task that is queued by the networking task source once the resource has been fetched (defined below) has been run.
This, unfortunately, can be used to perform a rudimentary port scan of the user's local network (especially in conjunction with scripting, though scripting isn't actually necessary to carry out such an attack). User agents may implement cross-origin access control policies that are stricter than those described above to mitigate this attack, but unfortunately such policies are typically not compatible with existing Web content.
If the resource is CORS-same-origin, each task
that is queued by the networking task source
while the image is being fetched must fire a progress
event named progress
at the img
element.
End the synchronous section, continuing the remaining steps asynchronously, but without missing any data from the fetch algorithm.
As soon as possible, jump to the first applicable entry from the following list:
multipart/x-mixed-replace
The next task that is queued by the networking task source while the image is being fetched must set the img
element's state to partially available.
Each task that is queued by the networking task source while the image is being fetched must update the presentation of the image, but as each new
body part comes in, it must replace the previous image. Once one body part has been completely
decoded, the user agent must set the img
element to the completely available state and queue a task to fire
a simple event named load
at the img
element.
The progress
and loadend
events are not fired for
multipart/x-mixed-replace
image streams.
The next task that is queued by the networking task source while the image is being fetched must set the img
element's state to partially available.
That task, and each subsequent task, that is queued by the networking task source while the image is being fetched must update the presentation of the image appropriately (e.g. if the image is a progressive JPEG, each packet can improve the resolution of the image).
Furthermore, the last task that is queued by the networking task source once the resource has been fetched must additionally run the steps for the matching entry in the following list:
Set the img
element to the completely
available state.
Add the image to the list of available images using the key key.
If the resource is CORS-same-origin: fire a progress event
named load
at the img
element.
If the resource is CORS-cross-origin: fire a simple event named
load
at the img
element.
If the resource is CORS-same-origin: fire a progress event
named loadend
at the img
element.
If the resource is CORS-cross-origin: fire a simple event named
loadend
at the img
element.
If the resource is CORS-same-origin: fire a progress event
named load
at the img
element.
If the resource is CORS-cross-origin: fire a simple event named
load
at the img
element.
If the resource is CORS-same-origin: fire a progress event
named loadend
at the img
element.
If the resource is CORS-cross-origin: fire a simple event named
loadend
at the img
element.
Either the image data is corrupted in some fatal way such that the image dimensions cannot
be obtained, or the image data is not in a supported file format; the user agent must set the
img
element to the broken state, abort the fetching algorithm, discarding any pending tasks generated by that algorithm, and then queue a
task to first fire a simple event named error
at the img
element and then fire a simple
event named loadend
at the img
element.
While a user agent is running the above algorithm for an element x, there
must be a strong reference from the element's Document
to the element x, even if that element is not in its
Document
.
When an img
element is in the completely available
state and the user agent can decode the media data without errors, then the
img
element is said to be fully decodable.
Whether the image is fetched successfully or not (e.g. whether the response code was a 2xx code or equivalent) must be ignored when determining the image's type and whether it is a valid image.
This allows servers to return images with error responses, and have them displayed.
The user agent should apply the image sniffing rules to determine the type of the image, with the image's associated Content-Type headers giving the official type. If these rules are not applied, then the type of the image must be the type given by the image's associated Content-Type headers.
User agents must not support non-image resources with the img
element (e.g. XML
files whose root element is an HTML element). User agents must not run executable code (e.g.
scripts) embedded in the image resource. User agents must only display the first page of a
multipage resource (e.g. a PDF file). User agents must not allow the resource to act in an
interactive fashion, but should honor any animation in the resource.
This specification does not specify which image types are to be supported.
What an img
element represents depends on the src
attribute and the alt
attribute.
src
attribute is set and the alt
attribute is set to the empty stringThe 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.
src
attribute is set and the alt
attribute is set to a value that isn't emptyThe 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.
src
attribute is set and the alt
attribute is notThere is no textual equivalent of the image available.
If the image is available and the user agent is configured to display that image, then the element represents the element's image data.
Otherwise, the user agent should display some sort of indicator that there is an image that is not being rendered, and may, if requested by the user, or if so configured, or when required to provide contextual information in response to navigation, provide caption information for the image, derived as follows:
If the image is a descendant of a figure
element that has a child
figcaption
element, and, ignoring the figcaption
element and its
descendants, the figure
element has no Text
node descendants other
than inter-element whitespace, and no embedded content descendant
other than the img
element, then the contents of the first such
figcaption
element are the caption information; abort these steps.
There is no caption information.
src
attribute is not set and either the alt
attribute is set to the empty string or the alt
attribute is not set at allThe element represents nothing.
The element represents the text given by the alt
attribute.
The alt
attribute does not represent advisory information.
User agents must not present the contents of the alt
attribute
in the same way as content of the title
attribute.
While user agents are encouraged to repair cases of missing alt
attributes, authors must not rely on such behavior. Requirements for providing text to act as an alternative for images are described
in detail below.
The contents of img
elements, if any, are ignored for the purposes of
rendering.
The usemap
attribute,
if present, can indicate that the image has an associated
image map.
The ismap
attribute, when used on an element that is a descendant of an
a
element with an href
attribute, indicates by its
presence that the element provides access to a server-side image
map. This affects how events are handled on the corresponding
a
element.
The ismap
attribute is a
boolean attribute. The attribute must not be specified
on an element that does not have an ancestor a
element
with an href
attribute.
The img
element supports dimension
attributes.
The alt
, src
IDL attributes must reflect the
respective content attributes of the same name.
The crossOrigin
IDL attribute must
reflect the crossorigin
content attribute,
limited to only known values.
The useMap
IDL attribute must
reflect the usemap
content attribute.
The isMap
IDL attribute must reflect
the ismap
content attribute.
width
[ = value ]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.
naturalWidth
naturalHeight
These attributes return the intrinsic dimensions of the image, or zero if the dimensions are not known.
complete
Returns true if the image has been completely downloaded or if no image is specified; otherwise, returns false.
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:
src
attribute is omitted.
img
element is completely available.
img
element is broken.
Otherwise, the attribute must return false.
The value of complete
can thus change while
a script is executing.
A constructor is provided for creating HTMLImageElement
objects (in addition to
the factory methods from DOM such as createElement()
): Image(width, height)
.
When invoked as a constructor, this must return a new HTMLImageElement
object (a new
img
element). If the width argument is present, the new object's
width
content attribute must be set to width. If the height argument is also present, the new object's
height
content attribute must be set to height. The element's document must be the active document of the
browsing context of the Window
object on which the interface object of
the invoked constructor is found.
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.
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:
<ul> <li><a href="red.html"><img src="red.jpeg" alt="Red"></a></li> <li><a href="green.html"><img src="green.jpeg" alt="Green"></a></li> <li><a href="blue.html"><img src="blue.jpeg" alt="Blue"></a></li> </ul>
In this example, a link contains a logo. The link points to the W3C web site from an external site. The text alternative is a brief description of the link target.
<a href="http://w3.org"> <img src="images/w3c_home.png" width="72" height="48" alt="W3C web site"> </a>
This example is the same as the previous example, except that the link is on the W3C web site. The text alternative is a brief description of the link target.
<a href="http://w3.org"> <img src="images/w3c_home.png" width="72" height="48" alt="W3C home"> </a>
Depending on the context in which an image of a logo is used it could be appropriate to provide an indication that the image is a logo as part of the text alternative. Refer to section 4.7.1.1.19 Logos, insignia, flags, or emblems.
In this example, a link contains a print preview icon. The link points to a version of the page with a print stylesheet applied. The text alternative is a brief description of the link target.
<a href="preview.html"> <img src="images/preview.png" width="32" height="30" alt="Print preview."> </a>
In this example, a button contains a search icon. The button submits a search form. The text alternative is a brief description of what the button does.
<button> <img src="images/search.png" width="74" height="29" alt="Search"> </button>
In this example, a company logo for the PIP Corporation has been split into the following two images,
the first containing the word PIP and the second with the abbreviated word CO. The images are the
sole content of a link to the PIPCO home page. In this case a brief description of the link target is provided.
As the images are presented to the user as a single entity the text alternative PIP CO home is in the
alt
attribute of the first image.
<a href="pipco-home.html"> <img src="pip.gif" alt="PIP CO home"><img src="co.gif" alt=""> </a>
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:
<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:
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.
<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.
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.
United Kingdom | Japan | Australia | |
---|---|---|---|
Spring | 5.3 (highest) | 2.4 | 2 (lowest) |
Summer | 4.5 (highest) | 3.4 | 2 (lowest) |
Autumn | 3.5 (highest) | 1.8 | 1.5 (lowest) |
Winter | 1.5 (highest) | 1.2 | 1 (lowest) |
<figure> <figcaption>Rainfall Data</figcaption> <img src="rainchart.gif" alt="Bar chart: Average rainfall in millimetres by Country and Season."> <table> <caption>Rainfall in millimetres by Country and Season.</caption> <tr><td><th scope="col">UK <th scope="col">Japan<th scope="col">Australia</tr> <tr><th scope="row">Spring <td>5.5 (highest)<td>2.4 <td>2 (lowest)</tr> <tr><th scope="row">Summer <td>4.5 (highest)<td>3.4<td>2 (lowest)</tr> <tr><th scope="row">Autumn <td>3.5 (highest) <td>1.8 <td>1.5 (lowest)</tr> <tr><th scope="row">Winter <td>1.5 (highest) <td>1.2 <td>1 lowest</tr> </table> </figure>
The figure
element is used to group the Bar Chart image and data table.
The figcaption
element provides a caption for the grouped content.
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!".
<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.
<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 5.99!
<p>Only <img src="euro.png" alt="euro ">5.99!
An image should not be used if Unicode characters would serve an identical purpose. Only when the text cannot be directly represented using Unicode, e.g. because of decorations or because the character is not in the Unicode character set (as in the case of gaiji), would an image be appropriate.
If an author is tempted to use an image because their default system font does not support a given character, then Web Fonts are a better solution than images.
An illuminated manuscript might use graphics for some of its letters. The text alternative in such a situation is just the character that the image represents.
nce upon a time and a long long time ago...
<p><img src="initials/fancyO.png" alt="O">nce upon a time and a long long time ago...
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.
<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.
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.
<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.
<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>
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.
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. -->
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>
When images are used inline as part of the flow of text in a sentence, provide a word or phrase as a text alternative which makes sense in the context of the sentence it is apart of.
I you.
I <img src="heart.png" alt="love"> you.
My breaks.
My <img src="heart.png" alt="heart"> breaks.
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.
<img src="pip.gif" alt="PIP CO"><img src="co.gif" alt="">
In the following example, a rating is shown as three filled
stars and two empty stars. While the text alternative could have
been "★★★☆☆", the author has
instead decided to more helpfully give the rating in the form "3
out of 5". That is the text alternative of the first image, and the
rest have empty alt
attributes.
<p>Rating: <meter max=5 value=3> <img src="1" alt="3 out of 5"> <img src="1" alt=""><img src="1" alt=""> <img src="0" alt=""><img src="0" alt=""> </meter></p>
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:
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...
<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>
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 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.
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>
Webcam images are static images that are automatically updated periodically. Typically the images are from a fixed viewpoint, the images may update on the page automatically as each new image is uploaded from the camera or the user may be required to refresh the page to view an updated image. Examples include traffic and weather cameras.
This example is fairly typical; the title and a time stamp are included in the image, automatically generated
by the webcam software. It would be better if the text information was not included in the image, but as it is part
of the image, include it part of the text alternative. A caption is also provided using the figure
and figcaption
elements. As the image is provided to give a visual indication of the current weather near a building,
a link to a local weather forecast is provided, as with automatically generated and uploaded webcam images it may be impractical to
provide such information as a text alternative.
The text of the alt
attribute includes a prose version of the timestamp, designed to make the text more
understandable when announced by text to speech software. The text alternative also includes a description of some aspects
of what can be seen in the image which are unchanging, although weather conditions and time of day change.
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>
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.
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.
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.
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.
An icon is usually a simple picture representing a program, action, data file or a concept. Icons are intended to help users of visual browsers to recognize features at a glance.
Use an empty alt
attribute when an icon is supplemental to
text conveying the same meaning.
In this example, we have a link pointing to a site's home page, the link contains a
house icon image and the text "home". The image has an empty alt
text.
Where images are used in this way, it would also be appropriate to add the image using CSS
<a href="home.html"><img src="home.gif" width="15" height="15" alt="">Home</a>
#home:before { content: url(home.png); } <a href="home.html" id="home">Home</a>
In this example, there is a warning message, with a warning icon. The word "Warning!" is in emphasized
text next to the icon. As the information conveyed by the icon is redundant the img
element is given an an empty alt
attribute.
Warning! Your session is about to expire.
<p><img src="warning.png" width="15" height="15" alt=""> <strong>Warning!</strong> Your session is about to expire</p>
When an icon conveys additional information not available in text, provide a text alternative.
In this example, there is a warning message, with a warning icon. The icon emphasizes the importance of the message and identifies it as a particular type of content.
Your session is about to expire.
<p><img src="warning.png" width="15" height="15" alt="Warning!"> Your session is about to expire</p>
Many pages include logos, insignia, flags, or emblems, which stand for a company, organization, project, band, software package, country, or other entity. What can be considered as an appropriate text alternative depends upon, like all images, the context in which the image is being used and what function it serves in the given context.
If a logo is the sole content of a link, provide a brief description of the link target in the alt
attribute.
This example illustrates the use of the HTML5 logo as the sole content of a link to the HTML specification.
<a href="http://dev.w3.org/html5/spec/spec.html"> <img src="HTML5_Logo.png" alt="HTML 5.1 specification"></a>
If a logo is being used to represent the entity, e.g. as a page heading, provide the name of the entity being represented by the logo as the text alternative.
This example illustrates the use of the WebPlatform.org logo being used to represent itself.
and other developer resources
<h2><img src="images/webplatform.png" alt="WebPlatform.org"> and other developer resources<h2>
The text alternative in the example above could also include the word "logo" to describe the type of image content.
If so, it is suggested that square brackets be used to delineate this information: alt="[logo] WebPlatform.org"
.
If a logo is being used next to the name of the what that it represents, then the logo is supplemental.
Include an empty alt
attribute as the text alternative is already provided.
This example illustrates the use of a logo next to the name of the organization it represents.
WebPlatform.org
<img src="images/webplatform1.png" alt=""> WebPlatform.org
If the logo is used alongside text discussing the subject or entity the logo represents, then provide a text alternative which describes the logo.
This example illustrates the use of a logo next to text discussing the subject the logo represents.
HTML5 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
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.
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 -->
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.
A conformance checker must report the lack of an alt
attribute as an error unless one of
the conditions listed below applies:
The img
element is in a figure
element that satisfies the
conditions described above.
The img
element has a (non-conforming) generator-unable-to-provide-required-alt
attribute whose value is the empty string. A conformance checker
that is not reporting the lack of an alt
attribute as an error must also not
report the presence of the empty generator-unable-to-provide-required-alt
attribute as an error. (This case does not represent a case where
the document is conforming, only that the generator could not
determine appropriate alternative text — validators are not
required to show an error in this case, because such an error might
encourage markup generators to include bogus alternative text
purely in an attempt to silence validators. Naturally, conformance
checkers may report the lack of an alt
attribute as an error even in the
presence of the generator-unable-to-provide-required-alt
attribute; for example, there could be a user option to report
all conformance errors even those that might be the more
or less inevitable result of using a markup generator.)
iframe
elementsrc
- Address of the resourcesrcdoc
- A document to render in the iframe
name
- Name of nested browsing contextsandbox
- Security rules for nested contentwidth
- Horizontal dimensionheight
- Vertical dimensionapplication
,
document
, img
or
presentation
.aria-*
attributes
applicable to the allowed roles.interface HTMLIFrameElement : HTMLElement {
attribute DOMString src;
attribute DOMString srcdoc;
attribute DOMString name;
[PutForwards=value] readonly attribute DOMSettableTokenList sandbox;
attribute DOMString width;
attribute DOMString height;
readonly attribute Document? contentDocument;
readonly attribute WindowProxy? contentWindow;
};
The iframe
element represents a nested browsing
context.
The src
attribute gives the address of a page
that the nested browsing context is to contain. The attribute, if present, must be a
valid non-empty URL potentially surrounded by spaces.
The srcdoc
attribute gives the content of
the page that the nested browsing context is to contain. The value of the attribute
is the source of an iframe
srcdoc
document.
For iframe
elements in HTML documents, the srcdoc
attribute, if present, must have a value using the
HTML syntax that consists of the following syntactic components, in the given order:
html
element.For iframe
elements in XML documents, the srcdoc
attribute, if present, must have a value that matches the
production labeled document
in the XML specification. [XML]
In the HTML syntax, authors need only remember to use """ (U+0022) characters to wrap the attribute contents and then to escape all """ (U+0022) and U+0026 AMPERSAND (&) characters, and to specify the sandbox
attribute, to ensure safe embedding of content.
Due to restrictions of the XHTML syntax, in XML the "<" (U+003C) character needs to be escaped as well. In order to prevent attribute-value normalization, some of XML's whitespace characters — specifically "tab" (U+0009), "LF" (U+000A), and "CR" (U+000D) — also need to be escaped. [XML]
If the src
attribute and the srcdoc
attribute are both specified together, the srcdoc
attribute takes priority. This allows authors to provide
a fallback URL for legacy user agents that do not support the srcdoc
attribute.
When an iframe
element is inserted
into a document, the user agent must create a nested browsing context, and
then process the iframe
attributes for the "first time".
When an iframe
element is removed
from a document, the user agent must discard the nested browsing context.
This happens without any unload
events firing
(the nested browsing context and its Document
are discarded, not unloaded).
Whenever an iframe
element with a nested browsing context has its
srcdoc
attribute set, changed, or removed, the user agent
must process the iframe
attributes.
Similarly, whenever an iframe
element with a nested browsing context
but with no srcdoc
attribute specified has its src
attribute set, changed, or removed, the user agent must
process the iframe
attributes.
When the user agent is to process the iframe
attributes, it must run
the first appropriate steps from the following list:
srcdoc
attribute is specifiedNavigate 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.
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.
If the value of the src
attribute is missing, or its value is the empty string,
let url be the string "about:blank
".
Otherwise, resolve the value of the src
attribute, relative to the iframe
element.
If that is not successful, then let url be the string
"about:blank
". Otherwise, let url be the resulting
absolute URL.
If there exists an ancestor browsing context whose active document's address, ignoring fragment identifiers, is equal to url, then abort these steps.
Navigate the element's child browsing context to url.
Any navigation required of the user agent in the process
the iframe
attributes algorithm must be completed as an explicit
self-navigation override and with the iframe
element's document's
browsing context as the source browsing context.
Furthermore, if the active document of the element's child browsing context before such a navigation was not completely loaded at the time of the new navigation, then the navigation must be completed with replacement enabled.
Similarly, if the child browsing context's session history contained
only one Document
when the process the iframe
attributes
algorithm was invoked, and that was the about:blank
Document
created
when the child browsing context was created, then any navigation required of the user agent in that algorithm must be completed
with replacement enabled.
When a Document
in an iframe
is marked as completely
loaded, the user agent must synchronously run the iframe load event steps.
A load
event is also fired at the
iframe
element when it is created if no other data is loaded in it.
Each Document
has an iframe load in progress flag and a mute
iframe load flag. When a Document
is created, these flags must be unset for
that Document
.
The iframe load event steps are as follows:
Let child document be the active document of the
iframe
element's nested browsing context.
If child document has its mute iframe load flag set, abort these steps.
Set child document's iframe load in progress flag.
Fire a simple event named load
at the
iframe
element.
Unset child document's iframe load in progress flag.
This, in conjunction with scripting, can be used to probe the URL space of the local network's HTTP servers. User agents may implement cross-origin access control policies that are stricter than those described above to mitigate this attack, but unfortunately such policies are typically not compatible with existing Web content.
When the iframe
's browsing context's active document is
not ready for post-load tasks, and when anything in the iframe
is delaying the load event of the iframe
's
browsing context's active document, and when the iframe
's
browsing context is in the delaying load
events
mode, the iframe
must delay the load event of its document.
If, during the handling of the load
event, the
browsing context in the iframe
is again navigated, that will further delay the load event.
If, when the element is created, the srcdoc
attribute is not set, and the src
attribute is either also not set or set but its value cannot be
resolved, the browsing context will remain at the initial
about:blank
page.
If the user navigates away from this page, the
iframe
's corresponding WindowProxy
object will proxy new
Window
objects for new Document
objects, but the src
attribute will not change.
The name
attribute, if present, must be a
valid browsing context name. The given value is used to name the nested
browsing context. When the browsing context is created, if the attribute
is present, the browsing context name must be set to the value of this attribute;
otherwise, the browsing context name must be set to the empty string.
Whenever the name
attribute is set, the nested
browsing context's name must be changed to
the new value. If the attribute is removed, the browsing context name must be set to
the empty string.
The sandbox
attribute, when specified,
enables a set of extra restrictions on any content hosted by the iframe
. Its value
must be an unordered set of unique space-separated tokens that are ASCII
case-insensitive. The allowed values are allow-forms
, allow-pointer-lock
, allow-popups
, allow-same-origin
, allow-scripts
, and allow-top-navigation
.
When the attribute is set, the content is treated as being from a unique origin,
forms, scripts, and various potentially annoying APIs are disabled, links are prevented from
targeting other browsing contexts, and plugins are secured.
The allow-same-origin
keyword causes
the content to be treated as being from its real origin instead of forcing it into a unique
origin; the allow-top-navigation
keyword allows the content to navigate its top-level browsing context;
and the allow-forms
, allow-pointer-lock
, allow-popups
and allow-scripts
keywords re-enable forms, the
pointer lock API, popups, and scripts respectively. [POINTERLOCK]
Setting both the allow-scripts
and allow-same-origin
keywords together when the
embedded page has the same origin as the page containing the iframe
allows the embedded page to simply remove the sandbox
attribute and then reload itself, effectively breaking out of the sandbox altogether.
These flags only take effect when the nested browsing context of
the iframe
is navigated. Removing them, or removing the
entire sandbox
attribute, has no effect on an
already-loaded page.
Potentially hostile files should not be served from the same server as the file
containing the iframe
element. Sandboxing hostile content is of minimal help if an
attacker can convince the user to just visit the hostile content directly, rather than in the
iframe
. To limit the damage that can be caused by hostile HTML content, it should be
served from a separate dedicated domain. Using a different domain ensures that scripts in the
files are unable to attack the site, even if the user is tricked into visiting those pages
directly, without the protection of the sandbox
attribute.
When an iframe
element with a sandbox
attribute has its nested browsing context created (before the initial
about:blank
Document
is created), and when an iframe
element's sandbox
attribute is set or changed while it
has a nested browsing context, the user agent must parse the sandboxing directive using the attribute's value as the input, and the iframe
element's nested browsing context's
iframe
sandboxing flag set as the output.
When an iframe
element's sandbox
attribute is removed while it has a nested browsing context, the user agent must
empty the iframe
element's nested browsing context's
iframe
sandboxing flag set as the output.
In this example, some completely-unknown, potentially hostile, user-provided HTML content is embedded in a page. Because it is served from a separate domain, it is affected by all the normal cross-site restrictions. In addition, the embedded page has scripting disabled, plugins disabled, forms disabled, and it cannot navigate any frames or windows other than itself (or any frames or windows it itself embeds).
<p>We're not scared of you! Here is your content, unedited:</p> <iframe sandbox src="http://usercontent.example.net/getusercontent.cgi?id=12193"></iframe>
It is important to use a separate domain so that if the attacker convinces the user to visit that page directly, the page doesn't run in the context of the site's origin, which would make the user vulnerable to any attack found in the page.
In this example, a gadget from another site is embedded. The gadget has scripting and forms enabled, and the origin sandbox restrictions are lifted, allowing the gadget to communicate with its originating server. The sandbox is still useful, however, as it disables plugins and popups, thus reducing the risk of the user being exposed to malware and other annoyances.
<iframe sandbox="allow-same-origin allow-forms allow-scripts" src="http://maps.example.com/embedded.html"></iframe>
Suppose a file A contained the following fragment:
<iframe sandbox="allow-same-origin allow-forms" src=B></iframe>
Suppose that file B contained an iframe also:
<iframe sandbox="allow-scripts" src=C></iframe>
Further, suppose that file C contained a link:
<a href=D>Link</a>
For this example, suppose all the files were served as text/html
.
Page C in this scenario has all the sandboxing flags set. Scripts are disabled, because the
iframe
in A has scripts disabled, and this overrides the allow-scripts
keyword set on the
iframe
in B. Forms are also disabled, because the inner iframe
(in B)
does not have the allow-forms
keyword
set.
Suppose now that a script in A removes all the sandbox
attributes in A and B.
This would change nothing immediately. If the user clicked the link in C, loading page D into the
iframe
in B, page D would now act as if the iframe
in B had the allow-same-origin
and allow-forms
keywords set, because that was the
state of the nested browsing context in the iframe
in A when page B was
loaded.
Generally speaking, dynamically removing or changing the sandbox
attribute is ill-advised, because it can make it quite
hard to reason about what will be allowed and what will not.
The iframe
element supports dimension attributes for cases where the
embedded content has specific dimensions (e.g. ad units have well-defined dimensions).
An iframe
element never has fallback content, as it will always
create a nested browsing context, regardless of whether the specified initial
contents are successfully used.
Descendants of iframe
elements represent nothing. (In legacy user agents that do
not support iframe
elements, the contents would be parsed as markup that could act as
fallback content.)
When used in HTML documents, the allowed content model
of iframe
elements is text, except that invoking the HTML fragment parsing
algorithm with the iframe
element as the context element and the text contents as the input must result in a list of nodes that are all phrasing content,
with no parse errors having occurred, with no script
elements being anywhere in the list or as descendants of elements in the list, and with all the
elements in the list (including their descendants) being themselves conforming.
The iframe
element must be empty in XML documents.
The HTML parser treats markup inside iframe
elements as
text.
The IDL attributes src
, srcdoc
, name
, and sandbox
must reflect the respective
content attributes of the same name.
The contentDocument
IDL attribute
must return the Document
object of the active document of the
iframe
element's nested browsing context, if any and if its
effective script origin is the same origin as the effective script
origin specified by the incumbent settings object, or null otherwise.
The contentWindow
IDL attribute must
return the WindowProxy
object of the iframe
element's nested
browsing context, if any, or null otherwise.
Here is an example of a page using an iframe
to include advertising from an
advertising broker:
<iframe src="http://ads.example.com/?customerid=923513721&format=banner" width="468" height="60"></iframe>
embed
elementsrc
type
width
height
interface HTMLEmbedElement : HTMLElement { attribute DOMString src; attribute DOMString type; attribute DOMString width; attribute DOMString height; legacycaller any (any... arguments); };
Depending on the type of content instantiated by the
embed
element, the node may also support other
interfaces.
The embed
element provides an integration point for an external (typically
non-HTML) application or interactive content.
The src
attribute gives the address of the
resource being embedded. The attribute, if present, must contain a valid non-empty URL
potentially surrounded by spaces.
The type
attribute, if present, gives the
MIME type by which the plugin to instantiate is selected. The value must be a
valid MIME type. If both the type
attribute and
the src
attribute are present, then the type
attribute must specify the same type as the explicit Content-Type metadata of the resource given by the src
attribute.
When the element is created with neither a src
attribute
nor a type
attribute, and when attributes are removed such
that neither attribute is present on the element anymore, and when the element has a media
element ancestor, and when the element has an ancestor object
element that is
not showing its fallback content, any plugin instantiated for
the element must be removed, and the embed
element then represents nothing.
An embed
element is said to be potentially
active when the following conditions are all met simultaneously:
Document
or was in a Document
the last time the event loop
reached step 1.Document
is fully active.src
attribute set or a type
attribute set (or both).src
attribute is either absent or its value
is not the empty string.object
element that is not showing its
fallback content.Whenever an embed
element that was not potentially active becomes potentially active, and whenever a potentially active embed
element that is
remaining potentially active and has its src
attribute set, changed, or removed or its type
attribute set, changed, or removed, the user agent must
queue a task using the embed task source to run the
embed
element setup steps.
The embed
element setup steps are as follows:
If another task has since been queued to run the
embed
element setup steps for this element, then abort these steps.
src
attribute setThe 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:
If another task has since been queued to run
the embed
element setup steps for this element, then abort these
steps.
Determine the type of the content being embedded, as follows (stopping at the first substep that determines the type):
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.
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
".
Otherwise, if the specified resource has explicit Content-Type metadata, then that is the content's type.
Otherwise, the content has no type and there can be no appropriate plugin for it.
If the previous step determined that the content's
type is image/svg+xml
, then run the following substeps:
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.
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.)
The embed
element now represents its associated
nested browsing context.
Otherwise, find and instantiate an appropriate plugin based on the content's type, and hand that plugin the
content of the resource, replacing any previously instantiated plugin for the element. The
embed
element now represents this plugin instance.
Whether the resource is fetched successfully or not (e.g. whether the response code was a 2xx code or equivalent) must be ignored when determining the content's type and when handing the resource to the plugin.
This allows servers to return data for plugins even with error responses (e.g. HTTP 500 Internal Server Error codes can still contain plugin data).
Fetching the resource must delay the load event of the element's document.
src
attribute setThe user agent should find and instantiate an appropriate plugin based on the
value of the type
attribute. The embed
element now represents this plugin instance.
The embed
element has no fallback content. If the user agent can't
find a suitable plugin when attempting to find and instantiate one for the algorithm above, then
the user agent must use a default plugin. This default could be as simple as saying "Unsupported
Format".
Whenever an embed
element that was potentially
active stops being potentially active, any
plugin that had been instantiated for that element must be unloaded.
When a plugin is to be instantiated but it cannot be secured and the sandboxed plugins browsing context
flag is set on the embed
element's Document
's active
sandboxing flag set, then the user agent must not instantiate the plugin, and
must instead render the embed
element in a manner that conveys that the
plugin was disabled. The user agent may offer the user the option to override the
sandbox and instantiate the plugin anyway; if the user invokes such an option, the
user agent must act as if the conditions above did not apply for the purposes of this element.
Plugins that cannot be secured are disabled in sandboxed browsing contexts because they might not honor the restrictions imposed by the sandbox (e.g. they might allow scripting even when scripting in the sandbox is disabled). User agents should convey the danger of overriding the sandbox to the user if an option to do so is provided.
Any namespace-less attribute other than name
, align
, hspace
, and vspace
may be
specified on the embed
element, so long as its name is XML-compatible
and contains no uppercase ASCII letters. These attributes are then passed as
parameters to the plugin.
All attributes in HTML documents get lowercased automatically, so the restriction on uppercase letters doesn't affect such documents.
The four exceptions are to exclude legacy attributes that have side-effects beyond just sending parameters to the plugin.
The user agent should pass the names and values of all the attributes of the embed
element that have no namespace to the plugin used, when one is instantiated.
The HTMLEmbedElement
object representing the element must expose the scriptable
interface of the plugin instantiated for the embed
element, if any. At a
minimum, this interface must implement the legacy caller
operation. (It is suggested that the default behavior of this legacy caller operation, e.g.
the behavior of the default plugin's legacy caller operation, be to throw a
NotSupportedError
exception.)
The embed
element supports dimension attributes.
The IDL attributes src
and type
each must reflect the respective
content attributes of the same name.
Here's a way to embed a resource that requires a proprietary plugin, like Flash:
<embed src="catgame.swf">
If the user does not have the plugin (for example if the plugin vendor doesn't support the user's platform), then the user will be unable to use the resource.
To pass the plugin a parameter "quality" with the value "high", an attribute can be specified:
<embed src="catgame.swf" quality="high">
This would be equivalent to the following, when using an object
element
instead:
<object data="catgame.swf"> <param name="quality" value="high"> </object>
object
elementusemap
attribute: Interactive content.param
elements, then, transparent.data
type
typemustmatch
name
usemap
form
width
height
interface HTMLObjectElement : HTMLElement { attribute DOMString data; attribute DOMString type; attribute boolean typeMustMatch; attribute DOMString name; attribute DOMString useMap; readonly attribute HTMLFormElement? form; attribute DOMString width; attribute DOMString height; readonly attribute Document? contentDocument; readonly attribute WindowProxy? contentWindow; readonly attribute boolean willValidate; readonly attribute ValidityState validity; readonly attribute DOMString validationMessage; boolean checkValidity(); void setCustomValidity(DOMString error); legacycaller any (any... arguments); };
Depending on the type of content instantiated by the
object
element, the node also supports other
interfaces.
The object
element can represent an external resource, which, depending on the
type of the resource, will either be treated as an image, as a nested browsing
context, or as an external resource to be processed by a plugin.
The data
attribute, if present, specifies the
address of the resource. If present, the attribute must be a valid non-empty URL potentially
surrounded by spaces.
Authors who reference resources from other origins
that they do not trust are urged to use the typemustmatch
attribute defined below. Without that
attribute, it is possible in certain cases for an attacker on the remote host to use the plugin
mechanism to run arbitrary scripts, even if the author has used features such as the Flash
"allowScriptAccess" parameter.
The type
attribute, if present, specifies the
type of the resource. If present, the attribute must be a valid MIME type.
At least one of either the data
attribute or the type
attribute must be present.
The typemustmatch
attribute is a
boolean attribute whose presence indicates that the resource specified by the data
attribute is only to be used if the value of the type
attribute and the Content-Type of the
aforementioned resource match.
The typemustmatch
attribute must not be
specified unless both the data
attribute and the type
attribute are present.
The name
attribute, if present, must be a
valid browsing context name. The given value is used to name the nested
browsing context, if applicable.
Whenever one of the following conditions occur:
Document
changes whether it is fully active,
object
elements changes to or from showing its
fallback content,
classid
attribute is set, changed, or removed,
classid
attribute is not present, and
its data
attribute is set, changed, or removed,
classid
attribute nor its
data
attribute are present, and its type
attribute is set, changed, or removed,
...the user agent must queue a task to run the following steps to (re)determine
what the object
element represents. The task source for this task is the DOM manipulation task source. This task being queued or actively
running must delay the load event of the element's document.
If the user has indicated a preference that this object
element's fallback
content be shown instead of the element's usual behavior, then jump to the step below labeled fallback.
For example, a user could ask for the element's fallback content to be shown because that content uses a format that the user finds more accessible.
If the element has an ancestor media element, or has an ancestor
object
element that is not showing its fallback content, or
if the element is not in a Document
with a
browsing context, or if the element's Document
is not fully
active, or if the element is still in the stack of open elements of an
HTML parser or XML parser, or if the element is not being
rendered, then jump to the step below labeled fallback.
If the classid
attribute is present, and has a value
that isn't the empty string, then: if the user agent can find a plugin suitable
according to the value of the classid
attribute, and
either plugins aren't being sandboxed or that
plugin can be secured, then that
plugin should be used, and the value of the data
attribute, if any, should be passed to the
plugin. If no suitable plugin can be found, or if the
plugin reports an error, jump to the step below labeled fallback.
If the data
attribute is present and its value is
not the empty string, then:
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.
Resolve the URL specified by the data
attribute, relative to the element.
If that failed, fire a simple event named error
at the element, then jump to the step below labeled
fallback.
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).
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.
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.
Determine the resource type, as follows:
Let the resource type be unknown.
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.
If the object
element has a typemustmatch
attribute, jump to the step below
labeled handler.
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.
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.
Run the appropriate set of steps from the following list:
Let binary be false.
If the type specified in the resource's Content-Type
metadata is "text/plain
", and the result of applying the rules for distinguishing if a resource is
text or binary to the resource is that the resource is not
text/plain
, then set binary to true.
If the type specified in the resource's Content-Type
metadata is "application/octet-stream
", then set binary to true.
If binary is false, then let the resource type be the type specified in the resource's Content-Type metadata, and jump to the step below labeled handler.
If there is a type
attribute present on the
object
element, and its value is not application/octet-stream
,
then run the following steps:
If the attribute's value is a type that a plugin supports, or the
attribute's value is a type that starts with "image/
" that is not also an
XML MIME type, then let the resource type be the type
specified in that type
attribute.
Jump to the step below labeled handler.
If there is a type
attribute present on the
object
element, then let the tentative type be the type
specified in that type
attribute.
Otherwise, let tentative type be the sniffed type of the resource.
If tentative type is not
application/octet-stream
, then let resource type be
tentative type and jump to the step below labeled
handler.
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.
Handler: Handle the content as given by the first of the following cases that matches:
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.
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.
image/
", and support
for images has not been disabledApply 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.
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.
The element's contents are not part of what the object
element
represents.
Once the resource is completely loaded, queue a task to fire a simple
event named load
at the element.
The task source for this task is the DOM manipulation task source.
If the data
attribute is absent but the type
attribute is present, and the user agent can find a
plugin suitable according to the value of the type
attribute, and either plugins
aren't being sandboxed or the plugin can be secured, then that plugin should be used. If these conditions cannot be met, or if the
plugin reports an error, jump to the step below labeled fallback.
Fallback: The object
element represents the element's
children, ignoring any leading param
element children. This is the element's
fallback content. If the element has an instantiated plugin, then
unload it.
When the algorithm above instantiates a plugin, the user agent
should pass to the plugin used the names and values of all the attributes on the
element, in the order they were added to the element, with the attributes added by the parser
being ordered in source order, followed by a parameter named "PARAM" whose value is null, followed
by all the names and values of parameters given by
param
elements that are children of the object
element, in tree
order. If the plugin supports a scriptable interface, the
HTMLObjectElement
object representing the element should expose that interface. The
object
element represents the plugin. The
plugin is not a nested browsing context.
Plugins are considered sandboxed for the purpose of an
object
element if the sandboxed plugins browsing context flag is set on
the object
element's Document
's active sandboxing flag
set.
Due to the algorithm above, the contents of object
elements act as fallback
content, used only when referenced resources can't be shown (e.g. because it returned a 404
error). This allows multiple object
elements to be nested inside each other,
targeting multiple user agents with different capabilities, with the user agent picking the first
one it supports.
Whenever the name
attribute is set, if the
object
element has a nested browsing context, its name must be changed to the new value. If the attribute is removed, if the
object
element has a browsing context, the browsing context
name must be set to the empty string.
The usemap
attribute, if present while the
object
element represents an image, can indicate that the object has an associated
image map. The attribute must be ignored if the
object
element doesn't represent an image.
The form
attribute is used to explicitly associate the
object
element with its form owner.
Constraint validation: object
elements are always barred
from constraint validation.
The object
element supports dimension attributes.
The IDL attributes data
, type
and name
each must reflect the respective
content attributes of the same name. The typeMustMatch
IDL attribute must
reflect the typemustmatch
content
attribute. The useMap
IDL attribute must
reflect the usemap
content attribute.
The contentDocument
IDL attribute
must return the Document
object of the active document of the
object
element's nested browsing context, if any and if its
effective script origin is the same origin as the effective script
origin specified by the incumbent settings object, or null otherwise.
The contentWindow
IDL attribute must
return the WindowProxy
object of the object
element's nested
browsing context, if it has one; otherwise, it must return null.
The willValidate
, validity
, and validationMessage
attributes, and the checkValidity()
, and setCustomValidity()
methods, are part of the
constraint validation API. The form
IDL attribute
is part of the element's forms API.
All object
elements have a legacy caller
operation. If the object
element has an instantiated plugin that
supports a scriptable interface that defines a legacy caller operation, then that must be the
behavior of the object's legacy caller operation. Otherwise, the object's legacy caller operation
must be to throw a NotSupportedError
exception.
In the following example, a Java applet is embedded in a page using the object
element. (Generally speaking, it is better to avoid using applets like these and instead use
native JavaScript and HTML to provide the functionality, since that way the application will work
on all Web browsers without requiring a third-party plugin. Many devices, especially embedded
devices, do not support third-party technologies like Java.)
<figure> <object type="application/x-java-applet"> <param name="code" value="MyJavaClass"> <p>You do not have Java available, or it is disabled.</p> </object> <figcaption>My Java Clock</figcaption> </figure>
In this example, an HTML page is embedded in another using the object
element.
<figure> <object data="clock.html"></object> <figcaption>My HTML Clock</figcaption> </figure>
The following example shows how a plugin can be used in HTML (in this case the Flash plugin,
to show a video file). Fallback is provided for users who do not have Flash enabled, in this case
using the video
element to show the video for those using user agents that support
video
, and finally providing a link to the video for those who have neither Flash
nor a video
-capable browser.
<p>Look at my video: <object type="application/x-shockwave-flash"> <param name=movie value="http://video.example.com/library/watch.swf"> <param name=allowfullscreen value=true> <param name=flashvars value="http://video.example.com/vids/315981"> <video controls src="http://video.example.com/vids/315981"> <a href="http://video.example.com/vids/315981">View video</a>. </video> </object> </p>
param
elementobject
element, before any flow content.name
value
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>
video
elementcontrols
attribute: Interactive content.src
attribute:
zero or more track
elements, then
transparent, but with no media element descendants.src
attribute: zero or more source
elements, then
zero or more track
elements, then
transparent, but with no media element descendants.src
crossorigin
poster
preload
autoplay
mediagroup
loop
muted
controls
width
height
interface HTMLVideoElement : HTMLMediaElement { attribute unsigned long width; attribute unsigned long height; readonly attribute unsigned long videoWidth; readonly attribute unsigned long videoHeight; attribute DOMString poster; };
A video
element is used for playing videos or movies, and audio files with
captions.
Content may be provided inside the video
element. User agents
should not show this content to the user; it is intended for older Web browsers which do
not support video
, so that legacy video plugins can be tried, or to show text to the
users of these older browsers informing them of how to access the video contents.
In particular, this content is not intended to address accessibility concerns. To
make video content accessible to the partially sighted, the blind, the hard-of-hearing, the deaf,
and those with other physical or cognitive disabilities, a variety of features are available.
Captions can be provided, either embedded in the video stream or as external files using the
track
element. Sign-language tracks can be provided, again either embedded in the
video stream or by synchronizing multiple video
elements using the mediagroup
attribute or a MediaController
object. Audio descriptions can be provided, either as a separate track embedded in the video
stream, or a separate audio track in an audio
element slaved to the same controller as the video
element(s), or in text
form using a WebVTT file referenced using the track
element and
synthesized into speech by the user agent. WebVTT can also be used to provide chapter titles. For
users who would rather not use a media element at all, transcripts or other textual alternatives
can be provided by simply linking to them in the prose near the video
element. [WEBVTT]
The video
element is a media element whose media data is
ostensibly video data, possibly with associated audio data.
The src
, preload
,
autoplay
, mediagroup
, loop
, muted
, and controls
attributes are the attributes common to all media
elements.
The poster
attribute gives the address of an
image file that the user agent can show while no video data is available. The attribute, if
present, must contain a valid non-empty URL potentially surrounded by spaces.
If the specified resource is to be used, then, when the element is created or when the poster
attribute is set, changed, or removed, the user agent must
run the following steps to determine the element's poster frame (regardless of the
value of the element's show poster flag):
If there is an existing instance of this algorithm running for this video
element, abort that instance of this algorithm without changing the poster
frame.
If the poster
attribute's value is the empty string
or if the attribute is absent, then there is no poster frame; abort these
steps.
Resolve the poster
attribute's value relative to the element. If this fails,
then there is no poster frame; abort these steps.
Fetch the resulting absolute URL, from the element's
Document
's origin. This must delay the load event of the
element's document.
If an image is thus obtained, the poster frame is that image. Otherwise, there is no poster frame.
The image given by the poster
attribute,
the poster frame, is intended to be a representative frame of the video (typically one of
the first non-blank frames) that gives the user an idea of what the video is like.
A video
element represents what is given for the first matching condition in the
list below:
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)video
element represents its poster frame, if any,
or else transparent black with no intrinsic dimensions.video
element is paused, the current playback position is the first frame of video,
and the element's show poster flag is setvideo
element represents its poster frame, if any,
or else the first frame of the video.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)video
element is neither potentially playing nor paused (e.g. when seeking or stalled)video
element represents the last frame of the video to have
been rendered.video
element is pausedvideo
element represents the frame of video corresponding to
the current playback position.video
element has a video channel and is potentially
playing)video
element represents the frame of video at the continuously
increasing "current" position. When the
current playback position changes such that the last frame rendered is no longer the
frame corresponding to the current playback position in the video, the new frame
must be rendered.Which frame in a video stream corresponds to a particular playback position is defined by the video stream's format.
The video
element also represents any text track cues whose text track cue active flag is set and whose
text track is in the showing mode, and any
audio from the media resource, at the current playback position.
Any audio associated with the media resource must, if played, be played synchronized with the current playback position, at the element's effective media volume.
In addition to the above, the user agent may provide messages to the user (such as "buffering", "no video loaded", "error", or more detailed information) by overlaying text or icons on the video or other areas of the element's playback area, or in another appropriate manner.
User agents that cannot render the video may instead make the element represent a link to an external video playback utility or to the video data itself.
When a video
element's media resource has a video channel, the
element provides a paint source whose width is the media resource's
intrinsic width, whose height is the
media resource's intrinsic
height, and whose appearance is the frame of video corresponding to the current playback position, if that is available, or else
(e.g. when the video is seeking or buffering) its previous appearance, if any, or else (e.g.
because the video is still loading the first frame) blackness.
videoWidth
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>
audio
elementcontrols
attribute: Interactive content.controls
attribute: Palpable content.src
attribute:
zero or more track
elements, then
transparent, but with no media element descendants.src
attribute: zero or more source
elements, then
zero or more track
elements, then
transparent, but with no media element descendants.src
crossorigin
preload
autoplay
mediagroup
loop
muted
controls
[NamedConstructor=Audio(optional DOMString src)] interface HTMLAudioElement : HTMLMediaElement {};
An audio
element represents a sound or audio stream.
Content may be provided inside the audio
element. User agents
should not show this content to the user; it is intended for older Web browsers which do
not support audio
, so that legacy audio plugins can be tried, or to show text to the
users of these older browsers informing them of how to access the audio contents.
In particular, this content is not intended to address accessibility concerns. To
make audio content accessible to the deaf or to those with other physical or cognitive
disabilities, a variety of features are available. If captions or a sign language video are
available, the video
element can be used instead of the audio
element to
play the audio, allowing users to enable the visual alternatives. Chapter titles can be provided
to aid navigation, using the track
element and a WebVTT file. And,
naturally, transcripts or other textual alternatives can be provided by simply linking to them in
the prose near the audio
element. [WEBVTT]
The audio
element is a media element whose media data is
ostensibly audio data.
The src
, preload
,
autoplay
, mediagroup
, loop
, muted
, and controls
attributes are the attributes common to all media
elements.
When an audio
element is potentially playing, it must have its audio
data played synchronized with the current playback position, at the element's
effective media volume.
When an audio
element is not potentially playing, audio must not play
for the element.
Audio
( [ url ] )Returns a new audio
element, with the src
attribute set to the value passed in the argument, if applicable.
A constructor is provided for creating HTMLAudioElement
objects (in addition to
the factory methods from DOM such as createElement()
): Audio(src)
. When invoked as a
constructor, it must return a new HTMLAudioElement
object (a new audio
element). The element must have its preload
attribute set
to the literal value "auto
". If the src argument is present, the object created must have its src
content attribute set to the provided value, and the user agent
must invoke the object's resource selection
algorithm before returning. The element's document must be the active document
of the browsing context of the Window
object on which the interface
object of the invoked constructor is found.
source
elementtrack
elements.src
type
media
interface HTMLSourceElement : HTMLElement { attribute DOMString src; attribute DOMString type; attribute DOMString media; };
The source
element allows authors to specify multiple alternative media resources for media
elements. It does not represent anything on its own.
The src
attribute gives the address of the
media resource. The value must be a valid non-empty URL potentially surrounded
by spaces. This attribute must be present.
Dynamically modifying a source
element and its attribute when the
element is already inserted in a video
or audio
element will have no
effect. To change what is playing, just use the src
attribute
on the media element directly, possibly making use of the canPlayType()
method to pick from amongst available
resources. Generally, manipulating source
elements manually after the document has
been parsed is an unnecessarily complicated approach.
The type
attribute gives the type of the
media resource, to help the user agent determine if it can play this media
resource before fetching it. If specified, its value must be a valid MIME
type. The codecs
parameter, which certain MIME types define, might be
necessary to specify exactly how the resource is encoded. [RFC4281]
The following list shows some examples of how to use the codecs=
MIME
parameter in the type
attribute.
<source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'>
<source src='video.mp4' type='video/mp4; codecs="avc1.58A01E, mp4a.40.2"'>
<source src='video.mp4' type='video/mp4; codecs="avc1.4D401E, mp4a.40.2"'>
<source src='video.mp4' type='video/mp4; codecs="avc1.64001E, mp4a.40.2"'>
<source src='video.mp4' type='video/mp4; codecs="mp4v.20.8, mp4a.40.2"'>
<source src='video.mp4' type='video/mp4; codecs="mp4v.20.240, mp4a.40.2"'>
<source src='video.3gp' type='video/3gpp; codecs="mp4v.20.8, samr"'>
<source src='video.ogv' type='video/ogg; codecs="theora, vorbis"'>
<source src='video.ogv' type='video/ogg; codecs="theora, speex"'>
<source src='audio.ogg' type='audio/ogg; codecs=vorbis'>
<source src='audio.spx' type='audio/ogg; codecs=speex'>
<source src='audio.oga' type='audio/ogg; codecs=flac'>
<source src='video.ogv' type='video/ogg; codecs="dirac, vorbis"'>
The media
attribute gives the intended media
type of the media resource, to help the user agent determine if this media
resource is useful to the user before fetching it. Its value must be a valid media
query.
The resource
selection algorithm is defined in such a way that when the media
attribute is omitted the user agent acts the same as if the
value was "all
", i.e. by default the media resource is suitable
for all media.
If a source
element is inserted as a child of a media element that
has no src
attribute and whose networkState
has the value NETWORK_EMPTY
, the user agent must invoke the media
element's resource selection
algorithm.
The IDL attributes src
, type
, and media
must reflect the respective content
attributes of the same name.
If the author isn't sure if user agents will all be able to render the media resources
provided, the author can listen to the error
event on the last
source
element and trigger fallback behavior:
<script> function fallback(video) { // replace <video> with its contents while (video.hasChildNodes()) { if (video.firstChild instanceof HTMLSourceElement) video.removeChild(video.firstChild); else video.parentNode.insertBefore(video.firstChild, video); } video.parentNode.removeChild(video); } </script> <video controls autoplay> <source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'> <source src='video.ogv' type='video/ogg; codecs="theora, vorbis"' onerror="fallback(parentNode)"> ... </video>
track
elementkind
src
srclang
label
default
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.
readyState
Returns the text track readiness state, represented by a number from the following list:
NONE
(0)The text track not loaded state.
LOADING
(1)The text track loading state.
LOADED
(2)The text track loaded state.
ERROR
(3)The text track failed to load state.
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)LOADING
(numeric value 1)LOADED
(numeric value 2)ERROR
(numeric value 3)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.)
Media elements (audio
and video
, in
this specification) implement the following interface:
enum CanPlayTypeEnum { "" /* empty string */, "maybe", "probably" };
interface HTMLMediaElement : HTMLElement {
// error state
readonly attribute MediaError? error;
// network state
attribute DOMString src;
readonly attribute DOMString currentSrc;
attribute DOMString crossOrigin;
const unsigned short NETWORK_EMPTY = 0;
const unsigned short NETWORK_IDLE = 1;
const unsigned short NETWORK_LOADING = 2;
const unsigned short NETWORK_NO_SOURCE = 3;
readonly attribute unsigned short networkState;
attribute DOMString preload;
readonly attribute TimeRanges buffered;
void load();
CanPlayTypeEnum canPlayType(DOMString type);
// ready state
const unsigned short HAVE_NOTHING = 0;
const unsigned short HAVE_METADATA = 1;
const unsigned short HAVE_CURRENT_DATA = 2;
const unsigned short HAVE_FUTURE_DATA = 3;
const unsigned short HAVE_ENOUGH_DATA = 4;
readonly attribute unsigned short readyState;
readonly attribute boolean seeking;
// playback state
attribute double currentTime;
readonly attribute unrestricted double duration;
Date getStartDate();
readonly attribute boolean paused;
attribute double defaultPlaybackRate;
attribute double playbackRate;
readonly attribute TimeRanges played;
readonly attribute TimeRanges seekable;
readonly attribute boolean ended;
attribute boolean autoplay;
attribute boolean loop;
void play();
void pause();
// media controller
attribute DOMString mediaGroup;
attribute MediaController? controller;
// controls
attribute boolean controls;
attribute double volume;
attribute boolean muted;
attribute boolean defaultMuted;
// tracks
readonly attribute AudioTrackList audioTracks;
readonly attribute VideoTrackList videoTracks;
readonly attribute TextTrackList textTracks;
TextTrack addTextTrack(TextTrackKind kind, optional DOMString label = "", optional DOMString language = "");
};
The media element attributes, src
, crossorigin
, preload
, autoplay
,
mediagroup
, loop
,
muted
, and controls
, apply to all media
elements. They are defined in this section.
Media elements are used to present audio data, or video and audio data, to the user. This is referred to as media data in this section, since this section applies equally to media elements for audio or for video. The term media resource is used to refer to the complete set of media data, e.g. the complete video file, or complete audio file.
A media resource can have multiple audio and video tracks. For the purposes of a
media element, the video data of the media resource is only that of the
currently selected track (if any) given by the element's videoTracks
attribute, and the audio data of the media
resource is the result of mixing all the currently enabled tracks (if any) given by the
element's audioTracks
attribute.
Both audio
and video
elements can be used for both audio
and video. The main difference between the two is simply that the audio
element has
no playback area for visual content (such as video or captions), whereas the video
element does.
Except where otherwise explicitly specified, the task source for all the tasks queued in this section and its subsections is the media element event task source of the media element in question.
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; };
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)MEDIA_ERR_NETWORK
(numeric value 2)MEDIA_ERR_DECODE
(numeric value 3)MEDIA_ERR_SRC_NOT_SUPPORTED
(numeric value 4)src
attribute was not suitable.The src
content attribute on media elements gives the address of the media resource (video, audio) to show. The
attribute, if present, must contain a valid non-empty URL potentially surrounded by
spaces.
The crossorigin
content attribute on
media elements is a CORS settings attribute.
If a src
attribute of a media element is set
or changed, the user agent must invoke the media element's media element load
algorithm. (Removing the src
attribute does
not do this, even if there are source
elements present.)
The src
IDL attribute on media elements must reflect the content attribute of the same
name.
The crossOrigin
IDL attribute must
reflect the crossorigin
content
attribute, limited to only known values.
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.
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 recognize the container type, or it doesn't support the listed codecs.
The MIME type "application/octet-stream
" with no parameters is never
a type that the user agent knows it cannot render. User agents must treat that type
as equivalent to the lack of any explicit Content-Type metadata
when it is used to label a potential media resource.
Only the MIME type "application/octet-stream
" with no
parameters is special-cased here; if any parameter appears with it, it will be treated just like
any other MIME type. This is a deviation from the rule that unknown MIME type parameters should be ignored.
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.
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)NETWORK_IDLE
(numeric value 1)NETWORK_LOADING
(numeric value 2)NETWORK_NO_SOURCE
(numeric value 3)The resource selection algorithm defined
below describes exactly when the networkState
attribute changes value and what events fire to indicate changes in this state.
load
()Causes the element to reset and start selecting and loading a new media resource from scratch.
All media elements have an autoplaying flag, which must begin in the true state, and a delaying-the-load-event flag, which must begin in the false state. While the delaying-the-load-event flag is true, the element must delay the load event of its document.
When the load()
method on a media
element is invoked, the user agent must run the media element load
algorithm.
The media element load algorithm consists of the following steps.
Abort any already-running instance of the resource selection algorithm for this element.
If there are any tasks from the media element's media element event task source in one of the task queues, then remove those tasks.
If there are any tasks that were queued by the resource selection algorithm (including the algorithms that it itself invokes) for this same media element from the DOM manipulation task source in one of the task queues, then remove those tasks.
Basically, pending events and callbacks for the media element are discarded when the media element starts loading a new resource.
If the media element's networkState
is set to NETWORK_LOADING
or NETWORK_IDLE
, queue a task to fire a
simple event named abort
at the media
element.
If the media element's networkState
is not set to NETWORK_EMPTY
, then run these
substeps:
Queue a task to fire a simple event named emptied
at the media element.
If a fetching process is in progress for the media element, the user agent should stop it.
If readyState
is not set to HAVE_NOTHING
, then set it to that state.
If the paused
attribute is false, then set it to
true.
If seeking
is true, set it to false.
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.
Set the initial playback position to 0.
Set the timeline offset to Not-a-Number (NaN).
Update the duration
attribute to Not-a-Number
(NaN).
The user agent will not fire a durationchange
event for this particular change of
the duration.
Set the playbackRate
attribute to the value of
the defaultPlaybackRate
attribute.
Set the error
attribute to null and the
autoplaying flag to true.
Invoke the media element's resource selection algorithm.
Playback of any previously playing media resource for this element stops.
The resource selection algorithm for a media element is as follows. This algorithm is always invoked synchronously, but one of the first steps in the algorithm is to return and continue running the remaining steps asynchronously, meaning that it runs in the background with scripts and other tasks running in parallel. In addition, this algorithm interacts closely with the event loop mechanism; in particular, it has synchronous sections (which are triggered as part of the event loop algorithm). Steps in such sections are marked with ⌛.
Set the element's networkState
attribute to
the NETWORK_NO_SOURCE
value.
Set the element's show poster flag to true.
Set the media element's delaying-the-load-event flag to true (this delays the load event).
Asynchronously await a stable state, allowing the task that invoked this algorithm to continue. The synchronous section consists of all the remaining steps of this algorithm until the algorithm says the synchronous section has ended. (Steps in synchronous sections are marked with ⌛.)
⌛ If the media element's blocked-on-parser flag is false, then populate the list of pending text tracks.
⌛ If the media element has a src
attribute, then let mode be attribute.
⌛ Otherwise, if the media element does not have a src
attribute but has a source
element child, then
let mode be children and let candidate
be the first such source
element child in tree order.
⌛ Otherwise the media element has neither a src
attribute nor a source
element child: set the
networkState
to NETWORK_EMPTY
, and abort these steps; the
synchronous section ends.
⌛ Set the media element's networkState
to NETWORK_LOADING
.
⌛ Queue a task to fire a simple event named loadstart
at the media element.
If mode is attribute, then run these substeps:
⌛ 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.
⌛ 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.
⌛ If absolute URL was obtained successfully, set the currentSrc
attribute to absolute
URL.
End the synchronous section, continuing the remaining steps asynchronously.
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.
Failed with attribute: Reaching this step indicates that the media resource failed to load or that the given URL could not be resolved. Queue a task to run the following steps, using the DOM manipulation task source:
Set the error
attribute to a new
MediaError
object whose code
attribute
is set to MEDIA_ERR_SRC_NOT_SUPPORTED
.
Set the element's networkState
attribute
to the NETWORK_NO_SOURCE
value.
Set the element's show poster flag to true.
Fire a simple event named error
at
the media element.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Wait for the task queued by the previous step to have executed.
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:
⌛ 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:
Other changes don't affect pointer.
⌛ 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.
⌛ 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.
⌛ If absolute URL was not obtained successfully, then end the synchronous section, and jump down to the failed with elements step below.
⌛ 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.
⌛ If candidate has a media
attribute whose value does not match the environment, then end the synchronous section, and
jump down to the failed with elements step below.
⌛ Set the currentSrc
attribute to absolute URL.
End the synchronous section, continuing the remaining steps asynchronously.
Run the resource fetch algorithm with absolute URL. If that algorithm returns without aborting this one, then the load failed.
Failed with elements: Queue a task, using the DOM
manipulation task source, to fire a simple event named error
at the candidate element.
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 ⌛.)
⌛ Forget the media element's media-resource-specific tracks.
⌛ Find next candidate: Let candidate be null.
⌛ Search loop: If the node after pointer is the end of the list, then jump to the waiting step below.
⌛ If the node after pointer is a source
element,
let candidate be that element.
⌛ 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.
⌛ If candidate is null, jump back to the search loop step. Otherwise, jump back to the process candidate step.
⌛ Waiting: Set the element's networkState
attribute to the NETWORK_NO_SOURCE
value.
⌛ Set the element's show poster flag to true.
⌛ Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
End the synchronous section, continuing the remaining steps asynchronously.
Wait until the node after pointer is a node other than the end of the list. (This step might wait forever.)
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 ⌛.)
⌛ 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).
⌛ Set the networkState
back to NETWORK_LOADING
.
⌛ Jump back to the find next candidate step above.
The resource fetch algorithm for a media element and a given absolute URL is as follows:
Let the current media resource be the resource given by the absolute URL passed to this algorithm. This is now the element's media resource.
Remove all media-resource-specific text tracks from the media element's list of pending text tracks, if any.
Optionally, run the following substeps. This is the expected behavior if the user agent
intends to not attempt to fetch the resource until the user requests it explicitly (e.g. as a way
to implement the preload
attribute's none
keyword).
Set the networkState
to NETWORK_IDLE
.
Queue a task to fire a simple event named suspend
at the element, using the DOM manipulation
task source.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Wait for the task to be run.
Wait for an implementation-defined event (e.g. the user requesting that the media element begin playback).
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).
Set the networkState
to NETWORK_LOADING
.
Perform a potentially CORS-enabled fetch of the current media resource's absolute URL, with the mode being
the state of the media element's crossorigin
content attribute, the origin
being the origin of the media element's Document
, and the
default origin behaviour set to taint.
The resource obtained in this fashion, if any, contains the media data. It can
be CORS-same-origin or CORS-cross-origin; this affects whether
subtitles referenced in the media data are exposed in the API and, for
video
elements, whether a canvas
gets tainted when the video is drawn
on it.
While the load is not suspended (see below), every 350ms (±200ms) or for every byte
received, whichever is least frequent, queue a task to fire a simple
event named progress
at the element.
The stall timeout is a user-agent defined length of time, which should be about
three seconds. When a media element that is actively attempting to obtain
media data has failed to receive any data for a duration equal to the stall
timeout, the user agent must queue a task to fire a simple
event named stalled
at the element.
User agents may allow users to selectively block or slow media data downloads. When a media element's download has been blocked altogether, the user agent must act as if it was stalled (as opposed to acting as if the connection was closed). The rate of the download may also be throttled automatically by the user agent, e.g. to balance the download with other connections sharing the same bandwidth.
User agents may decide to not download more content at any time, e.g.
after buffering five minutes of a one hour media resource, while waiting for the user to decide
whether to play the resource or not, while waiting for user input in an interactive resource, or
when the user navigates away from the page. When a media element's download has
been suspended, the user agent must queue a task, using the DOM manipulation
task source, to set the networkState
to NETWORK_IDLE
and fire a simple event named
suspend
at the element. If and when downloading of the
resource resumes, the user agent must queue a task to set the networkState
to NETWORK_LOADING
. Between the queuing of these tasks,
the load is suspended (so progress
events don't fire,
as described above).
The preload
attribute provides a hint
regarding how much buffering the author thinks is advisable, even in the absence of the autoplay
attribute.
When a user agent decides to completely stall a download, e.g. if it is waiting until the user starts playback before downloading any further content, the element's delaying-the-load-event flag must be set to false. This stops delaying the load event.
The user agent may use whatever means necessary to fetch the resource (within the constraints put forward by this and other specifications); for example, reconnecting to the server in the face of network errors, using HTTP range retrieval requests, or switching to a streaming protocol. The user agent must consider a resource erroneous only if it has given up trying to fetch it.
This specification does not currently say whether or how to check the MIME types of the media resources, or whether or how to perform file type sniffing using the actual file data. Implementors differ in their intentions on this matter and it is therefore unclear what the right solution is. In the absence of any requirement here, the HTTP specification's strict requirement to follow the Content-Type header prevails ("Content-Type specifies the media type of the underlying data." ... "If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource.").
The networking task source tasks to process the data as it is being fetched must, when appropriate, include the relevant substeps from the following list:
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:
The user agent should cancel the fetching process.
Abort this subalgorithm, returning to the resource selection algorithm.
Create an AudioTrack
object to represent the audio track.
Update the media element's audioTracks
attribute's AudioTrackList
object with the new AudioTrack
object.
Fire a trusted event with the name addtrack
, that does not bubble and is not cancelable, and
that uses the TrackEvent
interface, with the track
attribute initialized to the new
AudioTrack
object, at this AudioTrackList
object.
Create a VideoTrack
object to represent the video track.
Update the media element's videoTracks
attribute's VideoTrackList
object with the new VideoTrack
object.
Fire a trusted event with the name addtrack
, that does not bubble and is not cancelable, and that
uses the TrackEvent
interface, with the track
attribute initialized to the new
VideoTrack
object, at this VideoTrackList
object.
This indicates that the resource is usable. The user agent must follow these substeps:
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.
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).
Set the current playback position and the official playback position to the earliest possible position.
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.
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.
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.
Let jumped be false.
If the media element's default playback start position is greater than zero, then seek to that time, and let jumped be true.
Let the media element's default playback start position be zero.
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]
If either the media resource or the address of the current
media resource indicate a particular set of audio or video tracks to enable,
or if the user agent has information that would enable it to select specific tracks to improve the user's experience, then the
relevant audio tracks must be enabled in the element's audioTracks
object, and, of the relevant video tracks,
the one that is listed first in the element's videoTracks
object must be selected. All other tracks
must be disabled.
This could again be triggered by Media Fragments URI fragment identifier syntax, but it could also be triggered e.g. by the user agent selecting a 5.1 surround sound audio track over a stereo audio track. [MEDIAFRAG]
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.
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.
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:
The user agent should cancel the fetching process.
Set the error
attribute to a new
MediaError
object whose code
attribute
is set to MEDIA_ERR_NETWORK
.
Fire a simple event named error
at
the media element.
Set the element's networkState
attribute
to the NETWORK_IDLE
value.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Abort the overall resource selection algorithm.
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:
The user agent should cancel the fetching process.
Set the error
attribute to a new
MediaError
object whose code
attribute
is set to MEDIA_ERR_DECODE
.
Fire a simple event named error
at
the media element.
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.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Abort the overall resource selection algorithm.
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.
The user agent should cancel the fetching process.
Set the error
attribute to a new
MediaError
object whose code
attribute
is set to MEDIA_ERR_ABORTED
.
Fire a simple event named abort
at
the media element.
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.
Set the element's delaying-the-load-event flag to false. This stops delaying the load event.
Abort the overall resource selection algorithm.
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 data is CORS-same-origin, run the steps to expose a media-resource-specific text track with the relevant data.
Cross-origin videos do not expose their subtitles, since that would allow attacks such as hostile sites reading subtitles from confidential videos on a user's intranet.
When the networking task source has queued the last task as part of fetching the media resource (i.e. once the download has completed), if the fetching process completes without errors, including decoding the media data, and if all of the data is available to the user agent without network access, then, the user agent must move on to the next step. This might never happen, e.g. when streaming an infinite resource such as Web radio, or if the resource is longer than the user agent's ability to cache data.
While the user agent might still need network access to obtain parts of the media resource, the user agent must remain on this step.
For example, if the user agent has discarded the first half of a video, the
user agent will remain at this step even once the playback has
ended, because there is always the chance the user will seek back to the start. In fact,
in this situation, once playback has ended, the user agent
will end up firing a suspend
event, as described
earlier.
If the user agent ever reaches this step (which can only happen if the entire resource gets loaded and kept available): abort the overall resource selection algorithm.
When a media element is to forget the media element's media-resource-specific
tracks, the user agent must remove from the media element's list of text
tracks all the media-resource-specific
text tracks, then empty the media element's audioTracks
attribute's AudioTrackList
object,
then empty the media element's videoTracks
attribute's VideoTrackList
object. No events (in particular, no removetrack
events) are fired as part of this; the error
and emptied
events, fired by the algorithms that invoke this one, can be used instead.
The preload
attribute is an enumerated
attribute. The following table lists the keywords and states for the attribute — the
keywords in the left column map to the states in the cell in the second column on the same row as
the keyword. The attribute can be changed even once the media resource is being
buffered or played; the descriptions in the table below are to be interpreted with that in
mind.
Keyword | State | Brief description |
---|---|---|
none
| None | Hints to the user agent that either the author does not expect the user to need the media resource, or that the server wants to minimize unnecessary traffic. This state does not provide a hint regarding how aggressively to actually download the media resource if buffering starts anyway (e.g. once the user hits "play"). |
metadata
| Metadata | Hints to the user agent that the author does not expect the user to need the media resource, but that fetching the resource metadata (dimensions, track list, duration, etc), and maybe even the first few frames, is reasonable. If the user agent precisely fetches no more than the metadata, then the media element will end up with its readyState attribute set to HAVE_METADATA ; typically though, some frames will be obtained as well and it will probably be HAVE_CURRENT_DATA or HAVE_FUTURE_DATA .
When the media resource is playing, hints to the user agent that bandwidth is to be considered scarce, e.g. suggesting throttling the download so that the media data is obtained at the slowest possible rate that still maintains consistent playback.
|
auto
| Automatic | Hints to the user agent that the user agent can put the user's needs first without risk to the server, up to and including optimistically downloading the entire resource. |
The empty string is also a valid keyword, and maps to the Automatic state. The attribute's missing value default is user-agent defined, though the Metadata state is suggested as a compromise between reducing server load and providing an optimal user experience.
Authors might switch the attribute from "none
" or "metadata
" to "auto
" dynamically once the user begins playback. For
example, on a page with many videos this might be used to indicate that the many videos are not to
be downloaded unless requested, but that once one is requested it is to be downloaded
aggressively.
The preload
attribute is intended to provide a hint to
the user agent about what the author thinks will lead to the best user experience. The attribute
may be ignored altogether, for example based on explicit user preferences or based on the
available connectivity.
The preload
IDL attribute must
reflect the content attribute of the same name, limited to only known
values.
The autoplay
attribute can override the
preload
attribute (since if the media plays, it naturally
has to buffer first, regardless of the hint given by the preload
attribute). Including both is not an error, however.
buffered
Returns a TimeRanges
object that represents the ranges of the media
resource that the user agent has buffered.
The buffered
attribute must return a new
static normalized TimeRanges
object that represents the ranges of the
media resource, if any, that the user agent has buffered, at the time the attribute
is evaluated. Users agents must accurately determine the ranges available, even for media streams
where this can only be determined by tedious inspection.
Typically this will be a single range anchored at the zero point, but if, e.g. the user agent uses HTTP range requests in response to seeking, then there could be multiple ranges.
User agents may discard previously buffered data.
Thus, a time position included within a range of the objects return by the buffered
attribute at one time can end up being not included in
the range(s) of objects returned by the same attribute at later times.
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.
currentTime
[ = value ]Returns the official playback position, in seconds.
Can be set, to seek to the given time.
Will throw an InvalidStateError
exception if there is no selected media
resource or if there is a current media controller.
A media resource has a media timeline that maps times (in seconds) to positions in the media resource. The origin of a timeline is its earliest defined position. The duration of a timeline is its last defined position.
Establishing the media
timeline: If the media resource somehow specifies an explicit timeline whose
origin is not negative (i.e. gives each frame a specific time offset and gives the first frame a
zero or positive offset), then the media timeline should be that timeline. (Whether
the media resource can specify a timeline or not depends on the media resource's format.) If the media resource specifies an
explicit start time and date, then that time and date should be considered the zero point
in the media timeline; the timeline offset will be the time and date,
exposed using the getStartDate()
method.
If the media resource has a discontinuous timeline, the user agent must extend the timeline used at the start of the resource across the entire resource, so that the media timeline of the media resource increases linearly starting from the earliest possible position (as defined below), even if the underlying media data has out-of-order or even overlapping time codes.
For example, if two clips have been concatenated into one video file, but the video format exposes the original times for the two clips, the video data might expose a timeline that goes, say, 00:15..00:29 and then 00:05..00:38. However, the user agent would not expose those times; it would instead expose the times as 00:15..00:29 and 00:29..01:02, as a single video.
In the rare case of a media resource that does not have an explicit timeline, the zero time on the media timeline should correspond to the first frame of the media resource. In the even rarer case of a media resource with no explicit timings of any kind, not even frame durations, the user agent must itself determine the time for each frame in a user-agent-defined manner.
An example of a file format with no explicit timeline but with explicit frame
durations is the Animated GIF format. An example of a file format with no explicit timings at all
is the JPEG-push format (multipart/x-mixed-replace
with JPEG frames, often
used as the format for MJPEG streams).
If, in the case of a resource with no timing information, the user agent will nonetheless be able to seek to an earlier point than the first frame originally provided by the server, then the zero time should correspond to the earliest seekable time of the media resource; otherwise, it should correspond to the first frame received from the server (the point in the media resource at which the user agent began receiving the stream).
At the time of writing, there is no known format that lacks explicit frame time offsets yet still supports seeking to a frame before the first frame sent by the server.
Consider a stream from a TV broadcaster, which begins streaming on a sunny Friday afternoon in
October, and always sends connecting user agents the media data on the same media timeline, with
its zero time set to the start of this stream. Months later, user agents connecting to this
stream will find that the first frame they receive has a time with millions of seconds. The getStartDate()
method would always return the date that the
broadcast started; this would allow controllers to display real times in their scrubber (e.g.
"2:30pm") rather than a time relative to when the broadcast began ("8 months, 4 hours, 12
minutes, and 23 seconds").
Consider a stream that carries a video with several concatenated fragments, broadcast by a
server that does not allow user agents to request specific times but instead just streams the
video data in a predetermined order, with the first frame delivered always being identified as
the frame with time zero. If a user agent connects to this stream and receives fragments defined
as covering timestamps 2010-03-20 23:15:00 UTC to 2010-03-21 00:05:00 UTC and 2010-02-12 14:25:00
UTC to 2010-02-12 14:35:00 UTC, it would expose this with a media timeline starting
at 0s and extending to 3,600s (one hour). Assuming the streaming server disconnected at the end
of the second clip, the duration
attribute would then
return 3,600. The getStartDate()
method would return a
Date
object with a time corresponding to 2010-03-20 23:15:00 UTC. However, if a
different user agent connected five minutes later, it would (presumably) receive
fragments covering timestamps 2010-03-20 23:20:00 UTC to 2010-03-21 00:05:00 UTC and 2010-02-12
14:25:00 UTC to 2010-02-12 14:35:00 UTC, and would expose this with a media timeline
starting at 0s and extending to 3,300s (fifty five minutes). In this case, the getStartDate()
method would return a Date
object
with a time corresponding to 2010-03-20 23:20:00 UTC.
In both of these examples, the seekable
attribute
would give the ranges that the controller would want to actually display in its UI; typically, if
the servers don't support seeking to arbitrary times, this would be the range of time from the
moment the user agent connected to the stream up to the latest frame that the user agent has
obtained; however, if the user agent starts discarding earlier information, the actual range
might be shorter.
In any case, the user agent must ensure that the earliest possible position (as defined below) using the established media timeline, is greater than or equal to zero.
The media timeline also has an associated clock. Which clock is used is user-agent defined, and may be media resource-dependent, but it should approximate the user's wall clock.
All the media elements that share current media controller use the same clock for their media timeline.
Media elements have a current playback position, which must initially (i.e. in the absence of media data) be zero seconds. The current playback position is a time on the media timeline.
Media elements also have an official playback position, which must initially be set to zero seconds. The official playback position is an approximation of the current playback position that is kept stable while scripts are running.
Media elements also have a default playback start position, which must initially be set to zero seconds. This time is used to allow the element to be seeked even before the media is loaded.
Each media element has a show poster flag. When a media
element is created, this flag must be set to true. This flag is used to control when the
user agent is to show a poster frame for a video
element instead of showing the video
contents.
The currentTime
attribute must, on
getting, return the media element's default playback start position,
unless that is zero, in which case it must return the element's official playback
position. The returned value must be expressed in seconds. On setting, if the media
element has a current media controller, then the user agent must throw an
InvalidStateError
exception; otherwise, if the media element's readyState
is HAVE_NOTHING
, then it must set the media
element's default playback start position to the new value; otherwise, it must
set the official playback position to the new value and then seek to the new value. The new value must be interpreted as being in
seconds.
Media elements have an initial playback position, which must initially (i.e. in the absence of media data) be zero seconds. The initial playback position is updated when a media resource is loaded. The initial playback position is a time on the media timeline.
If the media resource is a streaming resource, then the user agent might be unable to obtain certain parts of the resource after it has expired from its buffer. Similarly, some media resources might have a media timeline that doesn't start at zero. The earliest possible position is the earliest position in the stream or resource that the user agent can ever obtain again. It is also a time on the media timeline.
The earliest possible position is not explicitly exposed in the API;
it corresponds to the start time of the first range in the seekable
attribute's TimeRanges
object, if any, or
the current playback position otherwise.
When the earliest possible position changes, then: if the current playback
position is before the earliest possible position, the user agent must seek to the earliest possible position; otherwise, if
the user agent has not fired a timeupdate
event at the
element in the past 15 to 250ms and is not still running event handlers for such an event, then
the user agent must queue a task to fire a simple event named timeupdate
at the element.
Because of the above requirement and the requirement in the resource fetch algorithm that kicks in when the metadata of the clip becomes known, the current playback position can never be less than the earliest possible position.
If at any time the user agent learns that an audio or video track has ended and all media
data relating to that track corresponds to parts of the media timeline that
are before the earliest possible position, the user agent may queue a
task to first remove the track from the audioTracks
attribute's AudioTrackList
object or the videoTracks
attribute's VideoTrackList
object as
appropriate and then fire a trusted event with the name removetrack
, that does not bubble and is not cancelable, and that
uses the TrackEvent
interface, with the track
attribute initialized to the AudioTrack
or
VideoTrack
object representing the track, at the media element's
aforementioned AudioTrackList
or VideoTrackList
object.
The duration
attribute must return the time
of the end of the media resource, in seconds, on the media timeline. If
no media data is available, then the attributes must return the Not-a-Number (NaN)
value. If the media resource is not known to be bounded (e.g. streaming radio, or a
live event with no announced end time), then the attribute must return the positive Infinity
value.
The user agent must determine the duration of the media resource before playing
any part of the media data and before setting readyState
to a value equal to or greater than HAVE_METADATA
, even if doing so requires fetching multiple
parts of the resource.
When the length of the media resource changes to a known value
(e.g. from being unknown to known, or from a previously established length to a new length) the
user agent must queue a task to fire a simple event named durationchange
at the media element. (The
event is not fired when the duration is reset as part of loading a new media resource.) If the
duration is changed such that the current playback position ends up being greater
than the time of the end of the media resource, then the user agent must also seek to the time of the end of the media resource.
If an "infinite" stream ends for some reason, then the duration would change
from positive Infinity to the time of the last frame or sample in the stream, and the durationchange
event would be fired. Similarly, if the
user agent initially estimated the media resource's duration instead of determining
it precisely, and later revises the estimate based on new information, then the duration would
change and the durationchange
event would be
fired.
Some video files also have an explicit date and time corresponding to the zero time in the media timeline, known as the timeline offset. Initially, the timeline offset must be set to Not-a-Number (NaN).
The getStartDate()
method must return a new Date
object representing the current
timeline offset.
The loop
attribute is a boolean
attribute that, if specified, indicates that the media element is to seek back
to the start of the media resource upon reaching the end.
The loop
attribute has no effect while the element has a
current media controller.
The loop
IDL attribute must reflect
the content attribute of the same name.
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:
Apply the first applicable set of substeps from the following list:
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.
HAVE_METADATA
and the new ready state is HAVE_CURRENT_DATA
or greaterIf 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.
HAVE_FUTURE_DATA
or more, and the new ready state is
HAVE_CURRENT_DATA
or lessIf 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.
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.
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:
paused
attribute to false.play
at the element.playing
at the element.User agents do not need to support autoplay, and it is suggested that user
agents honor user preferences on the matter. Authors are urged to use the autoplay
attribute rather than using script to force the
video to play, so as to allow the user to override the behavior if so desired.
In any case, the user agent must finally queue a task to fire a simple
event named canplaythrough
at the element.
If the media element has a current media controller, then report the controller state for the media element's current media controller.
It is possible for the ready state of a media element to jump between these states
discontinuously. For example, the state of a media element can jump straight from HAVE_METADATA
to HAVE_ENOUGH_DATA
without passing through the HAVE_CURRENT_DATA
and HAVE_FUTURE_DATA
states.
The readyState
IDL attribute must, on
getting, return the value described above that describes the current ready state of the
media element.
The autoplay
attribute is a boolean
attribute. When present, the user agent (as described in the algorithm
described herein) will automatically begin playback of the media resource as
soon as it can do so without stopping.
Authors are urged to use the autoplay
attribute rather than using script to trigger automatic playback, as this allows the user to
override the automatic playback when it is not desired, e.g. when using a screen reader. Authors
are also encouraged to consider not using the automatic playback behavior at all, and instead to
let the user agent wait for the user to start playback explicitly.
The autoplay
IDL attribute must
reflect the content attribute of the same name.
paused
Returns true if playback is paused; false otherwise.
ended
Returns true if playback has reached the end of the media resource.
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.
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.
played
Returns a TimeRanges
object that represents the ranges of the media
resource that the user agent has played.
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.
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:
readyState
attribute is HAVE_METADATA
or greater, and
Either:
loop
attribute specified, or the media element has
a current media controller.
Or:
The ended
attribute must return true if, the
last time the event loop reached step 1, the media element had
ended playback and the direction of playback was forwards, and false
otherwise.
A media element is said to have stopped due to errors when the
element's readyState
attribute is HAVE_METADATA
or greater, and the user agent encounters a non-fatal error during the processing of the
media data, and due to that error, is not able to play the content at the
current playback position.
A media element is said to have paused for user interaction when its
paused
attribute is false, the readyState
attribute is either HAVE_FUTURE_DATA
or HAVE_ENOUGH_DATA
and the user agent has reached a point
in the media resource where the user has to make a selection for the resource to
continue. If the media element has a current media controller when this
happens, then the user agent must report the controller state for the media
element's current media controller. If the media element has a
current media controller when the user makes a selection, allowing playback to
resume, the user agent must similarly report the controller state for the media
element's current media controller.
It is possible for a media element to have both ended playback and paused for user interaction at the same time.
When a media element that is potentially playing stops playing
because it has paused for user interaction, the user agent must queue a
task to fire a simple event named timeupdate
at the element.
A media element is said to have paused for in-band content when its
paused
attribute is false, the readyState
attribute is either HAVE_FUTURE_DATA
or HAVE_ENOUGH_DATA
and the user agent has suspended
playback of the media resource in order to play content that is temporally anchored
to the media resource and has a non-zero length, or to play content that is
temporally anchored to a segment of the media resource but has a length longer than
that segment. If the media element has a current media controller when
this happens, then the user agent must report the controller state for the
media element's current media controller. If the media
element has a current media controller when the user agent unsuspends
playback, the user agent must similarly report the controller state for the
media element's current media controller.
One example of when a media element would be paused for in-band content is when the user agent is playing audio descriptions from an external WebVTT file, and the synthesized speech generated for a cue is longer than the time between the text track cue start time and the text track cue end time.
When the current playback position reaches the end of the media resource when the direction of playback is forwards, then the user agent must follow these steps:
If the media element has a loop
attribute specified and does not have a current media controller, then seek to the earliest possible position of the
media resource and abort these steps.
As defined above, the ended
IDL attribute starts
returning true once the event loop's current task
ends.
Queue a task to fire a simple event named timeupdate
at the media element.
Queue a task that, if the media element does not have a
current media controller, and the media element has still ended
playback, and the direction of playback is still forwards, and paused is false, changes paused to true and fires a
simple event named pause
at the media
element.
Queue a task to fire a simple event named ended
at the media element.
If the media element has a current media controller, then report the controller state for the media element's current media controller.
When the current playback position reaches the earliest possible
position of the media resource when the direction of playback is
backwards, then the user agent must only queue a task to fire a simple
event named timeupdate
at the element.
The defaultPlaybackRate
attribute
gives the desired speed at which the media resource is to play, as a multiple of its
intrinsic speed. The attribute is mutable: on getting it must return the last value it was set to,
or 1.0 if it hasn't yet been set; on setting the attribute must be set to the new value.
The defaultPlaybackRate
is used
by the user agent when it exposes a user
interface to the user.
The playbackRate
attribute gives the
effective playback rate (assuming there is no current media controller
overriding it), which is the speed at which the media resource plays, as a multiple
of its intrinsic speed. If it is not equal to the defaultPlaybackRate
, then the implication is that the
user is using a feature such as fast forward or slow motion playback. The attribute is mutable: on
getting it must return the last value it was set to, or 1.0 if it hasn't yet been set; on setting
the attribute must be set to the new value, and the playback will change speed (if the element is
potentially playing and there is no current media controller).
When the defaultPlaybackRate
or playbackRate
attributes change value (either by
being set by script or by being changed directly by the user agent, e.g. in response to user
control) the user agent must queue a task to fire a simple event named
ratechange
at the media element.
The defaultPlaybackRate
and
playbackRate
attributes have no effect when the
media element has a current media controller; the namesake attributes on
the MediaController
object are used instead in that situation.
The played
attribute must return a new static
normalized TimeRanges
object that represents the ranges of points on the
media timeline of the media resource reached through the usual monotonic
increase of the current playback position during normal playback, if any, at the time
the attribute is evaluated.
When the play()
method on a media
element is invoked, the user agent must run the following steps.
If the media element's networkState
attribute has the value NETWORK_EMPTY
, invoke the media element's
resource selection algorithm.
If the playback has ended and the direction of playback is forwards, and the media element does not have a current media controller, seek to the earliest possible position of the media resource.
This will cause the user agent to queue a
task to fire a simple event named timeupdate
at the media element.
If the media element has a current media controller, then bring the media element up to speed with its new media controller.
If the media element's paused
attribute is
true, run the following substeps:
Change the value of paused
to false.
If the show poster flag is true, set the element's show poster flag to false and run the time marches on steps.
Queue a task to fire a simple event named play
at the element.
If the media element's readyState
attribute has the value HAVE_NOTHING
, HAVE_METADATA
, or HAVE_CURRENT_DATA
, queue a task to
fire a simple event named waiting
at the
element.
Otherwise, the media element's readyState
attribute has the value HAVE_FUTURE_DATA
or HAVE_ENOUGH_DATA
: queue a task to
fire a simple event named playing
at the
element.
Set the media element's autoplaying flag to false.
If the media element has a current media controller, then report the controller state for the media element's current media controller.
When the pause()
method is invoked, and when
the user agent is required to pause the media element, the user agent must run the
following steps:
If the media element's networkState
attribute has the value NETWORK_EMPTY
, invoke the media element's
resource selection algorithm.
Set the media element's autoplaying flag to false.
If the media element's paused
attribute
is false, run the following steps:
Change the value of paused
to true.
Queue a task to fire a simple
event named timeupdate
at the
element.
Queue a task to fire a simple
event named pause
at the element.
Set the official playback position to the current playback position.
If the media element has a current media controller, then report the controller state for the media element's current media controller.
The effective playback rate is not necessarily the element's playbackRate
. When a media element has a
current media controller, its effective playback rate is the
MediaController
's media controller playback rate. Otherwise, the
effective playback rate is just the element's playbackRate
. Thus, the current media
controller overrides the media element.
If the effective playback rate is positive or zero, then the direction of playback is forwards. Otherwise, it is backwards.
When a media element is potentially playing and
its Document
is a fully active Document
, its current
playback position must increase monotonically at effective playback rate units
of media time per unit time of the media timeline's clock. (This specification always
refers to this as an increase, but that increase could actually be a decrease if
the effective playback rate is negative.)
The effective playback rate can be 0.0, in which case the
current playback position doesn't move, despite playback not being paused (paused
doesn't become true, and the pause
event doesn't fire).
This specification doesn't define how the user agent achieves the appropriate playback rate — depending on the protocol and media available, it is plausible that the user agent could negotiate with the server to have the server provide the media data at the appropriate rate, so that (except for the period between when the rate is changed and when the server updates the stream's playback rate) the client doesn't actually have to drop or interpolate any frames.
Any time the user agent provides a stable state, the official playback position must be set to the current playback position.
When the direction of playback is backwards, any corresponding audio must be muted. When the effective playback rate is so low or so high that the user agent cannot play audio usefully, the corresponding audio must also be muted. If the effective playback rate is not 1.0, the user agent may apply pitch adjustments to the audio as necessary to render it faithfully.
Media elements that are potentially playing
while not in a Document
must not play any video, but should play any
audio component. Media elements must not stop playing just because all references to them have
been removed; only once a media element is in a state where no further audio could ever be played
by that element may the element be garbage collected.
It is possible for an element to which no explicit references exist to play audio, even if such an element is not still actively playing: for instance, it could have a current media controller that still has references and can still be unpaused, or it could be unpaused but stalled waiting for content to buffer.
Each media element has a list of newly introduced cues, which must be initially empty. Whenever a text track cue is added to the list of cues of a text track that is in the list of text tracks for a media element, that cue must be added to the media element's list of newly introduced cues. Whenever a text track is added to the list of text tracks for a media element, all of the cues in that text track's list of cues must be added to the media element's list of newly introduced cues. When a media element's list of newly introduced cues has new cues added while the media element's show poster flag is not set, then the user agent must run the time marches on steps.
When a text track cue is removed from the list of cues of a text track that is in the list of text tracks for a media element, and whenever a text track is removed from the list of text tracks of a media element, if the media element's show poster flag is not set, then the user agent must run the time marches on steps.
When the current playback position of a media element changes (e.g. due to playback or seeking), the user agent must run the time marches on steps. If the current playback position changes while the steps are running, then the user agent must wait for the steps to complete, and then must immediately rerun the steps. (These steps are thus run as often as possible or needed — if one iteration takes a long time, this can cause certain cues to be skipped over as the user agent rushes ahead to "catch up".)
The time marches on steps are as follows:
Let current cues be a list of cues, initialized to contain all the cues of all the hidden or showing text tracks of the media element (not the disabled ones) whose start times are less than or equal to the current playback position and whose end times are greater than the current playback position.
Let other cues be a list of cues, initialized to contain all the cues of hidden and showing text tracks of the media element that are not present in current cues.
Let last time be the current playback position at the time this algorithm was last run for this media element, if this is not the first time it has run.
If the current playback position has, since the last time this algorithm was run, only changed through its usual monotonic increase during normal playback, then let missed cues be the list of cues in other cues whose start times are greater than or equal to last time and whose end times are less than or equal to the current playback position. Otherwise, let missed cues be an empty list.
Remove all the cues in missed cues that are also in the media element's list of newly introduced cues, and then empty the element's list of newly introduced cues.
If the time was reached through the usual monotonic increase of the current playback
position during normal playback, and if the user agent has not fired a timeupdate
event at the element in the past 15 to 250ms and
is not still running event handlers for such an event, then the user agent must queue a
task to fire a simple event named timeupdate
at the element. (In the other cases, such as
explicit seeks, relevant events get fired as part of the overall process of changing the
current playback position.)
The event thus is not to be fired faster than about 66Hz or slower than 4Hz (assuming the event handlers don't take longer than 250ms to run). User agents are encouraged to vary the frequency of the event based on the system load and the average cost of processing the event each time, so that the UI updates are not any more frequent than the user agent can comfortably handle while decoding the video.
If all of the cues in current cues have their text track cue active flag set, none of the cues in other cues have their text track cue active flag set, and missed cues is empty, then abort these steps.
If the time was reached through the usual monotonic increase of the current playback position during normal playback, and there are cues in other cues that have their text track cue pause-on-exit flag set and that either have their text track cue active flag set or are also in missed cues, then immediately pause the media element.
In the other cases, such as explicit seeks, playback is not paused by going past the end time of a cue, even if that cue has its text track cue pause-on-exit flag set.
Let events be a list of tasks, initially empty. Each task in this list will be associated with a text track, a text track cue, and a time, which are used to sort the list before the tasks are queued.
Let affected tracks be a list of text tracks, initially empty.
When the steps below say to prepare an event named event for a text track cue target with a time time, the user agent must run these substeps:
Let track be the text track with which the text track cue target is associated.
Create a task to fire a simple event named event at target.
Add the newly created task to events, associated with the time time, the text track track, and the text track cue target.
Add track to affected tracks.
For each text track cue in missed
cues, prepare an event named enter
for the
TextTrackCue
object with the text track cue start time.
For each text track cue in other
cues that either has its text track cue active flag set or is in missed cues, prepare an event named exit
for the TextTrackCue
object with the later of the
text track cue end time and the text track cue start time.
For each text track cue in current
cues that does not have its text track cue active flag set, prepare an
event named enter
for the TextTrackCue
object with the text track cue start time.
Sort the tasks in events in ascending time order (tasks with earlier times first).
Further sort tasks in events that have the same time by the relative text track cue order of the text track cues associated with these tasks.
Finally, sort tasks in events that have
the same time and same text track cue order by placing tasks that fire enter
events before
those that fire exit
events.
Sort affected tracks in the same order as the text tracks appear in the media element's list of text tracks, and remove duplicates.
For each text track in affected tracks, in the list
order, queue a task to fire a simple event named cuechange
at the TextTrack
object, and, if the
text track has a corresponding track
element, to then fire a
simple event named cuechange
at the
track
element as well.
Set the text track cue active flag of all the cues in the current cues, and unset the text track cue active flag of all the cues in the other cues.
Run the rules for updating the text track rendering of each of the text tracks in affected tracks that are showing. For example, for text tracks based on WebVTT, the rules for updating the display of WebVTT text tracks. [WEBVTT]
For the purposes of the algorithm above, a text track cue is considered to be part of a text track only if it is listed in the text track list of cues, not merely if it is associated with the text track.
If the media element's Document
stops being a
fully active document, then the playback will stop
until the document is active again.
When a media element is removed
from a Document
, the user agent must run the following steps:
Asynchronously await a stable state, allowing the task that removed the media element from the
Document
to continue. The synchronous section consists of all the
remaining steps of this algorithm. (Steps in the synchronous section are marked with
⌛.)
⌛ If the media element is in a Document
,
abort these steps.
⌛ If the media element's networkState
attribute has the value NETWORK_EMPTY
, abort these steps.
⌛ Pause the media element.
seeking
Returns true if the user agent is currently seeking.
seekable
Returns a TimeRanges
object that represents the ranges of the media
resource to which it is possible for the user agent to seek.
The seeking
attribute must initially have the
value false.
When the user agent is required to seek to a particular new playback position in the media resource, optionally with the approximate-for-speed flag set, it means that the user agent must run the following steps. This algorithm interacts closely with the event loop mechanism; in particular, it has a synchronous section (which is triggered as part of the event loop algorithm). Steps in that section are marked with ⌛.
Set the media element's show poster flag to false.
If the media element's readyState
is HAVE_NOTHING
, abort these steps.
If the element's seeking
IDL attribute is true,
then another instance of this algorithm is already running. Abort that other instance of the
algorithm without waiting for the step that it is running to complete.
Set the seeking
IDL attribute to true.
If the seek was in response to a DOM method call or setting of an IDL attribute, then continue the script. The remainder of these steps must be run asynchronously. With the exception of the steps marked with ⌛, they could be aborted at any time by another instance of this algorithm being invoked.
If the new playback position is later than the end of the media resource, then let it be the end of the media resource instead.
If the new playback position is less than the earliest possible position, let it be that position instead.
If the (possibly now changed) new playback position is not in one of
the ranges given in the seekable
attribute, then let it
be the position in one of the ranges given in the seekable
attribute that is the nearest to the new
playback position. If two positions both satisfy that constraint (i.e. the new playback position is exactly in the middle between two ranges in the seekable
attribute) then use the position that is closest to
the current playback position. If there are no ranges given in the seekable
attribute then set the seeking
IDL attribute to false and abort these steps.
If the approximate-for-speed flag is set, adjust the new playback position to a value that will allow for playback to resume promptly. If new playback position before this step is before current playback position, then the adjusted new playback position must also be before the current playback position. Similarly, if the new playback position before this step is after current playback position, then the adjusted new playback position must also be after the current playback position.
For example, the user agent could snap to the nearest key frame, so that it doesn't have to spend time decoding then discarding intermediate frames before resuming playback.
Queue a task to fire a simple event named seeking
at the element.
Set the current playback position to the given new playback position.
If the media element was potentially playing
immediately before it started seeking, but seeking caused its readyState
attribute to change to a value lower than HAVE_FUTURE_DATA
, then a waiting
will be
fired at the element.
The currentTime
attribute does not
get updated asynchronously, as it returns the official playback position, not the
current playback position.
Wait until the user agent has established whether or not the media data for the new playback position is available, and, if it is, until it has decoded enough data to play back that position.
Await a stable state. The synchronous section consists of all the remaining steps of this algorithm. (Steps in the synchronous section are marked with ⌛.)
⌛ Set the seeking
IDL attribute to
false.
⌛ Run the time marches on steps.
⌛ Queue a task to fire a simple event
named timeupdate
at the element.
⌛ Queue a task to fire a simple event named seeked
at the element.
The seekable
attribute must return a new
static normalized TimeRanges
object that represents the ranges of the
media resource, if any, that the user agent is able to seek to, at the time the
attribute is evaluated.
If the user agent can seek to anywhere in the media resource, e.g.
because it is a simple movie file and the user agent and the server support HTTP Range requests,
then the attribute would return an object with one range, whose start is the time of the first
frame (the earliest possible position, typically zero), and whose end is the same as
the time of the first frame plus the duration
attribute's
value (which would equal the time of the last frame, and might be positive Infinity).
The range might be continuously changing, e.g. if the user agent is buffering a sliding window on an infinite stream. This is the behavior seen with DVRs viewing live TV, for instance.
Media resources might be internally scripted or interactive. Thus, a media element could play in a non-linear fashion. If this happens, the user agent must act as if the algorithm for seeking was used whenever the current playback position changes in a discontinuous fashion (so that the relevant events fire). If the media element has a current media controller, then the user agent must seek the media controller appropriately instead.
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.
audioTracks
Returns an AudioTrackList
object representing the audio tracks available in the
media resource.
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>
AudioTrackList
and VideoTrackList
objectsThe 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; };
audioTracks
. length
videoTracks
. length
Returns the number of tracks in the list.
audioTracks
[index]videoTracks
[index]Returns the specified AudioTrack
or VideoTrack
object.
audioTracks
. getTrackById
( id )videoTracks
. getTrackById
( id )Returns the AudioTrack
or VideoTrack
object with the given identifier, or null if no track has that identifier.
id
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]
kind
kind
Returns the category the given track falls into. The possible track categories are given below.
label
label
Returns the label of the given track, if known, or the empty string otherwise.
language
language
Returns the language of the given track, if known, or the empty string otherwise.
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.
videoTracks
. selectedIndex
Returns the index of the currently selected track, if any, or −1 otherwise.
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.
Category | Definition | Applies to... |
---|---|---|
"alternative "
| A possible alternative to the main track, e.g. a different take of a song (audio), or a different angle (video). | Audio and video. |
"captions "
| A version of the main video track with captions burnt in. (For legacy content; new content would use text tracks.) | Video only. |
"descriptions "
| An audio description of a video track. | Audio only. |
"main "
| The primary audio or video track. | Audio and video. |
"main-desc "
| The primary audio track, mixed with audio descriptions. | Audio only. |
"sign "
| A sign-language interpretation of an audio track. | Video only. |
"subtitles "
| A version of the main video track with subtitles burnt in. (For legacy content; new content would use text tracks.) | Video only. |
"translation "
| A translated version of the main audio track. | Audio only. |
"commentary "
| Commentary on the primary audio or video track, e.g. a director's commentary. | Audio and video. |
" " (empty string)
| No explicit kind, or the kind given by the track's metadata is not recognised by the user agent. | Audio and video. |
The AudioTrack.label
and VideoTrack.label
attributes must return the label
of the track, if it has one, or the empty string otherwise.
The AudioTrack.language
and VideoTrack.language
attributes must return the
BCP 47 language tag of the language of the track, if it has one, or the empty string otherwise. If
the user agent is not able to express that language as a BCP 47 language tag (for example because
the language information in the media resource's format is a free-form string without
a defined interpretation), then the method must return the empty string, as if the track had no
language.
Source attribute values for id, kind, label and language of multitrack audio and video tracks as described for the relevant media resource format. [INBANDTRACKS]
The AudioTrack.enabled
attribute, on
getting, must return true if the track is currently enabled, and false otherwise. On setting, it
must enable the track if the new value is true, and disable it otherwise. (If the track is no
longer in an AudioTrackList
object, then the track being enabled or disabled has no
effect beyond changing the value of the attribute on the AudioTrack
object.)
Whenever an audio track in an AudioTrackList
is enabled or disabled, the user
agent must queue a task to fire a simple event named change
at the AudioTrackList
object.
The VideoTrackList.selectedIndex
attribute
must return the index of the currently selected track, if any. If the VideoTrackList
object does not currently represent any tracks, or if none of the tracks are selected, it must
instead return −1.
The VideoTrack.selected
attribute, on
getting, must return true if the track is currently selected, and false otherwise. On setting, it
must select the track if the new value is true, and unselect it otherwise. If the track is in a
VideoTrackList
, then all the other VideoTrack
objects in that list must
be unselected. (If the track is no longer in a VideoTrackList
object, then the track
being selected or unselected has no effect beyond changing the value of the attribute on the
VideoTrack
object.)
Whenever a track in a VideoTrackList
that was previously not selected is selected,
the user agent must queue a task to fire a simple event named change
at the VideoTrackList
object. This task must be queued before the
task that fires the resize
event, if any.
The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes,
by all objects implementing the AudioTrackList
and VideoTrackList
interfaces:
Event handler | Event handler event type |
---|---|
onchange | change
|
onaddtrack | addtrack
|
onremovetrack | removetrack
|
The task source for the tasks listed in this section is the DOM manipulation task source.
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>
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.
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; };
MediaController
()Returns a new MediaController
object.
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.
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.
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.
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.
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.
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.
paused
Returns true if playback is paused; false otherwise. When this attribute is true, any media element slaved to this controller will be stopped.
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).
pause
()Sets the paused
attribute to true.
unpause
()Sets the paused
attribute to false.
play
()Sets the paused
attribute to false and
invokes the play()
method of each slaved media element.
played
Returns a TimeRanges
object that represents the union of the time ranges in all
the slaved media elements that have been played.
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.
playbackRate
[ = value ]Returns the current rate of playback.
Can be set, to change the rate of playback.
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.
muted
[ = value ]Returns true if all audio is muted (regardless of other attributes either on the controller or on any media elements slaved to this controller), and false otherwise.
Can be set, to change whether the audio is muted or not.
A media element can have a current media controller, which is a
MediaController
object. When a media element is created without a mediagroup
attribute, it does not have a current media
controller. (If it is created with such an attribute, then that attribute
initializes the current media controller, as defined below.)
The slaved media elements of a MediaController
are the media elements whose current media controller is that
MediaController
. All the slaved media elements of a
MediaController
must use the same clock for their definition of their media
timeline's unit time. When the user agent is required to act on each slaved media element in turn, they must be processed in the order that they
were last associated with the MediaController
.
The controller
attribute on a media
element, on getting, must return the element's current media controller, if
any, or null otherwise. On setting, the user agent must run the following steps:
Let m be the media element in question.
Let old controller be m's current media controller, if it currently has one, and null otherwise.
Let new controller be null.
Let m have no current media controller, if it currently has one.
Remove the element's mediagroup
content
attribute, if any.
If the new value is null, then jump to the update controllers step below.
Let m's current media controller be the new value.
Let new controller be m's current media controller.
Bring the media element up to speed with its new media controller.
Update controllers: If old controller and new controller are the same (whether both null or both the same controller) then abort these steps.
If old controller is not null and still has one or more slaved media elements, then report the controller state for old controller.
If new controller is not null, then report the controller state for new controller.
The MediaController()
constructor, when
invoked, must return a newly created MediaController
object.
The readyState
attribute must
return the value to which it was most recently set. When the MediaController
object
is created, the attribute must be set to the value 0 (HAVE_NOTHING
). The value is updated by the report the
controller state algorithm below.
The seekable
attribute must return
a new static normalized TimeRanges
object that represents the
intersection of the ranges of the media resources of the
slaved media elements that the user agent is able to seek to, at the time the
attribute is evaluated.
The buffered
attribute must return
a new static normalized TimeRanges
object that represents the
intersection of the ranges of the media resources of the
slaved media elements that the user agent has buffered, at the time the attribute is
evaluated. Users agents must accurately determine the ranges available, even for media streams
where this can only be determined by tedious inspection.
The duration
attribute must return
the media controller duration.
Every 15 to 250ms, or whenever the MediaController
's media controller
duration changes, whichever happens least often, the user agent must queue a
task to fire a simple event named durationchange
at the
MediaController
. If the MediaController
's media controller
duration decreases such that the media controller position is greater than the
media controller duration, the user agent must immediately seek the media
controller to media controller duration.
The currentTime
attribute must
return the media controller position on getting, and on setting must seek the
media controller to the new value.
Every 15 to 250ms, or whenever the MediaController
's media controller
position changes, whichever happens least often, the user agent must queue a
task to fire a simple event named timeupdate
at the
MediaController
.
When a MediaController
is created it is a playing media controller. It
can be changed into a paused media controller and back either via the user agent's user
interface (when the element is exposing a user
interface to the user) or by script using the APIs defined in this section (see below).
The paused
attribute must return
true if the MediaController
object is a paused media controller, and
false otherwise.
When the pause()
method is invoked,
if the MediaController
is a playing media controller then the user agent
must change the MediaController
into a paused media controller,
queue a task to fire a simple event named pause
at the MediaController
, and then
report the controller state of the MediaController
.
When the unpause()
method is
invoked, if the MediaController
is a paused media controller, the user
agent must change the MediaController
into a playing media controller,
queue a task to fire a simple event named play
at the MediaController
, and then
report the controller state of the MediaController
.
When the play()
method is invoked, the
user agent must invoke the play()
method of each slaved media element in turn, and then invoke the unpause
method of the MediaController
.
The playbackState
attribute
must return the value to which it was most recently set. When the MediaController
object is created, the attribute must be set to the value "waiting
". The value is updated by the report the
controller state algorithm below.
The played
attribute must return a
new static normalized TimeRanges
object that represents the union of the
ranges of points on the media timelines of the media resources of the slaved media elements that the
user agent has so far reached through the usual monotonic increase of their current playback positions during normal playback, at the time the
attribute is evaluated.
A MediaController
has a media controller default playback rate and a
media controller playback rate, which must both be set to 1.0 when the
MediaController
object is created.
The defaultPlaybackRate
attribute, on getting, must return the MediaController
's media controller
default playback rate, and on setting, must set the MediaController
's
media controller default playback rate to the new value, then queue a
task to fire a simple event named ratechange
at the
MediaController
.
The playbackRate
attribute, on
getting, must return the MediaController
's media controller playback
rate, and on setting, must set the MediaController
's media controller
playback rate to the new value, then queue a task to fire a simple
event named ratechange
at the
MediaController
.
A MediaController
has a media controller volume multiplier, which must
be set to 1.0 when the MediaController
object is created, and a media controller
mute override, much must initially be false.
The volume
attribute, on getting,
must return the MediaController
's media controller volume multiplier,
and on setting, if the new value is in the range 0.0 to 1.0 inclusive, must set the
MediaController
's media controller volume multiplier to the new value
and queue a task to fire a simple event named volumechange
at the
MediaController
. If the new value is outside the range 0.0 to 1.0 inclusive, then, on
setting, an IndexSizeError
exception must be thrown instead.
The muted
attribute, on getting, must
return the MediaController
's media controller mute override, and on
setting, must set the MediaController
's media controller mute override
to the new value and queue a task to fire a simple event named volumechange
at the
MediaController
.
The media resources of all the slaved media
elements of a MediaController
have a defined temporal relationship which
provides relative offsets between the zero time of each such media resource: for
media resources with a timeline offset, their
relative offsets are the difference between their timeline offset; the zero times of
all the media resources without a timeline offset
are not offset from each other (i.e. the origins of their timelines are cotemporal); and finally,
the zero time of the media resource with the earliest timeline offset
(if any) is not offset from the zero times of the media
resources without a timeline offset (i.e. the origins of media resources without a timeline offset are further cotemporal
with the earliest defined point on the timeline of the media resource with the
earliest timeline offset).
The media resource end position of a media resource in a media element is defined as follows: if the media resource has a finite and known duration, the media resource end position is the duration of the media resource's timeline (the last defined position on that timeline); otherwise, the media resource's duration is infinite or unknown, and the media resource end position is the time of the last frame of media data currently available for that media resource.
Each MediaController
also has its own defined timeline. On this timeline, all the
media resources of all the slaved media elements
of the MediaController
are temporally aligned according to their defined offsets. The
media controller duration of that MediaController
is the time from the
earliest earliest possible position, relative to this MediaController
timeline, of any of the media resources of the slaved
media elements of the MediaController
, to the time of the latest media
resource end position of the media resources of the
slaved media elements of the MediaController
, again relative to this
MediaController
timeline.
Each MediaController
has a media controller position. This is the time
on the MediaController
's timeline at which the user agent is trying to play the
slaved media elements. When a MediaController
is created, its
media controller position is initially zero.
When the user agent is to bring a media element up to speed with its new media controller, it must seek that media element to the
MediaController
's media controller position relative to the media
element's timeline.
When the user agent is to seek the media controller to a particular new playback position, it must follow these steps:
If the new playback position is less than zero, then set it to zero.
If the new playback position is greater than the media controller duration, then set it to the media controller duration.
Set the media controller position to the new playback position.
Seek each slaved media element to the new playback position relative to the media element timeline.
A MediaController
is a restrained media controller if the
MediaController
is a playing media controller, and none of its
slaved media elements are blocked media
elements, but either at least one of its slaved media elements whose
autoplaying flag is true still has its paused
attribute set to true, or, all of its slaved media elements have their paused
attribute set to true.
A MediaController
is a blocked media controller if the
MediaController
is a paused media controller, or if any of its
slaved media elements are blocked media
elements, or if any of its slaved media elements whose autoplaying
flag is true still have their paused
attribute set to
true, or if all of its slaved media elements have their paused
attribute set to true.
A media element is blocked on its media controller if the
MediaController
is a blocked media controller, or if its media
controller position is either before the media resource's earliest
possible position relative to the MediaController
's timeline or after the end
of the media resource relative to the MediaController
's timeline.
When a MediaController
is not a blocked media
controller and it has at least one slaved media
element whose Document
is a fully active Document
,
the MediaController
's media controller position must increase
monotonically at media controller playback rate units of time on the
MediaController
's timeline per unit time of the clock used by its slaved media
elements.
When the zero point on the timeline of a MediaController
moves relative to the
timelines of the slaved media elements by a time difference ΔT, the MediaController
's media controller
position must be decremented by ΔT.
In some situations, e.g. when playing back a live stream without buffering anything, the media controller position would increase monotonically as described above at the same rate as the ΔT described in the previous paragraph decreases it, with the end result that for all intents and purposes, the media controller position would appear to remain constant (probably with the value 0).
A MediaController
has a most recently reported readiness state, which
is a number from 0 to 4 derived from the numbers used for the media element readyState
attribute, and a most recently reported
playback state, which is either playing, waiting, or ended.
When a MediaController
is created, its most recently reported readiness
state must be set to 0, and its most recently reported playback state must be
set to waiting.
When a user agent is required to report the controller state for a
MediaController
, the user agent must run the following steps:
If the MediaController
has no slaved media elements, let new readiness state be 0.
Otherwise, let it have the lowest value of the readyState
IDL attributes of all of its slaved media
elements.
If the MediaController
's most recently reported readiness state is
less than the new readiness state, then run these substeps:
Let next state be the MediaController
's most
recently reported readiness state.
Loop: Increment next state by one.
Queue a task to run the following steps:
Set the MediaController
's readyState
attribute to the value next state.
Fire a simple event at the MediaController
object, whose
name is the event name corresponding to the value of next state given in
the table below.
If next state is less than new readiness state, then return to the step labeled loop.
Otherwise, if the MediaController
's most recently reported readiness
state is greater than new readiness state then queue a
task to fire a simple event at the MediaController
object,
whose name is the event name corresponding to the value of new readiness
state given in the table below.
Value of new readiness state | Event name |
---|---|
0 | emptied
|
1 | loadedmetadata
|
2 | loadeddata
|
3 | canplay
|
4 | canplaythrough
|
Let the MediaController
's most recently reported readiness state
be new readiness state.
Initialize new playback state by setting it to the state given for the first matching condition from the following list:
MediaController
has no slaved media elementsMediaController
's slaved media elements have
ended playback and the media controller playback rate is positive or
zeroMediaController
is a blocked media controllerIf the MediaController
's most recently reported playback state
is not equal to new playback state and the new playback
state is ended, then queue a task that, if the
MediaController
object is a playing media controller, and all of the
MediaController
's slaved media elements have still ended
playback, and the media controller playback rate is still positive or zero,
changes the MediaController
object to a paused media controller and
then fires a simple event named pause
at the MediaController
object.
If the MediaController
's most recently reported playback state is
not equal to new playback state then queue a task to run the
following steps:
Set the MediaController
's playbackState
attribute to the value given in
the second column of the row of the following table whose first column contains the new playback state.
Fire a simple event at the MediaController
object, whose name
is the value given in the third column of the row of the following table whose first column
contains the new playback state.
New playback state | New value for playbackState
| Event name |
---|---|---|
playing | "playing "
| playing
|
waiting | "waiting "
| waiting
|
ended | "ended "
| ended
|
Let the MediaController
's most recently reported playback state
be new playback state.
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.
The mediagroup
content attribute on media elements can be used to link multiple media elements together by implicitly creating a MediaController
. The
value is text; media elements with the same value are
automatically linked by the user agent.
When a media element is created with a mediagroup
attribute, and when a media element's
mediagroup
attribute is set, changed, or removed, the
user agent must run the following steps:
Let m be the media element in question.
Let old controller be m's current media controller, if it currently has one, and null otherwise.
Let new controller be null.
Let m have no current media controller, if it currently has one.
If m's mediagroup
attribute
is being removed, then jump to the update controllers step below.
If there is another media element whose Document
is the same as
m's Document
(even if one or both of these elements are not
actually in the Document
), and which
also has a mediagroup
attribute, and whose mediagroup
attribute has the same value as the new value of
m's mediagroup
attribute, then
let controller be that media element's current media
controller.
Otherwise, let controller be a newly created
MediaController
.
Let m's current media controller be controller.
Let new controller be m's current media controller.
Bring the media element up to speed with its new media controller.
Update controllers: If old controller and new controller are the same (whether both null or both the same controller) then abort these steps.
If old controller is not null and still has one or more slaved media elements, then report the controller state for old controller.
If new controller is not null, then report the controller state for new controller.
The mediaGroup
IDL attribute on media elements must reflect the mediagroup
content attribute.
Multiple media elements referencing the same media
resource will share a single network request. This can be used to efficiently play two
(video) tracks from the same media resource in two different places on the screen.
Used with the mediagroup
attribute, these elements can
also be kept synchronised.
In this example, a sign-languge interpreter track from a movie file is overlaid on the primary
video track of that same video file using two video
elements, some CSS, and an
implicit MediaController
:
<article> <style> div { margin: 1em auto; position: relative; width: 400px; height: 300px; } video { position; absolute; bottom: 0; right: 0; } video:first-child { width: 100%; height: 100%; } video:last-child { width: 30%; } </style> <div> <video src="movie.vid#track=Video&track=English" autoplay controls mediagroup=movie></video> <video src="movie.vid#track=sign" autoplay mediagroup=movie></video> </div> </article>
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:
track
element
children of the media element, in tree order.addTextTrack()
method, in the order they were added, oldest
first.A text track consists of:
This decides how the track is handled by the user agent. The kind is represented by a string. The possible strings are:
subtitles
captions
descriptions
chapters
metadata
The kind of track can change dynamically, in the case of
a text track corresponding to a track
element.
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.
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.
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.
One of the following:
Indicates that the text track's cues have not been obtained.
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.
Indicates that the text track has been loaded with no fatal errors.
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.
One of the following:
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.
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.
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 text track cues, along with rules for updating the text track rendering. For example, for WebVTT, the rules for updating the display of WebVTT text tracks. [WEBVTT]
The list of cues of a text track can change dynamically, either because the text track has not yet been loaded or is still loading, or due to DOM manipulation.
Each text track has a corresponding TextTrack
object.
Each media element has a list of pending text tracks, which must initially be empty, a blocked-on-parser flag, which must initially be false, and a did-perform-automatic-track-selection flag, which must also initially be false.
When the user agent is required to populate the list of pending text tracks of a media element, the user agent must add to the element's list of pending text tracks each text track in the element's list of text tracks whose text track mode is not disabled and whose text track readiness state is loading.
Whenever a track
element's parent node changes, the user agent must remove the
corresponding text track from any list of pending text tracks that it is
in.
Whenever a text track's text track readiness state changes to either loaded or failed to load, the user agent must remove it from any list of pending text tracks that it is in.
When a media element is created by an HTML parser or XML parser, the user agent must set the element's blocked-on-parser flag to true. When a media element is popped off the stack of open elements of an HTML parser or XML parser, the user agent must honor user preferences for automatic text track selection, populate the list of pending text tracks, and set the element's blocked-on-parser flag to false.
The text tracks of a media element are ready when both the element's list of pending text tracks is empty and the element's blocked-on-parser flag is false.
Each media element has a pending text track change notification flag, which must initially be unset.
Whenever a text track that is in a media element's list of text tracks has its text track mode change value, the user agent must run the following steps for the media element:
If the media element's pending text track change notification flag is set, abort these steps.
Set the media element's pending text track change notification flag.
Queue a task that runs the following substeps:
Unset the media element's pending text track change notification flag.
Fire a simple event named change
at
the media element's textTracks
attribute's TextTrackList
object.
If the media element's show poster flag is not set, run the time marches on steps.
The task source for the tasks listed in this section is the DOM manipulation task source.
A text track cue is the unit of time-sensitive data in a text track, corresponding for instance for subtitles and captions to the text that appears at a particular time and disappears at another time.
Each text track cue consists of:
An arbitrary string.
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.
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 boolean indicating whether playback of the media resource is to pause when the end of the range to which the cue applies is reached.
Additional fields, as needed for the format. For example, WebVTT has a text track cue writing direction and so forth. [WEBVTT]
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:
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]
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]
A media-resource-specific text track is a text track that corresponds to data found in the media resource.
Rules for processing and rendering such data are defined by the relevant specifications, e.g. the specification of the video format if the media resource is a video.
When a media resource contains data that the user agent recognises and supports as being equivalent to a text track, the user agent runs the steps to expose a media-resource-specific text track with the relevant data, as follows.
Associate the relevant data with a new text track and its corresponding new
TextTrack
object. The text track is a media-resource-specific
text track.
Set the new text track's kind, label, and language based on the semantics of the relevant data, as defined for the relevant format [INBANDTRACKS]. If there is no label in that data, then the label must be set to the empty string.
Associate the text track list of cues with the rules for updating the text track rendering appropriate for the format in question.
If the new text track's kind is metadata
, then set the text track in-band
metadata track dispatch type as follows, based on the type of the media
resource:
CodecID
element. [WEBMCG]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]
Populate the new text track's list of cues with the cues parsed so far, following the guidelines for exposing cues, and begin updating it dynamically as necessary.
Set the new text track's readiness state to loaded.
Set the new text track's mode to the mode consistent with the user's preferences and the requirements of the relevant specification for the data.
Add the new text track to the media element's list of text tracks.
Fire a trusted event with the name addtrack
, that does not bubble and is not cancelable, and that uses
the TrackEvent
interface, with the track
attribute initialized to the text track's TextTrack
object, at the
media element's textTracks
attribute's
TextTrackList
object.
When a track
element is created, it must be associated with a new text
track (with its value set as defined below) and its corresponding new
TextTrack
object.
The text track kind is determined from the state of the element's kind
attribute according to the following table; for a state given
in a cell of the first column, the kind is the string given
in the second column:
State | String |
---|---|
Subtitles | subtitles
|
Captions | captions
|
Descriptions | descriptions
|
Chapters | chapters
|
Metadata | metadata
|
The text track label is the element's track label.
The text track language is the element's track language, if any, or the empty string otherwise.
As the kind
, label
,
and srclang
attributes are set, changed, or removed, the
text track must update accordingly, as per the definitions above.
Changes to the track URL are handled in the algorithm below.
The text track readiness state is initially not loaded, and the text track mode is initially disabled.
The text track list of cues is initially empty. It is dynamically modified when the referenced file is parsed. Associated with the list are the rules for updating the text track rendering appropriate for the format in question; for WebVTT, this is the rules for updating the display of WebVTT text tracks. [WEBVTT]
When a track
element's parent element changes and the new parent is a media
element, then the user agent must add the track
element's corresponding
text track to the media element's list of text tracks, and
then queue a task to fire a trusted event with the name addtrack
, that does not bubble and is not cancelable, and that uses
the TrackEvent
interface, with the track
attribute initialized to the text track's TextTrack
object, at the
media element's textTracks
attribute's
TextTrackList
object.
When a track
element's parent element changes and the old parent was a media
element, then the user agent must remove the track
element's corresponding
text track from the media element's list of text tracks,
and then queue a task to fire a trusted event with the name removetrack
, that does not bubble and is not cancelable, and that
uses the TrackEvent
interface, with the track
attribute initialized to the text track's
TextTrack
object, at the media element's textTracks
attribute's TextTrackList
object.
When a text track corresponding to a track
element is added to a
media element's list of text tracks, the user agent must queue a
task to run the following steps for the media element:
If the element's blocked-on-parser flag is true, abort these steps.
If the element's did-perform-automatic-track-selection flag is true, abort these steps.
Honor user preferences for automatic text track selection for this element.
When the user agent is required to honor user preferences for automatic text track selection for a media element, the user agent must run the following steps:
Perform automatic text track selection for subtitles
and captions
.
If there are any text tracks in the media
element's list of text tracks whose text track kind is metadata
that correspond to track
elements with a default
attribute set whose text
track mode is set to disabled, then set the
text track mode of all such tracks to hidden
Set the element's did-perform-automatic-track-selection flag to true.
When the steps above say to perform automatic text track selection for one or more text track kinds, it means to run the following steps:
Let candidates be a list consisting of the text tracks in the media element's list of text tracks whose text track kind is one of the kinds that were passed to the algorithm, if any, in the order given in the list of text tracks.
If candidates is empty, then abort these steps.
If any of the text tracks in candidates have a text track mode set to showing, abort these steps.
If the user has expressed an interest in having a track from candidates enabled based on its text track kind, text track language, and text track label, then set its text track mode to showing.
For example, the user could have set a browser preference to the effect of "I want French captions whenever possible", or "If there is a subtitle track with 'Commentary' in the title, enable it", or "If there are audio description tracks available, enable one, ideally in Swiss German, but failing that in Standard Swiss German or Standard German".
Otherwise, if there are any text tracks in candidates that correspond to track
elements with a default
attribute set whose text track mode is
set to disabled, then set the text track
mode of the first such track to showing.
When a text track corresponding to a track
element experiences any of
the following circumstances, the user agent must start the track
processing
model for that text track and its track
element:
track
element is created.track
element's parent element changes and the new parent is a media
element.When a user agent is to start the track
processing model for a
text track and its track
element, it must run the following algorithm.
This algorithm interacts closely with the event loop mechanism; in particular, it has
a synchronous section (which is triggered as part of the event loop
algorithm). The steps in that section are marked with ⌛.
If another occurrence of this algorithm is already running for this text
track and its track
element, abort these steps, letting that other algorithm
take care of this element.
If the text track's text track mode is not set to one of hidden or showing, abort these steps.
If the text track's track
element does not have a media
element as a parent, abort these steps.
Run the remainder of these steps asynchronously, allowing whatever caused these steps to run to continue.
Top: Await a stable state. The synchronous section consists of the following steps. (The steps in the synchronous section are marked with ⌛.)
⌛ Set the text track readiness state to loading.
⌛ If the track
element's parent is a media element then
let CORS mode be the state of the parent media element's crossorigin
content attribute. Otherwise, let CORS mode be No CORS.
End the synchronous section, continuing the remaining steps asynchronously.
If URL is not the empty string, perform a potentially CORS-enabled
fetch of URL, with the mode being CORS mode, the origin being the origin of the
track
element's Document
, and the default origin behaviour set
to fail.
The resource obtained in this fashion, if any, contains the text track data. If any data is obtained, it is by definition CORS-same-origin (cross-origin resources that are not suitably CORS-enabled do not get this far).
The tasks queued by the fetching algorithm on the networking task source to process the data as it is being fetched must determine the type of the resource. If the type of the resource is not a supported text track format, the load will fail, as described below. Otherwise, the resource's data must be passed to the appropriate parser (e.g. the WebVTT parser) as it is received, with the text track list of cues being used for that parser's output. [WEBVTT]
The appropriate parser will synchronously (during these networking task source tasks) and incrementally (as each such task is run with whatever data has been received from the network) update the text track list of cues.
This specification does not currently say whether or how to check the MIME types of text tracks, or whether or how to perform file type sniffing using the actual file data. Implementors differ in their intentions on this matter and it is therefore unclear what the right solution is. In the absence of any requirement here, the HTTP specification's strict requirement to follow the Content-Type header prevails ("Content-Type specifies the media type of the underlying data." ... "If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource.").
If the fetching algorithm fails for any reason (network error, the server returns an error code, a cross-origin check fails, etc), if URL is the empty string, or if the type of the resource is not a supported text track format, then run these steps:
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.
Wait until the text track readiness state is no longer set to loading.
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.
Jump to the step labeled top.
If the fetching algorithm does not fail, then the final task that is queued by the networking task source must run the following steps after it has tried to parse the data:
Change the text track readiness state to loaded.
If the file was successfully processed, fire a simple event named load
at the track
element.
Otherwise, the file was not successfully processed (e.g. the format in question is an XML
format and the file contained a well-formedness error that the XML specification requires be
detected and reported to the application); fire a simple event named error
at the track
element.
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.
Jump back to the step labeled top.
If, while the fetching algorithm is active, either:
...then the user agent must run the following steps:
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).
Jump back to the step labeled top.
Until one of the above circumstances occurs, the user agent must remain on this step.
Whenever a track
element has its src
attribute
set, changed, or removed, the user agent must synchronously empty the element's text
track's text track list of cues. (This also causes the algorithm above to stop
adding cues from the resource being obtained using the previously given URL, if any.)
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:
Should be set to the empty string if the format has no obvious analogue to a per-cue identifier.
Should be set to false.
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; };
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.
textTracks[
n ]
Returns the TextTrack
object representing the nth text track in the media element's list of text tracks.
textTracks
. getTrackById
( id )Returns the TextTrack
object with the given identifier, or null if no track has that identifier.
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; };
addTextTrack
( kind [, label [, language ] ] )Creates and returns a new TextTrack
object, which is also added to the
media element's list of text tracks.
kind
Returns the text track kind string.
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).
language
Returns the text track language string.
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.
inBandMetadataTrackDispatchType
Returns the text track in-band metadata track dispatch type string.
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.
cues
Returns the text track list of cues, as a TextTrackCueList
object.
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.
addCue
( cue )Adds the given cue to textTrack's text track list of cues.
removeCue
( cue )Removes the given cue from textTrack's text track list of cues.
The addTextTrack(kind, label, language)
method of media elements, when invoked, must run the following steps:
Create a new TextTrack
object.
Create a new text track corresponding to the new object, and set its text track kind to kind, its text track label to label, its text track language to language, its text track readiness state to the text track loaded state, its text track mode to the text track hidden mode, and its text track list of cues to an empty list.
Initially, the text track list of cues is not associated with any rules for updating the text track rendering. When a text track cue is added to it, the text track list of cues has its rules permanently set accordingly.
Add the new text track to the media element's list of text tracks.
Queue a task to fire a trusted event with the name addtrack
, that does not bubble and is not cancelable, and
that uses the TrackEvent
interface, with the track
attribute initialized to the new text
track's TextTrack
object, at the media element's textTracks
attribute's TextTrackList
object.
Return the new TextTrack
object.
The kind
attribute must return the
text track kind of the text track that the TextTrack
object
represents.
The label
attribute must return the
text track label of the text track that the TextTrack
object represents.
The language
attribute must return the
text track language of the text track that the TextTrack
object represents.
The id
attribute returns the track's
identifier, if it has one, or the empty string otherwise. For tracks that correspond to
track
elements, the track's identifier is the value of the element's id
attribute, if any. For in-band tracks, the track's identifier is
specified by the media resource. If the media resource is in a format
that supports the Media Fragments URI fragment identifier syntax, the identifier
returned for a particular track must be the same identifier that would enable the track if used as
the name of a track in the track dimension of such a fragment identifier. [MEDIAFRAG]
The inBandMetadataTrackDispatchType
attribute must return the text track in-band metadata track dispatch type of the
text track that the TextTrack
object represents.
The mode
attribute, on getting, must return
the string corresponding to the text track mode of the text track that
the TextTrack
object represents, as defined by the following list:
disabled
"hidden
"showing
"On setting, if the new value isn't equal to what the attribute would currently return, the new value must be processed as follows:
disabled
"Set the text track mode of the text track that the
TextTrack
object represents to the text track disabled mode.
hidden
"Set the text track mode of the text track that the
TextTrack
object represents to the text track hidden mode.
showing
"Set the text track mode of the text track that the
TextTrack
object represents to the text track showing mode.
If the text track mode of the text track that the
TextTrack
object represents is not the text track disabled mode, then
the cues
attribute must return a
live TextTrackCueList
object that represents the subset of the
text track list of cues of the text track that the
TextTrack
object represents whose end
times occur at or after the earliest possible position when the script
started, in text track cue order. Otherwise, it must return null. When an
object is returned, the same object must be returned each time.
The earliest possible position when the script started is whatever the earliest possible position was the last time the event loop reached step 1.
If the text track mode of the text track that the
TextTrack
object represents is not the text track disabled mode, then
the activeCues
attribute must return a
live TextTrackCueList
object that represents the subset of the
text track list of cues of the text track that the
TextTrack
object represents whose active flag was set when the script
started, in text track cue order. Otherwise, it must return null. When an
object is returned, the same object must be returned each time.
A text track cue's active flag was set when the script started if its text track cue active flag was set the last time the event loop reached step 1.
The addCue(cue)
method
of TextTrack
objects, when invoked, must run the following steps:
If the text track list of cues does not yet have any associated rules for updating the text track rendering, then associate the text track list of cues with the rules for updating the text track rendering appropriate to cue.
If text track list of cues' associated rules for updating the text
track rendering are not the same rules for updating the text track rendering
as appropriate for cue, then throw an InvalidStateError
exception and abort these steps.
If the given cue is in a text track list of cues, then remove cue from that text track list of cues.
Add cue to the method's TextTrack
object's text
track's text track list of cues.
The removeCue(cue)
method of TextTrack
objects, when invoked, must run the following steps:
If the given cue is not currently listed in the method's
TextTrack
object's text track's text track list of cues,
then throw a NotFoundError
exception and abort these steps.
Remove cue from the method's TextTrack
object's
text track's text track list of cues.
In this example, an audio
element is used to play a specific sound-effect from a
sound file containing many sound effects. A cue is used to pause the audio, so that it ends
exactly at the end of the clip, even if the browser is busy running some script. If the page had
relied on script to pause the audio, then the start of the next clip might be heard if the
browser was not able to run the script at the exact time specified.
var sfx = new Audio('sfx.wav'); var sounds = sfx.addTextTrack('metadata'); // add sounds we care about function addFX(start, end, name) { var cue = new VTTCue(start, end, ''); cue.id = name; cue.pauseOnExit = true; sounds.addCue(cue); } addFX(12.783, 13.612, 'dog bark'); addFX(13.612, 15.091, 'kitten mew')) function playSound(id) { sfx.currentTime = sounds.getCueById(id).startTime; sfx.play(); } // play a bark as soon as we can sfx.oncanplaythrough = function () { playSound('dog bark'); } // meow when the user tries to leave window.onbeforeunload = function () { playSound('kitten mew'); return 'Are you sure you want to leave this awesome page?'; }
interface TextTrackCueList { readonly attribute unsigned long length; getter TextTrackCue (unsigned long index); TextTrackCue? getCueById(DOMString id); };
length
Returns the number of cues in the list.
Returns the text track cue with index index in the list. The cues are sorted in text track cue order.
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; };
Returns the TextTrack
object to which this
text track cue belongs, if any, or null
otherwise.
Returns the text track cue identifier.
Can be set.
Returns the text track cue start time, in seconds.
Can be set.
Returns the text track cue end time, in seconds.
Can be set.
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.
Chapters are segments of a media resource with a given title. Chapters can be nested, in the same way that sections in a document outline can have subsections.
Each text track cue in a text track being used for describing chapters has three key features: the text track cue start time, giving the start time of the chapter, the text track cue end time, giving the end time of the chapter, and the text track cue data giving the chapter title.
The rules for constructing the chapter tree from a text track are as follows. They produce a potentially nested list of chapters, each of which have a start time, end time, title, and a list of nested chapters. This algorithm discards cues that do not correctly nest within each other, or that are out of order.
Let list be a copy of the list of cues of the text track being processed.
Remove from list any text track cue whose text track cue end time is before its text track cue start time.
Let output be an empty list of chapters, where a chapter is a record consisting of a start time, an end time, a title, and a (potentially empty) list of nested chapters. For the purpose of this algorithm, each chapter also has a parent chapter.
Let current chapter be a stand-in chapter whose start time is negative infinity, whose end time is positive infinity, and whose list of nested chapters is output. (This is just used to make the algorithm easier to describe.)
Loop: If list is empty, jump to the step labeled end.
Let current cue be the first cue in list, and then remove it from list.
If current cue's text track cue start time is less than the start time of current chapter, then return to the step labeled loop.
While current cue's text track cue start time is greater than or equal to current chapter's end time, let current chapter be current chapter's parent chapter.
If current cue's text track cue end time is greater than the end time of current chapter, then return to the step labeled loop.
Create a new chapter new chapter, whose start time is current cue's text track cue start time, whose end time is current cue's text track cue end time, whose title is current cue's text track cue data interpreted according to its rules for rendering the cue in isolation, and whose list of nested chapters is empty.
For WebVTT, the rules for rendering the cue in isolation are the rules for interpreting WebVTT cue text. [WEBVTT]
Append new chapter to current chapter's list of nested chapters, and let current chapter be new chapter's parent.
Let current chapter be new chapter.
Return to the step labeled loop.
End: Return output.
The following snippet of a WebVTT file shows how nested chapters can be marked up. The file describes three 50-minute chapters, "Astrophysics", "Computational Physics", and "General Relativity". The first has three subchapters, the second has four, and the third has two. [WEBVTT]
WEBVTT 00:00:00.000 --> 00:50:00.000 Astrophysics 00:00:00.000 --> 00:10:00.000 Introduction to Astrophysics 00:10:00.000 --> 00:45:00.000 The Solar System 00:00:00.000 --> 00:10:00.000 Coursework Description 00:50:00.000 --> 01:40:00.000 Computational Physics 00:50:00.000 --> 00:55:00.000 Introduction to Programming 00:55:00.000 --> 01:30:00.000 Data Structures 01:30:00.000 --> 01:35:00.000 Answers to Last Exam 01:35:00.000 --> 01:40:00.000 Coursework Description 01:40:00.000 --> 02:30:00.000 General Relativity 01:40:00.000 --> 02:00:00.000 Tensor Algebra 02:00:00.000 --> 02:30:00.000 The General Relativistic Field Equations
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
|
The controls
attribute is a boolean
attribute. If present, it indicates that the author has not provided a scripted controller
and would like the user agent to provide its own set of controls.
If the attribute is present, or if scripting is disabled for the media element, then the user agent should expose a user interface to the user. This user interface should include features to begin playback, pause playback, seek to an arbitrary position in the content (if the content supports arbitrary seeking), change the volume, change the display of closed captions or embedded sign-language tracks, select different audio tracks or turn on audio descriptions, and show the media content in manners more suitable to the user (e.g. full-screen video or in an independent resizable window). Other controls may also be made available.
If the media element has a current media controller, then the user
agent should expose audio tracks from all the slaved media elements (although
avoiding duplicates if the same media resource is being used several times). If a
media resource's audio track exposed in this way has no known name, and it is the
only audio track for a particular media element, the user agent should use the
element's title
attribute, if any, as the name (or as part of the
name) of that track.
Even when the attribute is absent, however, user agents may provide controls to affect playback
of the media resource (e.g. play, pause, seeking, and volume controls), but such features should
not interfere with the page's normal rendering. For example, such features could be exposed in the
media element's context menu. The user agent may implement this simply by exposing a user interface to the user as
described above (as if the controls
attribute was
present).
If the user agent exposes a user interface to
the user by displaying controls over the media element, then the user agent
should suppress any user interaction events while the user agent is interacting with this
interface. (For example, if the user clicks on a video's playback control, mousedown
events and so forth would not simultaneously be fired at
elements on the page.)
Where possible (specifically, for starting, stopping, pausing, and unpausing playback, for seeking, for changing the rate of playback, for fast-forwarding or rewinding, for listing, enabling, and disabling text tracks, and for muting or changing the volume of the audio), user interface features exposed by the user agent must be implemented in terms of the DOM API described above, so that, e.g., all the same events fire.
When a media element has a current media controller, the user agent's
user interface for pausing and unpausing playback, for seeking, for changing the rate of playback,
for fast-forwarding or rewinding, and for muting or changing the volume of audio of the entire
group must be implemented in terms of the MediaController
API exposed on that
current media controller. When a media element has a current media
controller, and all the slaved media elements of that
MediaController
are paused, the user agent should also unpause all the slaved
media elements when the user invokes a user agent interface control for beginning
playback.
The "play" function in the user agent's interface must set the playbackRate
attribute to the value of the defaultPlaybackRate
attribute before invoking the play()
method. When a media element has a current media controller, the
attributes and method with those names on that MediaController
object must be used.
Otherwise, the attributes and method with those names on the media element itself
must be used.
Features such as fast-forward or rewind must be implemented by only changing the playbackRate
attribute (and not the defaultPlaybackRate
attribute). Again, when a media element has a current media controller,
the attributes with those names on that MediaController
object must be used;
otherwise, the attributes with those names on the media element itself must be used.
When a media element has a current media controller, seeking must be
implemented in terms of the currentTime
attribute on that MediaController
object. Otherwise, the user agent must directly
seek to the requested position in the media
element's media timeline. For media resources where seeking to an arbitrary
position would be slow, user agents are encouraged to use the approximate-for-speed flag
when seeking in response to the user manipulating an approximate position interface such as a seek
bar.
When a media element has a current media controller, user agents may
additionally provide the user with controls that directly manipulate an individual media
element without affecting the MediaController
, but such features are
considered relatively advanced and unlikely to be useful to most users.
The activation behavior of a media element that is exposing a user interface to the user must be to run the following steps:
If the media element has a current media controller, and that
current media controller is a restrained media controller, then invoke
the play()
method of the
MediaController
.
Otherwise, if the media element has a current media controller,
and that current media controller is a paused media controller, then
invoke the unpause()
method of the
MediaController
.
Otherwise, if the media element has a current media controller,
then that current media controller is a playing media controller;
invoke the pause()
method of the
MediaController
.
Otherwise, the media element has no current media controller; if
the media element's paused
attribute is true,
then invoke the play()
method on the media
element.
Otherwise, the media element has no current media controller,
and the media element's paused
attribute is
false; invoke the pause()
method on the media
element.
For the purposes of listing chapters in the media resource, only text tracks in the media element's list of text tracks
that are showing and whose text track kind is
chapters
should be used. Such tracks must be
interpreted according to the rules for constructing the chapter tree from a text
track. When seeking in response to a user maniplating a chapter selection interface, user
agents should not use the approximate-for-speed flag.
The controls
IDL attribute must
reflect the content attribute of the same name.
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.
muted
[ = value ]Returns true if audio is muted, overriding the volume
attribute, and false if the volume
attribute is being
honored.
Can be set, to change whether the audio is muted or not.
The volume
attribute must return the playback
volume of any audio portions of the media element, in the range 0.0 (silent) to 1.0
(loudest). Initially, the volume should be 1.0, but user agents may remember the last set value
across sessions, on a per-site basis or otherwise, so the volume may start at other values. On
setting, if the new value is in the range 0.0 to 1.0 inclusive, the playback volume of any audio
portions of the media element must be set to the new value. If the new value is
outside the range 0.0 to 1.0 inclusive, then, on setting, an IndexSizeError
exception
must be thrown instead.
The muted
attribute must return true if the
audio output is muted and false otherwise. Initially, the audio output should not be muted
(false), but user agents may remember the last set value across sessions, on a per-site basis or
otherwise, so the muted state may start as muted (true). On setting, if the new value is true then
the audio output should be muted and if the new value is false it should be unmuted.
Whenever either of the values that would be returned by the volume
and muted
attributes
change, the user agent must queue a task to fire a simple event named
volumechange
at the media element.
An element's effective media volume is determined as follows:
If the user has indicated that the user agent is to override the volume of the element, then the element's effective media volume is the volume desired by the user. Abort these steps.
If the element's audio output is muted, the element's effective media volume is zero. Abort these steps.
If the element has a current media controller and that
MediaController
object's media controller mute override is true, the
element's effective media volume is zero. Abort these steps.
Let volume be the playback volume of the audio portions of the media element, in range 0.0 (silent) to 1.0 (loudest).
If the element has a current media controller, multiply volume by that MediaController
object's media controller volume
multiplier.
The element's effective media volume is volume, interpreted relative to the range 0.0 to 1.0, with 0.0 being silent, and 1.0 being the loudest setting, values in between increasing in loudness. The range need not be linear. The loudest setting may be lower than the system's loudest possible setting; for example the user could have set a maximum volume.
The muted
attribute on media elements is a boolean attribute that controls the default state
of the audio output of the media resource, potentially overriding user
preferences.
When a media element is created, if it has a muted
attribute specified, the user agent must mute the
media element's audio output, overriding any user preference.
The defaultMuted
IDL attribute must
reflect the muted
content attribute.
This attribute has no dynamic effect (it only controls the default state of the element).
This video (an advertisement) autoplays, but to avoid annoying users, it does so without sound, and allows the user to turn the sound on.
<video src="adverts.cgi?kind=video" controls autoplay loop muted></video>
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); };
length
Returns the number of ranges in the object.
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.
end
(index)Returns the time for the end of the range with the given index.
Throws an IndexSizeError
exception if the index is out of range.
The length
IDL attribute must return the
number of ranges represented by the object.
The start(index)
method must return the position of the start of the indexth range represented
by the object, in seconds measured from the start of the timeline that the object covers.
The end(index)
method
must return the position of the end of the indexth range represented by the
object, in seconds measured from the start of the timeline that the object covers.
These methods must throw IndexSizeError
exceptions if called with an index argument greater than or equal to the number of ranges represented by the
object.
When a TimeRanges
object is said to be a normalized TimeRanges
object, the ranges it represents must obey the following criteria:
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.
[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; };
track
Returns the track object (TextTrack
, AudioTrack
, or
VideoTrack
) to which the event relates.
The track
attribute must return the value
it was initialized to. When the object is created, this attribute must be initialized to null. It
represents the context information for the event.
This section is non-normative.
The following events fire on media elements as part of the processing model described above:
Event name | Interface | Fired when... | Preconditions |
---|---|---|---|
loadstart
| Event
| The user agent begins looking for media data, as part of the resource selection algorithm. | networkState equals NETWORK_LOADING
|
progress
| Event
| The user agent is fetching media data. | networkState equals NETWORK_LOADING
|
suspend
| Event
| The user agent is intentionally not currently fetching media data. | networkState equals NETWORK_IDLE
|
abort
| Event
| The user agent stops fetching the media data before it is completely downloaded, but not due to an error. | error is an object with the code MEDIA_ERR_ABORTED . networkState equals either NETWORK_EMPTY or NETWORK_IDLE , depending on when the download was aborted.
|
error
| Event
| An error occurs while fetching the media data. | error is an object with the code MEDIA_ERR_NETWORK or higher. networkState equals either NETWORK_EMPTY or NETWORK_IDLE , depending on when the download was aborted.
|
emptied
| Event
| A media element whose networkState
was previously not in the NETWORK_EMPTY state has
just switched to that state (either because of a fatal error during load that's about to be
reported, or because the load() method was invoked while
the resource selection algorithm was already
running).
| networkState is NETWORK_EMPTY ; all the IDL attributes are in their
initial states.
|
stalled
| Event
| The user agent is trying to fetch media data, but data is unexpectedly not forthcoming. | networkState is NETWORK_LOADING .
|
loadedmetadata
| Event
| The user agent has just determined the duration and dimensions of the media resource and the text tracks are ready. | readyState is newly equal to HAVE_METADATA or greater for the first time.
|
loadeddata
| Event
| The user agent can render the media data at the current playback position for the first time. | readyState newly increased to HAVE_CURRENT_DATA or greater for the first time.
|
canplay
| Event
| The user agent can resume playback of the media data, but estimates that if playback were to be started now, the media resource could not be rendered at the current playback rate up to its end without having to stop for further buffering of content. | readyState newly increased to HAVE_FUTURE_DATA or greater.
|
canplaythrough
| Event
| The user agent estimates that if playback were to be started now, the media resource could be rendered at the current playback rate all the way to its end without having to stop for further buffering. | readyState is newly equal to HAVE_ENOUGH_DATA .
|
playing
| Event
| Playback is ready to start after having been paused or delayed due to lack of media data. | readyState is newly equal to or greater than
HAVE_FUTURE_DATA and paused is false, or paused is newly false and readyState is equal to or greater than HAVE_FUTURE_DATA . Even if this event fires, the
element might still not be potentially playing, e.g. if the element is
blocked on its media controller (e.g. because the current media
controller is paused, or another slaved media
element is stalled somehow, or because the media resource has no data
corresponding to the media controller position), or the element is paused
for user interaction or paused for in-band content.
|
waiting
| Event
| Playback has stopped because the next frame is not available, but the user agent expects that frame to become available in due course. | readyState is equal to or less than HAVE_CURRENT_DATA , and paused is false. Either seeking is true, or the current playback position
is not contained in any of the ranges in buffered . It
is possible for playback to stop for other reasons without paused being false, but those reasons do not fire this event
(and when those situations resolve, a separate playing
event is not fired either): e.g. the element is newly blocked on its media
controller, or playback ended, or playback
stopped due to errors, or the element has paused for user interaction
or paused for in-band content.
|
seeking
| Event
| The seeking IDL attribute changed to true, and the user agent has started seeking to a new position.
| |
seeked
| Event
| The seeking IDL attribute changed to false after the current playback position was changed.
| |
ended
| Event
| Playback has stopped because the end of the media resource was reached. | currentTime equals the end of the media
resource; ended is true.
|
durationchange
| Event
| The duration attribute has just been updated.
| |
timeupdate
| Event
| The current playback position changed as part of normal playback or in an especially interesting way, for example discontinuously. | |
play
| Event
| The element is no longer paused. Fired after the play()
method has returned, or when the autoplay attribute
has caused playback to begin.
| paused is newly false.
|
pause
| Event
| The element has been paused. Fired after the pause()
method has returned.
| paused is newly true.
|
ratechange
| Event
| Either the defaultPlaybackRate or the
playbackRate attribute has just been updated.
| |
resize
| Event
| One or both of the videoWidth and videoHeight attributes have just been updated.
| Media element is a video element; readyState is not HAVE_NOTHING
|
volumechange
| Event
| Either the volume attribute or the muted attribute has changed. Fired after the relevant
attribute's setter has returned.
|
The following events fire on MediaController
objects:
Event name | Interface | Fired when... |
---|---|---|
emptied
| Event
| All the slaved media elements newly have readyState set to HAVE_NOTHING or greater, or there are no longer any
slaved media elements.
|
loadedmetadata
| Event
| All the slaved media elements newly have readyState set to HAVE_METADATA or greater.
|
loadeddata
| Event
| All the slaved media elements newly have readyState set to HAVE_CURRENT_DATA or greater.
|
canplay
| Event
| All the slaved media elements newly have readyState set to HAVE_FUTURE_DATA or greater.
|
canplaythrough
| Event
| All the slaved media elements newly have readyState set to HAVE_ENOUGH_DATA or greater.
|
playing
| Event
| The MediaController is no longer a blocked media controller.
|
ended
| Event
| The MediaController has reached the end of all the slaved media
elements.
|
waiting
| Event
| The MediaController is now a blocked media controller.
|
ended
| Event
| All the slaved media elements have newly ended playback. |
durationchange
| Event
| The duration attribute has just been
updated.
|
timeupdate
| Event
| The media controller position changed. |
play
| Event
| The paused attribute is newly false.
|
pause
| Event
| The paused attribute is newly true.
|
ratechange
| Event
| Either the defaultPlaybackRate
attribute or the playbackRate attribute
has just been updated.
|
volumechange
| Event
| Either the volume attribute or the muted attribute has just been updated.
|
The following events fire on AudioTrackList
, VideoTrackList
, and
TextTrackList
objects:
Event name | Interface | Fired when... |
---|---|---|
change
| Event
| One or more tracks in the track list has been enabled or disabled. |
addtrack
| TrackEvent
| A track has been added to the track list. |
removetrack
| TrackEvent
| A track has been removed from the track list. |
The main security and privacy implications of the video
and audio
elements come from the ability to embed media cross-origin. There are two directions that threats
can flow: from hostile content to a victim page, and from a hostile page to victim content.
If a victim page embeds hostile content, the threat is that the content might contain scripted
code that attempts to interact with the Document
that embeds the content. To avoid
this, user agents must ensure that there is no access from the content to the embedding page. In
the case of media content that uses DOM concepts, the embedded content must be treated as if it
was in its own unrelated top-level browsing context.
For instance, if an SVG animation was embedded in a video
element,
the user agent would not give it access to the DOM of the outer page. From the perspective of
scripts in the SVG resource, the SVG file would appear to be in a lone top-level browsing context
with no parent.
If a hostile page embeds victim content, the threat is that the embedding page could obtain
information from the content that it would not otherwise have access to. The API does expose some
information: the existence of the media, its type, its duration, its size, and the performance
characteristics of its host. Such information is already potentially problematic, but in practice
the same information can more or less be obtained using the img
element, and so it
has been deemed acceptable.
However, significantly more sensitive information could be obtained if the user agent further
exposes metadata within the content such as subtitles or chapter titles. Such information is
therefore only exposed if the video resource passes a CORS resource sharing check.
The crossorigin
attribute allows authors to control
how this check is performed. [CORS]
Without this restriction, an attacker could trick a user running within a corporate network into visiting a site that attempts to load a video from a previously leaked location on the corporation's intranet. If such a video included confidential plans for a new product, then being able to read the subtitles would present a serious confidentiality breach.
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.
This section is non-normative.
How accurately various aspects of the media element API are implemented is considered a quality-of-implementation issue.
For example, when implementing the buffered
attribute,
how precise an implementation reports the ranges that have been buffered depends on how carefully
the user agent inspects the data. Since the API reports ranges as times, but the data is obtained
in byte streams, a user agent receiving a variable-bit-rate stream might only be able to determine
precise times by actually decoding all of the data. User agents aren't required to do this,
however; they can instead return estimates (e.g. based on the average bit rate seen so far) which
get revised as more information becomes available.
As a general rule, user agents are urged to be conservative rather than optimistic. For example, it would be bad to report that everything had been buffered when it had not.
Another quality-of-implementation issue would be playing a video backwards when the codec is designed only for forward playback (e.g. there aren't many key frames, and they are far apart, and the intervening frames only have deltas from the previous frame). User agents could do a poor job, e.g. only showing key frames; however, better implementations would do more work and thus do a better job, e.g. actually decoding parts of the video forwards, storing the complete frames, and then playing the frames backwards.
Similarly, while implementations are allowed to drop buffered data at any time (there is no requirement that a user agent keep all the media data obtained for the lifetime of the media element), it is again a quality of implementation issue: user agents with sufficient resources to keep all the data around are encouraged to do so, as this allows for a better user experience. For example, if the user is watching a live stream, a user agent could allow the user only to view the live video; however, a better user agent would buffer everything and allow the user to seek through the earlier material, pause it, play it forwards and backwards, etc.
When multiple tracks are synchronised with a MediaController
, it is possible for
scripts to add and remove media elements from the MediaController
's list of
slaved media elements, even while these tracks are playing. How smoothly the media
plays back in such situations is another quality-of-implementation issue.
When a media element that is paused is removed from a document and not reinserted before the next time the event loop spins, implementations that are resource constrained are encouraged to take that opportunity to release all hardware resources (like video planes, networking resources, and data buffers) used by the media element. (User agents still have to keep track of the playback position and so forth, though, in case playback is later restarted.)
map
elementname
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.
areas
Returns an HTMLCollection
of the area
elements in the
map
.
images
Returns an HTMLCollection
of the img
and object
elements that use the map
.
The areas
attribute must return an
HTMLCollection
rooted at the map
element, whose filter matches only
area
elements.
The images
attribute must return an
HTMLCollection
rooted at the Document
node, whose filter matches only
img
and object
elements that are associated with this map
element according to the image map processing model.
The IDL attribute name
must reflect
the content attribute of the same name.
Image maps can be defined in conjunction with other content on the page, to ease maintenance. This example is of a page with an image map at the top of the page and a corresponding set of text links at the bottom.
<!DOCTYPE HTML> <TITLE>Babies™: Toys</TITLE> <HEADER> <H1>Toys</H1> <IMG SRC="/images/menu.gif" ALT="Babies™ navigation menu. Select a department to go to its page." USEMAP="#NAV"> </HEADER> ... <FOOTER> <MAP NAME="NAV"> <P> <A HREF="/clothes/">Clothes</A> <AREA ALT="Clothes" COORDS="0,0,100,50" HREF="/clothes/"> | <A HREF="/toys/">Toys</A> <AREA ALT="Toys" COORDS="0,0,100,50" HREF="/toys/"> | <A HREF="/food/">Food</A> <AREA ALT="Food" COORDS="0,0,100,50" HREF="/food/"> | <A HREF="/books/">Books</A> <AREA ALT="Books" COORDS="0,0,100,50" HREF="/books/"> </MAP> </FOOTER>
area
elementmap
element ancestor or a template
element ancestor.alt
coords
shape
href
target
download
rel
hreflang
type
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.
The activation behavior of area
elements is to run the following
steps:
If the area
element's Document
is not fully active,
then abort these steps.
If the area
element has a download
attribute and the algorithm is not allowed to show a popup, or the element's target
attribute is present and applying the rules
for choosing a browsing context given a browsing context name, using the value of the
target
attribute as the browsing context name, would
result in there not being a chosen browsing context, then run these substeps:
If there is an entry settings object, throw an InvalidAccessError
exception.
Abort these steps without following the hyperlink.
Otherwise, the user agent must follow the
hyperlink or download the hyperlink created by
the area
element, if any, and as determined by the download
attribute and any expressed user
preference.
The IDL attributes alt
, coords
, target
, download
,
rel
,
hreflang
, and type
, each must reflect the respective
content attributes of the same name.
The IDL attribute shape
must
reflect the shape
content attribute.
The IDL attribute relList
must
reflect the rel
content attribute.
The area
element also supports the URLUtils
interface. [URL]
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.
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:
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>
If an img
element or an object
element representing an image has a
usemap
attribute specified, user agents must process it
as follows:
First, rules for parsing a hash-name reference to a map
element
must be followed. This will return either an element (the map) or
null.
If that returned null, then abort these steps. The image is not associated with an image map after all.
Otherwise, the user agent must collect all the area
elements that are
descendants of the map. Let those be the areas.
Having obtained the list of area
elements that form the image map (the areas), interactive user agents must process the list in one of two ways.
If the user agent intends to show the text that the img
element represents, then
it must use the following steps.
In user agents that do not support images, or that have images disabled,
object
elements cannot represent images, and thus this section never applies (the
fallback content is shown instead). The following steps therefore only apply to
img
elements.
Remove all the area
elements in areas that have no href
attribute.
Remove all the area
elements in areas that have no alt
attribute, or whose alt
attribute's value is the empty string, if there is another area
element in
areas with the same value in the href
attribute and with a non-empty alt
attribute.
Each remaining area
element in areas represents a
hyperlink. Those hyperlinks should all be made available to the user in a manner
associated with the text of the img
.
In this context, user agents may represent area
and img
elements
with no specified alt
attributes, or whose alt
attributes are the empty string or some other non-visible text, in a user-agent-defined fashion
intended to indicate the lack of suitable author-provided text.
If the user agent intends to show the image and allow interaction with the image to select
hyperlinks, then the image must be associated with a set of layered shapes, taken from the
area
elements in areas, in reverse tree order (so the last
specified area
element in the map is the bottom-most shape, and
the first element in the map, in tree order, is the top-most shape).
Each area
element in areas must be processed as follows to
obtain a shape to layer onto the image:
Find the state that the element's shape
attribute
represents.
Use the rules for parsing a list of integers to parse the element's coords
attribute, if it is present, and let the result be the
coords list. If the attribute is absent, let the coords
list be the empty list.
If the number of items in the coords list is less than the minimum number
given for the area
element's current state, as per the following table, then the
shape is empty; abort these steps.
State | Minimum number of items |
---|---|
Circle state | 3 |
Default state | 0 |
Polygon state | 6 |
Rectangle state | 4 |
Check for excess items in the coords list as per the entry in the
following list corresponding to the shape
attribute's
state:
If the shape
attribute represents the rectangle state, and the first number in the list is
numerically less than the third number in the list, then swap those two numbers around.
If the shape
attribute represents the rectangle state, and the second number in the list is
numerically less than the fourth number in the list, then swap those two numbers around.
If the shape
attribute represents the circle state, and the third number in the list is less than
or equal to zero, then the shape is empty; abort these steps.
Now, the shape represented by the element is the one described for the entry in the list
below corresponding to the state of the shape
attribute:
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.
The shape is a rectangle that exactly covers the entire image.
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]
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.
The math
element from the MathML namespace falls into the
embedded content, phrasing content, and flow content
categories for the purposes of the content models in this specification.
User agents must handle text other than inter-element whitespace found in MathML
elements whose content models do not allow straight text by pretending for the purposes of MathML
content models, layout, and rendering that that text is actually wrapped in an mtext
element in the MathML namespace. (Such text is not, however,
conforming.)
User agents must act as if any MathML element whose contents does not match the element's
content model was replaced, for the purposes of MathML layout and rendering, by an merror
element in the MathML namespace containing some appropriate
error message.
To enable authors to use MathML tools that only accept MathML in its XML form, interactive HTML user agents are encouraged to provide a way to export any MathML fragment as an XML namespace-well-formed XML fragment.
The semantics of MathML elements are defined by the MathML specification and other applicable specifications. [MATHML]
Here is an example of the use of MathML in an HTML document:
<!DOCTYPE html> <html> <head> <title>The quadratic formula</title> </head> <body> <h1>The quadratic formula</h1> <p> <math> <mi>x</mi> <mo>=</mo> <mfrac> <mrow> <mo form="prefix">−</mo> <mi>b</mi> <mo>±</mo> <msqrt> <msup> <mi>b</mi> <mn>2</mn> </msup> <mo>−</mo> <mn>4</mn> <mo></mo> <mi>a</mi> <mo></mo> <mi>c</mi> </msqrt> </mrow> <mrow> <mn>2</mn> <mo></mo> <mi>a</mi> </mrow> </mfrac> </math> </p> </body> </html>
The svg
element from the SVG namespace falls into the
embedded content, phrasing content, and flow content
categories for the purposes of the content models in this specification.
To enable authors to use SVG tools that only accept SVG in its XML form, interactive HTML user agents are encouraged to provide a way to export any SVG fragment as an XML namespace-well-formed XML fragment.
When the SVG foreignObject
element contains elements from the HTML
namespace, such elements must all be flow content. [SVG]
The content model for title
elements in the SVG namespace
inside HTML documents is phrasing content. (This further constrains the
requirements given in the SVG specification.)
The semantics of SVG elements are defined by the SVG specification and other applicable specifications. [SVG]
The SVG specification includes requirements regarding the handling of elements in the DOM that
are not in the SVG namespace, that are in SVG fragments, and that are not included in a foreignObject
element. This specification does not define any processing
for elements in SVG fragments that are not in the HTML namespace; they are considered neither
conforming nor non-conforming from the perspective of this specification.
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.