4.8.5. The Param Element - Categories:
- None.
- Contexts in which this element can be used:
- As a child of an
object element, before any flow content. - Content model:
- Empty.
- Content attributes:
- Global attributes
namevalue- DOM interface:
interface HTMLParamElement : HTMLElement { attribute DOMString name; attribute DOMString value;}; The param element defines parameters for plugins invoked by object elements. It does not represent anything on its own. The name attribute gives the name of the parameter. The value attribute gives the value of the parameter. Both attributes must be present. They may have any value. If both attributes are present, and if the parent element of the param is an object element, then the element defines a parameter with the given name-value pair. If either the name or value of a parameter defined by a param element that is the child of an object element that represents an instantiated plugin changes, and if that plugin is communicating with the user agent using an API that features the ability to update the plugin when the name or value of a parameter so changes, then the user agent must appropriately exercise that ability to notify the plugin of the change. The IDL attributes name and value must both reflect the respective content attributes of the same name. The following example shows how the param element can be used to pass a parameter to a plugin, in this case the O3D plugin. <!DOCTYPE HTML><html lang="en"> <head> <title>O3D Utah Teapot</title> </head> <body> <p> <object type="application/vnd.o3d.auto"> <param name="o3d_features" value="FloatingPointTextures"> <img src="o3d-teapot.png" title="3D Utah Teapot illustration rendered using O3D." alt="When O3D renders the Utah Teapot, it appears as a squat teapot with a shiny metallic finish on which the surroundings are reflected, with a faint shadow caused by the lighting."> <p>To see the teapot actually rendered by O3D on your computer, please download and install the <a href="http://code.google.com/apis/o3d/docs/gettingstarted.html#install">O3D plugin</a>.</p> </object> <script src="o3d-teapot.js"></script> </p> </body></html> 4.8.6 The video element - Categories:
- Flow content.
- Phrasing content.
- Embedded content.
- If the element has a
controls attribute: Interactive content. - Palpable content.
- Contexts in which this element can be used:
- Where embedded content is expected.
- Content model:
- If the element has a
src attribute: zero or more track elements, then transparent, but with no media element descendants. - If the element does not have a
src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants. - Content attributes:
- Global attributes
srccrossoriginposterpreloadautoplaymediagrouploopmutedcontrolswidthheight- DOM interface:
-
interface HTMLVideoElement : HTMLMediaElement { attribute unsigned long width; attribute unsigned long height; readonly attribute unsigned long videoWidth; readonly attribute unsigned long videoHeight; attribute DOMString poster;}; A video element is used for playing videos or movies, and audio files with captions. Content may be provided inside the video element. User agents should not show this content to the user; it is intended for older Web browsers which do not support video, so that legacy video plugins can be tried, or to show text to the users of these older browsers informing them of how to access the video contents. In particular, this content is not intended to address accessibility concerns. To make video content accessible to the partially sighted, the blind, the hard-of-hearing, the deaf, and those with other physical or cognitive disabilities, a variety of features are available. Captions can be provided, either embedded in the video stream or as external files using the track element. Sign-language tracks can be provided, again either embedded in the video stream or by synchronizing multiple video elements using the mediagroup attribute or a MediaController object. Audio descriptions can be provided, either as a separate track embedded in the video stream, or a separate audio track in an audio element slaved to the same controller as the video element(s), or in text form using a caption 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: 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.
When no video data is available (the element's readyState attribute is either HAVE_NOTHING, or HAVE_METADATA but no video data has yet been obtained at all, or the element's readyState attribute is any subsequent value but the media resource does not have a video channel), the video element represents the poster frame. When a video element is paused and the current playback position is the first frame of video, the element represents the poster frame, unless a frame of video has already been shown, in which case the element represents the frame of video corresponding to the current playback position. When a video element is paused at any other position, and the media resource has a video channel, the element represents the frame of video corresponding to the current playback position, or, if that is not yet available (e.g. because the video is seeking or buffering), the last frame of the video to have been rendered. When a video element whose media resource has a video channel is potentially playing, it 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. Similarly, 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. When a video element whose media resource has a video channel is neither potentially playing nor paused (e.g. when seeking or stalled), the element represents the last frame of the video to have been 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. In addition to the above, the user agent may provide messages to the user (such as "buffering", "no video loaded", "error", or more detailed information) by overlaying text or icons on the video or other areas of the element's playback area, or in another appropriate manner. User agents that cannot render the video may instead make the element represent a link to an external video playback utility or to the video data itself. When a video element's media resource has a video channel, the element provides a paint source whose width is the media resource's intrinsic width, whose height is the media resource's intrinsic height, and whose appearance is the frame of video corresponding to the current playback position, if that is available, or else (e.g. when the video is seeking or buffering) its previous appearance, if any, or else (e.g. because the video is still loading the first frame) blackness.
- video .
videoWidth - video .
videoHeight -
These attributes return the intrinsic dimensions of the video, or zero if the dimensions are not known. The intrinsic width and intrinsic height of the media resource are the dimensions of the resource in CSS pixels after taking into account the resource's dimensions, aspect ratio, clean aperture, resolution, and so forth, as defined for the format used by the resource. If an anamorphic format does not define how to apply the aspect ratio to the video data's dimensions to obtain the "correct" dimensions, then the user agent must apply the ratio by increasing one dimension and leaving the other unchanged. The videoWidth IDL attribute must return the intrinsic width of the video in CSS pixels. The videoHeight IDL attribute must return the intrinsic height of the video in CSS pixels. If the element's readyState attribute is HAVE_NOTHING, then the attributes must return 0. 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 video resource, if that is available; otherwise it is the intrinsic width of the poster frame, if that is available; otherwise it is 300 CSS pixels. The intrinsic height of a video element's playback area is the intrinsic height of the video resource, if that is available; otherwise it is the intrinsic height of the poster frame, if that is available; otherwise it is 150 CSS pixels.
User agents should provide controls to enable or disable the display of closed captions, audio description tracks, and other additional data associated with the video stream, though such features should, again, not interfere with the page's normal rendering. User agents may allow users to view the video content in manners more suitable to the user (e.g. full-screen or in an independent resizable window). As for the other user interface features, controls to enable this should not interfere with the page's normal rendering unless the user agent is exposing a user interface. In such an independent context, however, user agents may make full user interfaces visible, with, e.g., play, pause, seeking, and volume controls, even if the controls attribute is absent. User agents may allow video playback to affect system features that could interfere with the user's experience; for example, user agents could disable screensavers while video playback is in progress.
The poster IDL attribute must reflect the poster content attribute. This example shows how to detect when a video has failed to play correctly: <script> function failed(e) { // video playback failed - show a message saying why switch (e.target.error.code) { case e.target.error.MEDIA_ERR_ABORTED: alert('You aborted the video playback.'); break; case e.target.error.MEDIA_ERR_NETWORK: alert('A network error caused the video download to fail part-way.'); break; case e.target.error.MEDIA_ERR_DECODE: alert('The video playback was aborted due to a corruption problem or because the video used features your browser did not support.'); break; case e.target.error.MEDIA_ERR_SRC_NOT_SUPPORTED: alert('The video could not be loaded, either because the server or network failed or because the format is not supported.'); break; default: alert('An unknown error occurred.'); break; } }</script><p><video src="tgif.vid" autoplay controls onerror="failed(event)"></video></p><p><a href="tgif.vid">Download the video file</a>.</p> 4.8.7 The audio element - Categories:
- Flow content.
- Phrasing content.
- Embedded content.
- If the element has a
controls attribute: Interactive content. - If the element has a
controls attribute: Palpable content. - Contexts in which this element can be used:
- Where embedded content is expected.
- Content model:
- If the element has a
src attribute: zero or more track elements, then transparent, but with no media element descendants. - If the element does not have a
src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants. - Content attributes:
- Global attributes
srccrossoriginpreloadautoplaymediagrouploopmutedcontrols- DOM interface:
-
[NamedConstructor=Audio(), NamedConstructor=Audio(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. - audio = new
Audio( [ url ] ) -
Returns a new audio element, with the src attribute set to the value passed in the argument, if applicable. Two constructors are provided for creating HTMLAudioElement objects (in addition to the factory methods from DOM Core such as createElement()): Audio() and Audio(src). When invoked as constructors, these must return a new HTMLAudioElement object (a new audio element). The element must have its preload attribute set to the literal value "auto". If the src argument is present, the object created must have its src content attribute set to the provided value, and the user agent must invoke the object's resource selection algorithm before returning. The element's document must be the active document of the browsing context of the Window object on which the interface object of the invoked constructor is found. 4.8.8 The source element - Categories:
- None.
- Contexts in which this element can be used:
- As a child of a media element, before any flow content or
track elements. - Content model:
- Empty.
- Content attributes:
- Global attributes
srctypemedia- DOM interface:
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. - H.264 Constrained baseline profile video (main and extended video compatible) level 3 and Low-Complexity AAC audio in MP4 container
<source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'> - H.264 Extended profile video (baseline-compatible) level 3 and Low-Complexity AAC audio in MP4 container
<source src='video.mp4' type='video/mp4; codecs="avc1.58A01E, mp4a.40.2"'> - H.264 Main profile video level 3 and Low-Complexity AAC audio in MP4 container
<source src='video.mp4' type='video/mp4; codecs="avc1.4D401E, mp4a.40.2"'> - H.264 'High' profile video (incompatible with main, baseline, or extended profiles) level 3 and Low-Complexity AAC audio in MP4 container
<source src='video.mp4' type='video/mp4; codecs="avc1.64001E, mp4a.40.2"'> - MPEG-4 Visual Simple Profile Level 0 video and Low-Complexity AAC audio in MP4 container
<source src='video.mp4' type='video/mp4; codecs="mp4v.20.8, mp4a.40.2"'> - MPEG-4 Advanced Simple Profile Level 0 video and Low-Complexity AAC audio in MP4 container
<source src='video.mp4' type='video/mp4; codecs="mp4v.20.240, mp4a.40.2"'> - MPEG-4 Visual Simple Profile Level 0 video and AMR audio in 3GPP container
<source src='video.3gp' type='video/3gpp; codecs="mp4v.20.8, samr"'> - Theora video and Vorbis audio in Ogg container
<source src='video.ogv' type='video/ogg; codecs="theora, vorbis"'> - Theora video and Speex audio in Ogg container
<source src='video.ogv' type='video/ogg; codecs="theora, speex"'> - Vorbis audio alone in Ogg container
<source src='audio.ogg' type='audio/ogg; codecs=vorbis'> - Speex audio alone in Ogg container
<source src='audio.spx' type='audio/ogg; codecs=speex'> - FLAC audio alone in Ogg container
<source src='audio.oga' type='audio/ogg; codecs=flac'> - Dirac video and Vorbis audio in Ogg container
<source src='video.ogv' type='video/ogg; codecs="dirac, vorbis"'> The media attribute gives the intended media type of the media resource, to help the user agent determine if this media resource is useful to the user before fetching it. Its value must be a valid media query. The resource selection algorithm is defined in such a way that when the media attribute is omitted the user agent acts the same as if the value was "all", i.e. by default the media resource is suitable for all media. If the author isn't sure if the user agents will all be able to render the media resources provided, the author can listen to the error event on the last source element and trigger fallback behavior: <script> function fallback(video) { // replace <video> with its contents while (video.hasChildNodes()) { if (video.firstChild instanceof HTMLSourceElement) video.removeChild(video.firstChild); else video.parentNode.insertBefore(video.firstChild, video); } video.parentNode.removeChild(video); }</script><video controls autoplay> <source src='video.mp4' type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'> <source src='video.ogv' type='video/ogg; codecs="theora, vorbis"' onerror="fallback(parentNode)"> ...</video> 4.8.9 The track element - Categories:
- None.
- Contexts in which this element can be used:
- As a child of a media element, before any flow content.
- Content model:
- Empty.
- Content attributes:
- Global attributes
kindsrcsrclanglabeldefault- DOM interface:
interface HTMLTrackElement : HTMLElement { attribute DOMString kind; attribute DOMString src; attribute DOMString srclang; attribute DOMString label; attribute boolean default; const unsigned short NONE = 0; const unsigned short LOADING = 1; const unsigned short LOADED = 2; const unsigned short ERROR = 3; readonly attribute unsigned short readyState; readonly attribute TextTrack track;}; The track element allows authors to specify explicit external timed text tracks for media elements. It does not represent anything on its own. The kind attribute is an enumerated attribute. The following table lists the keywords defined for this attribute. The keyword given in the first cell of each row maps to the state given in the second cell. | Keyword | State | Brief description | subtitles | Subtitles | Transcription or translation of the dialogue, suitable for when the sound is available but not understood (e.g. because the user does not understand the language of the media resource's audio track). Overlaid on the video. | captions | Captions | Transcription or translation of the dialogue, sound effects, relevant musical cues, and other relevant audio information, suitable for when sound is unavailable or not clearly audible (e.g. because it is muted, drowned-out by ambient noise, or because the user is deaf). Overlaid on the video; labeled as appropriate for the hard-of-hearing. | descriptions | Descriptions | Textual descriptions of the video component of the media resource, intended for audio synthesis when the visual component is obscured, unavailable, or not usable (e.g. because the user is interacting with the application without a screen while driving, or because the user is blind). Synthesized as audio. | chapters | Chapters | Chapter titles, intended to be used for navigating the media resource. Displayed as an interactive (potentially nested) list in the user agent's interface. | metadata | Metadata | Tracks intended for use from script. Not displayed by the user agent. | The attribute may be omitted. The missing value default is the subtitles state. The src attribute gives the address of the text track data. The value must be a valid non-empty URL potentially surrounded by spaces. This attribute must be present. If the element has a src attribute whose value is not the empty string and whose value, when the attribute was set, could be successfully resolved relative to the element, then the element's track URL is the resulting absolute URL. Otherwise, the element's track URL is the empty string. If the element's track URL identifies a WebVTT resource, and the element's kind attribute is not in the metadata state, then the WebVTT file must be a WebVTT file using cue text. [WEBVTT] Furthermore, if the element's track URL identifies a WebVTT resource, and the element's kind attribute is in the chapters state, then the WebVTT file must be both a WebVTT file using chapter title text and a WebVTT file using only nested cues. [WEBVTT] The srclang attribute gives the language of the text track data. The value must be a valid BCP 47 language tag. This attribute must be present if the element's kind attribute is in the subtitles state. [BCP47] If the element has a srclang attribute whose value is not the empty string, then the element's track language is the value of the attribute. Otherwise, the element has no track language. The label attribute gives a user-readable title for the track. This title is used by user agents when listing subtitle, caption, and audio description tracks in their user interface. The value of the label attribute, if the attribute is present, must not be the empty string. Furthermore, there must not be two track element children of the same media element whose kind attributes are in the same state, whose srclang attributes are both missing or have values that represent the same language, and whose label attributes are again both missing or both have the same value. If the element has a label attribute whose value is not the empty string, then the element's track label is the value of the attribute. Otherwise, the element's track label is an empty string. The default attribute, if specified, indicates that the track is to be enabled if the user's preferences do not indicate that another track would be more appropriate. Each media element must have no more than one track element child whose kind attribute is in the subtitles or captions state and whose default attribute is specified. Each media element must have no more than one track element child whose kind attribute is in the description state and whose default attribute is specified. Each media element must have no more than one track element child whose kind attribute is in the chapters state and whose default attribute is specified. There is no limit on the number of track elements whose kind attribute is in the metadata state and whose default attribute is specified. - track .
readyState -
Returns the text track readiness state, represented by a number from the following list: - track .
NONE (0) -
The text track not loaded state. - track .
LOADING (1) -
The text track loading state. - track .
LOADED (2) -
The text track loaded state. - track .
ERROR (3) -
The text track failed to load state. - track .
track -
Returns the TextTrack object corresponding to the text track of the track element. 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="Fran�ais"> <track kind=subtitles src=brave.de.vtt srclang=de lang=de label="Deutsch"></video> Media elements (audio and video, in this specification) implement the following interface: 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(); DOMString canPlayType(DOMString type); // ready state const unsigned short HAVE_NOTHING = 0; const unsigned short HAVE_METADATA = 1; const unsigned short HAVE_CURRENT_DATA = 2; const unsigned short HAVE_FUTURE_DATA = 3; const unsigned short HAVE_ENOUGH_DATA = 4; readonly attribute unsigned short readyState; readonly attribute boolean seeking; // playback state attribute double currentTime; void fastSeek(double time); readonly attribute unrestricted double duration; readonly attribute Date startDate; 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(DOMString 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 specified, the task source for all the tasks queued in this section and its subsections is the media element event task source. 4.8.10.1 Error codes - media .
error -
Returns a MediaError object representing the current error state of the element. Returns null if there is no error. All media elements have an associated error status, which records the last error the element encountered since its resource selection algorithm was last invoked. The error attribute, on getting, must return the MediaError object created for this last error, or null if there has not been an error. interface MediaError { const unsigned short MEDIA_ERR_ABORTED = 1; const unsigned short MEDIA_ERR_NETWORK = 2; const unsigned short MEDIA_ERR_DECODE = 3; const unsigned short MEDIA_ERR_SRC_NOT_SUPPORTED = 4; readonly attribute unsigned short code;}; - media .
error . code -
Returns the current error's error code, from the list below. The code attribute of a MediaError object must return the code for the error, which must be one of the following: MEDIA_ERR_ABORTED (numeric value 1)- The fetching process for the media resource was aborted by the user agent at the user's request.
MEDIA_ERR_NETWORK (numeric value 2)- A network error of some description caused the user agent to stop fetching the media resource, after the resource was established to be usable.
MEDIA_ERR_DECODE (numeric value 3)- An error of some description occurred while decoding the media resource, after the resource was established to be usable.
MEDIA_ERR_SRC_NOT_SUPPORTED (numeric value 4)- The media resource indicated by the
src attribute was not suitable. 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. - media .
currentSrc -
Returns the address of the current media resource. Returns the empty string when there is no media resource. There are two ways to specify a media resource, the src attribute, or source elements. The attribute overrides the elements. 4.8.10.3 MIME types A media resource can be described in terms of its type, specifically a MIME type, in some cases with a codecs parameter. (Whether the codecs parameter is allowed or not depends on the MIME type.) [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. "application/octet-stream" is special-cased here; if any parameter appears with it, it should be treated just like any other MIME type. This is a deviation from the rule that unknown MIME type parameters should be ignored. - media .
canPlayType(type) -
Returns the empty string (a negative response), "maybe", or "probably" based on how confident the user agent is that it can play media resources of the given type. The canPlayType(type) method must return the empty string if type is a type that the user agent knows it cannot render or is the type "application/octet-stream"; it must return "probably" if the user agent is confident that the type represents a media resource that it can render if used in with this audio or video element; and it must return "maybe" otherwise. Implementors are encouraged to return "maybe" unless the type can be confidently established as being supported or not. Generally, a user agent should never return "probably" for a type that allows the codecs parameter if that parameter is not present. This script tests to see if the user agent supports a (fictional) new format to dynamically decide whether to use a video element or a plugin: <section id="video"> <p><a href="playing-cats.nfv">Download video</a></p></section><script> var videoSection = document.getElementById('video'); var videoElement = document.createElement('video'); var support = videoElement.canPlayType('video/x-new-fictional-format;codecs="kittens,bunnies"'); if (support != "probably" && "New Fictional Video Plugin" in navigator.plugins) { // not confident of browser support // but we have a plugin // so use plugin instead videoElement = document.createElement("embed"); } else if (support == "") { // no support from browser and no plugin // do nothing videoElement = null; } if (videoElement) { while (videoSection.hasChildNodes()) videoSection.removeChild(videoSection.firstChild); videoElement.setAttribute("src", "playing-cats.nfv"); videoSection.appendChild(videoElement); }</script> The type attribute of the source element allows the user agent to avoid downloading resources that use formats it cannot render. | 
