WebVTT API
Web Video Text Tracks (WebVTT) are text tracks providing specific text "cues" that are time-aligned with other media, such as video or audio tracks. The WebVTT API provides functionality to define and manipulate these text tracks. The WebVTT API is primarily used for displaying subtitles or captions that overlay with video content, but it has other uses: providing chapter information for easier navigation and generic metadata that needs to be time-aligned with audio or video content.
Concepts and usage
A text track is a container for time-aligned text data that can be played in parallel with a video or audio track to provide a translation, transcription, or overview of the content. A video or audio media element may define tracks of different kinds or in different languages, allowing users to display appropriate tracks based on their preferences or needs.
The different kinds of text data that can be specified are listed below. Note that browsers do not necessarily support all kinds of text tracks.
-
subtitles
provide a textual translation of spoken dialog. This is the default type of text track, and if used, the source language must be specified. -
captions
provide a transcription of spoken text, and may include information about other audio such as music or background noise. They are intended for hearing impaired users. chapters
provide high level navigation information, allowing users to more easily switch to relevant content.metadata
is used for any other kinds of time-aligned information.
The individual time-aligned units of text data within a track are referred to as "cues". Each cue has a start time, end time, and textual payload. It may also have "cue settings", which affect its display region, position, alignment, and/or size. Lastly, a cue may have a label, which can be used to select it for CSS styling.
A text track and cues can be defined in a file using the WebVTT File Format, and then associated with a particular <video>
element using the <track>
element.
Alternatively you can add a TextTrack
to a media element in JavaScript using HTMLMediaElement.addTextTrack()
, and then add individual VTTCue
objects to the track with TextTrack.addCue()
.
The ::cue
CSS pseudo-element can be used both in HTML and in a WebVTT file to style the cues for a particular element, for a particular tag within a cue, for a VTT class, or for a cue with a particular label.
The ::cue-region
pseudo-element is intended for styling cues in a particular region, but is not supported in any browser.
Most important WebVTT features can be accessed using either the file format or Web API.
Interfaces
VTTCue
-
Represents a cue, the text displayed in a particular timeslice of the text track associated with a media element.
VTTRegion
-
Represents a portion of a video element onto which a
VTTCue
can be rendered. TextTrack
-
Represents a text track, which holds the list of cues to display along with an associated media element at various points while it plays.
TextTrackCue
-
An abstract base class for various cue types, such as
VTTCue
. TextTrackCueList
-
An array-like object that represents a dynamically updating list of
TextTrackCue
objects. An instance of this type is obtained fromTextTrack.cues
in order to get all the cues in theTextTrack
object. TextTrackList
-
Represents a list of the text tracks defined for a media element, with each track represented by a separate
TextTrack
instance in the list.
Related interfaces
TrackEvent
-
Part of the HTML DOM API, this is the interface for the
addtrack
andremovetrack
events that are fired when a track is added or removed fromTextTrackList
(or more generally, when a track is added/removed from an HTML media element).
Related CSS extensions
These CSS pseudo-element are used to style cues in media with VTT tracks.
::cue
-
Matches cues within a selected element in media with VTT tracks.
Note: The specification defines another pseudo-element, ::cue-region
, but this is not supported by any browsers.
Examples
Using the WebVTT API to add captions
HTML
The following example adds a new TextTrack
to the video, then adds cues using TextTrack.addCue()
method calls, with constructed VTTCue
objects as arguments.
<video
controls
src="https://interactive-examples.mdn.mozilla.net/media/cc0-videos/friday.mp4"></video>
CSS
video {
width: 420px;
height: 300px;
}
JavaScript
let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";
track.addCue(new VTTCue(0, 0.9, "Hildy!"));
track.addCue(new VTTCue(1, 1.4, "How are you?"));
track.addCue(new VTTCue(1.5, 2.9, "Tell me, is the lord of the universe in?"));
track.addCue(new VTTCue(3, 4.2, "Yes, he's in - in a bad humor"));
track.addCue(new VTTCue(4.3, 6, "Somebody must've stolen the crown jewels"));
console.log(track.cues);
Result
Displaying VTT content defined in a file
This example demonstrates how to add the same set of captions to the video seen in the above Using the WebVTT API to add captions example. This time, however, we will do it declaratively using a <track>
element.
First, let's define the captions inside a "captions.vtt" file:
WEBVTT 00:00.000 --> 00:00.900 Hildy! 00:01.000 --> 00:01.400 How are you? 00:01.500 --> 00:02.900 Tell me, is the lord of the universe in? 00:03.000 --> 00:04.200 Yes, he's in - in a bad humor 00:04.300 --> 00:06.000 Somebody must've stolen the crown jewels
We can then add this to a <video>
element using the <track>
element.
The following HTML would result in the same text track as the previous example:
<video controls src="video.webm">
<track default kind="captions" src="captions.vtt" srclang="en" />
</video>
We can add multiple <track>
elements to specify different kinds of tracks in multiple languages, using the kind
and srclang
attributes. Note that, if kind
is specified, srclang
must be set too.
The default
attribute may be added to just one <track>
: this is the one that will be played if user preferences don't specify a particular language or kind.
<video controls src="video.webm">
<track default kind="captions" src="captions.vtt" srclang="en" />
<track kind="subtitles" src="subtitles.vtt" srclang="en" />
<track kind="descriptions" src="descriptions.vtt" srclang="en" />
<track kind="chapters" src="chapters_de.vtt" srclang="de" />
<track kind="subtitles" src="subtitles_en.vtt" srclang="en" />
</video>
Styling WebVTT in HTML or a stylesheet
You can style WebVTT cues by matching elements using the ::cue
pseudo-element.
This allows you to modify the appearance of all cue text, or just specific elements. In this example, we'll add some styling to the first example above.
Note: It is also possible to define styles in the WebVTT File Format.
HTML
The HTML for the video itself is the same as we saw previously:
<video
controls
src="https://interactive-examples.mdn.mozilla.net/media/cc0-videos/friday.mp4"></video>
CSS
First, we use the ::cue
pseudo-element to select all video text cues, giving them larger red and a gradient background.
video::cue {
font-size: 1.5rem;
background-image: linear-gradient(to bottom, yellow, lightyellow);
color: red;
}
We then use ::cue
to select text that has been marked up using the u
and b
elements and style them green and yellow, respectively.
video::cue(u) {
color: green;
}
video::cue(b) {
color: purple;
}
JavaScript
The JavaScript is the same as in the first example, except that we have marked up some of the cue text using <b>
(bold) and <u>
(underline) tags.
By default the marked text would be displayed as bold or underlined (depending on the tag) but we have used the ::cue
in the previous section to also style the text to be green and purple, respectively.
let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";
track.addCue(new VTTCue(0, 0.9, "Hildy!"));
track.addCue(new VTTCue(1, 1.4, "How are you?"));
track.addCue(
new VTTCue(1.5, 2.9, "Tell me, is the <u>lord of the universe</u> in?"),
);
track.addCue(new VTTCue(3, 4.2, "Yes, he's in - in a bad humor"));
track.addCue(
new VTTCue(4.3, 6, "Somebody must've <b>stolen</b> the crown jewels"),
);
console.log(track.cues);
Result
More cue styling examples
This example shows more examples of how you can mark up cue text with tags and then style them. The same markup and styles can be used in the WebVTT File Format.
The HTML and CSS for displaying the video itself is the same as for the first example above so here we only show the specific code for marking up and styling the text.
Styling by tag type
The first cue we create will be displayed for all 6 seconds of the video and display text marked up with b
, u
, i
and c
tags.
let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";
track.addCue(
new VTTCue(
0,
6,
"Styles: Normal <b>bold</b> <u>underlined</u> <i>italic</i> <c>class</c>",
),
);
First, we'll add a rule to make all cues 1.2 times bigger than normal.
video::cue {
font-size: 1.2rem;
}
Then we style each of the tags above with a different color.
video::cue(u) {
color: green;
}
video::cue(b) {
color: purple;
}
video::cue(i) {
color: red;
}
video::cue(c) {
color: lavender;
}
Styling by class
The second cue is displayed right after the first one and includes the same tags. However, they all have a class of myclass
applied to them.
track.addCue(
new VTTCue(
1,
6,
"Styles: Class markup: <b.myclass>bold</b> <u.myclass>underlined</u> <i.myclass>italic</i> <c.myclass>class</c>",
),
);
We style all items with the .myclass
class with a light blue text color, except for the specific case of c.myclass
, which is given a blue text color.
video::cue(.myclass) {
color: lightblue;
}
video::cue(c.myclass) {
color: blue;
}
Styling using attributes
The next two cues are displayed after two and then three seconds.
The first displays text marked up with the lang
tag for three locales of English, while the second displays a <v>
(voice) tag with the "Bob" attribute.
track.addCue(
new VTTCue(
2,
6,
"<lang en>Lang markup: 'en'</lang> <lang en-GB>Text: 'en-GB'</lang> <lang en-US>Text: 'en-US'</lang>",
),
);
track.addCue(new VTTCue(3, 6, "<v Bob>Bob's voice</v>"));
We use the lang
attribute selector to give each language variant a different text color.
video::cue([lang="en"]) {
color: lightgreen;
}
video::cue([lang="en-GB"]) {
color: darkgreen;
}
video::cue(:lang(en-US)) {
color: #6082b6;
}
Then we use the v
tag and attribute selector for voice
to color text in "Bob's voice" orange.
video::cue(v[voice="Bob"]) {
color: orange;
}
Result
The example should the cues with colour coding that matches the styling above (if the text is not colored, then ::cue
is not supported on your browser).
Specifications
Specification |
---|
WebVTT: The Web Video Text Tracks Format # the-vttcue-interface |
HTML Standard # texttrack |
WebVTT: The Web Video Text Tracks Format # the-vttregion-interface |
Browser compatibility
api.VTTCue
BCD tables only load in the browser
api.TextTrack
BCD tables only load in the browser
api.VTTRegion
BCD tables only load in the browser
See also
- The CSS
::cue
and::cue()
pseudo-elements