Using the track element to provide audio descriptions

Important Information about Techniques

See Understanding Techniques for WCAG Success Criteria for important information about the usage of these informative techniques and how they relate to the normative WCAG 2.1 success criteria. The Applicability section explains the scope of the technique, and the presence of techniques for a specific technology does not imply that the technology can be used in all situations to create content that meets WCAG 2.1.

Applicability

HTML5

This technique relates to:

Description

The objective of this technique is to use the HTML5 track element to specify a descriptions timed text track for a video element. Audio description timed text tracks contain textual descriptions of the video component of the media resource, intended for audio synthesis when the visual component is obscured, unavailable, or not usable. The user agent makes the cues available to the user in a non-visual fashion, for instance, by synthesizing them into speech.

The src attribute of the track element is an URL providing the text track data.

The audio description cues must fit into the gaps available in the audio component of the media resource. If there is not enough time to synthesize the description text in the track cue's time interval, user agents may truncate the speech. This limits the amount of supplementary information that can be added.

User agents may also support extended audio descriptions by halting the video until the description has been completely synthesized, then restarting the video.

As of February 2019 when this Advisory technique was last reviewed by the Working Group, there is no native support in user agents for this technique. However, support is available via JavaScript polyfills.

Examples

Example 1: Audio description in one language

A video element for a video in the English language. The audio descriptions are provided in the WebVTT format.

			 <video poster="myvideo.png" controls>
				<source src="myvideo.mp4" srclang="en" type="video/mp4">
				<track src="myvideo_en.vtt" kind="descriptions" srclang="en" label="English">
			  </video>
            

Example 2: Audio description in multiple languages

A video element for a video with both an English and French language source element, and with an English and a French audio description track using the WebVTT (vtt) file format.

			 <video poster="myvideo.png" controls>
				<source src="myvideo.mp4" srclang="en" type="video/mp4">
				<source src="myvideo.webm" srclang="fr" type="video/webm">
				<track src="myvideo_en.vtt" kind="descriptions" srclang="en" label="English">
				<track src="myvideo_fr.vtt" kind="descriptions" srclang="fr" label="French">
			  </video>            

Example 3: Captions in multiple languages

A video, "Google self-driving car". with an audio description track.

			<video controls tabindex="1">
				<source src="cdgQpa1pUUE.webm" type="video/webm">
				<source src="cdgQpa1pUUE.mp4" type="video/mp4">
				<track id="audesc" src="cdgQpa1pUUE.vtt" kind="descriptions" label="English descriptions" srclang="en-us"></track>
			</video>            

Resources

Resources are for information purposes only, no endorsement implied.

Tests

Procedure

For each video element used to play a video:

  1. Check that the video contains a track element of kind descriptions in the language of the video.

Expected Results

  • Check #1 is true.