Video element description track


Description

This rule checks that description tracks that come with non-streaming video elements are descriptive.

Applicability

The rule applies to every non-streaming video element that is visible where the video contains audio and a track element with a kind="descriptions" attribute.

Expectation

The visual information of each test target not available through its audio is described with a description track element.

Note: Multiple description track elements may be useful for different languages, but at least one must match the language of the video.

Assumptions

This rule assumes that a mechanism is available to start the video and that the video element is not simply used to display the poster.

Accessibility Support

Currently the description track is not supported by most assistive technology. Accessibility support for the description track attribute is relativly low to non-existent. Video players may be able to work around the lack of support for the description track by using aria-live but few do this today.

This means that the rule can only provide a pass for these success criteria if assistive technology support the description track or if the video player that is used has implemented such a work around.

Background

Test Cases

Passed

Passed Example 1

A video element with a track element that contains descriptions.

<video controls>
	<source src="../test-assets/rabbit-video/video.mp4" type="video/mp4" />
	<source src="../test-assets/rabbit-video/video.webm" type="video/webm" />
	<track kind="descriptions" src="../test-assets/rabbit-video/descriptions.vtt" />
</video>

Failed

Failed Example 1

A video element with a track element that contains incorrect descriptions.

<video controls>
	<source src="../test-assets/rabbit-video/video.mp4" type="video/mp4" />
	<source src="../test-assets/rabbit-video/video.webm" type="video/webm" />
	<track kind="descriptions" src="../test-assets/rabbit-video/incorrect-descriptions.vtt" />
</video>

Inapplicable

Inapplicable Example 1

A video element without a track element.

<video controls>
	<source src="../test-assets/rabbit-video/video.mp4" type="video/mp4" />
	<source src="../test-assets/rabbit-video/video.webm" type="video/webm" />
</video>

Inapplicable Example 2

A video element that is not visible on the page.

<video controls style="display: none;">
	<source src="../test-assets/rabbit-video/video.mp4" type="video/mp4" />
	<source src="../test-assets/rabbit-video/video.webm" type="video/webm" />
	<track kind="descriptions" src="../test-assets/rabbit-video/descriptions.vtt" />
</video>

Inapplicable Example 3

A video element without audio.

<video controls>
	<source src="../test-assets/rabbit-video/silent.mp4" type="video/mp4" />
	<source src="../test-assets/rabbit-video/silent.webm" type="video/webm" />
	<track kind="descriptions" src="../test-assets/rabbit-video/descriptions.vtt" />
</video>

Glossary

Outcome

key: outcome

A conclusion that comes from evaluating an ACT Rule on a test subject or one of its constituent test target. An outcome can be one of the three following types:

  • Inapplicable: No part of the test subject matches the applicability
  • Passed: A test target meets all expectations
  • Failed: A test target does not meet all expectations

Note: A rule has one passed or failed outcome for every test target. When there are no test targets the rule has one inapplicable outcome. This means that each test subject will have one or more outcomes.

Note: Implementers using the EARL10-Schema can express the outcome with the outcome property. In addition to passed, failed and inapplicable, EARL 1.0 also defined an incomplete outcome. While this cannot be the outcome of an ACT Rule when applied in its entirety, it often happens that rules are only partially evaluated. For example, when applicability was automated, but the expectations have to be evaluated manually. Such "interim" results can be expressed with the incomplete outcome.

Visible

key: visible

Content perceivable through sight.

Content is considered visible if making it fully transparent would result in a difference in the pixels rendered for any part of the document that is currently within the viewport or can be brought into the viewport via scrolling.

Content is defined in WCAG.


Changelog

Aug 21, 2019fix: update links to WCAG21 resources (#776)
Aug 21, 2019Editorial changes (#725)
Jul 19, 2019chore: run prettier (#688)
Jul 17, 2019fix: test if definition for all referenced glossary terms exist (#634)
May 9, 2019Chore: Adapt site to ACT Rules CR format (#547)
May 7, 2019Template update - Video element description track (#532)
Apr 29, 2019chore: rename files and update associations (#489)
Apr 16, 2019chore: add unique id to all rules (#478)
Apr 15, 2019chore: WCAG ACT RULES CG Website Update (#437)
Mar 30, 2019Rename SC1-2-Video-description-track.md to SC1-2-video-description-track.md
Jan 22, 2019rule: SC 1.2.3 & 1.2.5 Video alternatives (#204)
Oct 9, 2018Fix: testcase generation (#296)
Aug 30, 2018Chore: Update test cases format and descriptions (#230)
Aug 2, 2018Rule: SC1-2-3 video-description (#157)
Jun 6, 2018DRAFT: SC1-2-video-element (#106)

Useful Links



Acknowledgements