video element visual-only content has description track


Description

This rule checks that description tracks that come with non-streaming video elements, without audio, are descriptive.

Applicability

The rule applies to every non-streaming video element that is visible where the video does not contain audio and contains a track element with a kind attribute value of descriptions.

Expectation

The visual information of each test target is described with a description track element that has the same language as the video or the same language as the page.

Assumptions

  • A mechanism is available to start the video and that the video element is not simply used to display the poster.
  • The language of each test target can be correctly determined (either programmatically or by analyzing the content), and sufficiently understood.

Accessibility Support

Currently the description track is not supported by most assistive technologies. 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.

Background

Test Cases

Passed

Passed Example 1

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This video element, which has no audio, has a track element with descriptions.

<html lang="en">
	<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>
</html>

Failed

Failed Example 1

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This video element, which has no audio, has a track element with incorrect descriptions.

<html lang="en">
	<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/incorrect-descriptions.vtt" />
	</video>
</html>

Inapplicable

Inapplicable Example 1

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This video element has audio.

<html lang="en">
	<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>
</html>

Inapplicable Example 2

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This video element is not visible.

<html lang="en">
	<video controls style="display: none;">
		<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>
</html>

Inapplicable Example 3

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This video element, which has no audio, does not have a track element.

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

Glossary

Attribute value

The attribute value of a content attribute set on an HTML element is the value that the attribute gets after being parsed and computed according to specifications. It may differ from the value that is actually written in the HTML code due to trimming whitespace or non-digits characters, default values, or case-insensitivity.

Some notable case of attribute value, among others:

  • For enumerated attributes, the attribute value is either the state of the attribute, or the keyword that maps to it; even for the default states. Thus <input type="image" /> has an attribute value of either Image Button (the state) or image (the keyword mapping to it), both formulations having the same meaning; similarly, "an input element with a type attribute value of Text" can be either <input type="text" />, <input /> (missing value default), or <input type="invalid" /> (invalid value default).
  • For boolean attributes, the attribute value is true when the attribute is present and false otherwise. Thus <button disabled>, <button disabled="disabled"> and <button disabled=""> all have a disabled attribute value of true.
  • For attributes whose value is used in a case-insensitive context, the attribute value is the lowercase version of the value written in the HTML code.
  • For attributes that accept numbers, the attribute value is the result of parsing the value written in the HTML code according to the rules for parsing this kind of number.
  • For attributes that accept sets of tokens, whether space separated or comma separated, the attribute value is the set of tokens obtained after parsing the set and, depending on the case, converting its items to lowercase (if the set is used in a case-insensitive context).
  • For aria-* attributes, the attribute value is computed as indicated in the WAI-ARIA specification and the HTML Accessibility API Mappings.

This list is not exhaustive, and only serves as an illustration for some of the most common cases.

The attribute value of an IDL attribute is the value returned on getting it. Note that when an IDL attribute reflects a content attribute, they have the same attribute value.

Outcome

An outcome is 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: Implementations 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

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.

For more details, see examples of visible.


Useful Links


Implementations

This section is not part of the official rule. It is populated dynamically and not accounted for in the change history or the last modified date. This section will not be included in the rule when it is published on the W3C website.

No Implementations

Implementation reports are not provided for this rule.

Acknowledgments

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