SM2Sufficient technique WCAG 1.2.3 WCAG 1.2.5 WCAG 1.2.7 Level A Human testing required

Add extended audio description in SMIL 2.0

SM2 is a sufficient technique when used with G8, and it is SM1's method carried into SMIL 2.0. The behavior described is the same one. The video appears to play from end to end, and freezes in places while a longer audio description plays. Descriptions that could never fit between two lines of dialogue get the room they need. Which of the two you would reach for is a question about your player rather than about accessibility, since the accommodation is identical either way. What matters more than the version number is the decision underneath it. Extended description is the answer when the soundtrack has no usable gaps, and it costs the sighted viewer a few seconds of held picture. If you are not publishing SMIL at all, the modern equivalent is an HTML5 video that pauses under script at set points, or a second, described cut of the same film.

How we find it in an audit

The check is the same one we run on SM1 and it starts with a stopwatch rather than a specification. Watch the film with the screen turned away, note every moment where something visual carried the meaning, and measure the silence available to describe it. Short gaps and a lot of visual content means extended description. Then we test whether the implementation actually holds the picture, because a description track that plays over the narration has made the video harder to follow rather than easier. No scanner reaches any part of this.

How affected users experience it

The experience an extended description fixes is the sense of arriving late to something that already happened. The soundtrack is confident and continuous, the story clearly makes sense to everybody else, and every so often there is a beat where the room reacts to something you have no access to. Extended description does not add commentary on top of that. It makes space. The description and the film stop competing for the same seconds of a listener's attention.

Passes vs. fails

Passes: narration speaks the visuals. Fails: the offer is never spoken.

Passes

<seq>
  <par><video src="course-a.mp4"/></par>
  <par><audio src="describe-a.mp3"/></par>
  <par><video src="course-b.mp4"/></par>
</seq>
<!-- Playback freezes for each description, then resumes where it stopped. -->

Fails

<par>
  <video src="course.mp4"/>
  <audio src="narration.mp3"/>
</par>
<!-- Continuous narration over dense on-screen work. Nothing describes it. -->

How this gets tested

The W3C publishes test rules that define what a checker looks for here.

Video element visual content has accessible alternativeA tool finds candidates, you decide
Video element visual content has strict accessible alternativeA tool finds candidates, you decide

Other ways to satisfy this rule

13 guides on this site are filed under 1.2.3 Audio Description or Media Alternative (Prerecorded). W3C lists this one as sufficient for that rule only alongside G8, so the pair is what passes and neither half does on its own.

This guide is our interpretation of W3C technique SM2: Adding extended audio description in SMIL 2.0. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.