[whatwg] Alt attribute for <video> and <audio>

Sun Aug 9 23:53:09 PDT 2009

On 10/08/2009 04:05, Remco wrote:
> A title is a short description, and could be the movie title in the
> case of a video element.

WCAG 2 1.1.1 requires that:

"If non-text content is time-based media, then text alternatives at 
least provide descriptive identification of the non-text content."

"title" and "aria-labelledby" seem sufficient for this purpose.

So do "figure" and "legend":

http://dev.w3.org/html5/spec/Overview.html#the-figure-element

> An alt is a textual alternative for the content.

[snip]

> For video, audio, object, iframe, this is a little sparse.

[snip]

> But Elephants Dream may not be a good example for a video where an alt
> text would be useful. It's simply too complicated to replace with
> alternative text. But if you have a short video that explains
> something on Wikipedia, it would be tremendously helpful if the alt
> text would convey the same meaning. A video of a ball falling to show
> what gravity is, could have the alt text: "A ball accelerates as it
> moves down. Next to the ball's trajectory, a speedometer increases
> with 9.8 m/s per second.".

If you want to provide an "alternative for time-based media" (in WCAG 
2's phrase), then you want a method that can scale to contain semantic 
information, such as indicating language changes ("lang") or changes of 
speaker ("dialog").

Here's how WCAG 2 defines "alternative for time-based media":

"document including correctly sequenced text descriptions of time-based 
visual and auditory information and providing a means for achieving the 
outcomes of any time-based interaction

"Note: A screenplay used to create the synchronized media content would 
meet this definition only if it was corrected to accurately represent 
the final synchronized media after editing."

http://www.w3.org/TR/WCAG20/#alt-time-based-mediadef

Here's just three ways you could do this without changing HTML5, 
assuming the incorporation of WAI-ARIA:

1. <figure><legend>Ball acceleraton.<details>A ball accelerates as it 
moves down. Next to the ball's trajectory, a speedometer increases with 
9.8 m/s per second.</details></legend><video>...</video></figure>

2. <video title="Ball acceleration" 
aria-describedby="alternative"...</video><p id="alternative">A ball 
accelerates as it moves down. Next to the ball's trajectory, a 
speedometer increases with 9.8 m/s per second.</p>

3. <video title="Interview with Barack Obama" 
aria-describedby="transcript-link"...</video><a href="transcript.html" 
id="transcript-link">Transcript of Interview with Barack Obama</a>