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

From: Charles McCathieNevile <chaals@opera.com>
Date: Tue, 11 Aug 2009 14:20:37 -0400
Message-ID: <op.uyifwnk8wxe0ny@widsith.local>
On Mon, 10 Aug 2009 10:42:36 -0400, Remco <remco47 at gmail.com> wrote:

> On Mon, Aug 10, 2009 at 8:53 AM, Benjamin
> Hawkes-Lewis<bhawkeslewis at googlemail.com> wrote:
>> 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."

"Must at least" means that if you can't do it right, here is how to get it  
only half-wrong (i.e. not good practice, but enough to be useful anyway)

>> "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 ant to provide an alternative, then I suggest that is not a good  
one. It is a description of the video, rather than a replacement.

An alternative is something more like

"As a ball falls, every second it goes 9.8m/s faster than it was going a  
second before, because gravity makes it accelerate. In practice, effects  
such as "drag" (wind resistance) will affect the actual speed - and the  
value of 9.8 is specific to the average sea-level of earth - it depends on  
the mass of the earth and the ball."

If you already have this in the preceeding or following text, something  
like Benjamin's examples, then the job is done and repeating it in alt is  
redundant and bad practice.

To be clear, your example text is better than nothing - and in  
accesibility that is often as good as it gets :( But there's value to  
explaining how to do things better - and this particular issue is  
something that has gathered a *huge* amount of discussion and explanation  
over the last decade and a half.

[snip]

To be clear, I think that the existing options for video in HTML5 are  
better than anything we would gain by adding an alt attribute. The only  
value I can see in adding one is that some people who are careful enough  
to use it with images will also use it effectively even if they will do  
nothing better - but this would require careful large-scale study of  
authors *as they work* to verify, and I doubt that is going to be done.

>> --
>> Benjamin Hawkes-Lewis

> One advantage of this is that the alternative content is now by
> default always visible (or can be made visible in the case of
> <details>). That makes it much more useful for normal use cases (no
> network problems or disabled audience), which means it would be
> provided a lot more. This is a lot better than the current situation
> with alt.
>
> The question now is: why would we need both <figure> and  
> aria-describedby?

Because many designers will not put sufficiently complete text  
explanations of everything in the ordinary flow of a document. This is  
actually a valuable accessibility practice - the large number of people  
who have difficulty reading can often find that their ability to use  
content is significantly reduced by the presence of large blocks of text.  
This is something that most usability engineers and communication experts  
understand instinctively, actually.

So we need some way that allows for more than one presentation scenario.  
Either we ask people to make multiple pages (something that is well-known  
to suffer from the "out of sight out of mind" problem, or we provide ways  
to manage more information in a single page (which also suffers from the  
problem, but it is believed less so. I don't know how many careful studies  
have been done of this, if any).

cheers

Chaals

-- 
Charles McCathieNevile  Opera Software, Standards Group
     je parle fran?ais -- hablo espa?ol -- jeg l?rer norsk
http://my.opera.com/chaals       Try Opera: http://www.opera.com
Received on Tuesday, 11 August 2009 11:20:37 UTC

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