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Re: Acessibility of <audio> and <video>

From: Lachlan Hunt <lachlan.hunt@lachy.id.au>
Date: Tue, 02 Sep 2008 13:19:23 +0200
Message-ID: <48BD213B.2030807@lachy.id.au>
To: Leif Halvard Silli <lhs@malform.no>
Cc: public-html@w3.org

Leif Halvard Silli wrote:
> Lachlan Hunt 2008-09-01 23.36:
>> Leif Halvard Silli wrote:
>>> Justin James 2008-09-01 19.42:
>>>> I can imagine the furor if we also applied this logic to
>>>> images, by saying, "if you want accessible images, use a format
>>>> that natively supports metadata of alternate text, or put a
>>>> subtitle/caption/legend/etc. in your image."
>> No. Unlike video, images have no way embedding accessibility features 
>> within them, and their meaning is very often depending on the context. 
>> However, videos do have various ways of including native accessibility 
>> features, such as subtitles/captions or audio descriptions.
> Our recent debate about EXIF proves opposite.

EXIF data is just metadata and is not supported in PNG or GIF. It is 
certainly not optimised for accessibility purposes and only provides 
very limited benefit.  Although it can contain descriptions, that's not 
particularly useful when the alternate text needs to be context sensitive.

Alternative content for a video could also be context sensative to some 
extent too, and that kind of alternative content should be made 
available externally to everyone, as I have argued all along.  However, 
the type of accessibility features that I was referring to, which would 
be embedded within a video like subtitles don't change depending on the 

>>> Add to that that movie formats are *very* often used for displaying 
>>> photos.
>> Could you elaborate on what you mean, provide some sort of evidence to 
>> support this claim, and also explain why it's relevant?
> Dagbladet.no publishes vidoes which serves both a photo role and a video 
> role almost daily. Here is one article from yesterday:
> Link: http://www.dagbladet.no/kultur/2008/09/01/545443.html

> One could have taken away the video functionality, and it would still be 
> a photo. (I, for one, did not care to look at the video.)

That's just using the poster frame for its intended purpose: providing a 
visual, iconic representation of the video to indicate what the video is 
about.  It is not a substitue for actually watching the video.  Since it 
isn't just an image, it's a video, the most suitable additional 
information to provide is not alternative text for the poster frame, but 
rather an brief summary of what the video is about.

One possibility worth considering is using the title attribute, or the 
surrounding text (possibly the caption) to provide additional 
information.  Conceptually, it would be like video description in the 
sidebar of a YouTube video.

In other words, the poster frame is like the picture on the front cover 
of a DVD, and the summary is like the blurb on the back that describes 
what the movie is about.  Clearly neither of them are a substitute for 
actually watching it, but both provide advisory information about its 
content, which is useful to anyone.

>>> Plus the fact the <video> elment itself has a poster image to be 
>>> displayed before the video is started.
>> I'm not sure why that is relevant.  The poster frame can be thought of 
>> as being more like an icon representing the video.  It's the content 
>> of the video that is important
> The Dagbladet.no example I gave above shows that the poster image  can 
> serve the role of a photo.

No, it serves as a visual representation of the video in order to give a 
hint about what the video is about.

> If the poster image shows Putin looking at a sleeping siberian tiger, 
> then why do you need a separate <img> with the same motif?

I never suggested that you would.

> And just as I was more than satisfied from seeing the photo, likewise 
> could anyone incapable of seeing the video be more satisfied with an 
> @alt text for the poster photo. (Rather than having to be bothered with 
> listening/seeing the entire video before getting any clue.)

No, what would you rather have? Alt text for just the poster frame or a 
summary of the actual video?  Surely, the former is relatively useless 
compared with the latter, and the summary is most certainly more 
appropriate and useful for everyone.

> This has me asking: Why couldn't the poster image just be a regular 
> <img> element, rather than an attribute?

I can't recall the rationale for that.

>> * Transcript of spoken content.
>> * Textual descriptions of relavant non-spoken content.
>>   e.g. descriptions of significant actions or sounds in the video.
>> * Still images illustrating significant moments from the video.
>>   e.g. images of presentation slides, if the video was of someone giving
>>   a presentation.
> "Still images illustrating significan moments" - that is exactly the 
> usecase we have in the Dagbladet.no newsarticle I sited.

Correct, and it does it's job quite well.  But what you seem to be 
asking for is transcripts, textual descriptions and a short summary, but 
I already explained why all of that should be available to everyone, 
rather than hidden away from all but assistive technology, which would 
be the practical effect of including it within the element or its 

> The poster image should be viewed as <video> fallback!

Sure, it has some benefit for accessibility too, but it's not fallback. 
It's available to everyone and shown regardless of whether or not they 
choose to watch the video.  That's like arguing that seeing a movie 
poster is fallback for going to the cinema and watching the film.

> It is when looking at the fallback, that we should be offered the choice 
> between how we want the video served: as slides, transcript or the very 
> video.

Right, and I'm ok with providing all of that in a way that is available 
to everyone, and providing it or a link to it from within the 
surrounding content seems most appropriate for that.

>> * A link to download the video, possibly in alternative formats, to
>>   watch in an external media player, perhaps in several formats.
> When it comes to getting the video in another video format, then - if we 
> are talking /fallback/ - this requires that the user /knows/ that he 
> does not want the default format - and that he also knows that he 
> doesn't want any of the (textual) fallback either.

What? I really don't understand what you are trying to say.  In 
practice, it's somewhat common to provide links to media in alternative 

There are lots of podcasts that provide high quality or low quality 
alternatives, in MP3, Ogg, streaming via a flash based player in the 
page or subscribing via itunes.  Similarly, with videos, it has been 
quite common to provide links to various formats targetted at QuickTime 
or Windows Media player, and to provide high and low quality alternatives.

For example, Ask a Ninja provides both Flash and MOV (h.264/AAC), and 
Diggnation provides both high and low quality streaming in the flash 
player; links to small, large and high def versions in various codecs 
and an audio only MP3.

Lachlan Hunt - Opera Software
Received on Tuesday, 2 September 2008 11:20:04 UTC

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