Re: Proposal for <canvas src> to allow images with structured fallback by Tab Atkins Jr.

Rich Schwerdtfeger
CTO Accessibility Software Group

public-html-request@w3.org wrote on 03/10/2011 05:01:38 PM:

> From: "Tab Atkins Jr." <jackalmage@gmail.com>
> To: Richard Schwerdtfeger/Austin/IBM@IBMUS
> Cc: Leif Halvard Silli <xn--mlform-iua@xn--mlform-iua.no>, Steve
> Faulkner <faulkner.steve@gmail.com>, Laura Carlson
> <laura.lee.carlson@gmail.com>, HTML WG LIST <public-html@w3.org>,
> HTML Accessibility Task Force <public-html-a11y@w3.org>, public-
> html-request@w3.org
> Date: 03/10/2011 05:04 PM
> Subject: Re: Proposal for <canvas src> to allow images with
> structured  fallback by Tab Atkins Jr.
> Sent by: public-html-request@w3.org
>
> On Thu, Mar 10, 2011 at 2:51 PM, Richard Schwerdtfeger
> <schwer@us.ibm.com> wrote:
> > So, we discussed this on the html accessibility call today.
> >
> > The concern about using canvas to do this is that for canvas
> accessibility we have a one for one mapping between the fallback
> content and the corresponding visual renderings on the physical
> canvas. It sounds like what people are asking for is the ability to
> associate rich descriptive content for an image. If we were to use
> that as the fallback content we would totally hose ATs who expect
> the content to be a one for one mapping.
> >
> > One of the things we did in XHTML 2 was to use the fallback
> content of <img> and <object> to provide this augmented content.
> Here you would use the fallback content to provide the equivalent of
> a long description. Something on the order of:
> >
> > <img src="eiffeltower.jpg">
> >
> > This is a picture of the Eiffel tower. The tower is composed of
> ... The tower is open from ...
> >
> > <h1>Entrance Fees</h1>
> > The tower entrance fees are as follows:
> >
> > <table>
> > <th> ....
> >
> > </table>
> > </img>
> >
> > Is this what you were thinking of? I mean, we could do this for
> <img> and <obj>
>
> If the page is *extremely* badly authored such that all of that
> content is exposed to sighted users via the image, then sure.  The
> fallback content should still be a one-to-one replacement.
>
> I would hope that we still *strongly* advocate against such horrible
> usability.  Having all that information exposed solely as an image
> means that sighted users can't copypaste or find-in-page on it, etc..
>
> In general, actual content should still be in the normal page,
> accessible to all users.  Rich fallback (like what @longdesc and
> <canvas src> are designed to expose) should only be necessary in rare
> circumstances, when the author has a particularly information-dense
> graphic for some reason, like a webcomic or a chart.
>
Yes, but you are proposing overloading fallback content in canvas to
support one for one mappings or alternative text.

The user agent needs to map the accessible objects in the fallback content
to the platform accessibility API and also include the fallback content in
the keyboard navigation order for <canvas>

If we overload this to say that this is also for text equivalents for
images then we have a problem. That is the issue as I see it.

I agree with your assertion about images and bad authoring practices. This,
to me, is a user agent discussion/AT interaction discussion.


> ~TJ
>