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RE: Canvas and ARIA alternatives

From: Jonathan Avila <jon.avila@ssbbartgroup.com>
Date: Wed, 1 Aug 2012 08:45:43 -0400
Message-ID: <e5d874ccd556480da56aaa849df36ccd@mail.gmail.com>
To: WAI Interest Group <w3c-wai-ig@w3.org>
> Do you have any examples of the use of @alt on <canvas>? It's the only
> usage you mention that would be definitely incorrect.

Benjamin, thank you for the additional information.  Unfortunately, I am not
able to share the example with the use of alt.

I've discovered inconsistencies though with support for tabIndex directly on
canvas.  It would appear that IE 9 does not focus the canvas and focuses the
next keyboard accessible element but Firefox 13 does and includes a visual
focus indicator.  IE 9 appears to show fallback content and visual focus
through an empty canvas as one would expect given it's transparent nature --
however Firefox does not appear to display the fallback content through an
empty canvas (with default styles).

I've also noticed with other ARIA implementation screen readers work
differently depending where in the structure the ARIA roles and properties
are placed.  For example, it would appear sometimes they are more effective
on a div surrounding content and sometimes directly on descendant HTML
elements like list items.  In this case - perhaps it would be a better
practice to place the ARIA on a surrounding DIV rather than the canvas or on
a div within the canvas.  Additional testing will be necessary to determine
the best approach for different browsers and assistive technologies.

I am very glad to see that a method is being worked out to share location
information about fallback content with assistive technologies.   I hope
assistive technologies can also work out location for elements based on the
aria-activedescendant property too!  I haven't check that one in a few
months though.


-----Original Message-----
From: Benjamin Hawkes-Lewis [mailto:bhawkeslewis@googlemail.com]
Sent: Tuesday, July 31, 2012 3:55 PM
To: Jonathan Avila
Cc: WAI Interest Group
Subject: Re: Canvas and ARIA alternatives

On Tue, Jul 31, 2012 at 6:11 PM, Jonathan Avila <jon.avila@ssbbartgroup.com>
> The proper way to provide an equivalent to canvas content is to use
> fallback content that is a descendant of the canvas element.  Lately I
> have seen authors applying ARIA markup including roles, aria-labels,
> title attributes, and alt attributes directly to canvas elements.

Do you have any examples of the use of @alt on <canvas>? It's the only usage
you mention that would be definitely incorrect.

> What are peoples thoughts on the canvas element and ARIA.

How "canvas" is backed by accessibility structures, including ARIA annotated
HTML, is a matter of active work at a spec level.

See for example:


Benjamin Hawkes-Lewis
Received on Wednesday, 1 August 2012 12:46:15 UTC

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