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

Hi Leif,

>Viable solution to which problem?
What I think is important is that a robust , programmatic method for
providing structured text alternatives for non text content is provided in
Whether that is via longdesc or some other mechanism is immaterial. If
longdesc is included in HTML5 but fails to be specced/implemented to be
useful and robust then it will be a failure.

your wrote:
"But, once again, which @role would the canvas@src in Tab's example get?

<canvas src="complex-chart.png">
   -data that the chart represents-

Why do you have this obsession with what the role of canvas should be? I
think the role will depend entirely on the context it is used in.
Note that HTML5  like HTML 4 etc has many elements are not mappable to
specific ARIA roles or accessibility API roles and ARIA itself only covers a
subset of existing accessibility API roles.

canvas itself has no defined default ARIA role in HTML5 and it may well be
that it is appropriate to not have a defined role for it. Or it may be given
a generic grouping roles or a role of animation or a role of img by browsers
when they map it to accessibility APIs, what IS importanat is that there is
uniformity in how it is mapped.

As to the img roles children being presentational, this is a requirement
from ARIA, it does not follow that it is a requirement on the roles that
ARIA maps to in accessibility APIs. In other words, when the ARIA role="img"
is explicitly set on an element

<div role=img></div> the requirement stands.

when an element is mapped to the MSAA ROLE_SYSTEM_GRAPHIC by browsers it
does not mean that all required children have to be presentational.

In the example above if the content represented in the canvas is wholely
provided in the structured content alternative, then the canvas itself could
be left out of the accessible tree.


On 8 March 2011 14:45, Leif Halvard Silli

> Steve Faulkner, Tue, 8 Mar 2011 10:06:24 +0000:
> > Hi Laura
> >
> > How did you come to the conclusion it is not a viable solution?
> Viable solution to which problem?
> * Did you mean a "viable replacement-for-<img@longdesc solution"?
> * Or "viable solution to <object@data=image's real-world problems"?
> If you meant the former, then the reasoning in Laura's change proposal
> explains very well why it isn't a viable solution. [1] But if you meant
> the latter, then it looks as if it could be a viable solution. Laura's
> change proposal is however about the former and not the altter. Hence I
> choose to believe that Laura did not express herself about wether it is
> a solution to the latter problem.
> But, once again, which @role would the canvas@src in Tab's example get?
> <canvas src="complex-chart.png">
>  <table>
>    -data that the chart represents-
>  </table>
> </canvas>
> According to Tab: [2] "The <canvas> element's situation is *directly*
> analogous, as <canvas> represents an image", and I would thus guess
> that many would say it should get role=img, not? However, according to
> ARIA, the children of elements with a role=img SHOULD NOT be presented
> to the user.
> I had planned to claim that for <img@longdesc, then the element would
> retain its role=img. In other words: @longdesc doesn't conflict with
> <img's default role - which seems to me like an advantage.
> However, this is not quite true. Because BOTH canvas@src AND
> <img@longdesc are outside ARIA's role=img model, which NEITHER has room
> for non-presentational children NOR for the @longdesc link.
> It would be great if some of those who specced ARIA could shed light on
> the @role problem of <img@longdesc and <canvas@src ... Isn't it ARIA
> which needs to change/clarified here?
> [1]
> [2]
> --
> leif halvard silli

with regards

Steve Faulkner
Technical Director - TPG | |
HTML5: Techniques for providing useful text alternatives -
Web Accessibility Toolbar -

Received on Thursday, 10 March 2011 13:49:09 UTC