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[text] starter draft of clarification on alt validation, for discussion

From: Judy Brewer <jbrewer@w3.org>
Date: Mon, 25 Apr 2011 11:55:00 -0400
Message-Id: <E1QEO8R-0006yO-C2@lisa.w3.org>
To: HTML Accessibility Task Force <public-html-a11y@w3.org>
DRAFT for discussion purposes only.... partly for approach, party for 


Dear All,

With regard to the HTML Working Group Co-Chairs' decisions, as 
described in the following email...


...which discussed the following information...

>There is a basic disagreement in the group on the validity
>requirements for alt.  The result was two issues, six change
>proposals, and a straw poll for objections:

...and which arrived at the following six conclusions...

>Therefore, the HTML Working Group hereby decides that:
>    * The presence of aria-labelledby does not make missing alt conforming.
>    * The presence of role=presentation does not make missing alt conforming.
>    * The presence of <meta name=generator> makes missing alt conforming.
>    * Use of private communications does not, in itself, make 
> missing alt conforming.
>    * The presence of title makes missing alt conforming.
>    * The presence of figcaption makes missing alt conforming.

...and which furthermore proposed addressing these through 
implementation of a combination of the following two Change Proposals...

>The two Change Proposals closest to these results are those identified
>as Requirement Set 1 and Requirement Set 4:
>     http://lists.w3.org/Archives/Public/public-html/2010Jul/0050.html
>     http://www.w3.org/html/wg/wiki/ChangeProposals/ImgElement20100707
>These Change Proposals agree with each other and with the WG decision
>on aria-labeldby, role=presentation and figcaption.
>On the generator mechanism and the title attribute, Requirement Set 1
>aligns with the WG decision:
>     http://lists.w3.org/Archives/Public/public-html/2010Jul/0050.html
>On the email exception, Requirement Set 4 aligns with the WG decision:
>     http://www.w3.org/html/wg/wiki/ChangeProposals/ImgElement20100707
>Thus, overall, the WG adopts the Requirement Set 1 proposal with
>regards to aria-labelledby, role=presentation, <meta name=generator>,
>title and figcaption; but Requirement Set 4 with regards to the email

...we note that the following information was not considered. The 
respondents on the surveys mentioned above had not anticipated that 
this information would be unknown to the Co-Chairs, and so have 
described this information in some detail within this mail, and 
presented test samples to illustrate failures associated with the 
proposed approaches. These clarifications follow.

On the Co-Chair's decision on aria-labelledby:

>    * The presence of aria-labelledby does not make missing alt conforming.

The purpose of alt is to provide alternative text on images, and to 
allow a user agent to render text in place of the images when images 
are turned off.

As an example, applications such as Yahoo! mail render alt text for 
images when web page content is embedded in a mail message. This 
allows applications and browsers to only fetch images if a user 
really needs them, improving download performance; and providing a 
label to explain to the sighted user what is missing.

aria-labelledby is used to reference a label that is already visible 
on the page, similar to an image caption. Authors put these captions 
or labels in order to assist the sighted user in providing context 
about the user.

Both the label and the alt text serve the same purpose when images 
are turned off; and both provide a label. It does not seem 
appropriate  to force the author to provide two labels for the same 
image, and to have two labels rendered when images are turned of. 
Both will supply a label or "name" for the image in the accessibility API.

Consequently, we request that the Co-Chairs consider allowing 
aria-labelledby to be used to point to a label as a suitable 
alternative to alt.

On the Co-Chair's decision on role=presentation:

 > * The presence of role=presentation does not make missing alt conforming.

Alt having a value of "" tells the user and assistive technology that 
the image is presentational. Yet, with alt="" an assistive technology 
(AT) must still filter out the image when it has no intrinsic value 
other than to be decorative or be used as a spacer.

Unlike alt="", role="presentation" has the added value of removing 
the image from the accessibility API object tree, effectively 
filtering out the image and improving assistive technology 
performance. Furthermore, a role of presentation is to state the 
intent of the author in a declarative fashion. For these reasons, 
role="presentation" should be considered a suitable alternative to 
requiring alt when it adds no meaningful information to an AT.

As background, note that in most cases, browsers map browser content 
to platform accessibility APIs, and an accessible object with a 
standard API interface is created for each DOM object in a web page. 
These objects are referenced and communicated with by an AT to 
process accessibility information about visible objects in the web page.

On the Co-Chair's decision on the presence of title making missing 
alt conforming:

 > * The presence of title makes missing alt conforming.

Title has a completely different function from alt in HTML.

Title is used to generate a tooltip, and is invisible when images are 
turned off. Alt does not generate a tooltip, and is visible when 
images are turned off.

If title is allowed as alternative text over alt it will break 
applications such as Yahoo! mail; it will also break a commonly-used 
feature, in less powerful mobile phones, where images are turned off 
to improve performance.

If title were to be used in place of alt then when images are turned 
off in the browser, nothing meaningful will be shown in the browser.

Furthermore, having title take precedence over alt will result in 
tooltips being generated on decorative images and spacers, which 
would do tremendous harm to the user experience.

It should be noted that title is used as a last resort when other 
measures cannot be employed to compute the label or "name" of an 
object in the accessibility API mapping for browsers.

Please note the following demonstrations of failures resulting from 
the proposed approach:


On the Co-Chair's decision on the presence of figcaption making 
missing alt conforming:

* The presence of figcaption makes missing alt conform

[clarification pending]

Please let us know if additional clarification is needed, and thank 
you in advance for your re-consideration.


Received on Monday, 25 April 2011 15:55:41 UTC

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