Re: Difference in behavior between HTML label and aria-labelledby

Yes, it is, that's right, which is why its a conformance error in HTML and
WCAG 2.0 to have duplicate id values in a document.

--

Regards

SteveF
HTML 5.1 <http://www.w3.org/html/wg/drafts/html/master/>


On 25 April 2014 08:00, Mark Rogers <mark.rogers@powermapper.com> wrote:

> Is the problem here not caused by duplicate IDs?
>
>
>
> The behaviour of the DOM method getElementByID is specified as undefined
> if there are duplicate IDs in the DOM:
>
> http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-getElBId
>
>
>
> Anything built on the DOM then suffers from undefined behaviour when
> duplicate IDs are present. I’d be surprised if browser accessibility APIs
> didn’t use the same underlying ID mapping as as getElementByID  (otherwise
> you’d get weird inconsistencies between ACC APIs and DOM APIs)
>
>
>
> Worth thinking about this from the browser implementation perspective. The
> HTML spec defines IDs as unique, so a browser implementation is likely to
> use a fast hash lookup to map unique IDs to a single DOM node. If duplicate
> IDs are present then lookup for a duplicated ID is likely to return,
> depending on the exact implementation:
>
> -          First matching node in HTML parse order
>
> -          Last matching node in HTML parse order
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> I believe this is the motivation for F77
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> http://www.w3.org/TR/2014/NOTE-WCAG20-TECHS-20140311/F77
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>
>
> Best Regards
>
> Mark
>
>
>
> Mark Rogers - mark.rogers@powermapper.com
> PowerMapper Software Ltd - www.powermapper.com
>
> Registered in Scotland No 362274 Quartermile 2 Edinburgh EH3 9GL
>
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>
> *From:* Karl Groves [mailto:kgroves@paciellogroup.com]
> *Sent:* 22 April 2014 16:01
> *To:* public-pfwg-comments@w3.org; w3c-wai-ig@w3.org
> *Subject:* Difference in behavior between HTML label and aria-labelledby
>
>
>
> Working on an audit recently I observed a client who had reused ID
> attributes for form controls in a tabpanel interface. They do this because
> some form fields are shared across panels. The duplicate fields are hidden
> with a CSS declaration of display:none.  I came across this on the heels of
> a discussion with my good friend and co-worker, Steve Faulkner about this
> same topic and I wanted to bring it up here.
>
>
>
> I've created a test file and uploaded it at:
>
> http://karlgroves-sandbox.com/a11yVisible.html
>
>
>
> The first field is a normally labelled field, using label[for] pointing to
> the field's ID
>
>
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> The second example is a single label with 'for' with 3 fields sharing the
> same ID.  The first two fields are hidden with display:none and
> visibility:hidden, respectively.  JAWS and VoiceOver recognize the third
> field, skipping the first two.
>
>
>
> The third example uses aria-labelledby to reference two IDREFs, one of
> which is hidden with display:none. JAWS and VO read both. This behavior is
> correct according to ARIA spec[1]
>
>
>
> The fourth example uses a label[for] and aria-describedby.  JAWS and VO
> behave differently, from what I can tell. JAWS behaves as according top
> ARIA spec[1] but VO does not.
>
>
>
>
>
> This raises a few questions for me.  First, for those who may also be
> inclined to test the URL above: are your results similar.
>
>
>
> Second, and more importantly, why the difference between the behavior of
> label[for] and aria-labelledby?  The ARIA spec says "Skip hidden elements
> unless the author specifies to use them via an aria-labelledby or
> aria-describedby being used in the current computation."   If the
> definition of "hidden" says "Indicates that the element is not visible or
> perceivable to any user. "  and "Authors are reminded that
> visibility:hidden and display:none apply to all CSS media types; therefore,
> use of either will hide the content from assistive technologies that access
> the DOM through a rendering engine."
>
>
>
> If that's the case, why allow an exception when authors use
> 'aria-labeledby' and 'aria-describedby'?  Further, why the difference in
> behavior from how the label element behaves?
>
>
>
> Last, how will the proposed labelfor behave?
>
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> 1 - http://www.w3.org/WAI/PF/aria-implementation/#mapping_additional_nd
>
> 2 - http://www.w3.org/WAI/PF/aria-implementation/#def_hidden
>
>
>
> --
>
> Karl Groves
> Senior Technical Lead Accessibility Software Consultant & Director of
> Training
> The Paciello Group
> @karlgroves
> Phone: +1 443-875-7343
>
>
>