alternate text for image using aria-labelledby throws error

[robfentress]

An image that is given alternate text using aria-labelledby is incorrectly flagged as an error. This is the code that threw the error:

<p><img src="x.jpg" aria-labelledby="monkey"/></p>
<div id="monkey">My monkey</div>

The error given is:

Img element missing an alt attribute. Use the alt attribute to specify a short text alternative.

[luketw]

I don't agree that this is a false positive, but that's because I had a different interpretation on what the ARIA attributes are meant to be equivalents of.

HTMLCS does check for aria-labelledby, but it considers it the equivalent of the "title" attribute or a label element - what the ARIA definitions call an "accessible name" - and not the "alt" attribute.

When not used in an anchor (like "a" or image map "area"s), "alt" text is intended for an actual description of the image involved, effectively as if describing it to a visually impaired person... similar to an audio description of a video (one of the success criteria in the Audio/Video guideline, 1.2). So in this case, the "alt text" might be "a baboon sitting on a branch, amongst the leaves, in a tree" or something like that.

If #monkey was a fuller description instead, you could potentially put a blank alt attribute (alt="") and place the description in body text, optionally using aria-describedby to link #monkey with the image - but then you'd need a title or aria-label attribute. But having a totally omitted "alt" tag is always going to throw an error at this stage.

Same reasoning applies for #157, too.

However, I'm also well aware that the ARIA and WCAG2 documents are a bit vague on whether they're meant to be a label or a text alternative, and in theory could be both. Hence why I want to leave this open along with #157 #158 #159.

[robfentress]

This makes some sense.

However, I am confused, then, why, in the accessible name calculation algorithm, aria-label when provided together with the alt attribute, is supposed to override the alt attribute as the accessible name. I guess I view aria-label as a more general case of alt. Indeed 'F65: Failure of Success Criterion 1.1.1 due to omitting the alt attribute or text alternative on img elements, area elements, and input elements of type "image"' seems to pretty specifically allow for the use of aria-label and aria-labelledby as alternatives for alt. Jonathan Avila discusses the issue in his post "Is the Alt Attribute Dead? and, though not crystal clear, I view it as supporting this interpretation.

Personally, I'm not sure I see the use case for why it would not be better to just use the alt attribute, but I also, as yet, remain unconvinced that using aria-label for images is an out-and-out failure.