Over in  Steve proposes moving the UA requirements for accessibility mapping
out of W3C HTML. The corresponding section in WHATWG HTML is .
Recently I tried to do implementer-ey things  but found the delineation
between authoring and UA requirements in that section very confusing. So I am
supportive of anything that clears that up.
In the replies to both  and  Steve points out that the HTML - ARIA
mappings given in  are not a complete description of how UAs expose HTML
elements to accessibility technologies, and that the document at  is more
accurate. (For example, he notes that figcaption is exposed as a caption to
certain accessibility technologies, but there is no way of expressing that in
ARIA.) So in addition to being confusing, I am not sure the content at 
actually has any normative value; at the very least it is incomplete.
In the future I would like to work toward a system where HTML maps to ARIA,
which maps to platform-specific accessibility stuff. So for example ARIA would
grow to have a caption role, and figcaption's accessibility aspects would be
defined entirely by saying it has ARIA role caption. But apparently that is not
the world we live in (yet?!), so I think  just confuses the issue by
pretending that it is. In the meantime I think I agree with Steve that 
seems to be a better place for implementers to consult.
Thus I think we should remove all UA requirements from  and instead refer to
 as the authoritative source for UA requirements for accessibility in HTML.
I do not have a strong opinion on whether we should keep the authoring
requirements of  intact or remove them as well. This is probably because I
do not have a strong opinion about authoring requirements in general.
What do you think? I could very well be missing something; this world of
accessibility specs is pretty new to me...