Accessibility mapping of <a> element

[tannal]

Closes w3c/mathml-aam#39

This change map mathml a element to link role in atspi and axapi.
Leave TBD for MSAA and UIA api.
See the mathml-aam issue in detail.

Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• "author MUST" tests: N/A
• "user agent MUST" tests: Add MathML-AAM tests infrastructure and comprehensive mapping tests for mathml elements web-platform-tests/wpt#61018
• Browser implementations (link to issue or commit):
  • WebKit: will file an issue later
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2054819
  • Blink: https://issues.chromium.org/issues/510487697
• ACT review? N/A
• Does this need AT implementations? N/A
• Related APG Issue/PR: N/A
• MDN Issue/PR: N/A

I got 503 when accessing webkit bug tracker, so for now I can't file the issue. :-)

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[spectranaut]

I'm a little worried that this is so similar t

[spectranaut]

I'm a little worried that this is so similar to the core-aam definition of an a element -- other than the AXSubrole.

For example: The "tag" object attribute is actually always exposed, for all elements, that should be made more clear in the Core-AAM, and the linux mappings in the Core-AAM mention HyperlinkImpl, which is not mentioned here.

I think it might be better to say that there is a corresponding role, and that you should follow it, but that there is an AXSubrole override. So basically, for ATK write "Use the WAI-ARIA mappings" like here https://w3c.github.io/html-aam/#el-a, but for AX AXPI write "Use the WAI-ARIA mappings, but override subrole with..."

[tannal]

Done.

[spectranaut]

This is looking good, one small change request.

[spectranaut]

Looks good to me, thanks @tannal !

[spectranaut]

I've marked this as "waiting for implementations" because we require two implementations to merge.