Add ATK and AX API mapping for <mphantom>

[fred-wang]

See discussion in [1]. Per MathML Core [2], <mphantom> has style
visibility: hidden by default and so is not exposed to ATs [3].
This commit specifies how to map <mphantom> when the element is
exposed (e.g. by reverting visibility: hidden), essentially
following what exists for the <mrow> element.

[1] https://github.com/w3c/mathml-aam/issues/9
[2] https://w3c.github.io/mathml-core/#making-sub-expressions-invisible-mphantom
[3] https://www.w3.org/TR/wai-aria-1.2/#tree_exclusion

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[fred-wang]

cc @asurkov

Note that I'm keeping MathML 3 links for now, although we probably want to move to MathML Core at some point. This is separate from this issue.

[asurkov]

I might be missing the context, but where it says the default styling must be overridden in order to be exposed?

[fred-wang]

Isn't it something that is defined in "another spec"? I mean isn't that the case that alll "visibility: hidden" node are not exposed by default?

[spectranaut]

In general, in these AAMs, if the role is only mapped in some scenarios, we do list the scenario in the description. For example: https://w3c.github.io/html-aam/#el-aside-ancestorbodymain

Would be nice to see this land because I want to make an editorial change that will cause a conflict with this branch!

[fred-wang]

OK, so the general rule to exclude elements (including the one with visibility: hidden) from the Accessibility tree is here: https://www.w3.org/TR/wai-aria-1.2/#tree_exclusion

mphatom's default "visibility: hidden" style is here: https://w3c.github.io/mathml-core/#making-sub-expressions-invisible-mphantom

This means that by default the mphantom element is not included in the accessibility tree. This PR just says to use the same roles as the <mrow> element in case the element is included (e.g. when "visibility: hidden" is reverted).

I don't think we need to specify precisely here when the element is included as that's explained in other specs. Otherwise, I guess we would need to be explicit that e.g. MathML elements are excluded if visibility: hidden, etc which would be redundant.

However, we can probably add a note that by default mpantom has visibility: hidden and so is not included in the tree. How do you suggest to do that? HTML AAM has a "comments" row which would work, but the layout is a bit different in MathML AAM. I'm also not sure I understand what you mean with the <aside> example?

[spectranaut]

Ah I see your point. By the <aside> example, I meant we often put in the first row, the element row, something like <code>aside</code> under these conditions so I imagined at first saying something like <code>mpantom</code> when exposed -- but on second thought, and looking at the mathml table, that doesn't seem to make sense. I think this PR is fine as is.

[fred-wang]

@spectranaut Thanks! FWIW, I don't have write access to merge this.