AXMathSubscripts/AXMathSupscripts and ignored accessibility nodes

[fred-wang]

Currently, AXMathSubscripts/AXMathSupscripts returns an array of subscript/superscript pairs, each pair represented by a dictionnary of keys AXMathSubscript/AXMathSupscript.

It seems WebKit allows key to be optional, they are not present if the child corresponds to an "ignored" accessibility node: https://webkit-search.igalia.com/webkit/source/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.mm#244

I'm not sure how it works. I imagine it happens if someone uses a <mspace> (not mapped). Maybe it's to handle <none> (see #12 )?