sync MathML spacing with MathML Core

[xworld21]

Is your feature request related to a problem? Please describe.
MathML Core has changed dictionary slightly as well as the form detection algorithm. I believe MathJax currently implements the old MathML rules (for instance: the infix/postfix/prefix detection should check all 'grouping elements' and mpadded, msqrt, but MathJax checks mrow only, and it should only look at 'in-flow' children).

This makes MathJax with MathML spacing slightly inconsistent with browser implementations, although I do not know how often that happens in practice.

I suspect the dictionary is also from an old version of the MathML (non-core) spec and might need updating too.

Describe the solution you'd like
Follow new MathML Core spec more closely.

Describe alternatives you've considered
I can override getMathMLSpacing in configuration. I'll post the code here when I can make it presentable.

Additional context
Dictionary-based attributes (including how to determine the default forms): https://w3c.github.io/mathml-core/#dictionary-based-attributes
New operator dictionary: https://w3c.github.io/mathml-core/#operator-tables

[dpvc]

MathJax implements MathML3 (so you are right that it is an older spec than MathML-Core). MathML-Core does not include some features that are used by MathJax (e.g., mlabeledtr elements), and so MathJax is not going to implement the MathML-Core specification. MathML4 seems the next likely version that MathJax would work on, and it is current a FPWD (first public working draft), which is not yet far enough along for us to start making significant changes. While MathML-Core is supposed to be a subset of MathML4, I have found in the past that this isn't actually the case, as the two specifications didn't agree on some issues (though I'm afraid I no longer remember what that was), at least at the time I looked (the drafts may have changed since then). For now, MathJax is likely to stay at MathML3.

I'm not particularly concerned about not matching the browser MathML output exactly. MathML-Core does make much more explicit what the positioning should be for things like fractions, roots, and so on, and MathJax doesn't implement those at this point either (MathJax is over 15 years old, so its output long precedes the MathJax-Core specification), as MathJax implements (to the best of its ability) the TeX layout for these constructs, even when MathML spacing is requested (which really only affects the horizontal spacing).

[xworld21]

While MathML-Core is supposed to be a subset of MathML4

You might see me going back and forth between different repositories about this. My understanding from the outside (and quite far out) is that MathML Core is the spec being actively developed and that browsers are aligning to it. Updates to MathML 4 seem to happen whenever some decision is made about stuff that goes beyond the scope of Core. I was assuming that MathML 4 would be brought back in sync once MathML Core 'level 1'(?) is stabilised.

At any rate: I will post a configuration that implements MathML Core spacing for MathJax, as it might be useful for people in my situation, and it is easy to tweak (for spec updates or e.g. to work around quirks of MathML authoring tools).