One thing that continues to puzzle me is the lack of support for elementary math in many editors despite it being used by every student in every math class for at least five years of their life. Textbook publishers really need it and there are tons of (often poorly displayed) examples and tutorials on the web that use it. It was added to MathML 3 although support in many implementations of MathML has been poor to non-existent (including in MathJax).

I don't know if this is a sign that implementers don't care about elementary math (even Knuth left out support in TeX and someone had to write a complicated macro package to make it work). Or maybe this is a sign that it is hard. I implemented it in MathPlayer by using the table implementation and spreading the digits out into their own columns. Hence, it wasn't that complicated, but I did need to tweak MathPlayer's internal table code to add support for partial lines between rows (i.e., lines that start in column i and end in column j). Perhaps this can be done by using a colspan and the border property for where the mline should go.

The other alternative (and I suspect the likely one) is to exclude elementary math from core and write some JS that implements it via table. It loses some semantics that way, but maybe whatever solution is developed to help disambiguate presentation MathML (#64) can be used if such a translation is done.