Support Math Tags (e.g. mfrac)

I hope this support math tag, such as mfrac. Can you give me some advices?

The issue names no files, tests, or entry points. First clarify which MathML tags and target renderers are required, then identify the parser and rendering locations and a regression test; done means the agreed tags render correctly across the stated targets.

索引モデルが issue の本文から書いたものです。