I think this element is only partially implemented in Webkit (in Firefox too).
MathJax relies on this attribute to implement various LaTeX commands. For example the values "script", "double-struck" or "fraktur" are respectively needed for \mathcal, \mathbb and \mathfrak.
I think this can be closed. There are some edge cases we do not support yet (e.g. we only apply mathvariant transform on token elements with a single char which corresponds to the cases used in practice) but other follow-bugs can be opened if necessary.