Polyfills

I went through the spec and created an issue
<https://github.com/w3c/mathml/issues/534>that lists all the
elements/attributes that are not in core. I marked all the ones that
already have polyfills.

Please take a look at the issue and add anything that I missed (edit the
list).

I think that some of the entries in the list don't merit a polyfill. For
example, I don't see the point of supporting "xref" since it hasn't been
valid in a long time. At the next meeting, we can hopefully reach a
decision as to which polyfills we don't think are needed.

Looking at the list, the biggest holes are polyfills mtable/mtr/mtd
attributes. It would be good to get some priorities on which are most
important to support.

   Neil