I filed a Chromium issue today, which came up while naturally doing some CSS work over MathML:
https://issues.chromium.org/issues/442636151

with the following codepen example
https://codepen.io/dginev/pen/KwdrjKN

Basically, attr(mathcolor) and attr(mathbackground) don't resolve to their values in Chrome or Firefox. I wanted to use them to set color properties in CSS, but that may be independently limited.

I did a quick wpt search for mathcolor and I think there are no tests using them with attr().

Maybe we should add a couple of tests, if we are keeping the attributes? I am specifically interested in CSS color mixing.