You could always clean up the test element in the interim using the callback and take the workaround out when Modernizr includes the fix.

Also should we use MathML test extensions that we have in v3.1? The problem with the Modernizr test (at least back then) was that it let very buggy MathML implementations through where it couldn't do more than the most basic MathML. The extensions we added in v3.1 tested for more advanced MathML as well. Should probably go to Modernizr upstream if the test extensions are a good idea.