Downgrading "may" tests

Since higher level CSS specs can have more stringent requirements
than CSS2.1 tests, some of these tests are "may" in CSS2.1 but
shouldn't be flagged as such for their Level 3 version.

I think we'd talked about having levelled "may" flags as a possible
solution. In any case, do we have a way forward here? Can we make one?

~fantasai