Please consider making the spec state that the value of the "sizes" attribute must not contain CSS comments. (In other words, that authors must not use CSS comments in anywhere in the the "sizes" attribute.)

I recognize there are some hypothetical use cases for allowing CSS comments in "sizes" but I've yet to see any actual examples of anybody putting comments in there. 

So while UAs should still need to support parsing of "media" values that contain comments, conformance checkers would for now report the comments as invalid.

Then if we eventually end up finding that actual web authors are finding it unreasonable that CSS comments in sizes are not allowed, we can drop the requirement from the spec. But I'd suggest we start by trying out the requirement and seeing what happens.