Our codebase assumes that a selector only contains a single
pseudo-element, and that it's in the final compound selector. If there
are multiple of them, or they're somewhere else in the selector, we
just silently pretend the others aren't there, which is *not* what we
want, and causes the selector to match things it shouldn't.
A proper fix is quite involved, so as a temporary fix, just reject any
selector that doesn't fit our assumptions during parsing. That way we
get false negatives instead of false positives.
