Note that this one in particular is _not_ an EVENT_STATE pseudo-class, so it's not a dupe of bug 1350140. But it's trivial to implement, so I might as well do it right after that bug lands.
Oh I see - I saw it in the list in your patch, but realized that that was the list of "Pending pseudo-classes", which presumably means the unimplemented ones. So I had it backwards. :-)