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You need to do some kind of validation here, no? Pseudo-elements cannot just be any code unit sequence. Apart from that if this is supposed to match CSS ::{pseudo} then there's another problem as currently CSS requires such unknown pseudo-elements to be dropped per its parsing rules (WebKit might have a bug there).
"pseudo" is now "part", and it needs to be rewritten anyway. Duping against "part" bug. *** This bug has been marked as a duplicate of bug 22461 ***