See bug 1337068 comment 3. In general, Gecko's CSS engine is much more feature-complete than Servo's, but we have no reason to believe it's a strict super-set. We need to audit before shipping.
I don't think stylo still supports unprefixed :read-only and :read-write now.
Adding dev-doc-needed keyword and ni'ing teoli (who works on CSS and DOM docs IIRC) to keep him informed. Is there or will these be a way to know the complete list of features stylo has implemented that Gecko didn't have?