I saw that "subset" term as well but I couldn't find a list that defined which ones were/were not included in the subset - it just went on to section 3 where it said ALL pseudo elements/classes must be supported.
Do you know where that list could be found if there is such a beast?
 ok now that I'm awake, let me clarify the drivel in my previous statement...
Jellby's reference on how ePub spec relates to CSS declares that the spec only requires a compliance with a subset of properties
- and the link points to section 3 where it describes the subset of properties among the many tables. But the only reference I can see to selectors
in section 3.1 is that ALL pseudo elements/classes MUST be included.
Is there a list that describes a subset of selectors??