[CSS21] Generated Content - properties that can be applied to :before/:after

I seem to remember that in the Level 2 spec, there was some guidance  
on which properties could be applied to :before and :after. However,  
I've looked at both L 2.1 and L 3 specs, and it seems the guidance has  
been removed - has this restriction been lifted? (I did do some  
digging around in the mail archive and couldn't find anything relating  
to this)