Generated content and replaced inlines

I'm told that CSS3 will not allow generated :before/:after content on 
replaced elements.  CSS2.1 doesn't explicitly say anything about it one 
way or another, though a reasonable reading of the spec would imply that 
something with no children can't have generated content.

CSS2 is like CSS2.1, except its example of generated content uses an <img>.

As a result, authors and browser implementors have tended to assume that 
replaced elements can have :before/:after generated content, from what I 
can see.

Would it be at all possible to have CSS2.1 explicitly say that this is 
not supported?  Otherwise, by the time the relevant CSS3 module is ready 
there will be enough UAs out there supporting it and enough author 
expectations that it works that changing the behavior will effectively 
be impossible....

-Boris