[selectors] Matching of :first-child and the like for elements whose parent is not an element

The selectors spec says, at 
http://dev.w3.org/csswg/selectors4/#the-first-child-pseudo :

   The :first-child pseudo-class represents an element that is the first
   child of some other element.

and similar for other structural pseudo-classes.

On the face of it that means such selectors never match root elements or 
children of document fragments.

Is this restriction purposeful?  It seems like sane structural 
pseudo-class matching (e.g. nth-of-type) might be desirable with 
querySelector on DocumentFragment at the very least...

-Boris