Some things that need testing:
   :nth-child(2n+3)
   :nth-child(2n-2)
   :nth-child(2n-3)
   :nth-child(-2n+1)
   :nth-child(-1n+6)
   :nth-child(-2)
   :nth-child(-2n-2)
   :nth-child(-n-2)
   summary file

See also:
   http://www.w3.org/mid/40B8E7FA.3040409@mit.edu