The newest versions of :nth-child() and :nth-last-child() accept an optional of S clause which filters the children to only those which match the selector list S. For example, :nth-child(1 of .foo) selects the first child among the children that have the foo class (ignoring any non-foo children which precede that child). Similar to :nth-of-type, but for arbitrary selectors instead of only type selectors.