[2] Selectors Level 4 ( (11/14/2011, 7:03:20 PM 版)) <http://dev.w3.org/csswg/selectors4/#past-pseudo>
[3] Selectors Level 4 ( (11/14/2011, 7:03:20 PM 版)) <http://dev.w3.org/csswg/selectors4/#future-pseudo>
Google search: :past