仕様書

[1] WebVTT Standard (2011-12-06 00:38:23 +09:00 版) http://dev.w3.org/html5/webvtt/#the-':past'-and-':future'-pseudo-classes

[2] Selectors Level 4 ( (2011-11-15 04:03:20 +09:00 版)) http://dev.w3.org/csswg/selectors4/#past-pseudo

[3] Selectors Level 4 ( (2011-11-15 04:03:20 +09:00 版)) http://dev.w3.org/csswg/selectors4/#future-pseudo