* 仕様書

[REFS[
- [1] [CITE@en-US-x-hixie[WebVTT Standard]] ([TIME[2011-12-06 00:38:23 +09:00]] 版) <http://dev.w3.org/html5/webvtt/#the-':past'-and-':future'-pseudo-classes>
]REFS]

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