* 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-'::cue'-pseudo-element>
]REFS]

[2] [CITE@en[Web Applications 1.0 r7452     Allow 'opacity' and 'visibility' on ::cue]]
( ([TIME[2012-10-11 08:20:00 +09:00]] 版))
<http://html5.org/tools/web-apps-tracker?from=7451&to=7452>