In your WebVTT implementation, please add support for the ::cue pseudo element as specified at http://dev.w3.org/html5/webvtt/#the-'::cue'-pseudo-element . This will allow authors to use CSS to style captions and subtitles that are displayed through the <track> element.
This works just fine, e.g. <style> video::cue {color: lime;} video::cue(#\31\ first\ cue) {background-color: blue;} video::cue(v[voice="Tay Zonday"]) {color: red !important;} video::cue(:past) {color: lime;} video::cue(:future) {color: gray;} video::cue(c.brown) {color:brown; background-color: white;} </style> On a file such as: WEBVTT 1 first cue 00:00:10.000 --> 00:00:21.710 <v Tay Zonday>Chocolate Rain</v> 2 00:00:12.210 --> 00:00:21.710 <b>Some </b><i>stay </i><u>dry </u><c>and </c>others feel the pain 3 00:00:15.920 --> 00:00:21.170 <c.brown>Chocolate </c><u>Rain</u> 4 00:00:18.000 --> 00:00:21.170 <00:00:18.250><c>A </c><00:00:18.500><c>baby </c><00:00:19.000><c>born </c><00:00:19.250><c>will </c><00:00:19.500><c>die </c><00:00:19.750><c>before </c><00:00:20.500><c>the </c><00:00:20.750><c>sin</c> 6 00:00:21.180 --> 00:00:23.000 <00:00:21.200><c>Chocolate </c><00:00:21.500><c>Rain</c> See: http://html5videoguide.net/NEW/CH4/code_c4_14.html
I lost track of this bug. the ::cue pseudo element was added in r137955 for bug 10419