This bug is here to ensure that all the aspects in the WebVTT spec related to basic CSS styling (section 3.5.1) of the text track cues are completely supported. http://dev.w3.org/html5/webvtt/#applying-css-properties-to-webvtt-node-objects
The ::cue(#id) selector should match a WebVTT cue with the given id. It's seems it's not supported in WebKit. Presto supports it. Test: http://w3c-test.org/webvtt/rendering/cues-with-video/processing-model/selectors/cue_function/id_color.html http://w3c-test.org/webvtt/rendering/cues-with-video/processing-model/selectors/cue_function/id_color-ref.html Spec: "For the purposes of ID selector matching, lists of WebVTT Node Objects have the ID given by the cue's text track cue identifier, if any." http://dev.w3.org/html5/webvtt/#dfn-pseudo-cue-selector
All browsers are failing this 'WPT Test': https://wpt.fyi/results/webvtt/rendering/cues-with-video/processing-model/selectors/cue_function/id_color.html?label=experimental&label=master&aligned= Adding 'WPTImpact' tag and also changing from 'Unconfirmed' to 'New' to track it better.