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.
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.
"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."