We fail the Opera-submitted test at http://w3c-test.org/html/tests/submission/Opera/media/interfaces/TextTrackCue/onenter.html. We fail the first test because it doesn't appear that we dispatch the "enter" event on the TextTrackCue, possibly because it's not attached to a TextTrack or any sort of tree. The DOM events spec doesn't seem to force the element to be participating in a tree to allow an event to be dispatched to it, so I think the test is accurate and we have a bug (see http://dom.spec.whatwg.org/#concept-event-dispatch).
<rdar://problem/12758086>
We run into the same issue for the "exit" event as highlighted in the test at http://w3c-test.org/html/tests/submission/Opera/media/interfaces/TextTrackCue/onexit.html.