The Closed Captions button in the YouTube “HTML5” player jiggles when the timeline animates. To reproduce: navigate to the URL, and as the video begins to play, hover over the video, then hover off, causing the timeline and playhead at the bottom to animate in and out.
Please retry with a recent WebKit version. It might be the same as bug #115304 and fixed.
The CC button is a <button>. Duplicated.
*** This bug has been marked as a duplicate of bug 101848 ***