A description of compositing/video-page-visibility.html from jamesr:
"That test is (effectively) a repaint test. The way repaint tests work is, on each paint we basically draw a 50% opaque gray rectangle over the whole viewport, then paint the rect(s) that were invalidated since the last paint at 100% alpha. On this test, we do a bunch of paints with no damage, so you see the first paint at 1/2^N alpha, where N is the number of repaints.
"Unfortunately, on this test I don't think we care at all about N, and I think N isn't entirely stable. File a new bug to rework the test -- in its current form it isn't telling us anything except that we aren't hitting any ASSERTs (which is somewhat useful)."