Another thing to consider is that whenever a format works in "all" browsers (for some small values of "all" that devs care to test in), it becomes a de-facto pa

Another thing to consider is that whenever a format works in "all" browsers (for some small values of "all" that devs care to test in), it becomes a de-facto part of the Web platform, and browsers end up having to support it forever. This makes browser vendors very cautious about adding anything, because even if it's easy now, it's may haunt them later. Maybe they could use HW accell on current-gen GPUs, but what if the next-gen GPUs move to a newer codec and drop H.265? Browsers will still be expected to play existing web content encoded in the old codec, but now with a patent liability and no hardware to subsidize it.