The YouTube developer blog lists several things Flash can do that HTML5 video tags cannot:
Flash cuts down on the number of formats YouTube needs to encode. With browsers divided over which video codecs to use, YouTube would need to re-encode most of its content. With YouTube users uploading 24 hours of video to the site every minute, that’s no small task. The new WebM video codec offers some hope here, but it isn’t universally supported yet.
YouTube already does this anyway. The codec issue is therefore already resolved with the exception of a few mozilla hippies who were insisting on ogg but may now settle for VP8 in any case.
Flash offers “fine control over buffering and dynamic quality control.”
Hadn’t really noticed, it all looks pretty bad to me a lot of the time.
The HTML5 video tag doesn’t cover live streaming, nor does it allow for adaptive video quality when streaming long movies. However, as the post points out, “a number of vendors and organizations are working to improve the experience of delivering video over HTTP,” meaning there’s hope this problem will eventually be solved.
Hopefully quicker than flash is adapted for mobile. I was under the impression streaming HTTP was already used in iPhone but could be wrong.
Flash offers content protection.
While not the top of the list when it comes to features a user is looking for, without a means of protecting content from being distributed illegally, most of YouTube’s content partners would likely jump ship.
Name me a single user who wants “copy protection” or a “copy protection” that actually works.
Encapsulation and embedding.
Guess what? The same browser that delivers HTML5 likely supports JS too.
This one makes the least sense. Firefox and WebKit both offer rudimentary support for fullscreen HTML5 video, though there is no hardware acceleration or other extras you’d get with Flash.
Hardware acceleration in flash on a mac would be nice too one day.
Camera and microphone access.
The ability to record video directly to YouTube requires the site to be able to access your computer’s camera and microphone, something HTML5 video on its own cannot do.
Hardly a mainstream requirement, but it will be interesting to see how HTML5 integrates with device hardware features as it evolves.
YouTube also doesn’t mention a couple of other areas where HTML5 video lags well behind Flash: accessibility and translation tools.
Accessible flash? Really? I’ve tried to find out more about this and usually find precious little evidence even though I now people who are paid to enable this. Yet to see a flash translation, but I don’t doubt it can be done any more than it can be done in JS.