Speaking of that, HTML5 is still in its infancy. Browsers have still problems in getting it properly implemented, and some have different behavior. Cross-browser compatibility suffers as a result, making it further difficult for a web developer to get a working site with advanced features in plain HTML. Flash on the other hand is a mature thing, and differences between versions are minimal, which simplifies development.
Cross-browser compatibility, broken of course by Google and their stupid VP9 standard for HTM5 videos (H264 is faster) just to save bandwidth...
Now, being serious, yeah, this is still in diapers, I mean, it's impossible that video cards capable of decoding H264 are blacklisted by Chrome or Firefox, and of course, it's more ridiculous that this is the only solution to fix their problems with HTML5 playback, in hopes you buy another card/laptop... I just wonder why programmers are so lazy this days, specially talking about browsers (again).
I mean, I can decode a full HD 60fps video locally, even with VLC (that may exhibit some problems unless you configure it to use OpenGL, which is faster than DirectX) and Firefox just derps with it, simply by setting the flag to "You can't decode because my detection routines don't work properly and I'm lazy as **** to fix it, ah! almost forgot, I force down your throat Ciscos's plugin for H264, which clearly demonstrates is way too slow for this, and gives problems with decoding detection, so your CPU will suffer the consequences, just instead of making t optional for people that can't decode anything to save their lives - same for me, Google Chrome"... Yeah, latest Nighly build seem to fix many issues, but not all, and things go on...
The ony thing Flash seems to not allow is 60 fps playback (which is a shame), but most of the times is way more stable than HTML5... And it's just because of lazyness...
So yeah, Flash is to stay at least a couple of years more, until browsers can do something decent with HTML5 multimedia handling...