I switched to Youtube HTML5 after Aurora 15.0a2 consistently crash if I load more than 2-3 Flash applications (ex: Flash game and standard Youtube).
However, I noticed that upon loading a HTML5 video with a Flash game, Task Manager reported Flash plugin eating up more memory. Upon loading the fourth video, Flash application crashed and took down Youtube and the game.
I haven't tested on IE9 or Google Chrome yet.
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HTML5 still does not have all the features of flash. sometimes it is necessary to use flash still.