HTML5 doesn't work with all video's, It doesn't work with any video which has ads for example.
It also doesn't work very well IMHO. stuff like fullscreen etc. suck. In my experience, the flash player works better, at least on Windows.
IMHO using
youtube_dl is still the best solution. And since UNIX doesn't mandatory lock *everything*, you can just start playing with mplayer/vlc/... after a few seconds without downloading the whole file first (Doesn't work on Windows).