In my view, the great advantage of HTML5 at the user level over Flash is that the consumption of the CPU is drastically reduced, so so does the battery -if you have a MacBook- and the computer does not drown us by watch a couple of videos.
FlashToHTML5 is a Safari extension which converts whenever it can will present us with an HTML5 player instead of the corresponding one in Flash, although it always depends on YouTube having the version of the video available for HTML5, something that does not always happen.
They are 9 measly kilobytes that, honestly, for me they are essential.
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