I thought there was a way to play youtube full screen in webkit by using the Safari 11 UA? the older UA's in webkit won't allow certain features like full screen. I can't remember where to find switch UA's but I've known other people who have watched youtube in fullscreen using webkit, including the user MrPilot on here who claimed to do it perfectly at just 1ghz.
It's definitely possible, from memory I can't remember which user agent it is.