Seems like it isn't perfect yet. At least on Firefox I don't get to watch videos...

Nexxxeh · on Jan 28, 2015

I watched a 1080p video using HTML5 on Opera yesterday.

As a test, I just popped open the first interesting looking video on my signed-in YouTube front page (my account has been set to HTML5 for a while).

I'm not a fan of the Yeah Yeah Yeahs but the video does have Lily Cole is in 1080p. Opera 27.0.1689.54 on Win 8.1 x64.

spain · on Jan 28, 2015

I think it might be an issue with codecs which is why it works with some browsers and doesn't with others.

jconnop · on Jan 30, 2015

TL;DR: For now you need to use beta/aurora/nightly releases of firefox to enable 1080p/4K youtube with the HTML5 player.