Chrome has this and I truly and completely hate the way Youtube uses it. The only reason I switched to HTML5 video on YT was the fact that it resized to the browser window, therefore not wasting massive amounts of screen estate on my 24" monitor. The way I'd use it was to have the video on 2/3 of the screen and Vim on the other 1/3.

Alas, those days are gone, unless I figure out a way to override requestFullscreen to always return false.