So, a member on my site pointed out that if you click play on an embedded YouTube video that is being shown in a quote, trying to play in full screen mode in Chrome only loads the small player in the top left corner
Tested on 2 sites now, different themes. Tested in Firefox, and full screen works fine.
I thought this was just down to the size of the video bugging out, but it's actually because we have a max-width/height specified and the browser definition only overrides width/height themselves. I have managed to workaround this in Webkit/Chrome with their -webkit-full-screen pseudo-class.
IE 11 appears to have the same issue, and despite having a -ms-fullscreen pseudo-class, I'm unable to workaround this there. (-ms-fullscreen doesn't seem to do anything, so presumably it's not being applied to the iframe.) Note that IE has problems with just a plain width/height specified on the iframe, so there's something more that they're not properly handling here.