The builtin fullscreen button for html games misaligns the iframe in fullscreen mode. It puts it 20px lower than it should be, leaving a black bar at the top and cutting off the bottom of the game.

Adding 'margin: 0 !important' to the '.game_frame:fullscreen' css is enough to fix this.

I realise this is long into the future, but how would you add this CSS into the appropriate space? and where?