this may have been possible in early ie editions but i can remember one, and with microsoft pushing out ie 8 to xp and 9 to vista and 7 it most likely changed then.
i dont think its possible now, once you change focus (i.e. alt tab back to wow) it takes it from full screen and shrinks it.
maybe try 3/4 mode you will notice below you tube 3 squares try the one to the left of full screen. its ont quite full screen but hey