Full-screen mode is an experimental feature, and may not work well with some games.
To get the most out of full-screen mode, it's not enough to click the full-screen icon—you must also put your browser into full-screen mode.
In most browsers, you can do this by pressing F11. (You may have to click somewhere on the page other than the game itself before this will work.) Apple Safari does not offer a full-screen mode.