.centered {
	position: fixed ;
	width: 600px ;
	height: 400px ;
	/* Position the top-left corner at center of screen. */
	top: 50% ;
	left: 50% ;
	/* Translate element back-up and back-over. */
	margin-top: -200px ; /* Half of 400. */
	margin-left: -300px ; /* Half of 600. */
}