#contenedor {
	width: 800px;
	height: 400px;
	position: absolute;
	top: 50%;
	left: 50%;
	margin-top: -200px;   /* height/2 = 500px / 2 */
	margin-left: -400px;  /* width/2 = 500px / 2 */
}
body{
	background-color:#333;
}