.center_div {
	width: 300px;
	height: 300px;
	background-color: red;

	position: absolute;
	top:0;
	bottom: 0;
	left: 0;
	right: 0;

	margin: auto;
}

#example {
	top: 50%;
	left: 50%;
	width: 50em;
	height: 16em;
	margin-top: -8em;
	margin-left: -25em;
	position: fixed;
}