CSS/常に中央に表示

#sampleDiv {
	width: 300px;
	height: 200px;
	text-align: center;
	line-height: 200px;
	background: green;
	position: fixed;
	top: 0px;
	right: 0px;
	bottom: 0px;
	left: 0px;
	margin: auto;
}