让一个块级元素在某区域内上下左右居中

	.container {
	border: 1px solid #000;
	width: 300px;
	height: 300px;
	position: relative;
	}
	
	.box {
	border: 1px solid #000;
	width: 50px;
	height: 50px;
	
	position: absolute;
	margin: auto;
	left: 0;
	right: 0;
	top: 0;
	bottom: 0;
}

效果图：