居中显示DIV

.loginBG{
	position:absolute;
	top:50%;
	left:50%;
	margin-top:-250px;
	margin-left:-250px;
	/*此时宽和高都要固定*/
	width:500px;
	height:350px;
	text-align: center;
	border:0px solid red;
	padding:0;
	background: url(images/background.png);
	border-radius: 10px;
}