DIV水平和垂直居中问题

.className{
width:270px;
height:150px;
margin:0 auto;
}

.className{
width:270px;
height:150px;
position:absolute;
left:50%;
top:50%;
margin:-75px 0 0 -135px;
}

$(document).ready(function(){

$(window).resize(function(){

$('.className').css({
position:'absolute',
left: ($(window).width()
- $('.className').outerWidth())/2,
top: ($(window).height()
- $('.className').outerHeight())/2
});

});