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

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jquery方法



$(document).ready(function(){
						   
 $(window).resize(function(){

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

});

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