控制div始终居中显示

 

//下面这段是随屏幕大小改变而改变

window.onresize = window.onload = function(){
     var w,h
     if(!!(window.attachEvent && !window.opera))
     {
      h = document.documentElement.clientHeight;
      w = document.documentElement.clientWidth;
     }else{
      h = window.innerHeight;
      w = window.innerWidth;
     }
    var bgImg = document.getElementById('kuaijie');
    bgImg.width = (w - 5);
    bgImg.height= (h-5) ; 
         
   }  

//控制DIV居中显示

top:50%; left:50%;  margin-top:-200px; margin-left:-350px;position:absolute;