function vmiddle(){
var middlediv=document.getelementbyid("wrap");
var divheight=middlediv.offsetheight;
var bodyheight=document.body.offsetheight ;
if(bodyheight>divheight)
middlediv.style.margintop=-divheight/2+"px";
else{
middlediv.style.margintop=0;
middlediv.style.top=0;
}
}
html,body{height:100%;padding:0;margin:0;}
#wrap{width:800px;margin:0 auto;border:1px solid red;position:absolute;top:50%;left:50%;margin-left:-400px;}
jb51.net
[ctrl+a 全选 注:如需引入外部js需刷新才能执行]