var oDiv = document.getElementById("show");
var sTop = document.documentElement.scrollTop || document.body.scrollTop;
var cHeight= document.documentElement.clientHeight || document.body.clientHeight;
var mid = (cHeight - oDiv.offsetHeight) / 2;
// oDiv.style.top = parseInt(sTop + mid) + "px"; //理论上应该只设这句就可以居中吧
//实际上的 各大浏览器只认识下面这if判断
if(navigator.appVersion.indexOf("MSIE 6")> -1){
oDiv.style.top = parseInt(sTop + mid) + "px";
}else{
oDiv.style.top =mid + "px";
}
}