让createElement创建的div层，始终在屏幕的中央，位置怎么控制啊！

Question

   
function CreateWaitElement() 
{
    var elem = document.getElementById('__AjaxCall_Wait');
    if (!elem) 
    {
       elem = document.createElement("div");
                             elem.id = '__AjaxCall_Wait';
                             elem.style.position = 'absolute';
                             //
                             elem.style.height = 25;
                             elem.style.width = 120;
                             elem.border = "1px";
                             elem.style.fontSize = "18px";
                             //elem.style.align="center";
                             elem.style.borderColor = "#990000";
                             elem.style.borderWidth = "0";
                             elem.style.borderStyle="solid";
                             elem.style.backgroundColor = "#990000";
                             elem.style.color = "#FFFFFF";
                             elem.innerHTML = "Load Date...";
                             alert(screen.availWidth-elem.style.pixelWidth);
                             alert(screen.availHeight-elem.style.pixelHeight);                             //alert(elem.style.pixelLeft=(screen.availWidth-elem.style.pixelWidth)/2); 
                             //alert(elem.style.pixelTop=(screen.availHeight-elem.style.pixelHeight)/2); 
                             elem.style.visibility = 'hidden';
      document.body.insertBefore(elem, document.body.firstChild);
    }
    waitElement = elem;
}
＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝下面的2行代码可以正确获得当前窗口的高度和宽度
alert(screen.availWidth-elem.style.pixelWidth);
alert(screen.availHeight-elem.style.pixelHeight);
－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－
下面的代码用来设置层的左边和上边的高度，把高度和宽度除2，使div层始终在窗口的中央
elem.style.pixelLeft=(screen.availWidth-elem.style.pixelWidth)/2; 
elem.style.pixelTop=(screen.availHeight-elem.style.pixelHeight)/2; 可实际执行结果是：
层横向始终在中央，但是top高度确始终在窗口的顶层，＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝想问一下高手们，层的那个属性是控制，层位置的！～我已经整了一周了  

Answer 1

DIV的STYLE属性中的LEFT和TOP就是控制层的位置的..

Answer 2

function CreateWaitElement() 
{
    var elem = document.getElementById("__AjaxCall_Wait");
    if (!elem) 
    {
       elem = document.createElement("div");
                             elem.id = '__AjaxCall_Wait';
                             elem.style.position = 'absolute';
                             //
                             elem.style.height = 25;
                             elem.style.width = 120;
                             elem.border = "1px";
                             elem.style.fontSize = "18px";
                             //elem.style.align="center";
                             elem.style.borderColor = "#990000";
                             elem.style.borderWidth = "0";
                             elem.style.borderStyle="solid";
                             elem.style.backgroundColor = "#990000";
                             elem.style.color = "#FFFFFF";
                             elem.innerHTML = "Load Date...";
                             alert(screen.availWidth-elem.style.pixelWidth);
                             alert(screen.availHeight-elem.style.pixelHeight);
                             elem.style.pixelLeft=(screen.availWidth-elem.style.pixelWidth)/2; 
                             elem.style.pixelTop=(screen.availHeight-elem.style.pixelHeight)/2; 
                             //alert(elem.style.pixelLeft=(screen.availWidth-elem.style.pixelWidth)/2); 
                             //alert(elem.style.pixelTop=(screen.availHeight-elem.style.pixelHeight)/2); 
                             //elem.style.visibility = 'hidden';
      document.body.insertBefore(elem, document.body.firstChild);
    }
    waitElement = elem;
}
试了一下，没出现你说的问题。

Answer 3

这是MagicAjax控件里面的一个js文件，我也测了一下，在单独做成文件，测试可以，但是放到MagicAjax文件里面执行的时候就是不能居中！