DIV的绝对居中

<script type='text/javascript'>
        function myPopupRelocate() {
            var scrolledX, scrolledY;
            if (self.pageYOffset) {//IE9 起作用
                scrolledX = self.pageXOffset;
                scrolledY = self.pageYOffset;
                alert("self.pageYOffset");
            } else if (document.documentElement && document.documentElement.scrollTop) {// IE 6 ,360浏览器等起作用
                scrolledX = document.documentElement.scrollLeft; 
                scrolledY = document.documentElement.scrollTop;
                alert("document.documentElement && document.documentElement.scrollTop");
            } else if (document.body) {//Chrome... IE9 Firfox....IE 5.5起作用
                scrolledX = document.body.scrollLeft;
                scrolledY = document.body.scrollTop;
                alert("document.body");
            }
           //以上是浏览器滚动的距离
           // alert("scrolledX:" + scrolledX);
           // alert("scrolledY:" + scrolledY);

            var centerX, centerY;
            if (self.innerHeight) {
                centerX = self.innerWidth;
                centerY = self.innerHeight;
            } else if (document.documentElement && document.documentElement.clientHeight) {
                centerX = document.documentElement.clientWidth;
                centerY = document.documentElement.clientHeight;

            } else if (document.body) {
                centerX = document.body.clientWidth;
                centerY = document.body.clientHeight;
            }

            alert("centerX:" + centerX);
            alert("centerY:" + centerY);

            var leftOffset = scrolledX + (centerX - 250) / 2;
            var topOffset = scrolledY + (centerY - 200) / 2;
            document.getElementById("mypopup").style.top = topOffset + "px";
            document.getElementById("mypopup").style.left = leftOffset + "px";
        }
        function fireMyPopup() {
            myPopupRelocate();
            document.getElementById("mypopup").style.display = "block";
            //            document.body.onscroll = myPopupRelocate;
            //            window.onscroll = myPopupRelocate;
        }
    </script>

<div id='mypopup' name='mypopup' style='position: absolute; width: 250px; height: 200px;
        display: none; background: #ddd; border: 1px solid #000; z-index: 100'>
        <p>
            我现在的位置是居中状态<br>
            </p>
        <input type='submit' value=' 关闭窗口! (2) ' onclick='document.getElementById("mypopup").style.display="none"'>
    </div>
    <input type='submit' value=' Fire! (2) ' onclick='fireMyPopup()'>

var leftOffset = scrolledX + (centerX - 250) / 2;
var topOffset = scrolledY + (centerY - 200) / 2;

var leftOffset = scrolledX + 100;
var topOffset = scrolledY + 100;