一个简单的拖动层(兼容IE,FF)

Code
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>拖动层</title>
    <style type="text/css">
    .main
    {
        position: absolute;
        background-color: #fff;
        top: 100px;
        left: 100px;
        z-index: 101;
        border: solid 1px #ccc;
    }
    .titlebar
    {
        background-color: #ccc;
        cursor: move;
        height: 20px;
        color: #fff;
        font-size: 13px;
        padding-top: 5px;
        padding-left: 10px;
    }
    </style>
</head>
<body>
    <div id="main" class="main" style=" 500px; height: 350px;">
        <div id="titlebar" class="titlebar">
            拖动层
        </div>
        <div id="Div1" class="main" style=" 200px; height: 50px;">
            <div id="Div2" class="titlebar">
                拖动层2
            </div>
        </div>
    </div>

    <script type="text/javascript">
        new drag("main", "titlebar");
        new drag("Div1", "Div2");

        function drag(main, titlebar) {
            var obj = document.getElementById(main);
            var title = document.getElementById(titlebar);
            if(obj && title)
            {
                var posX;
                var posY;
                var offset = 10;
                var mousemove = function(evt) {
                    if (evt == null) evt = window.event;
                    var left = evt.clientX - posX;
                    var top = evt.clientY - posY;
                    var limitLeft = 0;
                    var limitTop = 0;
                    if(obj.parentNode.nodeName != "BODY")
                    {
                        limitLeft = obj.parentNode.clientWidth - obj.clientWidth - offset;
                        if(limitLeft < left)
                        {
                            left = limitLeft;
                        }
                        limitTop = obj.parentNode.clientHeight - obj.clientHeight - offset;
                        if(limitTop < top)
                        {
                            top = limitTop;
                        }
                    }
                    else
                    {
                        limitLeft = window.screen.availWidth - obj.clientWidth - offset;
                        if(limitLeft < left)
                        {
                            left = limitLeft;
                        }
                        limitTop = window.screen.availHeight - obj.clientHeight - offset;
                        if(limitTop < top)
                        {
                            top = limitTop;
                        }
                    }
                    if(left < offset)
                    {
                        left = offset;
                    }
                    if(top < offset)
                    {
                        top = offset;
                    }
                    obj.style.left = left + "px";
                    obj.style.top = top + "px";
                }

                title.onmousedown = function(evt) {
                    if (!evt) evt = window.event;
                    posX = evt.clientX - obj.offsetLeft;
                    posY = evt.clientY - obj.offsetTop;
                    document.onmousemove = mousemove;
                }

                document.onmouseup = function() {
                    document.onmousemove = null;
                }
            }
            else
            {
                alert("参数错误!");
            }
        }
    </script>

</body>
</html>