js实现div在页面拖动效果

<style type="text/css">
body {
 margin: 0px;
}
#div1 {
 display: none;
 position: absolute;
 z-index: 1000;
 height: 100%;
 width: 100%;
 background: #000000;
 filter:Alpha(opacity=30);
}
#div2 {
 display: none;
 position: absolute;
 height: 100%;
 width: 100%;
 padding-top: 10%;
 z-index: 1001;
}
#div3 {
 display: block;
 position: absolute;
 z-index: 999;
}
</style>
<script type="text/javascript">
//定义移动对象和移动坐标
var Mouse_Obj="none",_x,_y;
//拖动对象函数(自动)
document.onmousemove=function()
{
 if(Mouse_Obj!=="none")
 {
 document.getElementById(Mouse_Obj).style.left=_x+event.x;
 document.getElementById(Mouse_Obj).style.top=_y+event.y;
 event.returnValue=false;
 }
}
//停止拖动函数(自动)
document.onmouseup=function()
{
 Mouse_Obj="none";
}
//确定被拖动对象函数 o为被拖动对象
function m(o)
{
 Mouse_Obj=o;
 _x=parseInt(document.getElementById(Mouse_Obj).style.left)-event.x;
 _y=parseInt(document.getElementById(Mouse_Obj).style.top)-event.y;
}
</script>
<div id="div1"></div>
<div id="div2" onmousedown="m(this.id)" style="left: 0px;top: 0px;">
<table width="50%" border="0" cellpadding="3" cellspacing="1"
style="background: #ff7300;
position:static;filter:progid:DXImageTransform.Microsoft.DropShadow
(color=#666666,offX=4,offY=4,positives=true)" align="left">
 <tr style="cursor: move;">
  <td><font color="#FFFFFF">温馨提示：</font></td>
  <td align="right"><input type="button" value="ｘ"
onClick="document.getElementById
('div1').style.display='none';document.getElementById
('div2').style.display='none';" style="cursor: hand;"></td>
 </tr>
 <tr>
  <td colspan="2" width="100%" bgcolor="#FFFFFF" height="150"
align="middle">欢迎访问　<a href="http://www.jb51.net">http://www.jb51.net</a></td>
 </tr>
</table>
</div>
<div id="div3"><input type="button" value="打开层"
onClick="document.getElementById
('div1').style.display='block';document.getElementById
('div2').style.display='block';"></div>