菜鸟问问题

Question

<style>
#myDiv {position:absolute;left:100px;top:100px;width:100px;height:100px;border:1px solid #F00;background-color:#F90;display:none;}
</style><div id="myDiv" onmousedown="startMove(this)"><img src="1.gif" width="100" height="100" alt="图片"></div>
<input type="button" value="显示层" onclick="ctrDiv(this)"><script language="javascript">
function ctrDiv(bt)
{
var div=document.getElementById("myDiv");
if(div.style.display=="")
{
div.style.display="block";
bt.value="隐藏层";
}
else
{
div.style.display="";
bt.value="显示层";
}
}var objDiv,startX,startY,posX,posY;
function startMove(obj)
{
document.attachEvent("onmousemove",moveDiv);
document.attachEvent("onmouseup",endMove);
objDiv = obj;
startX = event.x;
startY = event.y;
posX = obj.getBoundingClientRect().left;
posY = obj.getBoundingClientRect().top;
obj.style.cursor="move";
}function moveDiv()
{
if(objDiv)
{
objDiv.style.left = posX + event.x-startX;
objDiv.style.top  = posY + event.y-startY;
}
}function endMove()
{
document.detachEvent("onmousemove",moveDiv);
document.detachEvent("onmouseup",endMove);
}</script>