兼容FF/IE跟随鼠标的层的效果

#level{
border: 1px solid #ccc;
background: #CCFFCC;
width:100px;height:30px;
position:absolute;
}



function addEvent(elm, evType, fn, useCapture)
{
if (elm.addEventListener){
elm.addEventListener(evType, fn, useCapture);
return true;
} else if (elm.attachEvent){
var r = elm.attachEvent("on"+evType, fn);
return r;
} else {
alert("Handler could not be removed");
}
}
var x,y,t;
function doit(evt){
var event=evt||window.event;
var level=document.getElementById("level");
/*如果是第一次调用该函数level.style.left和level.style.top都为空，所以要初始化*/
if(!level.style.left)level.style.left="1px";
if(!level.style.top)level.style.top="1px";
//用setTimeout函数调用该函数时event对象不存在，只有移动事件触发时event才存在
if(event){x=(event.x||event.pageX)};
if(event){y=(event.y||event.pageY)};
var theLeft=parseInt(level.style.left);
var theTop=parseInt(level.style.top);
if(x!=theLeft){
if(theLeft