js的图片无缝上下滚动

<div id=demo style=”overflow:hidden; width:475; height:80px;”> 
 <div id=demo1> 
 <a href=”#” charset=”utf-8″ target=”_blank”><img src=”images/ad1.jpg”></a>
 <br>
 <img src=”images/ad2.jpg”>
 <br>
 <img src=”images/ad1.jpg”>
 <br>
 </div> 
 <div id=demo2></div> 
 </div> 
 <script language=”javascript”> 
 var speed=20
 document.getElementById(“demo2”).innerHTML=document.getElementById(“demo1”).innerHTML 
 function Marquee(){ 
 if(document.getElementById(“demo2”).offsetTop-document.getElementById(“demo”).scrollTop<=0) 
 document.getElementById(“demo”).scrollTop-=document.getElementById(“demo1”).offsetHeight 
 else{ 
 document.getElementById(“demo”).scrollTop++ 
 } 
 } 
 var MyMar=setInterval(Marquee,speed) 
 document.getElementById(“demo”).onmouseover=function() {clearInterval(MyMar)} 
 document.getElementById(“demo”).onmouseout=function() {MyMar=setInterval(Marquee,speed)} 
 </script>