自己定义的模块实现先下滚动

<div id="demo" style=" height:120px; ovflow:hidden;">
<div id="demo1">
[loop]
<div class="Ant-Cl" >
<div class="NewsUpfile">
<div><a href="{url}"{target}><img src="{pic}" width="{picwidth}" height="{picheight}" alt="{title}" /></a></div>
</div>
<div class="New">{summary} <a href="{url}" title="{title}"{target}>[详细]</a></div>
</div>
[/loop]
</div>
<div class="Ant-Cl" id="demo2"></div>
</div>

<script type="text/javascript">
var speed=10;
var tab=document.getElementById("demo");
var tab1=document.getElementById("demo1");
var tab2=document.getElementById("demo2");
tab2.innerHTML=tab1.innerHTML;
function Marquee(){
if(tab2.offsetTop-tab.scrollTop<=0)
tab.scrollTop-=tab1.offsetHeight
else{
tab.scrollTop++;
}
}
var MyMar=setInterval(Marquee,speed);
tab.onmouseover=function() {clearInterval(MyMar)};
tab.onmouseout=function() {MyMar=setInterval(Marquee,speed)};

</script>

但是不动  怎么回事  哪有问题  谢谢！