这个filter有啥作用

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>t透明度动画</title>
</head>

<style>
body,div{
margin:0;
padding:0;
}
#div1{
width:200px;
height:200px;
background:red;
filter:alpha(opacity:30);
opacity:0.3;

}
</style>
<script>
window.onload=function(){
var oDiv=document.getElementById('div1');
oDiv.onmouseover=function(){
startMove(100);
}
oDiv.onmouseout=function(){
startMove(30);
}
}
var timer=null;
var alpha=30;
function startMove(iTarget){
var oDiv=document.getElementById('div1');
clearInterval(timer);
timer=setInterval(function(){
var speed=0;
if(alpha>iTarget){speed=-10;}
else{speed=10;}
if(alpha==iTarget){clearInterval(timer);}
else{alpha+=speed;
oDiv.style.filter='alpha(opacity:'+alpha+')';
oDiv.style.opacity=alpha/100;
}
},30)

}
</script>
<body>
<div id="div1"></div>
</body>
</html>
其中，oDiv.style.filter='alpha(opacity:'+alpha+')';
filter:alpha(opacity:30);
去掉后没有变化啊，求赐教，我实在没看懂