网页层变成半透明的方法

<style type="text/css">
<!--
#apDiv1 {
 position:absolute;
 left:20px;
 top:92px;
 width:505px;
 height:445px;
 z-index:1;
 background-color: #000000;
}
-->
</style>
 
 
<span id="apDiv1" style="filter:Alpha(Opacity=40);"> </span>