<div id="Layer1" style="position:absolute; width:200px; height:115px; left:50px;top:50px;filter:alpha(opacity=30);opacity: 0.6;background-color:blue"></div>
opacity值越大，透明度越高
FF
<script>document.getElementByIdx_xx_x("Layer1").style.opacity
= "0.3";</script>
IE
<script>document.getElementByIdx_xx_x("Layer1").style.filter
= "alpha(opacity=30)";</script>
