Opacity

Old Method:

<style type="text/css">
.box {
opacity: .6; // all modern browsers (this is CSS3)
-ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=50)"; // IE 8
filter: alpha(opacity=60); // IE 5-7
}
</style>
 
<div class="box">
    <!--CONTENT-->
</div>

CSS3 Method:

<style type="text/css">
.box {
opacity: .6;
}
</style>
 
<div class="box">
    <!--CONTENT-->
</div>