CSS Transparency Settings for All Browsers

.transparent {
 /* Required for IE 5, 6, 7 */
 /* ...or something to trigger hasLayout, like zoom: 1; */
 width: 100%; 

 /* Theoretically for IE 8 & 9 (more valid) */
 /* ...but not required as filter works too */
 /* should come BEFORE filter */
 -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";

 /* This works in IE 8 & 9 too */
 /* ... but also 5, 6, 7 */
 filter: alpha(opacity=50);

 /* Older than Firefox 0.9 */
 -moz-opacity:0.5;

 /* Safari 1.x (pre WebKit!) */
 -khtml-opacity: 0.5;

 /* Modern!
 /* Firefox 0.9+, Safari 2?, Chrome any?
 /* Opera 9+, IE 9+ */
 opacity: 0.5;
}

(http://css-tricks.com/css-transparency-settings-for-all-broswers/)