CSS - Opacity

-khtml-opacity: .5;  // old safari
-moz-opacity: .5;  // old firefox
-ms-filter: alpha(opacity=50);  // new ie
-ms-filter: progid:DXImageTransform.Microsoft.Alpha(Opacity=50);  // new ie
filter: alpha(opacity=50);  // old ie
opacity: 0.5;  // modern gecko and webkit

NOTE: Most old browsers are not used anymore, so this could be simplified to only:

filter: alpha(opacity=50);
opacity: 0.5;