I had to rip this off of Chris Coyier’s www.css-tricks.com because I want to be able to find it more easily.
Also from the same post, this info defines what the different parts of this code do:
opacity: 0.5: most versions of Firefox, Safari, and Opera
This is the “most important” one because it is the current standard in CSS. This will work in most versions of Firefox, Safari, and Opera. This would be all you need if all browsers supported current standards. Which, of course, they don’t.
filter:alpha(opacity=50): Internet Explorer
You need this one to support way old school versions of the Mozilla browsers like Netscape Navigator.
-khtml-opacity: 0.5: old versions of Safari (1.x)
This is for way old versions of Safari (1.x) when the rendering engine it was using was still referred to as KTHML, as opposed to the current WebKit.