A fast example (basicly what LessCSS does).
Creates the following code:
opacity: .5; filter: alpha(opacity=50); /* Internet Explorer */ -khtml-opacity: .5; /* Safari */ -moz-opacity: .5; /* Firefox */
More.less is my LessCSS library, which helps building cross-browser compatible CSS3 code. The default hacks and tweaks for supporting all browsers. Download it, use it for whatever you like. It's free!