It’s been trendy for a while now to play with transparency to simulate layers, but guess what? Yeap, you know it. It’s not valid under CSS2.

Stuff like opacity: .4 -moz-opacity: .4 filter: alpha(opacity=40) for the moment out of the question. We all have to wait patiently until CSS3. Yeah you heard me CSS3. If you did’t know, CSS2 is the current cascading style sheet language recognized by most of your browsers and it’s getting a face lift. Alas, the due date is unknown at this point.

If you want more information on this check out these important websites:
www.css3.info
www.w3.org/Style/CSS/current-work