A (CSS) Horse of a Different Color: A quick look at the upcoming opacity features in CSS3.
“The draft CSS3 color module takes this a step further, adding an alpha channel to the RGB value, effectively allowing you to change the opacity of an object. This additional alpha value takes a range from ‘0.0’, which is fully transparent, to ‘1.0’, which is fully opaque. To specify an RGBA color value, you set it explicitly by using ‘rgba’ and add the alpha value to the end of the RGB statement…”
You can do opacity now, but browsers handle it differently — alpha in IE, and moz-opacity in Gecko-based browsers. The W3C wants to consolidate and simplify it.