Transparency
Once more, a long-needed feature will become much more ubiquitous with CSS3 - transparency / opacity.
The opacity declaration has been available for some time across several browsers - Firefox, Safari/Chrome and Opera all support this property. The flaw with it is that it automatically inherits - in other words, if you want a <div> to be partially transparent, all its content will also be partially transparent, no matter what CSS is applied. An example:
Container with opacity applied - text hard to read
div#example1
{opacity:0.7;}
div#example1 p
{opacity:1;} /*--- doesn't work! See above --- */
This declaration is supported by Firefox 1+, Safari 1.2+, Chrome 2+ and Opera 9+.
The latest solution, courtesy of CSS3, is to use RGBA to declare colours. RGBA is a colour standard that determines the usual red, green, blue hues, plus an alpha channel for transparency. Importantly, it isn't inherited, so child elements aren't forced to be equally opaque:
Container with an RGBA background colour - and readable text
div#example2
{background:rgba(238,238,238,0.7);}
This declaration is supported by Firefox 3+, Safari 3.1+, Chrome 2+ and Opera 10+.
Converting Hex colours to their RGB equivalents can be done via any number of online tools - the final part of the property is just a decimal from 0 to 1.0, representing opacity.