HSLA Colors
As RGBA is to RGB, HSLA is to HSL; that is, it allows a fourth value, which sets the Opacity (via the Alpha channel) of the element.
At the time of writing, only Safari 3 and the Firefox 3 beta support the HSLA style of color declaration.
An example:
The top row (if you are one of the few brave souls who have downloaded the Firefox 3 beta you can see it) uses the HSLA declaration;
<div style="background-color: hsla(0,100%,50%,0.2);"></div> <div style="background-color: hsla(0,100%,50%,0.4);"></div> <div style="background-color: hsla(0,100%,50%,0.6);"></div> <div style="background-color: hsla(0,100%,50%,0.8);"></div> <div style="background-color: hsla(0,100%,50%,1);"></div>
The second row approximates the output using RGB.
<div style="background-color: rgb(243,191,189);"></div> <div style="background-color: rgb(246,143,142);"></div> <div style="background-color: rgb(249,95,94);"></div> <div style="background-color: rgb(252,47,47);"></div> <div style="background-color: rgb(255,0,0);"></div>
A fuller explanation of HSLA colors can be found in the W3C CSS3 Color Candidate Recommendation.