This article covers the method for varying the opacity of elements in a Web document:
There are two general methods for creating partially transparent elements on a Web page. The first method uses transparent PNGs with the opacity pre-set through a digital imaging application. While this technique works, making it cross-browser compatible is a bit complicated. Internet Explorer 6 and below don’t support PNG alpha-transparency natively, so you need to incorporate Microsoft’s AlphaImageLoader filter and some scripting to correct for this deficiency.
The second method for creating translucent elements—the technique we’ll explore in this article—is to use CSS properties to directly control the transparency of an element.