CSS 3 - Opacity

CSS3 has a specific property that controls the transparency of an element, aptly named "opacity". It takes as its value a number from zero to one, representing the degree to which the element is opaque. A value of zero means completely transparent and one means completely solid.
This is not yet a CSS standard. However, it works in all modern browsers, and is a part of the W3C CSS 3 recommendation.
Firefox/Opera/Safari uses the property opacity:x for transparency, while IE uses filter:alpha(opacity=x).

Move the mouse over the thundercats image to see the opacity effect.

Current Browser Support:
FireFox (3.0.5)
Google Chrome (1.0.154.36)
Opera (9.6)
Safari (3.2.1, Windows)