The W3C specification for CSS 3 has added a new opacity property. Previous to this, Internet Explorer used
filter:progid:DXImageTransform.Microsoft.Alpha(opacity=50)
and Mozilla-based browsers used
-moz-opacity
Of course, this was not an ideal situation since it relied on coding that was not based on standards, something the W3C has been trying to reduce and something that was at the root of the browser wars of the 1990s.
The W3C introduced the opacity property in the 5 March 2001 working draft version of the CSS3 specification. Currently, the W3C specification for CSS3 is only at “candidate recommendation” status and has been for nearly two years. This means, technically, it is not a “standard”.
That being said, Firefox supports the opacity property of the CSS3 specification. This is important because it proves that Firefox is still committed to web standards and is forward thinking.
Check out these two boxes below. In Firefox 1.0, the top one is black and the bottom one is grey.
UPDATE: (31 March 2005) I just checked Safari and it supports the opacity property of CSS3 as well. Very cool.