The current Candidate Recommendation for CSS3 Color [1] proposes
a CSS property called opacity that would offer a straightforward
means of setting an element's opacity. Current Mozilla browsers,
Opera, and Safari 1.2 support this syntax:

#elementid { 
  opacity: 0.5; 
}

Older Mozilla browsers require a nonstantard property name:

#elementid { 
  -moz-opacity: 0.5; 
}

Older versions of Safari and Konqueror use a different
nonstandard property name:

#elementid { 
  -khtml-opacity: 0.5; 
}

Internet Explorer, in its infinite wisdom, still requires that
you use DirectX filters to achieve this effect:

#elementid { 
  filter: alpha(opacity=50); 
}

This will work in Internet Explorer 4 or later, but Microsoft
would prefer you use the more cumbersome Internet Explorer 5.5
version:

#elementid { 
  filter: progid:DXImageTransform.Microsoft.Alpha(opacity=50); 
}

To get the broadest possible effect, you can apply all of these
in tandem:

#elementid { 
  opacity: 0.5; 
  -moz-opacity: 0.5; 
  -khtml-opacity: 0.5; 
  filter: alpha(opacity=50); 
}