For IE, you can use:
(I can never remember which). That sets it to 50% opacity.
CSS3 supplies a property called "opacity". When Gecko (NS6, NS7, Mozilla, etc), don't fully support a CSS property to specs, the append a -moz- in front of it. Because of Gecko's partial supoort for opacity, you can also use:
to achieve the same thing in NS6+.