How to make the background DIV only transparent using CSS

Question

I am using CSS attrubutes :

filter: alpha(opacity=90);    

opacity: .9;

to make the DIV transparent,

Answer 1

I don't know if this has changed. But from my experience. nested elements have a maximum opacity equal to the fathers.

Which mean:

Div#a has 0.6 opacity
div#b has 1 opacity

Has #b is within #a then it's maximum opacity is always 0.6

If #b would have 0.5 opacity. In reallity it would be 0.6*0.5 == 0.3 opacity