Q:

How to make the background DIV only transparent using CSS

I am using CSS attrubutes :

filter: alpha(opacity=90);    

opacity: .9;

to make the DIV transparent, but when I add another DIV inside this DIV it makes it transparent also.
I want to make the outer(background) DIV only transparent. How ?

A:

Fiddle: http://jsfiddle.net/uenrX/1/
The opacity property of the outer DIV cannot be undone by the inner DIV. If you want to achieve transparency, use rgba or hsla:
Outer div:
background-color: rgba(255, 255, 255, 0.9); /* Color white with alpha 0.9*/

Inner div:
background-color: #FFF; /* Background white, to override the background propery*/

EDIT
Because you've added filter:alpha(opacity=90) to your question, I assume that you also want a working solution for (older versions of) IE. This should work (-ms- prefix for the newest versions of IE):
/*Padded for readability, you can write the following at one line:*/
filter: progid:DXImageTransform.Microsoft.Gradient(
    GradientType=1,
    startColorStr="#E6FFFFFF",
    endColorStr="#E6FFFFFF");

/*Similarly: */
filter: progid:DXImageTransform.Microsoft.Gradient(
    GradientType=1,
    startColorStr="#E6FFFFFF",
    endColorStr="#E6FFFFFF");

I've used the Gradient filter, starting with the same start- and end-color, so that the background doesn't show a gradient, but a flat colour. The colour format is in the ARGB hex format. I've written a JavaScript snippet to convert relative opacity values to absolute alpha-hex values:
var opacity = .9;
var A_ofARGB = Math.round(opacity * 255).toString(16);
if(A_ofARGB.length == 1) A_ofARGB = "0"+a_ofARGB;
else if(!A_ofARGB.length) A_ofARGB = "00";
alert(A_ofARGB);

A:

I had the same problem, this is the solution i came up with, which is much easier!
Make a little 1px x 1px transparent image and save it as a .png file.
In the CSS for your DIV, use this code:
background:transparent url('/images/trans-bg.png') repeat center top;

Remember to change the file path to your transparent image. 
I think this solution works in all browsers, maybe except for IE 6, but I haven't tested.

