I've tried to work on transparent CSS. I put all 3 types of opacity setting below :

filter: alpha(opacity='70');
-moz-opacity: 0.7;
opacity: 0.7

With opacity setting, it works fine with Firefox. However, I couldn't make the same code to work with IE 6 and 7. Then, I searched online and found the following page -> http://labs.adobe.com/wiki/index.php/Talk:Spry#Spry.Effects.Opacity

I just followed what they suggested which is to put width and height as below :

style="opacity: 0.5; width: 100%; height: 100%; filter:alpha(opacity=50);"

Then, the transparent came to work as it should be on IE.