This week, when I was doing cross-browser testing, a new CSS problem came across to me - rgba supporting issue in IE.

The a means alpha, which specifies opacity value. However, it is not supported in some older browsers e.g. IE8 and below.

For example:

.classname {
    background: rgba(255, 255, 255, 0.5);  
}

In IE specific CSS file, you can define the color in the following way.

.classname {
    background:transparent;
    filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#7FFFFFFF, endColorstr=#7FFFFFFF);
}

The trick is: the two digits of transparent percentage (say 50%) 50 should be at the start of the string; and you need to use hexadecimal to represent it but not the decimal.

A good article to read: RGBa Browser Support.