How to make a div's background color translucent?

Question

I only want the background color of white in my div to be translucent roughly 50%. The content should be fully opaque. What's the proper way to do this? I imagined when I looked up the background CSS property, I'd find an opacity setting, but didn't. Don't care about IE6.

UPDATE: solving with the rgba solution given below in conjunction with CSS3PIE's solution for getting rgba to work in IE browsers.

Answer 1

You can use the background-color: rgba() notation:

#theIdofYourElement,
.classOfElements {
    background-color: #fff;
    background-color: rgba(255,255,255,0.5);
}

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Edited to add the default background-color (for browsers that don't understand the rgba() notation). Albeit I was under the impression that all but IE do understand it (but I could be wrong, and haven't tested to be sure...).

Edit with thanks to @akamike.

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Edited to address question from OP (in comments):

which browsers don't understand rgba? will they all in the future, is this part of css3?

The best information I could find is the CSS Tricks' rgba() browser support table, with a link to a demo and 'more complete' compatibility table.