How do you add different opacities to nested items?

Question

I have two nested items in HTML and I want to give the wrraping one opacity 0.8 and the one inside it opacity 1.
I think I understand why it does not work, but, how can I mimick that effect?
Here is a simplified HTML that shows the problem, I want the green square to be solid.

<div style='background-color:red;
            width: 500px;
            height: 500px;
            border: 1px solid black;
            position: absolute;
            top:0;
            left:0;
            opacity:0.8'>

    <div style='width:150px; height:150px; background-color:green; opacity:1'>
      Some content
    </div>
</div>

Answer 1

If you use the rgba CSS property instead of the opacity property you can achieve this:

<div style='background-color:rgba(0, 255, 0, 0.8) ;width: 500px; height: 500px; border: 1px solid black; position: absolute; top: 0; left: 0'>
<div style='position: relative; width: 150px; height: 150px; background-color: rgba(0, 0, 255, 1);'>aaaaaaaaa<br>aaaaaaaaa<br></div>
</div>

Demo: http://jsfiddle.net/ScHgC/