Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<style> 
div
{
background-color:red;
opacity:0.5;
filter:Alpha(opacity=50); /* IE8 and earlier */
}
</style>
</head>
<body>

<div>This element's opacity is 0.5! Note that both the text and the background-color are affected by the opacity level!</div>

</body>
</html>

Result:

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