How to create a blur effect under a div?

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I try to make a div look like the windows 7 window background.I've found a blur effect, but it makes the content of the div blur, not the background of it.\[code\]background-color: rgba(100, 100, 250, 0.4);-webkit-filter: blur(2px);filter: blur(2px);\[/code\]jsfiddle: http://jsfiddle.net/gw6Zq/