How can I make a CSS glass/blur effect work for an overlay?

I am having trouble applying a blur effect on a semi-transparent overlay div. I'd like everything behind the div the be blurred, like this:

Here is a jsfiddle which doesn't work: http://jsfiddle.net/u2y2091z/

Any ideas how to make this work? I'd like to keep this as uncomplicated as possible and have it be cross-browser. Here is the CSS I'm using:

#overlay {
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;

    background:black;
    background:rgba(0,0,0,0.8);

    filter:blur(4px);
    -o-filter:blur(4px);
    -ms-filter:blur(4px);
    -moz-filter:blur(4px);
    -webkit-filter:blur(4px);
}

I was able to piece together information from everyone here and further Googling, and I came up with the following which works in Chrome and Firefox: http://jsfiddle.net/xtbmpcsu/. I'm still working on making this work for IE and Opera.

The key is putting the content inside of the div to which the filter is applied:

body {
    background: #300000;
    background: linear-gradient(45deg, #300000, #000000, #300000, #000000);
    color: white;
}
#mask {
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
    background-color: black;
    opacity: 0.5;
}
img {
    filter: blur(10px);
    -webkit-filter: blur(10px);
    -moz-filter: blur(10px);
  -o-filter: blur(10px);
  -ms-filter: blur(10px);
    position: absolute;
    left: 100px;
    top: 100px;
    height: 300px;
    width: auto;
}

<div id="mask">
    <p>Lorem ipsum ...</p>
    <img src="http://www.byui.edu/images/agriculture-life-sciences/flower.jpg" />
</div>

So mask has the filters applied. Also, note the use of url() for a filter with an <svg> tag for the value -- that idea came from http://codepen.io/AmeliaBR/pen/xGuBr. If you happen to minify your CSS, you might need to replace any spaces in the SVG filter markup with "%20".

So now, everything inside the mask div is blurred.