CSS Box Shadow - Top and Bottom Only
After some experimentation I found that a fourth value in the line controls the spread (at least in FF 10). I opposed the vertical offsets and gave them a negative spread.
Here's the working pen: http://codepen.io/gillytech/pen/dlbsx
<html>
<head>
<style type="text/css">
#test {
width: 500px;
border: 1px #CCC solid;
height: 200px;
box-shadow:
inset 0px 11px 8px -10px #CCC,
inset 0px -11px 8px -10px #CCC;
}
</style>
</head>
<body>
<div id="test"></div>
</body>
</html>
This works perfectly for me!
As Kristian has pointed out, good control over z-values will often solve your problems.
If that does not work you can take a look at CSS Box Shadow Bottom Only on using overflow hidden to hide excess shadow.
I would also have in mind that the box-shadow property can accept a comma-separated list of shadows like this:
box-shadow: 0px 10px 5px #888, 0px -10px 5px #888;
This will give you some control over the "amount" of shadow in each direction.
Have a look at http://www.css3.info/preview/box-shadow/ for more information about box-shadow.
Hope this was what you were looking for!