nb Development

Whatever the value of the third value (the blur), set the fourth value (the spread) to the negative of it and voila! You have a bottom side only drop shadow with css.

-moz-box-shadow: 0 6px 4px -4px gray;
 
-webkit-box-shadow: 0 6px 4px -4px gray;
 
box-shadow: 0 6px 4px -4px gray;