#head.

#head{position: relative;}

#box

Children of #head. Should stack above all all elements
#box {position: absolute;z-index: 3;}

#main

#main{position: relative;}

All positioned elements should have a z-index with value "auto".
The view result differs from IE to the other browsers. IE cuts #box
IE sets the z-index basically to "0" on positioned elements. That's wrong:
W3C
To reach the same behaviour set the z-index to "0" on all elements.
Click here

#head.

#box

#main

#footer