Problem
CSS property z-index not working.
Introduction
If you are trying to use z-index and it's not working then you have to make sure that you are using position attribute.
CSS position attribute can have following values
position:absolute position:relative position:fixed
Erroneous Code
<div style="width:500px; height:500px; z-index:5px"></div>
Solution
<div style="width:500px; height:500px; z-index:5px; position:relative"></div>