It is common to have element stacking at bottom no matter how large the z-index is. There are 2 things to check when it happens.

The position property in CSS

z-index only affect elements with position property set to “relative”, “absolute”, and “fixed”.The z-index values of the all the parent

Parent elements up the DOM tree

If you want a child element stack on top, all its parent elements must have an z-index that’s large enough to make itself stacking on top. That means if an element(Element A) stack below something, all the child nodes of Element A will stack below it.

Extra favor for IE6

There is a serious bug in IE6, element with position set to “relative”, “absolute”, or “fixed” without explicitly specify a z-index, the z-index of that element will be ZERO. This is 90% of the cause that makes z-index not working on IE6. When you found high z-index does not stack on top in IE6, but works well in firefox, you probably want to check all the parent elements with position set to the above 3 values.