Z-index - how to use it?

From CSS Standards FAQ

z-index applies to positioned elements, in other words elements with ``position property set to either, ``fixed, ``absolute or relative. If you just want 'z-index', { position: relative; } will suffice.

It's not necessary to set enormous z-index values. Generally setting in increments of '10' is a good enough. Increments in '1' will also work, but using '10' is a better idea in case later you'll want to stick something in the middle.

If you find yourself using z-index on virtually every element, you are probably going the wrong road -- rethink your approach.