 <p>My tests show that <code>z-index: 2147483647</code> is the maximum value, tested on FF 3.0.1 for OS X. I discovered a integer overflow bug: if you type <code>z-index: 2147483648</code> (which is 2147483647 + 1) the element just goes behind all other elements. At least the browser doesn't crash.</p> <p>And the lesson to learn is that you should beware of entering too large values for the <code>z-index</code> property because they wrap around.</p>