Z-index is the CSS property governing how high in the stack an element is, if you visualise the elements as appearing in a 3D stack coming out of the page. The actual value of an element’s z-index doesn’t matter; just its value relative to other elements on the page. Elements with higher z-indexes appear on top of elements of lower z-indexes.

I was just designing a bookmark for Scrumptious (a TiddlyWeb powered tool to have discussions around websites; I’ll talk about it in a future article). And with a bookmarklet, I need it to appear above everything else on the page. So it needs a higher z-index than everything else. Maybe not the highest z-index possible, since there might be apps that need to sit above my app’s bookmarklet (Design For Extensibility). But I still need to know the maximum z-index.

It would be nice if the standards publshed the maximum allowed z-index. Every reference always makes a comment like “there’s no real limit”. For good reason too, since the W3C standards don’t really cover this. The CSS Spec (2.1) goes so far as to include an “Elaborate description of Stacking Contexts”, and it’s even called “zindex.html”, but even here, omits to pin down the max z-index value.

A handy summary was stated on StackOverflow (which has fast become the central resource for programming FAQs and a site I have quickly come to adore for its clear mission, clean design, and community feel):

So basically there are no limitations for z-index value in the CSS standard, but I guess most browsers limit it to signed 32-bit values (−2147483648 to +2147483647) in practice (64 would be a little off the top, and it doesn't make sense to use anything less than 32 bits these days)

Looking further, I came across the most comprehensive summary of the situation, published recently. It also highlights the fact that it’s not just the maximum value we want, but what happens if we exceed it.

I made a simple test page to find these limits and figure out what happens when you exceed them.

Browser Max z-index value When exceeded, value changes to:
Internet Explorer 6 2147483647 2147483647
Internet Explorer 7 2147483647 2147483647
Internet Explorer 8 2147483647 2147483647
Firefox 2 2147483647 *element disappears*
Firefox 3 2147483647 0
Safari 3 16777271 16777271
Safari 4 2147483647 2147483647
Opera 9 2147483647 2147483647

So the best way would be to use browser detection and pick the max from there. But if not, use 2147483647.