For reference (yours and mine) the maximum allowed z-index is 2147483647. That is, 27 – 1, the highest single byte signed int. Don’t ask me where I found that value (I don’t really remember)… but I did a lot of research and figuring when I was writing my Chrome Status Bar extension as I really hated when some site’s bottom toolbar overlapped it.
Please though, if you do use this number, only use it for browser extensions, and not on your website itself. Browser extensions should always take precedence over the site content, so you don’t want your site to be arm wrestling with the browser for top spot.
Note however that this does not seem to have any affect on embedded YouTube videos, which will still overlap it. To get around that, add the wmode=”opaque” value to the <embed> tag:
<embed type="application/x-shockwave-flash" width="425" height="344" src="http://www.youtube.com/v/zrqRcMRuhUM?hl=en&fs=1" allowscriptaccess="always" allowfullscreen="true" wmode="opaque">
And voila! YouTube videos now roll underneath other site content instead of above it.