Maximum value of z-index in css: depends on each browser. The latest version of every browser use the same value which is 2147483647 that come from sign 32-bit values (−2147483648 to +2147483647)

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