I’ve been using cssnano for compressing my CSS on some projects recently. Apparently, it rebases z-index values, ie reduces them to the minimum values that have the same stacking context within the file. If you have other stylesheets or JS involved, though, this can cause problems. I was very confused why whatever change I made to the number seemed to not be showing up. Per an issue on GitHub, it can be disabled by setting "zindex": false.