Using grunt to compile sass, concatenate and minify, I discovered z-index entries were being set to 1. This is a known issue with cssnano. See https://github.com/ben-eb/gulp-cssnano/issues/10 and https://github.com/ben-eb/gulp-cssnano/issues/14. In the readme for cssnano, it says: Note that cssnano enables aggressive optimisations by default, which might not always be what you want. Set `options.safe` to `true` if [...]