See the optimisations section.

#options.safe (bool)

Set this to true to disable advanced optimisations that are not always safe. Currently, this disables custom identifier reduction, z-index rebasing, unused at-rule removal & conversion between absolute length values.

cssnano --safe in.css out.css

#options.sourcemap (bool)

Set this to true to generate an inline source map.

cssnano --sourcemap in.css out.css