• All cssnano plugins and cssnano itself have migrated to PostCSS 5.x. Please
  make sure that when using the 3.x releases that you use a 5.x compatible
  PostCSS runner.
• cssnano will now compress inline SVG through SVGO. Because of this change,
  interfacing with cssnano must now be done through an asynchronous API. The
  main process method has the same signature as a PostCSS processor instance.
• The old options such as merge & fonts that were deprecated in
  release 2.5.0 were removed. The new architecture allows you to specify any
  module name to disable it.
• postcss-minify-selectors' at-rule compression was extracted out into
  postcss-minify-params (thanks to @TrySound).
• Overall performance of the module has improved dramatically, thanks to work
  by @TrySound and input from the community.
• Improved selector merging/deduplication in certain use cases.
• cssnano no longer compresses hex colours in filter properties, to better
  support old versions of Internet Explorer (thanks to @faddee).
• cssnano will not merge properties together that have an inherit keyword.
• postcss-minify-font-weight & postcss-font-family were consolidated into
  postcss-minify-font-values. Using the old options will print deprecation
  warnings (thanks to @TrySound).
• The cssnano CLI was extracted into a separate module, so that dependent
  modules such as gulp-cssnano don't download unnecessary extras.