Here is an online generator: https://www.sitelocity.com/critical-path-css-generator that destroyed my font paths.
Here is a better one: https://pegasaas.com/critical-path-css-generator/
Drupal module: https://www.drupal.org/project/critical_css
If you use critical css your page will load faster and you will get better scores for search engine optimization (SEO).
Interesting tool: https://purgecss.com/