'Clean URLs' in Drupal 8

In Drupal 8, 'Clean URLs' are enabled by default and can't be disabled.

Check your browser's address-bar. Your site's URLs should not contain ?q= within the URL.

Example of proper 'Clean URLs'
http://www.example.com/node/83

Example of 'Clean URLs' not working
http://www.example.com/?q=node/83

Fix Drupal 8 'Clean URLs' problems

Potential problems and solutions for 'Clean URLs' in Drupal 8.

Guide maintainers