A frustrating thing for webmasters is often that they cannot see the eventual lay-out of text styling in their CKEditor. This snippet fixes this issue.

/**
 * Implements hook_ckeditor_css_alter().
 *
 * Injects our CSS sheets anytime CKEditor has loaded.
 *
 * @param array $css
 * @param Drupal\editor\Entity\Editor $editor
 */
function MYMODULE_ckeditor_css_alter(array &$css, Editor $editor) {
  if (!$editor->hasAssociatedFilterFormat()) {
    return;
  }

  $known_formats = [
    'basic_html',
    'full_html',
  ];

  if (in_array($editor->getFilterFormat()->id(), $known_formats)) {
    $css[] = drupal_get_path('theme', 'MYTHEME') . '/css/main.css';
  }
}