Drupal 9: How to add meaningfull css classes to body

This is a generic solution that i use in almost every custom theme that i build.

function mytheme_preprocess_html(&$variables) {

  $route = \Drupal::routeMatch();
  $user = \Drupal::currentUser();

  $route_parts = explode('.', $route->getRouteName());

  $is_front = \Drupal::service('path.matcher')->isFrontPage();

  $variables['attributes']['class'][] = $is_front ? 'frontpage' : 'not-frontpage';

  if (in_array('taxonomy_term', $route_parts)) {
    $variables['attributes']['class'][] = 'taxonomy-term--page';
  }

  if (in_array('node', $route_parts)) {
    $variables['attributes']['class'][] = 'node--page';
  }

  $variables['attributes']['class'][] = 'page--' . implode('-', $route_parts);

  $variables['attributes']['class'][] = $user->id() ? 'user-authenticated' : 'user-anonymous';
}