drupal fresh install no css Layout was forced before the page was fully loaded

The problem with the fresh install of Drupal 9 on Firefox and Chrome:

Layout was forced before the page was fully loaded. If stylesheets are not yet loaded this may cause a flash of unstyled content

The site looks like the CSS doesn't work. The solution: go to the Appearance settings (https://example.com/admin/appearance) and simply switch to a different theme. As is turns out, this no CSS thing is the expected behaviour:

Stark 9.1.9 An intentionally plain theme with no styling to demonstrate default Drupal’s HTML and CSS.