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.
