By “CSS” I meant to some possible bleeding-edge technology CSS command that will duplicate the CSS classes from element A to element B automatically but probably there is no such command yet
When my content management system Drupal will get updated, the default theme change may be deleted but I might just duplicate the CSS of region--content to region--content_below in the sub-theme which Drupal updates should never modify.
Drupal’s default theme has some complex grid based CSS so it may be a bit frustrating for me to duplicate and maintain that CSS but maybe there is no other way.