Last modified: 2014-06-08 00:46:15 UTC
It's currently explicitly excluded in : if ( in_array( $group, array( '*', 'user' ) ) ) { continue; } But having user supported should be useful, to distinguish logged-in users and anons. Of course '*' is redundant to MediaWiki:Common.css.
... excluded in includes/resourceloader/ResourceLoaderUserGroupsModule.php.
*** This bug has been marked as a duplicate of bug 47812 ***