code: private void ruleset( String[] sel, CssFormatter formatter ) {
          formatter = formatter.startBlock( sel );
          appendPropertiesTo( formatter );
  
          for( Formattable prop : properties ) {
              if( prop instanceof Mixin ) {
                  ((Mixin)prop).appendSubRules( null, formatter );
              }
          }
  
          for( Rule rule : subrules ) {
              rule.appendTo( formatter );
          }
          formatter.endBlock();
      }
docs: Directives like @media in the root.
  
  @param sel the selectors
  @param formatter current formatter