code: boolean hasOnlyInlineProperties( CssFormatter formatter ) {
          for( Formattable prop : properties ) {
              if( prop instanceof Mixin ) {
                  return false;
              }
          }
  
          for( Rule rule : subrules ) {
              if( rule.isValidCSS( formatter ) && rule.isInlineRule( formatter) ) {
                  return false;
              }
          }
          return true;
      }
docs: If there are only properties that should be inlined.
  
  @param formatter current formatter
  @return true, if only inline