code: private boolean isValidCSS( CssFormatter formatter ) {
          if( params == null ) {
              if( guard != null ) {
                  //CSS Guards
  //                guard = ValueExpression.eval( formatter, guard );
                  return guard.booleanValue( formatter );
              }
              return true;
          }
          return false;
      }
docs: If this rule is a CSS rule (not a mixin declaration) and if an guard exists if it true.
  
  @param formatter the CCS target
  @return true, if a CSS rule