code: public function attr($keys = [])
      {
          if (!is_array($keys)) {
              $keys = [$keys];
          }
          
          $out = '';
          foreach ($keys as $key) {
              $value = $this->getValue($key)->attr();
              if (!empty($value)) {
                  $out .= ' ' . $value;
              }
          }
          return ltrim($out, ' ');
      }
docs: Renders the all the given keys as HTML element attributes.
  
  @param array|string $keys
  @return string