 this.elements[i], i < this.elements.length; i++) {
        // Custom elements that extend a native element will also appear in
        // this list, but they've already been validated.
        if (!el.hasAttribute('is') && el.willValidate && el.checkValidity) {
          valid = el.checkValidity() && valid;
        }
      }

      return valid;
    },

    /**
     * Returns whether the given element is a radio-button or a checkbox.
     * @return {boolean} True if the element has a `checked` property.
  