Button::processButton

public static Button::processButton(&$element, FormStateInterface $form_state, &$complete_form)

public static function processButton(&$element, FormStateInterface $form_state, &$complete_form) {
  // If this is a button intentionally allowing incomplete form submission
  // (e.g., a "Previous" or "Add another item" button), then also skip
  // client-side validation.
  if (isset($element['#limit_validation_errors']) && $element['#limit_validation_errors'] !== FALSE) {
    $element['#attributes']['formnovalidate'] = 'formnovalidate';
  }
  return $element;
}