Issue #1967112 by nod_: Fixed HTML 5 validation breaks the cancel button in Views UI dialogs.

core/modules/views/views_ui/lib/Drupal/views_ui/ViewUI.php

@@ -322,6 +322,7 @@ public function getStandardButtons(&$form, &$form_state, $form_id, $name = NULL)
       '#value' => empty($form_state['ok_button']) ? t('Cancel') : t('Ok'),
       '#submit' => array($cancel_submit),
       '#validate' => array(),
+      '#attributes' => array('formnovalidate' => ''),
     );
 
     // Compatibility, to be removed later: // TODO: When is "later"?