reverted:
--- b/core/includes/form.inc
+++ a/core/includes/form.inc
@@ -4837,7 +4837,7 @@
   // form_builder().
   if (!empty($element['#required'])) {
     $element['#attributes']['class'][] = 'required';
+    $element['#attributes']['required'] = 'required';
-    //$element['#attributes']['required'] = 'required';
     $element['#attributes']['aria-required'] = 'true';
   }
   if (isset($element['#parents']) && form_get_error($element) !== NULL && !empty($element['#validated'])) {