Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/themes/stable/templates/form/details.html.twig

@@ -15,8 +15,14 @@
  */
 #}
 <details{{ attributes }}>
+  {%
+    set summary_classes = [
+      required ? 'js-form-required',
+      required ? 'form-required',
+    ]
+  %}
   {%- if title -%}
-    <summary{{ summary_attributes }}>{{ title }}</summary>
+    <summary{{ summary_attributes.addClass(summary_classes) }}>{{ title }}</summary>
   {%- endif -%}
 
   {% if errors %}