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

core/themes/stable/templates/block/block--system-menu-block.html.twig

@@ -38,7 +38,7 @@
     {% set title_attributes = title_attributes.addClass('visually-hidden') %}
   {% endif %}
   {{ title_prefix }}
-  <h2{{ title_attributes }}>{{ configuration.label }}</h2>
+  <h2{{ title_attributes.setAttribute('id', heading_id) }}>{{ configuration.label }}</h2>
   {{ title_suffix }}
 
   {# Menu. #}

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 %}

core/themes/stable/templates/navigation/breadcrumb.html.twig

@@ -9,7 +9,7 @@
 #}
 {% if breadcrumb %}
   <nav role="navigation" aria-labelledby="system-breadcrumb">
-    <h2 class="visually-hidden">{{ 'Breadcrumb'|t }}</h2>
+    <h2 id="system-breadcrumb" class="visually-hidden">{{ 'Breadcrumb'|t }}</h2>
     <ol>
     {% for item in breadcrumb %}
       <li>