diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index e7b48dc..dcefab4 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -204,13 +204,23 @@ public static function preRenderGroup($element) {
       }
     }
 
-    // Set attributes to indicate that there is a child with an error.
+    // Set attributes (and modify a details summary) to indicate that there is
+    // a child with an error.
     if (!empty($element['#children_errors'])) {
+      $error_count = count($element['#children_errors']);
       $element['#attributes']['class'][] = 'form-wrapper--child-error';
       if ($element['#type'] === 'details') {
         $element['#attributes']['class'][] = 'details--child-error';
+        $element['#title'] = \Drupal::translation()->formatPlural(
+          $error_count,
+          '@title <span class="visually-hidden child-error-indicator">(contains an error)</span>',
+          '@title <span class="visually-hidden child-error-indicator">(contains @count errors)</span>',
+          [
+            '@title' => $element['#title']
+          ]
+        );
       }
-      $element['#attributes']['data-children-errors-count'] = count($element['#children_errors']);
+      $element['#attributes']['data-children-errors-count'] = $error_count;
     }
 
     return $element;
diff --git a/core/misc/vertical-tabs.es6.js b/core/misc/vertical-tabs.es6.js
index ce73e5b..785f686 100644
--- a/core/misc/vertical-tabs.es6.js
+++ b/core/misc/vertical-tabs.es6.js
@@ -56,7 +56,7 @@
         $details.each(function () {
           const $that = $(this);
           const vertical_tab = new Drupal.verticalTab({
-            title: $that.find('> summary').text(),
+            title: $that.find('> summary').html(),
             details: $that,
             children_errors: $that.data('children-errors-count')
           });
@@ -240,7 +240,7 @@
     const tab = {};
     tab.item = $('<li class="vertical-tabs__menu-item" tabindex="-1"></li>')
       .append(tab.link = $('<a href="#"></a>')
-        .append(tab.title = $('<strong class="vertical-tabs__menu-item-title"></strong>').text(settings.title))
+        .append(tab.title = $('<strong class="vertical-tabs__menu-item-title"></strong>').html(settings.title))
         .append(tab.summary = $('<span class="vertical-tabs__menu-item-summary"></span>'),
         ),
       );
diff --git a/core/misc/vertical-tabs.js b/core/misc/vertical-tabs.js
index 09982bf..0414613 100644
--- a/core/misc/vertical-tabs.js
+++ b/core/misc/vertical-tabs.js
@@ -31,7 +31,7 @@
         $details.each(function () {
           var $that = $(this);
           var vertical_tab = new Drupal.verticalTab({
-            title: $that.find('> summary').text(),
+            title: $that.find('> summary').html(),
             details: $that,
             children_errors: $that.data('children-errors-count')
           });
@@ -131,7 +131,7 @@
 
   Drupal.theme.verticalTab = function (settings) {
     var tab = {};
-    tab.item = $('<li class="vertical-tabs__menu-item" tabindex="-1"></li>').append(tab.link = $('<a href="#"></a>').append(tab.title = $('<strong class="vertical-tabs__menu-item-title"></strong>').text(settings.title)).append(tab.summary = $('<span class="vertical-tabs__menu-item-summary"></span>')));
+    tab.item = $('<li class="vertical-tabs__menu-item" tabindex="-1"></li>').append(tab.link = $('<a href="#"></a>').append(tab.title = $('<strong class="vertical-tabs__menu-item-title"></strong>').html(settings.title)).append(tab.summary = $('<span class="vertical-tabs__menu-item-summary"></span>')));
 
     if (settings.children_errors) {
       tab.item.addClass('vertical-tab__menu-item--child-error');