Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

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

@@ -5,6 +5,7 @@
  *
  * Available variables
  * - attributes: A list of HTML attributes for the details element.
+ * - errors: (optional) Any errors for this details element, may not be set.
  * - title: (optional) The title of the element, may not be set.
  * - description: (optional) The description of the element, may not be set.
  * - children: (optional) The children of the element, may not be set.
@@ -18,6 +19,11 @@
     <summary{{ summary_attributes }}>{{ title }}</summary>
   {%- endif -%}
   <div class="details-wrapper">
+    {% if errors %}
+      <div class="form-item--error-message">
+        <strong>{{ errors }}</strong>
+      </div>
+    {% endif %}
     {%- if description -%}
       <div class="details-description">{{ description }}</div>
     {%- endif -%}

core/themes/classy/templates/form/field-multiple-value-form.html.twig

@@ -11,17 +11,19 @@
  *
  * Available variables when there are multiple fields.
  * - table: Table of field items.
- * - description: Description text for the form element.
+ * - description: The description element containing the following properties:
+ *   - content: The description content of the form element.
+ *   - attributes: HTML attributes to apply to the description container.
  * - button: "Add another item" button.
  *
  * @see template_preprocess_field_multiple_value_form()
  */
 #}
 {% if multiple %}
-  <div class="form-item">
+  <div class="js-form-item form-item">
     {{ table }}
-    {% if description %}
-      <div class="description">{{ description }}</div>
+    {% if description.content %}
+      <div{{ description.attributes.addClass('description') }} >{{ description.content }}</div>
     {% endif %}
     {% if button %}
       <div class="clearfix">{{ button }}</div>

core/themes/classy/templates/form/fieldset.html.twig

@@ -22,6 +22,7 @@
 #}
 {%
   set classes = [
+    'js-form-item',
     'form-item',
     'js-form-wrapper',
     'form-wrapper',

core/themes/classy/templates/form/form-element.html.twig

@@ -46,9 +46,11 @@
 #}
 {%
   set classes = [
+    'js-form-item',
     'form-item',
     'js-form-type-' ~ type|clean_class,
     'form-type-' ~ type|clean_class,
+    'js-form-item-' ~ name|clean_class,
     'form-item-' ~ name|clean_class,
     title_display not in ['after', 'before'] ? 'form-no-label',
     disabled == 'disabled' ? 'form-disabled',