Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/system/templates/fieldset.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation for a fieldset element and its children.
+ *
+ * Available variables:
+ * - attributes: HTML attributes for the fieldset element.
+ * - errors: (optional) Any errors for this fieldset element, may not be set.
+ * - required: Boolean indicating whether the fieldeset element is required.
+ * - legend: The legend element containing the following properties:
+ *   - title: Title of the fieldset, intended for use as the text of the legend.
+ *   - attributes: HTML attributes to apply to the legend.
+ * - description: The description element containing the following properties:
+ *   - content: The description content of the fieldset.
+ *   - attributes: HTML attributes to apply to the description container.
+ * - children: The rendered child elements of the fieldset.
+ * - prefix: The content to add before the fieldset children.
+ * - suffix: The content to add after the fieldset children.
+ *
+ * @see template_preprocess_fieldset()
+ *
+ * @ingroup themeable
+ */
+#}
+{%
+  set classes = [
+    'form-item',
+    'js-form-wrapper',
+    'form-wrapper',
+  ]
+%}
+<fieldset{{ attributes.addClass(classes) }}>
+  {%
+    set legend_span_classes = [
+      'fieldset-legend',
+      required ? 'form-required',
+    ]
+  %}
+  {#  Always wrap fieldset legends in a SPAN for CSS positioning. #}
+  <legend{{ legend.attributes }}>
+    <span{{ legend_span.attributes.addClass(legend_span_classes) }}>{{ legend.title }}</span>
+  </legend>
+  <div class="fieldset-wrapper">
+    {% if errors %}
+      <div>
+        {{ errors }}
+      </div>
+    {% endif %}
+    {% if prefix %}
+      <span class="field-prefix">{{ prefix }}</span>
+    {% endif %}
+    {{ children }}
+    {% if suffix %}
+      <span class="field-suffix">{{ suffix }}</span>
+    {% endif %}
+    {% if description.content %}
+      <div{{ description.attributes.addClass('description') }}>{{ description.content }}</div>
+    {% endif %}
+  </div>
+</fieldset>