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

core/modules/system/templates/status-messages.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation for status messages.
+ *
+ * Displays status, error, and warning messages, grouped by type.
+ *
+ * An invisible heading identifies the messages for assistive technology.
+ * Sighted users see a colored box. See http://www.w3.org/TR/WCAG-TECHS/H69.html
+ * for info.
+ *
+ * Add an ARIA label to the contentinfo area so that assistive technology
+ * user agents will better describe this landmark.
+ *
+ * Available variables:
+ * - message_list: List of messages to be displayed, grouped by type.
+ * - status_headings: List of all status types.
+ * - display: (optional) May have a value of 'status' or 'error' when only
+ *   displaying messages of that specific type.
+ * - attributes: HTML attributes for the element, including:
+ *   - class: HTML classes.
+ *
+ * @see template_preprocess_status_messages()
+ *
+ * @ingroup themeable
+ */
+#}
+{% for type, messages in message_list %}
+  <div role="contentinfo" aria-label="{{ status_headings[type] }}"{{ attributes|without('role', 'aria-label') }}>
+    {% if type == 'error' %}
+      <div role="alert">
+    {% endif %}
+      {% if status_headings[type] %}
+        <h2 class="visually-hidden">{{ status_headings[type] }}</h2>
+      {% endif %}
+      {% if messages|length > 1 %}
+        <ul>
+          {% for message in messages %}
+            <li>{{ message }}</li>
+          {% endfor %}
+        </ul>
+      {% else %}
+        {{ messages|first }}
+      {% endif %}
+    {% if type == 'error' %}
+      </div>
+    {% endif %}
+  </div>
+{% endfor %}