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

core/modules/book/templates/book-node-export-html.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation for a single node in a printer-friendly outline.
+ *
+ * Available variables:
+ * - node: Fully loaded node.
+ * - depth: Depth of the current node inside the outline.
+ * - title: Node title.
+ * - content: Node content.
+ * - children: All the child nodes recursively rendered through this file.
+ *
+ * @see template_preprocess_book_node_export_html()
+ *
+ * @ingroup themeable
+ */
+#}
+<article>
+  <h1>{{ title }}</h1>
+  {{ content }}
+  {{ children }}
+</article>