core/themes/stable/templates/layout/book-export-html.html.twig

{#
/**
 * @file
 * Theme override for printed version of book outline.
 *
 * Available variables:
 * - title: Top level node title.
 * - head: Header tags.
 * - language: Language object.
 * - language_rtl: A flag indicating whether the current display language is a
 *   right to left language.
 * - base_url: URL to the home page.
 * - contents: Nodes within the current outline rendered through
 *   book-node-export-html.html.twig.
 *
 * @see template_preprocess_book_export_html()
 */
#}
<!DOCTYPE html>
<html{{ html_attributes }}>
  <head>
    <title>{{ title }}</title>
    {{ page.head }}
    <base href="{{ base_url }}" />
    <link type="text/css" rel="stylesheet" href="misc/print.css" />
  </head>
  <body>
    {#
      The given node is embedded to its absolute depth in a top level section.
      For example, a child node with depth 2 in the hierarchy is contained in
      (otherwise empty) div elements corresponding to depth 0 and depth 1. This
      is intended to support WYSIWYG output - e.g., level 3 sections always look
      like level 3 sections, no matter their depth relative to the node selected
      to be exported as printer-friendly HTML.
    #}

  {% for i in 1..depth-1 if depth > 1 %}
    <div>
  {% endfor %}
  {{ contents }}
  {% for i in 1..depth-1 if depth > 1 %}
    </div>
  {% endfor %}
  </body>
</html>