code: def gen_html(self, book_idx):
          
          toc = self._gen_toc(book_idx)
          start, end = self._start_end_of_index(book_idx)
          content = '\n'.join([chap.txt2html() for chap in self._chapters[start: end]])
          html = """<!DOCTYPE html
  PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  <html xmlns="http://www.w3.org/1999/xhtml" lang="zh" xml:lang="zh">
  <head>
      <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
      <title>{book_title}</title>
      <style type="text/css">
      p {{ margin-top: 1em; text-indent: 0em; }}
      h1 {{margin-top: 1em}}
      h2 {{margin: 2em 0 1em; text-align: center; font-size: 2.5em;}}
      h3 {{margin: 0 0 2em; font-weight: normal; text-align:center; font-size: 1.5em; font-style: italic;}}
      .center {{ text-align: center; }}
      .pagebreak {{ page-break-before: always; }}
      </style>
  </head>
  <body>
  <a name="toc"/>
  {toc}
  <!-- Your book goes here -->
  {content}
  </body>
  </html>
          """.format(
              book_title=self._title,
              toc=toc,
              content=content
          )
          return html
docs: 生成HTML文件
          :return:
          :rtype: