docs: def get_lang_tags(index_page):
      """
      Collect informations about language of the page from HTML and Dublin core
      tags and langdetect guesses.
  
      Args:
          index_page (str): HTML content of the page you wish to analyze.
  
      Returns:
          list: List of :class:`.SourceString` objects.
      """
      dom = dhtmlparser.parseString(index_page)
  
      lang_tags = [
          get_html_lang_tags(dom),
          get_dc_lang_tags(dom),
          [detect_language(dom)],
          get_html_tag_lang_params(dom),
      ]
  
      return list(sorted(set(
          SourceString(normalize(lang), source=lang.source)
          for lang in sum(lang_tags, [])
      )))