code: def _html_class_str_from_tag(self, tag):
          
          if "html-classes" not in self.extras:
              return ""
          try:
              html_classes_from_tag = self.extras["html-classes"]
          except TypeError:
              return ""
          else:
              if tag in html_classes_from_tag:
                  return ' class="%s"' % html_classes_from_tag[tag]
          return ""
docs: Get the appropriate ' class="..."' string (note the leading
          space), if any, for the given tag.