docs: def codemirror_html(self, config_name, varname, element_id):
          """
          Render HTML for a CodeMirror instance.
  
          Since a CodeMirror instance have to be attached to a HTML element, this
          method requires a HTML element identifier with or without the ``#``
          prefix, it depends from template in
          ``settings.CODEMIRROR_FIELD_INIT_JS`` (default one require to not
          prefix with ``#``).
  
          Arguments:
              config_name (string): A registred config name.
              varname (string): A Javascript variable name.
              element_id (string): An HTML element identifier (without
                  leading ``#``) to attach to a CodeMirror instance.
  
          Returns:
              string: HTML to instanciate CodeMirror for a field input.
          """
          parameters = json.dumps(self.get_codemirror_parameters(config_name),
                                  sort_keys=True)
          return settings.CODEMIRROR_FIELD_INIT_JS.format(
              varname=varname,
              inputid=element_id,
              settings=parameters,
          )