formatHtmlOutputHeaders example

 $headers);
    return '<hr />Headers: <pre>' . Html::escape(var_export($flattened_headers, TRUE)) . '</pre>';
  }

  /** * Returns headers in HTML output format. * * @return string * HTML output headers. */
  protected function getHtmlOutputHeaders() {
    return $this->formatHtmlOutputHeaders($this->getSession()->getResponseHeaders());
  }

  /** * Logs a HTML output message in a text file. * * The link to the HTML output message will be printed by the results printer. * * @param string|null $message * (optional) The HTML output message to be stored. If not supplied the * current page content is used. * * @see \Drupal\Tests\Listeners\VerbosePrinter::printResult() */