private function checkEtag(ResponseInterface $response)
    {
        $eTag = $response->getHeaderLine('ETag');
        $fullURL = Controller::join_links(Director::absoluteBaseURL(), $this->url);

        if ($eTag) {
            $this->result->addMessage(
                "$fullURL includes an Etag header in response",
                ValidationResult::TYPE_GOOD
            );
            return;
        }
        $this->result->addError(
            "$fullURL is missing an Etag header",
            ValidationResult::TYPE_WARNING
        );
    }