private function getValidationData($markup)
    {
        $this->httpRequestParameters['body'] = $markup;

        $reponse = $this->httpClient->post(
            $this->configuration[self::ENDPOINT_CONFIG_KEY],
            $this->httpRequestParameters
        );
        $responseData = $reponse->getBody()->getContents();
        $validationData = json_decode($responseData, true);
        if ($validationData === null) {
            throw new Exception('Unable to parse W3C Markup Validation Service response.');
        }

        return $validationData;
    }