protected function normaliseViewport($prefix, array $valid, $value)
    {
        $value = $this->convertToArray($value);

        foreach ($value as $i => $viewport) {
            if (!\in_array($viewport, $valid)) {
                throw new InvalidConfigurationException(sprintf('The "%s" %s viewport option does not exist. Known options are: "%s"', $viewport, $prefix, implode('", "', $valid)));
            }

            $value[$i] = sprintf('%s-%s', $prefix, $viewport);
        }

        return $value;
    }