protected function getHeaderKeys(bool $legacy = true): array
    {
        // We always want this
        $return = [
            $this->reportOnly
                ? 'Content-Security-Policy-Report-Only'
                : 'Content-Security-Policy'
        ];

        // If we're supporting legacy devices, include these too:
        if ($legacy) {
            $return []= $this->reportOnly
                ? 'X-Content-Security-Policy-Report-Only'
                : 'X-Content-Security-Policy';
            $return []= $this->reportOnly
                ? 'X-Webkit-CSP-Report-Only'
                : 'X-Webkit-CSP';
        }
        return $return;
    }