public function resolveUrlInCss($url)
    {
        if (substr($url, 0, 2) === '//') {
            // double slashed indicate a fully fledged url like //typo3.org
            return $url;
        }
        if (strpos($url, '://') !== false) {
            // http://, ftp:// etc. should not be touched
            return $url;
        }
        if (substr($url, 0, 1) === '/') {
            if (substr($url, 0, strlen(PATH_site)) === PATH_site) {
                return '../../'.substr($url, strlen(PATH_site));
            }

            return $url;
        }
        if (substr($url, 0, 5) === 'data:') {
            // data:image/svg+xml;base64,... should not be touched
            return $url;
        }
        // anything inside TYPO3 has to be adjusted
        return '../../../../'.dirname($this->removePrefixFromString(PATH_site, $this->inputFilename)).'/'.$url;
    }