public static function extractImports($css)
    {
        $imports = [];
        preg_match_all(static::IMPORT_PATTERN, $css, $matches, PREG_SET_ORDER);
        if (empty($matches)) {
            return $imports;
        }
        foreach ($matches as $match) {
            $url = empty($match[2]) ? $match[4] : $match[2];
            $imports[] = $url;
        }
        return $imports;
    }