protected function filterCssUrlCb(array $m)
    {
        if (mb_stripos($m['url'], 'data:') === 0) {
            return $m[0]; // Don't filter `data:` URIs.
        }
        return $m['url_'].$m['open_bracket'].$m['open_encap'].$this->hook_api->applyFilters('css_url()', $m['url']).$m['close_encap'].$m['close_bracket'];
    }