protected function resolveCssRelatives($css, $base = '')
    {
        if (!($css = (string) $css)) {
            return $css; // Nothing to do.
        }
        $this->current_base = $base; // Make this available to callback handlers (possible empty string here).

        $import_without_url_regex = '/(?P<import>@(?:\-(?:'.$this->regex_vendor_css_prefixes.')\-)?import\s*)(?P<open_encap>["\'])(?P<url>.+?)(?P<close_encap>\\2)/ui';
        $any_url_regex            = '/(?P<url_>url\s*)(?P<open_bracket>\(\s*)(?P<open_encap>["\']?)(?P<url>.+?)(?P<close_encap>\\3)(?P<close_bracket>\s*\))/ui';

        $css = preg_replace_callback($import_without_url_regex, [$this, 'resolveCssRelativesImportCb'], $css);
        $css = preg_replace_callback($any_url_regex, [$this, 'resolveCssRelativesUrlCb'], $css);

        return $css;
    }