protected function stripExistingCssCharsets($css)
    {
        if (!($css = (string) $css)) {
            return $css; // Nothing to do.
        }
        $css        = preg_replace('/@(?:\-(?:'.$this->regex_vendor_css_prefixes.')\-)?charset(?:\s+[^;]*?)?;/ui', '', $css);
        return $css = $css ? trim($css) : $css;
    }