protected function forceAbsRelativePathsInCss($css)
    {
        if (!($css = (string) $css)) {
            return $css; // Nothing to do.
        }
        $regex = '/(?:[a-z0-9]+\:)?\/\/'.$this->pregQuote($this->currentUrlHost()).'\//ui';

        return preg_replace($regex, '/', $css); // Absolute relative paths.
    }