public static function css($content) {
        $error = 'unknown';
        try {
            $minifier = new MatthiasMullie\Minify\CSS($content);
            return $minifier->minify();
        } catch (Exception $e) {
            $error = $e->getMessage();
        }

        $return = <<<EOD

/* Error: $error */
/* Problem detected during CSS minimisation, please review the following code */
/* ========================================================================== */


EOD;

        return $return.$content;
    }