public function css($header = true) {
        $out = '';
        if (count($this->errors) > 0) {
            if ($header)
                $out .= '<style type="text/css" media="screen">';
            $out .= "#" . implode(", #", array_keys($this->errors)) . " { {$this->style} }";
            if ($header)
                $out .= '</style>';
        }
        echo $out;
    }