private function appendAttribute($attrName, $attrValue)
    {
        $xml1 = defined('ENT_XML1') ? ENT_XML1 : 16;

        $attrName = htmlspecialchars($attrName, $xml1 | ENT_COMPAT);
        $attrValue = htmlspecialchars($attrValue, $xml1 | ENT_COMPAT);

        $this->outString .= ' '.$attrName.'="'.$attrValue.'"';
    }