public static function createZone($zoneName, $tag='div')
    {
        if (self::$_page && !isset(self::$_zones[$zoneName])) {
            $element = new Html\Element($tag);
            self::$_zones[$zoneName] = $element;
            self::append(self::$_outputZone, $element);
            return $element;
        }
        return null;
    }