Hi i found the answer after studying http://www.zkoss.org/smalltalks/rssxml/

ZK XML Output

While a ZUML page is requested, ZK parser will use the suffix of this request's URL(e.g. *.zul, *.mil, *.zhtml etc) to choose the right way to generate the response document. And in ZK-2.5.0 we add a new way for parser to handle the request from such client side application which requires a XML over HTTP response. In this demo, this client side application is an RSS reader.

This means that you should add a language-mapping to zk.xml (WEB-INF) in the example:

<language-mapping> <language-name>xml</language-name> <extension>zrss</extension> </language-mapping>

Also in web.xml this extension should appear

<servlet-mapping> <servlet-name>zkLoader</servlet-name> <url-pattern>*.zrss</url-pattern> </servlet-mapping>

So for the <x:transformer/> to work you can choose any extension for the ZK page as long as it's configured in this way.

Regards, Huub