Work with Existent Servlets
From Documentation
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By enclosing ZUL components with FORM, you could post the content of ZUML components as the request parameters to an existent servlet. Refer to the Work with HTML FORM and Java Servlets section in the ZUML with the XUL Component Set chapter for details.
With this approach, you could design rich user interfaces without modifying the existent servlet by simply replacing some of them with ZUL components.