XHTML Components
From Documentation
This section describes how to use the XHTML component set (i.e., XHTML components). XHTML components are in a different component set than ZUL components. You have to specify XML namespace to distinguish them if you want to use them in the same ZUML document. For example,
<window xmlns:h="xhtml">
<h:ul>
<h:li>Click <button/></h:li>
</h:ul>
</window>
Notice that the HTML component set is one of the approaches to use HTML tags directly in a ZUML document. In most cases, it is not the best approach. For information please refer to ZK Developer's Reference: HTML tags.