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.
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.