The ZK User Interface Markup Language (ZUML) is a XML-based language used by developers to describe the visual presentation. ZUML is aimed to separate the dependency of the set of components to use. In other words, different set of components^[28], such as XUL and XHTML, could be used simultaneously in the same ZUML page. Different markup languages could be added transparently. If two or more set of components are used in the same page, developers have to use the XML namespaces to distinguish them. Refer to the Component Sets and XML Namespaces section in the ZK User Interface Markup Language chapter if you want to mix multiple component sets, say XUL and XHTML, in the same page.

Tip: Using XML namespaces in ZUML is optional. You need it only if you want to mix two or more.