Syntax:

<engine-class>a_class_name</engine-class>

[Default: org.zkoss.zk.ui.impl.UiEngineImpl]

It specifies which class is used to implement the UI Engine. The class must have a default constructor (without any argument), and implement the org.zkoss.zk.ui.sys.UiEngine interface.

One instance of the UI engine is created and shared for each Web application, so you have to synchronize the access properly.