Syntax:

<web-app-class>a_class_name</web-app-class>

[Default: org.zkoss.zk.ui.http.SimpleWebApp]

It specifies which class is used to implement the Web application. The class must have a default constructor (without any argument), and implement both the org.zkoss.zk.ui.WebApp and org.zkoss.zk.ui.sys.WebAppCtrl interfaces. Instead of implementing from scratch, you can extend it from the org.zkoss.zk.ui.impl.AbstractWebApp or org.zkoss.zk.ui.http.SimpleWebApp classes.

<system-config>
  <web-app-class>foo.MyKillerApp</web-app-class>
</system-config>

Alternatively, you could use the web-app-factory-class element instead. Notice, if both specified, the web-app-factory-class element has the higher priority.