The ZK team managed to provide a framework where the developer gets a number of choices of how to do things: ZUML, Java, JSP/XUL/HTML and all kinds of combinations.  The visual editor is just a web application – packed in a '*.war' file' - that can be launched on an application server. It provides a visual interface with a toolkit that holds all the visual components that the ZK framework has implemented so far and is designed to use all kinds of custom components as well. This means that a developer can add to the toolkit components that he/she has designed, as far as they are compliant with the framework's protocol.

Full Article