The next image shows a class diagram of the main classes of the WebKit2 API with the relationships among them.
Not dashed lines means association relationships usually implemented as getter methods or as properties. The dashed lines represents use relationships and means that one class is used by the another usually as a parameter of some first class method of signal callback.
This is not an exhaustive class diagram of the WebKit2 API, it doesn't have many classes and types and there are some relationship that are missing but it tries to give a general view of the API. This general view could be quite helpful for newcommers. If you want to modify the diagram you can find the Dia file here.