Preview shows you a real-time, 100% preview of the Canvas. Additionally, by clicking on a point in the Preview, you can center the Canvas on that particular point. Moreover, it is also possible to click and drag in the Preview to move around the Canvas smoothly.
You can choose between WebCode and WebKit preview modes at the bottom of the Preview window. The WebKit preview renders the generated code using WebKit, the same engine that is used by Safari, Chrome and other web browsers.