Just like the other webkits such as Yahoo's YUI and Google's GWT, Pyjamas' AJAX library provides you with all of the tricks of the Web trade (from Opera, Netscape, IE, Firefox and Safari) so that you don't have to know any of them. You can therefore focus on developing your application as if the user's browser was truly a platform-independent Desktop development environment, rather than a web browser.
Pyjamas' underlying widget set is very similar in concept to that of KiWi, with the difference being that it targets the Web rather than Desktop. If you want your Pyjamas application to run on the desktop, you can - unmodified! - using PyjamasDesktop.
As Pyjamas supports every major browser, if those browsers have SVG support, then there has even been a Widget contributed which provides access to the SVG "Canvas", allowing you to develop exceptionally powerful graphics applications.
Pyjamas was originally a port of Google Web Toolkit, to python.
Information on Pyjamas is available here: