I made some changes to the XULPlanet.com stylesheet so it works better in IE, although there are still some issues with tutorial pages that have wider examples. It should be OK if you widen or maximize your browser window. The pages should also work in Mozilla (naturally), Netscape 6/7 and Opera.
Even though most people visiting XULPlanet are using Mozilla or Mozilla-based browsers, many first-time visitors come to learn more about XUL because they are interested in the technology and don't want to commit themselves yet. So, there is a need to handle other browsers also. This is why mozilla.org uses a very simple layout. Part of the goal of their site is to help existing developers and help new people who have just discovered Mozilla but are using a different browser.
One thing I have noticed though is that it is easier to support browsers which support standards well or browsers that don't support them at all. Anything in-between is harder to deal with. A properly structured document seems to render in a readable fashion better in Mozilla, Opera, Netscape 3/4 and text browsers than it does in IE.