E4X means that at the moment my engine will only run in Firefox. But then this is a hobby project and I have better things to do then deal with the Pandora's Box of cross browser issues. Especially as Firefox 3 already implements so many goodies from HTML5 and ECMAScript 5th Edition.
Though I can't wait for the next beta of Firefox 3.5 and the @font-face CSS property. That and border images Should really come in handy when I get around to putting an interface page together.