XUL "The piece of XUL that Safari implements is "(b) implement some additional layout primitives." XUL basically introduces four new layout primitives to CSS: the flexible box, the grid (flexible boxes in 2 dimensions), rich popups/tooltips, and stacks. Safari in Panther has implemented the first (and most useful) of those layout primitives, the flexible box model. Since the box layout primitives are defined via CSS, you can even use them in HTML (in either Safari or Mozilla)."
See also, Safari 1.1 and XUL renderer for KDE.
IE Development: XAML "The operating system lock-in created and perpetuates the browser lock-in. Now the browser will give that extra boost to the OS. “Our application only works on Windows, using IE. No, sorry, extra development costs are too much. You’ll just have to use a Windows desktop.”
Apple has a tremendous amount of momentum right now, and about three years to innovate and compete against an OS that was released two years ago tomorrow. If they embrace a development platform like XUL and actually make inroads with the customers who will be deploying applications using it, will it be enough?
All this can be somewhat negated by the opportunity that exists for Microsoft to play fair. If, as Simon postulates, XAML can be transformed server-side to XUL (using XSLT), then we all win. But Eric remembers his history, which suggests we shouldn’t rely on that happening. I’ll throw this into the ring then: even if it’s possible, will it be cost-effective? For organizations to spend an extra 20% of a development budget to support a 3% share of the market is a bit of a stretch. But it’s obviously do-able, seeing as how many still bend over backward for NN4.x."