Web Applications with Mozilla's XUL?

An Anonymous Coward's idle musings inspires this query: "Web-based applications are a hot topic, right now, but sometimes HTML is too simple for your applications. Using a cross-plataform, more powerful and efficient UI like Mozilla's XUL would be great." XUL is more an interface description language rather than an application language, as it still uses Javascript to handle application processing. It would be interesting to see if future browsers (or future versions of existing browsers) would add XUL bindings for other languages like PHP, Perl, or even Visual Basic if such a thing interests you.