An Anonymous Coward
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
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.