From the XPToolkit website :
The XPToolkit is an open source SDK for building common cross-platform user interfaces using existing cross-platform standards (e.g., XML, RDF, HTML, CSS, DOM, JavaScript) and technologies (e.g., Gecko, XPConnect) to minimize platform-specific code, and therefore time to market. We want to provide for massively parallel development of XP UI. We are particularly focussed on building widgets and services to support the common needs of the mozilla browser, editor and mail/news apps.
We are providing for intermixing of native dialogs and widgets, but we aren't trying to trying to be the Alpha and Omega of UI toolkits. In particular, we currently have no plans to do widgets that aren't universally needed (like Date/Time pickers). We aren't trying to provide for writing the most Mac-like Windows-like or Linux-like (whatever that means) applications; for that you'll probably always have to go native. The platforms we are committed to shipping on are Linux, MacPPC? and Win32; but we have an internal need for Solaris in order to do Purify builds.
XPToolkit project is staffed by nine full-time Netscape engineers (the mercenaries), one full-time manager (as pointy-haired as they come), and several developers contributing via mozilla.org (Olympians, every one).
Vision
We will make UIs as easy to build as web pages, and we will make applications easier to write and to customize along the way.