I've been wanting to start discovering cross-platform development with Mono, MonoDevelop, Gecko#, C++, XPCOM, XUL, XULRunner, WebKit, et al. I have a couple vaporware apps in mind and I have just purchased a Mac Mini and an iPhone mainly for this purpose. And meanwhile since Silverlight happens to be cross-platform as well, I was curious about its licensing. Theoretically, one can accomplish an Adobe AIR-like cross-platform application implementation using Silverlight and XUL or WebKit. Problem is, I had heard that Silverlight was explicitly written to disallow it from being used on anything but a standard HTML web browser (Safari, Firefox, Internet Explorer).

After spending an hour or so poking at the n00b tutorials on XUL and XPCOM, I went to the Silverlight site and spent several minutes looking everywhere for the darn EULA. (Sadly, after finding it, once I hopped on this blog editor I lost it and it took another 15 minutes to find it again.)

http://www.microsoft.com/silverlight/resources/LicenseWin.aspx and http://msdn.microsoft.com/en-us/silverlight/cc307279.aspx

I didn't see any such limitation there, nothing about "thou shalt only use Silverlight in a 'standard web browser', namely Internet Explorer, Firefox, or Safari".  There are some limitations, of course, such as perhaps redistribution, which theoretically if the app is a XUL app can be deal with using HTML+JS+XUL+XPI, getting Silverlight into thinking it's downloading and installing itself through and onto Firefox. (All theory, of course.)

On a side topic, if anyone out there is reading this, can someone tell me why there are almost no open discussions correlating XUL and XAML/WPF? They seem to attempt to do the same basic function--create apps using XML and components--albeit WPF is far more powerful and versatile in itself as a tool in its niche, whereas XUL is Javascript/HTML friendly and is cross-platform.

UPDATE: After discussing with a buddy who's done cross-platform .NET programming with Mono, apparently Glade + GTK# has an XML markup language that also meets the same objective.

On second thought, maybe I just wasn't searching hard enough. I see a lot of hits here: http://www.google.com/search?q=xul+vs+xaml