The Qt Labs Blog was the Trolltech and Nokia developer blog on Qt news, including QML, LGPL licensing, and Qt releases.
HN commenters point to it for Qt's declarative UI work, stylesheets, and Kinetic, praising Qt's progress over competing toolkits like XUL.
I'd further add that in the time since I wrote that, where XUL has crept forward, GTK and especially QT have blazed forward. XUL just isn't remotely competitive unless your app looks like a web app, in which case, it probably should be a web app. The range where you need something that looks like a web app but isn't one, and absolutely positively won't ever need any of the extra functionality the other toolkits offer (all of which is there for a reason) is a pretty narrow one.
Between bindings and scriptability built into some of the toolkits such as QT, there's almost nothing left unique to the XUL platform. You can see an example of that in http://labs.trolltech.com/blogs/2008/12/02/widgets-enter-the-third-dimension-wolfenqt/ around 2:40, where the script driving the little Nazi is actually the scripting language QT ships with based on Ecmascript.
Although not a direct answer to your question, I do think Qt's Kinetic (http://labs.trolltech.com/page/Projects/Graphics/Kinetic) might be a good source for inspiration. It contains a declarative UI markup and scripting language called QML, that looks roughly like JS/JSON. An old example can be seen here: http://labs.trolltech.com/blogs/2009/05/13/qt-declarative-ui/
I am very curious about how Apple would react to something like this though. It's obviously a good idea, but they have been really frantic about alternative solutions to these kinds of problems :)
They probably use WebKit to render the HTML to a buffer and then compress it, save to disk.
If you were asking more generally, see this discussion[1] on the WebKit mailing list, and these Qt links[2, 3] that I found in that thread.
[1] http://www.nabble.com/Webkit-capture-web-page-content-API-td23516755.html
[2] http://labs.trolltech.com/blogs/2008/11/03/thumbnail-preview-of-web-page/
[3] http://labs.trolltech.com/blogs/2009/01/15/capturing-web-pages/
I don't think they're sitting on their hands. They bought Trolltech, and if you follow the work they're doing[1], there's lots of emphasis on small device functionality like multi-touch support, optimizing for small devices with embedded graphics processors, and rich declarative UI stuff that smells like CoreAnimation.
[1] http://labs.trolltech.com/blogs/
Shameless plug to my favorite toolkit: Qt has had this for ages now: http://doc.qt.nokia.com/4.6/stylesheet.html
The Qt guys also have been trying to take this a step further with the declarative UI / QML (basically a tiny DSL for UIs) project:
http://labs.trolltech.com/blogs/2009/05/13/qt-declarative-ui/