They seem to be using their own javascript engine, so it might need separate testing because of that. Although if it becomes popular enough, most javascript libraries and frameworks will surely support it.
The hard part is DOM interaction and styling. But as they're using WebKit and are putting a lot of effort into compatibility - this should be a breeze.
The really hard work is support for IE6. 1990s JS assumptions, very bad and inconsistent DOM behaviour, etc.
New, open JS engines with a different take on executing JS (Tamarin, V8) are wonderful news for JS developers everywhere. And consumers too, of course.
I wonder at what point it is justified to return a 'please update your browser, preferrably to one of these: [list of reasonable browsers]', just because of the effort it takes you to keep IE6 compatibility.