Well, assuming it uses a pristine WebKit backend, you should only have to test in one browser with the same WebKit build. :) I guess that might be idealistic thinking, but hopefully it pans out.
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.