We could probably rewrite the entire Firefox UI in HTML nowadays, but that'd mostly be busywork (and would break a bunch of extensions). I would bet that Servo's UI will just be HTML.
Servo is just the browser engine, like Gecko. The UI/browser-chrome will be a separate project (named Crow, but not started yet afaik). Also relevant is the fact that Servo wants to be embeddable, like Webkit.