Gecko is not exactly separable from Firefox (unlike Blokc/Webkit and I'm guessing Trident/Spartan). The firefox UI depends heavily on things like XUL and XPCOM

Gecko is not exactly separable from Firefox (unlike Blokc/Webkit and I'm guessing Trident/Spartan). The firefox UI depends heavily on things like XUL and XPCOM and whatnot which are nonstandard features in Gecko and would be a pain to support in Servo. I guess you could run two browser engines -- one for the UI, one for the content, but I'm not sure how well that would work.