The idea is to start being dogfoodable for the team by the end of the year, but that only covers as tiny subset of sites that are used by the team, maybe Etherpad (which is used for meetings) and /r/rust. The main focus with Servo so far has been to look at the real performance bottlenecks that other engines face during layout and parallelize them. The plethora of other features that need to be supported will come later after these problems have been solved (they mostly have been). That said, replacing Gecko is a looong way off.