If Servo becomes the default layout engine for Firefox, I really hope that having one more engine in the mix forces everyone towards standards compliance, or at least uniformity. Obviously, they're still working out basic functionality, so they've got a way to go before that becomes a concern. But if the project is successful, it's going to be one more browser type for web designers to worry about. Either that or Servo's multi-threaded speed and safety are so awesome that all the major browser developers decide to ditch their own engines and everybody settles on Servo.