I personally haven't much work on embedding Servo, however I do think there has been some progress the last few years, Servo is being embedded in a Hololens applications(see https://github.com/servo/servo/tree/master/support/hololens), as well as a Magicleap application(see https://github.com/servo/servo/tree/master/support/magicleap).

The embedding API used can be found at https://github.com/servo/servo/tree/master/ports/libsimplese...

I think it's a pretty low-level embedding API, so perhaps not comparable with embedding Webkit(I have no experience with that either), it does seem to do the trick.