This algorithm that I'm adding in #106 is very similar to the Open Window Algorithm in the Service Workers specification (because I've copied and pasted most of it). Should we refer to the Service Workers specification instead of copying their steps?

The thing is, our algorithm is not exactly the same, so we might have to say something inelegant, like "run the open window algorithm, but skip step number four and set some value to the internal slot [[windowClient]] just before returning".