XRSessionInit takes a sequence of DOMString but wouldn't it be better if it takes an enumeration? There's a predefined set of allowed values so it seems it should be restricted to those.

I think XRSessionInit should also be made a bit more forward looking. We might want to ask for additional features (anchors, meshing, hit testing, etc) and this would be a good place to do it.
What if we change the IDL to the following:

enum XRSpaceType {
  "local",
  "local-floor",
  "bounded-floor",
  "unbounded"
};

dictionary XRSessionFeatures {
  XRSpaceType space;
}

dictionary XRSessionInit {
  XRSessionFeatures requiredFeatures;
  XRSessionFeatures optionalFeatures;
};

Future extension spec can then extend the 'XRSessionFeatures' object with more than just a simple value ie

dictionary XRWorldMeshFeatures {
  readonly attribute FrozenArray<DOMPointReadOnly> boundsGeometry; 
  XRWorldMeshQuality quality;
};

dictionary XRSessionFeatures {
  XRSpaceType space;
  XRWorldMeshFeatures worldMesh;
};

Any thoughts @toji ?

Sorry that I didn't chime in earlier. I was on PTO in early July and didn't notice all these changes going in.