The spec says

The originOffset attribute is a XRRigidTransform that describes an additional translation and rotation to be applied to any poses queried using the XRReferenceSpace.

However, it's not exactly clear how exactly the rotation is applied: is it applied after the pose (like a periscope)? Or before the pose (as if the user is standing on a platform or something)? Or is it an offset being applied to the space?

I would have assumed it's a premultiplication with the pose, however the chromium impl seems to indicate that its inverse is premultiplied, which makes me think it's logically a transformation applied to the entire space.

Could this be clarified? (May be worth doing as part of #565)