Hi! Coming back to this thread after a while to see if anything has changed. Now that Babylon 5 has just been released 
To summarize the thread above, (and pls correct me if I’m wrong), it’s possible to update the XRCamera position continuously with another moving mesh (like a boat) by copying positions on every frame BUT because the position of “head” to boat is fixed, the illusion of riding on the boat with freedom to shift side to side, stand-up or sit down in the boat is nullified. The illusion would only be maintained if you didn’t translate your head and only rotated it.
One question I have is, does WebXR provide any information regarding the local offset to our own room / local VR play area (the Oculus guardian boundary). If we could somehow get that, it seems like that’s the position we wish we could be be updating and not the camera directly?
(Forgive my crude drawing to illustrate my point – no pun intended).
That way the room’s is fixed to the boat, but I can still walk around the boat. But I dunno if that’s possible.