XRRigidTransform: orientation property
  The read-only XRRigidTransform property
  orientation is a DOMPointReadOnly
  containing a normalized quaternion (also called a unit
quaternion or versor)
  specifying the rotational component of the transform represented by the object.
  If you specify a quaternion whose length is not exactly 1.0 meters, it will be
  normalized for you.
Value
  A DOMPointReadOnly object which contains a unit quaternion providing the
  orientation component of the transform. As a unit quaternion, the length of the returned
  quaternion is always 1.0 meters.
Examples
To create a reference space which is oriented to look straight up, positioned 2 meters off of ground level:
js
xrReferenceSpace = refSpace.getOffsetReferenceSpace(
  new XRRigidTransform({ y: -2 }, { x: 0.0, y: 1.0, z: 0.0, w: 1.0 })
);
The unit quaternion specified here is [0.0, 1.0, 0.0, 1.0] to indicate that the object should be facing directly along the y axis.
Specifications
| Specification | 
|---|
| WebXR Device API # dom-xrrigidtransform-orientation | 
Browser compatibility
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