The read-only XRRigidTransform property position is a DOMPointReadOnly object which provides the 3D point, specified in meters, describing the translation component of the transform.

A read-only DOMPointReadOnly indicating the 3D position component of the transform matrix. The units are meters.

To create a reference space which can be used to place an object at eye level (assuming eye level is 1.5 meters):

function onSessionStarted(xrSession) {
  xrSession.addEventListener("end", onSessionEnded);

  gl = initGraphics(xrSession);

  const glLayer = new XRWebGLLayer(xrSession, gl);
  xrSession.updateRenderState({ baseLayer: glLayer });

  if (immersiveSession) {
    xrSession
      .requestReferenceSpace("bounded-floor")
      .then((refSpace) => {
        refSpaceCreated(refSpace);
      })
      .catch(() => {
        session.requestReferenceSpace("local-floor").then(refSpaceCreated);
      });
  } else {
    session.requestReferenceSpace("viewer").then(refSpaceCreated);
  }
}

function refSpaceCreated(refSpace) {
  xrReferenceSpace = immersiveSession
    ? refSpace
    : refSpace.getOffsetReferenceSpace(new XRRigidTransform({ y: -1.5 }));
  xrSession.requestAnimationFrame(onFrame);
}

After setting up the graphics context for WebXR use, this begins by looking to see if a variable immersiveSession is true; if so, we first request a bounded-floor reference space. If that fails (probably because bounded-floor isn't supported), we try requesting a local-floor reference space.

If we're not in an immersive session, we instead request a viewer reference space.

In all cases, once the space has been obtained, it gets passed into the refSpaceCreated() function. For immersive spaces, the specified space is saved for future use. However, for inline sessions, we know we're in a space not automatically adjusted for floor level, so we request an offset reference space to shift the viewer's height to 1.5 meters above the presumed floor level of 0 meters. That new reference space is used instead of the one initially received.

Finally, an animation frame request is submitted.