If your browser supports WebXR, the polyfill won't load. However, if your browser is a phone that just supports immersive-ar (and possibly inline), it would be nice to have a version of the Cardboard device loaded up that supports immersive-vr by starting an immersive-ar session or inline (if inline support 6DOF) and rendering the dual view over the video (using the immersive-ar 6DOF tracking).
If your browser supports WebXR, the polyfill won't load. However, if your browser is a phone that just supports immersive-ar (and possibly inline), it would be nice to have a version of the Cardboard device loaded up that supports immersive-vr by starting an immersive-ar session or inline (if inline support 6DOF) and rendering the dual view over the video (using the immersive-ar 6DOF tracking).