I would say - careful when changing the fov for VR/XR.
Just like @JCPalmer said, WebVR and WebXR provide the information needed to display the scene correctly. Amending it might cause the user to… not feel well when seeing you scene.
I would recommend not to do that. fov is being ignored, since we are using the projection matrix directly. If you want to do it , you will need to calculate the projection matrix yourself.