Fallback for VR Mode

I originated this question in the Declarative WebVR group, but wanted to 
get input from people here too.

Some devices cannot go into WebVR mode (think Chromebook). There are 
just no VR devices.

Some devices have VR added (e.g., desktop with Vive).

Many devices can go into VR (e.g., Pixel phones), but the user does not 
have the appropriate device (Daydream for Pixel) to complete the 
transition. The user may have cardboard and wish to use that instead of 
a fancier headset.

The WebVR JS code that THREE uses bases its decision on the existence of 
'navigator.getVRDisplays' having length > 0.

What do people here think the expected/default fallback should be when 
the device

1) Is not VR capable but can do stereographic display on 3D content
   OR
2) The user wishes to use a non-VR headset (e.g., cardboard) instead of 
the associated VR device

I can see several answers here including required user-prompt, 
auto-fallback, do exactly what's requested. What are peoples thoughts or 
comments as to what they would expect?

Thanks.

-- 
*Leonard Daly*
3D Systems Architect & Cloud Consultant
President, Daly Realism - /Creating the Future/