If the browser chooses the stronger protocol it should force the user to opt-in to any fallback behavior which might downgrade security. The current behavior of the browser in most cases is if you enter an 'https' url and the request fails or the cert is invalid you get a failure or warning message. I'd like to see this behavior kept in this case. It communicates what the browser is doing instead of silently downgrading the protocol based upon fuzzy failure signals.