The "xr-standard" Gamepad Mapping section (added by #553) says, "Additional buttons or axes may be exposed after these reserved indices, and SHOULD appear in order of decreasing importance." However, the values and/or behavior of the buttons or axes at the reserved indices is not defined.

@toji says there is established behavior for Gamepad API, but it doesn't appear to be documented there either. It would be helpful to have some sort of text in the WebXR spec giving a hint of the intended behavior. It might be ideal if this pointed to general guidance in the Gamepad API spec.