It would be nice if content could add features to the XRSession in the middle of the session. For example, a complex app may not require hand-tracking by default (and may not wish to appear creepy by asking for it up front), but might have a "hand tracking mode" that can be enabled in the settings.

The Permissions Request API exists, but is not implemented, and with #1189 , it will not be sufficient for granting permissions to a given session. We should add a requestAdditionalFeatures() API to XRSession that does the same required/optional stuff as requestSession() that can update the session's set of granted features mid-session