[Disclaimer: This issue is one of several being filed to capture discussions that began either on #638, on #689, or at the most recent F2F]

This issue is NOT to discuss the requirements for what Trusted Immersive UI should look or behave like. That conversation is happening in #718

Instead, this issue tracks the question of whether a Trusted Immersive UI is a requirement for a UA to be considered conformant with WebXR.

(Note: this is also related to issue #702 which discusses figuring out what to do with non-XR feature permissions which developers may request during an immersive session)