WebXR

- [ ] DeviceOrientationCamera, #4608
- [ ] Future [WEBGL_multiview](https://github.com/Oletus/WebGL/blob/multiview-strawman/extensions/proposals/WEBGL_multiview/extension.xml) extension?
- [ ] The `VRButton` should search for VR devices. If none, are available default to Cardboard.
- [ ] The [HMDVRDevice](https://developer.mozilla.org/en-US/docs/Web/API/HMDVRDevice) should be passed to `Fullscreen.requestFullscreen` and has viewing parameters that should be used for the left and right eye cameras.
- [ ] The [PositionSensorVRDevice](https://developer.mozilla.org/en-US/docs/Web/API/PositionSensorVRDevice) should be passed to `DeviceOrientationCameraController` to set the camera orientation. If the parameter is `undefined`, default to using the `deviceorientation` event.
- [ ] Public API for multiple viewports, each potentially with its own camera

This issue is a broad WebXR checklist covering DeviceOrientationCamera, VRButton, Fullscreen.requestFullscreen, DeviceOrientationCameraController, and multiple viewports. Start by selecting and separately scoping one checklist item, then inspect the relevant existing entry point and its tests; completion criteria are not defined for the issue as a whole.

Written by the indexing model from the issue text.