WebXR is now supported on both the Oculus Browser and Firefox Reality (you can test with the Hello WebXR demo from Mozilla)
I am currently seeing two issues using Typescript, 4.1.0-rc9 preview packages, webxr-polyfill and the Default XR Experience:
On Oculus Browser, the first time I enter the immersive mode on a new page load it has very bad rendering issues (only in one eye, screen is tiny and out of view). The second time everything looks perfect.
On Firefox Reality, it launches the browser into a strange 3DOF mode. I never can get it into immersive mode.
The Hello WebXR code may be helpful to look at?