A few years ago I helped build a WebXR (Well, I think it was the WebVR specification) site for both Android and iOS, back when Motion & Orientation Access was possible on the latter platform. It worked pretty well, until it was no longer possible to access that API and we had to spend countless hours on a static fallback for iOS. The gyroscope is all you need for a relatively novel experience, ThreeJS had a VR shooter example you could easily extend, and not needing to download an app was great for getting more people to check it out.
