Accessing the iPhone Accelerometer with Javascript in iOS 14 and 13

TL;DR: Demo here. Hit the button at the top to start.

I recently tried out a bunch of different Javascript accelerometer demos that claimed to able to access the sensor in-browser. It was usually a simple demo: dots rolling across a screen due to gravity, or perspective shifting a 3D scene based on what your orientation was. However, none of these demos worked at all — no popup asking about permissions or anything. What gives?