Test the motion sensors in your phone or tablet: a bubble level on the accelerometer, an axis check, a compass sweep and a gyroscope drift test.
Waiting for the first reading
Pick up the device and move it. A sensor counts only after it sends a reading: browsers expose the same events on desktops with no sensors and fill them with nulls.
No accelerometer reading yet
The bubble level needs the accelerometer. Move the device or check that motion access is allowed above.
Open this page on the device you want to test. Desktop computers don't have these sensors.
Phone motion sensing uses three separate chips: an accelerometer senses gravity, a gyroscope senses rotation, and a magnetometer senses the earth's magnetic field. Each can fail on its own, so each is checked separately above.
The accelerometer measures force; at rest that's gravity, so the phone knows which way is up. That alone is enough for screen rotation and step counting. The gyroscope measures rotation and reacts within milliseconds, so it handles fast camera pans and head movement in VR. The magnetometer measures the earth's field and gives the other two a north. Budget phones often have only an accelerometer; when a game says your device is unsupported, the gyroscope is usually what's missing.
Every accelerometer has a small built-in offset, and the phone subtracts a factory value for it. After a drop or a screen replacement the sensor can shift relative to the housing, and the correction no longer matches. The level then reads a degree or two off on a flat surface. One reading can't show this; turning the device 180° can, and the check above does that.
A gyroscope at rest reports a small rotation, and the phone keeps subtracting it. When that offset grows with age, heat or a knock, the correction falls behind: a game camera creeps, a compass turns while the phone lies still, a VR view slowly rotates. 20 seconds of rest is enough to measure it.
A magnetometer measures a weak field, about half a gauss, so nearby objects easily overpower it: a magnetic case clasp, a car mount, a speaker, a laptop lid, steel in the floor. Most compass problems are interference. The figure-eight gesture in map apps makes the phone recalibrate. A heading that jumps instead of turning smoothly points to a real fault, so the test above uses a full turn.
The sensors sit on the mainboard but are calibrated against the housing. A screen or battery replacement, or a case that was opened, can shift them. After a repair, check all three: a level that reads off, a compass that keeps asking for calibration, and a gyroscope that drifts.
Motion data can reveal what someone types and identify a device, so on iPhone and iPad a page must ask for it after a tap, over https. Chrome on Android doesn't ask, so the button appears only on Apple devices.