Accelerometer.z

The z read-only property of the Accelerometer interface returns a double precision integer containing the acceleration of the device along its z axis.

If a feature policy blocks use of a feature it is because your code is inconsistent with the policies set on your server. This is not something that would ever be shown to a user. The Feature-Policy HTTP header article contains implementation instructions.

A Number.

Acceleration is typically read in the reading event callback. In the example below this occurs sixty times a second.

let accelerometer = new Accelerometer({frequency: 60});

accelerometer.addEventListener('reading', e => {
  console.log("Acceleration along the X-axis " + accelerometer.x);
  console.log("Acceleration along the Y-axis " + accelerometer.y);
  console.log("Acceleration along the Z-axis " + accelerometer.z);
});
accelerometer.start();