How to get heading accuracy of mobile devices? - mobile-website

Question

I offer a service which needs the correct location and heading of the mobile device the user is accessing my service with. It was relatively easy to get the information but now I am stuck at finding out how accurate the heading I received is.
Since I'm not using any framework for this or develop in Android/iOS, where there are solutions for that problem, I'd need a solution only depending on javascript (+ 3rd party libraries).
I've found the Generic Sensor API from W3C but couldn't find any sensor which hold that information. Neither the Accelerometer-, nor the AbsoluteOrientationSensor held the needed information.
My current code looks like that..
let accelerometer = null;
try {
accelerometer = new Accelerometer({ frequency: 60 });
accelerometer.addEventListener('error', event => {
// Handle runtime errors.
if (event.error.name === 'NotAllowedError') {
console.log('Permission to access sensor was denied.');
} else if (event.error.name === 'NotReadableError') {
console.log('Cannot connect to the sensor.');
}
});
accelerometer.addEventListener('reading', (event) => {
console.log(accelerometer);
console.log(event);
});
accelerometer.start();
.. but since the problem if more of a 'finding the right tool to do the job'-type, it won't help much in better depicting my problems. Further, Google uses kind of the same functionality I'm aiming for in Google Maps, so theoretically there must be a way to get the heading accuracy.
So, in conclusion, is there any way to retrieve the heading accuracy in a simple javascript environment?
Thanks!