UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36

Steps to reproduce the problem:
1. Enable the 'Generic Sensor', 'Generic Sensor Extra Classes' and 'Experimental Web Platform features' in chrome://flags.
2. Run test case 'https://kaixinjxq.github.io/generic-sensor-demos/sensor-info/build/bundled/'.
3. Click the button at the top right corner
4. A popup shows, and choose the "AbsoluteOrientation Screen" Sensor type, click "ADD" button
5. Turn on the switch button 
 6. After rotating the screen, then make sure the +Y axis of screen coordinates points to North.
7. Check the value of sensor's quaternion.

What is the expected behavior?
sensor reports quaternion approach to [0,0,0,1.0] when device is at rest on a leveled surface and dom screen's +Y axis is pointing magnetic north.

What went wrong?
Rotate screen by clockwise:
when rotating screen to 0°/90°/180°/270°, the value of AbsoluteOrientation's quaternion is as below:

Note:
0° means that +X axis of screen coordinates  =  +X axis of device coordinates.
90° means that  +X axis screen coordinates  =  -Y axis of device coordinates.
180° means that  +X axis screen coordinates  =  -X axis device coordinates.
270° means that  +X axis screen coordinates  =  +Y axis device coordinates.

Result:
                                AbsoluteOrientation Screen's quaternion
0°(portrait)                    x -0.011 y -0.003 z -0.183 w 0.976   (PASS)
90°(landscape)                  x: -0.001 y -0.012 z -0.073 w 0.997  (PASS)
180°(clockwise portrait)        x: -0.023 y -0.007 z -0.194 w 0.981  (PASS)
270°(clockwise landscape)       x: 0.005 y 0.019 z -0.116 w -0.993   (expected value:[0,0,0,1.0], but got [0,0,0,-1.0])

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 66.0.3355.0  Channel: canary
OS Version: 10.0
Flash Version: