UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36

Example URL:
https://www.shadertoy.com/view/XdtczB

Steps to reproduce the problem:
1. pickup shader color value with your favorite tool.
2. compare value obtained with Angle vs OpenGl
3. option: compare with true texture average (e.g. with gimp. #if 0 to show the texture.)

What is the expected behavior?
texture is 256x256 uniform color noise. Theoretical average is 127.5, real average is 127.16, 127.16, 127.55.
But ultimate MIPmap level is  123, 123, 124 on Windows/Angle and 126, 126, 127 on OpenGL (either windows or linux).

What went wrong?
Bias on texture average (MIPmap0), small on OpenGL but quite larger on Angle.
MIPmap computation by 8bits cascading yields imprecisions, but 3.6 * more in Angle for some strange reason.

Does it occur on multiple sites: N/A

Is it a problem with a plugin? No 

Did this work before? N/A 

Does this work in other browsers? No
 Firefox

Chrome version: 62.0.3202.94  Channel: n/a
OS Version: 
Flash Version: Shockwave Flash 28.0 r0

More that 3% relative error can cause thresholding and bias issues (e.g. when MIPmap0 is used to normalize the texture values).

May somebody please set the flags :	
Blink>WebGL
Internals>GPU>ANGLE
? thanks !