It depends on what the various different layers in between you and the gfx hardware are doing. For instance, see the discussion on this "monitor calibration" s

It depends on what the various different layers in between you and the gfx hardware are doing. For instance, see the discussion on this "monitor calibration" shader: https://www.shadertoy.com/view/NsS3Rw https://www.shadertoy.com/view/NsS3Rw In that case, FireFox does not 100% provide some of the needed APIs in order to get pixel-perfect rendering. There's nothing you can do to fix that yourself. "...Anyways, long story short, we need Firefox to implement 'devicePixelContentBoxSize' before we can have pixel exact rendering, deterministically, in that browser." That's just one example. For something delicate like pixel-perfect rendering, every layer in the chain from hardware to OS -> libraryX -> libraryY -> libraryZ -> browser -> your code, all need to handle coordinates and thigs _perfectly_. If any one of those links in the chain messes things up, or doesn't provide the correct rounding, etc. then you are stuck. It's not so terribly hard to do, but it has subtleties and edge cases, and everyone involved must handle them correctly. Also, if they handle them not-correctly, the result is still "good enough" for many purposes, so the fix doesn't get prioritized.