I'm running some WebGL triangles in different environments, and was surprised to see a difference in this test with one triangle:

Firefox:

headless-gl:

diff:

Scaling up the images in GIMP reveals that the issue is the antialiasing in the triangle (presumably due to differences in the environment):

headless-gl:

Firefox:

I've manually checked the value of the alpha channels in the pixels of the diagonal, and it's the same in all: 39.5%.

I've noticed that the antialias-detection algorithm clamps a 3x3 pixel bounding box along the edges, so I assume that the pixels that show as red in my diff image have a 2x2 bounding box, and therefore the calculations differ, ref: