@noleli Hmm, in Safari on a Mac, I think I do get your expected results? Sampling the pixels in Acorn, I see #363636 (21%) and #d9d9d9 (85%).
I wondered if your browser might be blending in linear rather than sRGB, for arguably visually better results. I don't think so, because your first example would then be more like 50% gray (but that's just based on plugging 0.21 into an SRGB converter so I might not have it quite right)
