Now I wonder how much it depends on the gamma. I understand there's some differences depending on the hardware, browser and image format/metadata. By default we may all see a different shade of grey, affecting how much of an illusion we get.
Edit: checked Firefox and Chrome on Android and they look exactly the same (work on me). Low brightness works better than full brightness.
That’s certainly a possible factor. I’m on an iPhone 14 Pro with brightness at medium levels in an averagely lit room using Safari. I just tried on my Mac’s XDR display and it’s similar: I can either see or not see the illusion (now that I know what the illusion looks like I can turn it on/off in my head).
Edit: checked Firefox and Chrome on Android and they look exactly the same (work on me). Low brightness works better than full brightness.