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I had to check. It's Next.js /React and the images are all CSS using Chakra. Just looking at the payload, it's sending the byte sizes which tells me they are counting bytes and checking for the difference. Pretty clever.

They are probably counting pixels but you could for example use destination-out compositing operation on canvas and resize to 1x1 pixel and read brightness of single pixel.

I'd guess SVG. It's not a complex integral. They can do a boolean intersection in SVG.