Even then, visual diffs were pretty flakey for web development, because one's OS and browser choice would slightly alter the exact pixels blitted to the screen. At least this was the case for the tests that would simply match pixels instead of computing a sort of visual hash.

It's also partly why some people preferred snapshot tests that compared the DOM tree instead, though that was brittle in other ways (e.g. tests would break if an application's frontend used a major UI library and an update to the library permuted the order of classes in some part of the HTML).

For web UI tests this is mainly solved, at least when using Playwright. It allows setting thresholds, percentages and some other config items to allow some small differences in pixels. https://playwright.dev/docs/test-snapshots#options