> It’s not 100 % clean either, but much better. Granted, this is WebP re-encoding of an already lossy compressed JPEG, so we stack 2 steps of destructive compression. But this is what Google Page Speed insights encourage you to do and what a shitload of plugins enable you to do, while pretending it’s completely safe. It’s not.

> I have seen a similar effect in other similar pictures : always pictures with large, smooth, gradients in the background, which happens a lot when some punctual-ish light falls off a wall. That’s not something accidental, smooth fall-off are actively built by photographers to create organic-looking backgrounds with just enough of texture to not get boring, yet discrete enough to not draw attention off the foreground/subject.

I think this rant could have highlighted these paragraphs a lot more, because these are indeed problems. The first paragraph probably refers to [1] where it doesn't say too much about recompression artifacts, and the second paragraph is indeed a well-known issue of the lossy WebP format---it tends to create gradient bands that are particularly significant when viewed on big and bright screens. It is far-fetched to claim that this requires somehow trained eyes, rather it is more or less device-specific in my opinion.

[1] https://developer.chrome.com/docs/lighthouse/performance/use...