You can't easily use that with a plain* CDN though - you would need to add a Vary: Accept header and cache misses will increase as every variant of Accept: will

You can't easily use that with a plain* CDN though - you would need to add a Vary: Accept header and cache misses will increase as every variant of Accept: will cache a different version * it seems fixable with things like Lambda@Edge and I wouldn't be surprised if a smart CDN which already does on-the-fly image compression had implemented this too