A page can carry the same picture in four ways: as a plain JPEG, as WebP served to everyone, as a negotiated file that depends on the Accept header, or as a picture element that lets the browser choose. Only the second and third are invisible in the markup — they show up in the response headers. This check asks for each image twice, once with a modern Accept header and once without, and compares what comes back.
From this server: one request for the page and, for each of the first six images, one GET capped at 256 KB plus one HEAD. No cookies are sent, no scripts are executed, nothing is stored beyond the rate counter.
Limits worth knowing: only images in the served HTML are seen — anything a script inserts later stays invisible, as do CSS background images. The first six distinct addresses are checked, and the real file size comes from Content-Length, because the download stops at 256 KB.