A browser announces what it can decode in Accept-Encoding, and the server picks one. Which one, and how much it saves, is invisible in the page itself. This check fetches the same address five times from this server, once uncompressed as the reference, once each asking only for gzip, Brotli and zstd, and once with the combined header current browsers send. For every answer it counts the bytes on the wire against the unpacked size and reads Content-Encoding, Vary and ETag.
Redirects and five full downloads of up to 3 MB each go from this server to the address entered. Hosts in private, loopback and link-local networks are refused. The query is protected by reCAPTCHA v3; the calling IP address and the target are stored for one hour to limit the rate.
Limits worth knowing: only one of the resolved addresses is tested; a CDN may compress differently per edge or only after a first miss; the times are single measurements and mostly reflect transfer size and distance; bodies above 3 MB are cut off, so their ratios are approximate; and a server that compresses on the fly at request time may show slightly different byte counts on each run.