Brotli, Zstandard and gzip
Text - HTML, CSS, JavaScript - compresses extremely well, and the browser decompresses it for free. gzip has done this since the 1990s. Brotli typically produces files 15-25% smaller than gzip for web text and works in every current browser over HTTPS. Zstandard (zstd) is newer, accepted by Chrome and Firefox, and fast to compress on the fly. Serve Brotli first, keep gzip as the fallback.
How the test works
The page is requested four times, each time asking for one encoding only - Brotli, Zstandard, gzip and none - and the checker records what the server actually sent and how many bytes went over the wire. Servers that ignore the request or send the wrong encoding show up immediately. It also checks for Vary: Accept-Encoding, without which a shared cache can hand a compressed page to a client that cannot read it.
HTTP/2 and HTTP/3
HTTP/2 lets a browser fetch many files over one connection. HTTP/3 runs over QUIC instead of TCP, which recovers from packet loss without stalling every file - the gain is biggest on mobile. Servers announce HTTP/3 in an Alt-Svc header and browsers switch to it on the next request, so the checker reports whether it is advertised. On Cloudflare, Brotli and HTTP/3 are single switches; we compared all three encodings on our own site in Brotli vs Zstd vs gzip.
