HTTP/3 & Compression Checker

Capabilities and processing

Enter a web address, or paste response headers. For an address, our server requests the page once each with Accept-Encoding set to br, zstd, gzip and identity, requests the http:// address and the main domain for their HSTS headers, and asks the HSTS preload list (hstspreload.org) about the main domain. Pasted headers are read in your browser.

The result distinguishes the HTTP/3 offer in Alt-Svc from a direct QUIC connection test. When the direct probe is available, it measures each encoded response body and the identity response before decompression, and the HSTS header against the preload list's rules: a redirect from HTTP to HTTPS on the same host, a max-age of at least a year, includeSubDomains and preload.

Example workflow

The sample shop offers HTTP/3 and serves Brotli and gzip, but its HSTS header lasts six months without includeSubDomains or preload, so it cannot join the preload list. Break it shows a site with gzip only, one with no compression, a short Alt-Svc, a site that is ready to preload, and your own headers without Alt-Svc.

Limits and interpretation

The direct HTTP/3 test sends a HEAD request to the final HTTPS origin and does not fall back to HTTP/2. It does not test alternate hosts or ports advertised by Alt-Svc. A failed connection describes this probe location, not every visitor. Encoded byte counts exclude HTTP headers, TLS/QUIC framing and retransmissions. Transfers above 2 MiB or interrupted transfers have unknown complete sizes. Separate encoding requests can return different content. When the direct probe is unavailable, the tool reports only advertisement and encoding headers; unknown byte counts stay unknown. Preloading is a commitment that takes months to undo; check every subdomain serves HTTPS first. Nothing is stored.

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