QUIC started as a Google experiment to fix TCP's head-of-line blocking and slow start. It runs on UDP, integrates TLS 1.3 directly, and supports 0-RTT connection resumption.
It's how HTTP/3 ships. Most modern browsers and CDNs (Cloudflare, Google, Fastly) speak it. IPFerret advertises HTTP/3 via the Alt-Svc header when running behind Caddy.
See also