nginx 1.28, and enabling HTTP/3 on purpose

HTTP/3 support has been present and experimental for a while, and the April stable release made enabling it a decision rather than a gamble.

listen 443 ssl;
listen 443 quic reuseport;

http2 on;
http3 on;

add_header Alt-Svc 'h3=":443"; ma=86400' always;

# the Alt-Svc header is what tells a browser to try
# QUIC on the NEXT request. without it, nothing uses
# HTTP/3 and the configuration appears to do nothing.

The Alt-Svc header is the piece everybody misses, because the connection upgrade is advertised rather than negotiated — the first request is always HTTP/2. The measured gain on our traffic was about thirty milliseconds at the mobile p75, which is real and is a rounding error against a four-hundred-millisecond server render.