HTTP/3 и QUIC: пример кода

# The server advertises HTTP/3 with Alt-Svc; the browser uses it on the NEXT visit
Alt-Svc: h3=":443"; ma=86400

# Verify negotiation
curl -I --http3 https://cloudflare.com        # "HTTP/3 200"

// DevTools: Network panel → right-click column headers → enable "Protocol"
//   h3        = HTTP/3 (QUIC)
//   h2        = HTTP/2 (TCP)
//   http/1.1  = HTTP/1.1

// First-visit reality: the FIRST request to an origin usually goes over h2/h1,
// because the browser only learns h3 is available from the Alt-Svc header in
// that response. Subsequent requests upgrade to h3. Don't expect h3 on a
// cold cache from a synthetic single-request test.