HTTP/1.1 vs HTTP/2 vs HTTP/3

hard

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Why this matters

HTTP/1.1 head-of-line blocking and the 6-connection-per-host limit dictate why your site loads fonts in waves; HTTP/2 multiplexing fixes one HOL blocking layer but TCP retransmits still stall every stream. HTTP/3 over QUIC is now standard on every major CDN and has different debugging tools — Wireshark won't decode QUIC payloads without keylogs.

Demo

Inspect which HTTP versions a cloud load balancer negotiates per request.

Try it yourself

Hit your domain three times forcing --http1.1, --http2, --http3 — record which the server actually returns.
Inspect the alt-svc response header on an HTTP/2 response — note the h3 advertisement that triggers QUIC for the next visit.
Reproduce HTTP/1.1 head-of-line blocking by requesting a slow endpoint then a fast one over the same connection with curl -Z.

References

# Force HTTP/1.1
curl -v --http1.1 https://example.com -o /dev/null 2>&1 | grep -i "HTTP/"

# Force HTTP/2 (negotiated via ALPN)
curl -v --http2 https://example.com -o /dev/null 2>&1 | grep -i "HTTP/"

# CloudFront supports HTTP/3; check distribution config
aws cloudfront get-distribution-config --id <dist-id> \
  --query 'DistributionConfig.HttpVersion'

# Probe HTTP/3 support
curl --http3 https://cloudflare.com -o /dev/null -w "%{http_version}\n"