Monitoring

    HTTP/2 & HTTP/3 support check

    Confirms your server negotiates HTTP/2 and advertises HTTP/3 via Alt-Svc — a free performance win especially for mobile.

    Updated 2026-07-10
    1 min read
    http2
    http3
    quic

    Quick answer#

    We negotiate ALPN and read the Alt-Svc header to detect HTTP/2 and HTTP/3 (QUIC) support per site.

    What it checks#

  1. ALPN negotiation to h2
  2. Alt-Svc advertising h3
  3. Server push disabled (Chrome dropped support)
  4. 0-RTT indication on HTTP/3 (bonus)

    5. Where to find it in Lemwatch#

    Site details → Performance → Protocols.

    How to fix#

  5. Enable HTTP/2 in nginx (listen 443 ssl http2;).
  6. If on Cloudflare, HTTP/3 is one toggle away.
  7. For self-hosted HTTP/3, use Caddy or nginx-quic branch.
  8. Re-run the check.

    9. FAQ#

    How much does HTTP/3 help?#

    5-15% median TTFB improvement on lossy mobile networks.

    Do I need HTTP/3 if I have HTTP/2?#

    Not critical, but it's a cheap win behind a CDN.

    Related#

  9. Performance budgets
  10. Core Web Vitals
  11. Gzip/Brotli compression check
    12. Was this article helpful?

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