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.
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
- ALPN negotiation to h2
- Alt-Svc advertising h3
- Server push disabled (Chrome dropped support)
- 0-RTT indication on HTTP/3 (bonus)
Where to find it in Lemwatch
Site details → Performance → Protocols.
How to fix
- Enable HTTP/2 in nginx (
listen 443 ssl http2;).
- If on Cloudflare, HTTP/3 is one toggle away.
- For self-hosted HTTP/3, use Caddy or nginx-quic branch.
- Re-run the check.
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.
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