A server that speaks HTTP/3 has to tell clients about it: through an HTTPS record in DNS, which a browser reads before the first connection, or through an Alt-Svc header, which only helps from the second connection on. This check runs four requests against the same address from this server: HTTP/1.1 only, HTTP/2 as a first visit that learns the Alt-Svc header, HTTP/3 only over QUIC without fallback, and a second visit with the cache from the first. Each row shows the negotiated version and the time to the first byte.
Redirects and four short requests go from this server to the address entered; the response body is cut off after the first packet. The HTTPS record is asked of 1.1.1.1 and 8.8.8.8. Hosts in private, loopback and link-local networks are refused. The query is protected by reCAPTCHA v3; the calling IP address and the target are stored for one hour to limit the rate.
Limits worth knowing: only one of the resolved addresses is tested; the times are a single measurement from one server in Europe with a warm resolver and say nothing about a visitor's network; whether a real browser uses the HTTPS record depends on the browser and its DNS path; and a middlebox between this server and the target can block UDP 443 while a visitor elsewhere gets through.