What this tool checks
HTTP/2 and HTTP/3 both replace parts of how a browser talks to your server, mainly to eliminate the head-of-line blocking and connection overhead that HTTP/1.1 carries. This tool tests HTTP/2 support directly — a real TLS handshake offering both h2 and http/1.1 via ALPN, reporting exactly which one your server picked — and checks for HTTP/3 support indirectly, since there's no equivalent way to perform a live QUIC handshake here.
Why HTTP/3 can only be inferred, not proven
HTTP/3 runs over QUIC, which is built on UDP rather than TCP. Testing it live would mean implementing a QUIC client, which Node.js doesn't ship. Instead, servers that support HTTP/3 advertise it declaratively via the Alt-Svc response header (something like h3=":443"; ma=86400), telling browsers "you can try QUIC on this port." This tool reads that header — a reliable signal in practice, since browsers themselves rely on the exact same mechanism to decide whether to attempt HTTP/3, but not a live connection test the way the HTTP/2 check is.
A mismatch between your CDN and this test is common
Many sites run behind a CDN that terminates TLS and speaks HTTP/2 or HTTP/3 to real visitors, while the origin server behind it only speaks HTTP/1.1. Since this tool connects to whatever IP your domain resolves to — typically the CDN edge, if one is in front — the result should usually reflect what visitors actually get. If it doesn't match your CDN dashboard, double-check that the domain being scanned is the CDN-fronted hostname, not an internal origin address.