FITstagram

FITstagram was configured to allow HTTP/3. Not only that, it advertises the HTTP/3 capability with a Alt-Svc header. This is whats called Application Layer Protocol Negotiation (ALPN). This means that the browser first accesses the site via HTTP/1.1 or HTTP/2, but after that, the follow-up resources are accessed via HTTP/3. Upon reload, the whole site is accessed via HTTP/3.

FITstagram's Alt-Svc header can be seen here https://iis.vitapavlik.cz/p/9849f4b689324304b286924cab056961

Alt-Svc header on MDN docs https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Alt-Svc

ALPN on MDN docs https://developer.mozilla.org/en-US/docs/Glossary/ALPN