This page demonstrates the differences in network activity when serving a page over different versions of HTTP. You can load it over:
To see the protocol used for all requests, open your browser's developer console, go to the network tab, right click the table header and enable the "Protocol" column:
The first request over HTTP/3 will use HTTP/2. The Alt-Svc ma value is set to 10 sec, so that you can see the reversion to HTTP/2 more easily. To make the browser forget the HTTP/3 Alt-Svc max-age for this domain, open it in a private browsing window.
Some resources to load so that you can see the behaviour of multiple TCP connections and pipelining for 1.1, and multiplexing for 2 and 3:
Note that caching is disabled for all requests - gotta make that network do its thing!
Check your domain's HTTP/3 config using http3check.net.