From Monterey v 12.2 (21D49) I'm running Safari Technology Preview Release 140 (Safari 15.4, WebKit 17614.1.1.4) and I installed and verified I can get HTTP/3 connections to work through CURL via the quiche client there. For testing, I also disabled all router firewalls and restarted the browser. I'm still not able to see h3 connections in the web inspector tool with the HTTP/3 flag enabled under the Develop > Experimental Features menu. Has anyone been able to see http/3 traffic in any of the safari builds? I'm not sure why I never do.