Plenty of browsers allow self-signed certs—Firefox and Safari, to the best of my knowledge, treat HTTP/3 certs exactly the same as they treat HTTP/2 and HTTP/1.1 certs. Chrome has taken the position that it will no longer allow self-signed Root CAs for HTTP/3 websites, to prevent SSL interception with companies intercepting all of your traffic. For personal use, you can always whitelist an individual certificate using a CLI flag without allowing a trusted root CA