If a company creates a self signed Root CA certificate and then install that in their computers’ store for trusted root certificates, then create a leaf certificate and install that on their internal server, then that certificate will be trusted by browsers visiting that server (note that the server might not be public reachable, so can’t use official CA certificates).

However when sites from that server is shown in browser, although the connection uses TLS 1.3, it will not be using HTTP/3.

I have for a long time tried to find out why that is, but have not been able to find a technical reason for that (I can easily find political reasons).

I might be wrong, so I ask here: Is there a technical reason why HTTP/3 is only supported when you use a certificate from a public CA?