[nickf]

Publicly-trusted client authentication does nothing. It's not a thing that should exist, or is needed.

[account42]

It does if the "client" in the TLS sense is really a public server in a federated network. Like for example in XMPP which you may have heard of.

[direwolf20]

Then you specify your protocol to accept server certs from clients

[ahmedtd]

I don't think this is true. It's something that could be useful, with some sort of ACME-like automated issuance, but should definitely be issued from a non-WebPKI certificate authority.