If someone does that you’ve already been pwned. In reality you limit the CA to be domain scoped. I don’t know why domain-scoped CAs aren’t a thing.
> If someone does that you’ve already been pwned
Then why use encryption at all when your threat model for encrypted communication can't handle a malicious actor on the network?
Because there are various things in HTML and JS that require https.
(Though getting the browser to just assume http to local domains is secure like it already does for http://localhost would solve that)