[JonathonW]

CAA records (which specify which CAs are allowed to issue certificates for a given domain) are intended to be enforced by CAs, not by browsers. This prevents unintentional misissuance of a certificate, but not deliberate MITM attempts if the CA is actively involved.

Their counterpart for browsers are TLSA records, which associate specific keys or certificates with a domain name. This is the part that actually prevents MITM attacks on the client side (assuming the client's getting a complete and accurate DNS response, which is a whole other issue), since it'll cause a compliant client to reject any other keys or certificates. (No idea how widespread the implementation of this is on the client side, though.)

[mmalone]

Neat. I had never heard of TLSA.

I’ll also add that certificate transparency (CT) is another mechanism designed to mitigate malicious cert issuance by a CA. A CT log is an public, append-only data structure. It doesn’t actively prevent anything, but it does ensure that a malicious issuance is easily detectable. In practice it seems to be a pretty effective deterrent against nation-state attacks: they won’t go undetected for long.