HTTP Public Key Pinning

2.6

Declare that a website's HTTPS certificate should only be treated as valid if the public key is contained in a list specified over HTTP to prevent MITM attacks that use valid CA-issued certificates.

Further reading:

1. Can I use page for: HTTP Public Key Pinning
2. MDN Web Docs - Public Key Pinning
3. Scott Helme article on the issues of HPKP