This part is wrong, sorry. An attacker can sign his own certificate for your domain, and send you that. Then he can communicate with the real site on your behal

This part is wrong, sorry. An attacker can sign his own certificate for your domain, and send you that. Then he can communicate with the real site on your behalf. Your only protection would be if you pre-installed said self-signed certificate before going to the coffee shop. Your warning would be that the certificate was suddenly untrusted again. It also wouldn't prevent an attack by someone that could forge a certificate from a trusted intermediate CA. HPKP could help there; but if you have someone like that targeting you in a coffee shop, you have much bigger problems. The important thing though is that this is still not worse than plain HTTP. And in fact, it's better: it makes the attacker's job a lot more difficult. Especially when done at large scale (eg ISP ad injection.)