I never understood why a website served using a self-signed (and untrusted) certificate would throw up more warnings than a website served without any encryption at all. Even today, a page served over HTTP just gets an unobtrusive bit of text saying "Not secure", but if a page is served over HTTPS with a cert that expired yesterday you will get a very scary full-page warning that entirely blocks you from accessing th…
An analogy may help, imagine the website as a door. A website using HTTP is a normal door and using HTTPS is a door with a lock, where the keyring in this analogy are the trusted CAs by your browser. A website using HTTPS with an expired certificate is a door that should have a lock, but the lock no longer latches; and a self-signed certificate is a locked door with a key left in the doorknob. From a security perspec…
Put a third way: to the average user, a website behind an SSL-stripping MITM proxy is going to look more trustworthy than a website that forgot to renew their cert.