Basically, on any ordinary HTTPS site, every browser can allow you to proceed despite a certificate error. Such as knowing the certificate is expired or revoked

Basically, on any ordinary HTTPS site, every browser can allow you to proceed despite a certificate error. Such as knowing the certificate is expired or revoked or not from a known CA or something. HSTS by specification prohibits browsers from offering that option. So I can know a server is serving valid, authentic traffic via HTTPS, be aware of why the certificate has an issue, and HSTS requires that my browser prohibit me from connecting to the server.