This specification defines a mechanism enabling web sites to declare themselves accessible only via secure connections and/or for users to be able to direct their user agent(s) to interact with given sites only over secure connections. This overall policy is referred to as HTTP Strict Transport Security. The policy is declared by web sites via the
Strict-Transport-SecurityHTTP response header field and/or by other means, such as user agent configuration, for example.
I only actually heard about this recently, though the RFC’s been published since last November. To read.