Google HTTP defines proactive content negotiation to allow servers to select the appropriate response for a given request, based upon the user agent's characteristics, as expressed in request headers. In practice, clients are often unwilling to send those request headers, because it is not clear whether they will be used, and sending them impacts both performance and privacy. This document defines two response headers, Accept-CH and Accept-CH- Lifetime, that servers can use to advertise their use of request headers for proactive content negotiation, along with a set of guidelines for the creation of such headers, colloquially known as "Client Hints." It also defines an initial set of Client Hints.