Expiration semantics
	A cache may keep an entry past its expiration time, but
	MUST NOT provide this value as a response unless it has
	been revalidated with the origin server.
	A user or cache administrator may override that "MUST NOT"
	with an explicit request, and if the resulting known-stale
	response is explicitly flagged to the user as being stale.

Propose to add: No agreement has been reached on syntax for the overriding facility, and no agreement has been reached on how explicit the staleness flag must be. The opinions range from 'you have to do the equivalent of signing a waiver form [when you want to override max-age=0 or no-cache].' (caching meeting) to 'Requiring a visible/noticeable warning be presented when semantic transparency is disabled [i.e. when overriding is enabled] is reasonable, provided that it does not actively interfere with people's work.' (Roy).