What does Cache-Control: no-cache actually require of a cache?

Why this is the answer

The name is the single most misleading thing in HTTP caching: no-cache permits storage and demands revalidation, which is exactly what you want for HTML that must always be current but is usually unchanged. Refusing to store anything is no-store. Serving only during an origin outage is closer to stale-if-error. And unrestricted reuse is what happens with no directive at all.