Re: [whatwg/fetch] {cache: "no-cache"} vs Cache-Control: immutable (#308)

If we think of cache-control fields as being, not facts, but something else, the murkiness may shift in a useful way:

- These fields are not so much facts, but predictions or promises about the future: in other words, guesses.
- Sometimes these guesses, taken together, can be reconciled. Sometimes they can't.

Given where we are in time, I'd make the following assumptions for now:

- "immutable" is very new and only exists in response headers from sites where someone was paying attention to it quite recently. 
- This means it's more intentional and less forgotten than other cache-control decisions.
- As a good citizen, I'd pretend "immutable" was gospel, ignore evidence to the contrary, and alert responsible parties whenever it blew up.

Given the difficulty in taming the crummy headers coming out of my own Wordpress-powered sites, a certain amount of psychosis in Cache-Control headers is probably to be expected, even in the long term. Back in the RFC 2616 days, the presence of no-cache with max-age etc. served as a bonehead alert, since that wasn't supposed to happen, but the newer spec seems to have dropped this handy indicator.

Robert


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