Zero At The Browser

A pricing endpoint is served through a CDN with these headers:

HTTP/1.1 200 OK
Date: Mon, 17 Aug 2026 11:00:00 GMT
Cache-Control: max-age=0, s-maxage=600, stale-while-revalidate=86400
ETag: "b71e-2"

An edge stored the response at 11:00:00. A user requests the URL at 11:11:40, which is 700 seconds later, and the browser still holds its own copy of this URL from two days ago. What happens?

The edge holds the user's request while it revalidates with the origin, so this request pays full origin latency

The browser treats its copy as stale and asks the edge; the edge's copy went stale at 600 s but stale-while-revalidate lets it answer immediately from storage and revalidate in the background

The browser serves its own copy without asking, because s-maxage=600 gives it a 600 second freshness window

The edge returns 504, since its stored copy is stale and it has no permission to serve it