SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/tech-http-304-etag

Your browser asks if its cached file changed and gets 304 Not Modified. What happens?

The browser reuses its cached copy; the 304 has no representation body.

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  1. The browser downloads the file again
  2. The page shows an error
  3. The browser reuses its cached copy ✔

Why: The browser sends the cached ETag value in an If-None-Match header. If it still matches, the server replies 304 without a representation body. If the file changed, it sends 200 OK with the new version.

Huh, didn't know that: An ETag is an arbitrary server-generated value, such as a content hash or version number.

The dad joke
I asked my teenager if anything changed. I got a 304: no body, just a status.
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References

Answer: The browser reuses its cached copy. The browser sends the cached ETag value in an If-None-Match header. If it still matches, the server replies 304 without a representation body. If the file changed, it sends 200 OK with the new version.

  1. MDN: HTTP caching developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Caching
    the client reverts the stored stale response back to being fresh and can reuse it
    Checked 2026-10-10.

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