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While looking back over HTTP response codes today I noticed that the 301, 302, and 304 response codes were defined in HTTP/1.0 but not the 303 response code. Does anyone know why the number 303 was skipped in HTTP/1.0?

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MDN states that the 303 code was defined in HTTP/0.9, although all other docs I've seen state that it was introduced with HTTP/1.1 (to resolve an issue with 302 implementations in HTTP/1.0). There are other gaps in the numbering, 203 for instance was inly introduced HTTP/1.1. I would guess that a 303 was included in an initial working draft but simply didn't make the final cut?! Status codes wouldn't be renumbered since implementations would have already been developed I suspect. – w3dk Dec 13 '12 at 11:04
up vote 3 down vote accepted

It was not skipped right away. As you can see in this diff between version 1 and 2 of the RFC draft they dropped it. At the beginning it was in there.

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