Substantive comment: The third paragraph from the end of Section 2 says: When adding a value to the Proxy-Status field, intermediaries SHOULD preserve the existing members of the field, to allow debugging of the entire chain of intermediaries handling the request. Under what circumstances would an intermediary need to mess with existing Proxy-Status entries? Please consider upgrading this requirement to a MUST, and perhaps also require that the ordering of existing entries be preserved. Minor comments: The descriptions of errors in sections 2.3.13, 2.3.14, 2.3.28, 2.3.30, and 2.3.31 all contain a paragraph saying, "Note that additional information...(as is the case for all errors)." It isn't clear whether this text belongs in the "Notes" column of the registry entry being created since it is separate from the bulleted list describing the entry. Please consider removing this text in these sections and instead including it as introductory text describing the error parameter. The Notes for the entries either are empty or contain the text, "Responses with this error type might not have been generated by the intermediary." I wonder if this might more efficiently be represented by a Boolean flag in the registry entries, but probably preserving the Notes field in the registry for future use. The introduction uses both the phrase "towards the origin server" and the term "upstream". It might be helpful to some readers to make it clear that they're synonymous, e.g., "towards the origin server (upstream)" Section 1.1 says that this specification uses ABNF, but I find only structured fields. Consider removing the mention of ABNF and the informative reference to RFC 5234.