Code change:
10.3. <del> The HTTP/2 field block encoding allows the expression of names that are not valid field names in HTTP.  Requests or responses containing invalid field names MUST be treated as malformed.  An intermediary therefore cannot translate an HTTP/2 request or response containing an invalid field name into an HTTP/1.1 message. Similarly, HTTP/2 allows field values that are not valid.  While most of the values that can be encoded will not alter header field parsing, carriage return (CR, ASCII 0xd), line feed (LF, ASCII 0xa), and the zero character (NUL, ASCII 0x0) might be exploited by an attacker if they are translated verbatim.  Any request or response that contains a character not permitted in a field value MUST be treated as malformed.  Valid characters are defined by the ABNF rule in HTTP. </del> <ins> HPACK permits encoding of field names and values that might be treated as delimiters in other HTTP versions.  An intermediary that translates an HTTP/2 request or response MUST validate fields according to the rules in HttpHeaders roles before translating a message to another HTTP version.  Translating a field that includes invalid delimiters could be used to cause recipients to incorrectly interpret a message, which could be exploited by an attacker. An intermediary can reject fields that contain invalid field names or values for other reasons, in particular those that do not conform to the HTTP ABNF grammar from HTTP.  Intermediaries that do not perform any validation of fields other than the minimum required by HttpHeaders could forward messages that contain invalid field names or values. </ins> 10.4.

Related discussion:
text: -semantics gives the answer to this: you MUST NOT emit (§ 2.2), but as a receiver you are cautioned that (§ 2.3): Additionally (§ 2.4): This gives you the permission you want: you are encouraged to parse the field defensively and you may attempt to recover, which implies that by default you may fail, and that -semantics does not specify exactly how you will handle these failures. HTTP/2 continues to grant your application that freedom. The specification does not bind you: you can handle this as you want.
OK True. Which is actually worse, as it leaves the set of undefined other than we know it is a subset of the compliment of the set of invalid fields define here. Also worse, as it means the document only vaguely defines validity. There is actually no definition of what is a valid h2-field. This reason could be used to say that there should be no additional invalidity check in this document. Just allow any hpack-valid field and leave the rest to be a matter of semantics. The fields excluded by this definition are only dangerous if forwarded, which is already covered by "Intermediary Encapsulation Attacks" section that suggests that full HTTP ABNF grammar should be applied (in addition to the vaguely defined exclusions of "HTTP Fields"). So I'm back to why is this document in-precisely defining valid/invalid h2-fields. It is not for intermediaries as there is a section in the document dedicated to that. It is not for locally interpreted HTTP as you say HTTP semantics should be applied. Why does this document say that a field name containing space is invalid, yet one containing a double quote is not? Neither are valid HTTP, both are valid hpack. There is no good reason I can see for either and whilst I don't know of any specific attack that either could be used for, I would not be surprised if both could be used to some evil ends. This vague definition is going to have a significant carbon footprint.