Code change:
This change is done while ignoring the fact that we might want a bigger document reshuffle when bringing in a "content-digest" header. But it helps illustrate how the abstract / intro could be editorialized to address some off-list feedback.
LGTM :)

Related discussion:
text: 1. <del> The core specification of HTTP does not define a means to protect the integrity of resources.  When HTTP messages are transferred between </del> <ins> HTTP does not define a means to protect the integrity of representations.  When HTTP messages are transferred between </ins> endpoints, the protocol might choose to make use of features of the lower layer in order to provide some integrity protection; for instance TCP checksums or TLS records RFC2818. <del> However, there are cases where relying on this alone is insufficient. An HTTP-level integrity mechanism that operates independent of transfer can be used to detect programming errors and/or corruption of data in flight or at rest, be used across multiple hops in order to provide end-to-end integrity guarantees, can aid fault diagnosis across hops and system boundaries, and can be used to validate </del> <ins> This document defines the Digest HTTP integrity mechanism that acts on representation data.  It operates independent of transport integrity, offering the potential to detect programming errors and corruption of data in flight or at rest.  It can be used across multiple hops in order to provide end-to-end integrity guarantees, which can aid fault diagnosis when resources are transferred across hops and system boundaries.  Finally, it can be used to validate </ins> integrity when reconstructing a resource fetched using different HTTP connections. <del> This document defines a mechanism that acts on HTTP representation- data.  It can be combined with other mechanisms that protect representation-metadata, such as digital signatures, in order to protect the desired parts of an HTTP exchange in whole or in part. </del> <ins> This document obsoletes RFC3230. </ins> 1.1. <del> The Content-MD5 header field was originally introduced to provide integrity, but HTTP/1.1 (RFC7231, Appendix B) obsoleted it: </del> <ins> This document describes Digest integrity for HTTP and is structured as follows: </ins> <del> The Content-MD5 header field has been removed because it was inconsistently implemented with respect to partial responses. </del> <ins> representation-digest describes concepts related to representation digests, </ins> <del> RFC3230 provided a more flexible solution introducing the concept of "instance", and the fields "Digest" and "Want-Digest". </del> <ins> digest defines the Digest request and response header and trailer field, </ins> <del> 1.2. The concept of "selected representation" defined in Section 3.2 of SEMANTICS makes RFC3230 definitions inconsistent with current HTTP semantics.  This document updates the "Digest" and "Want-Digest" field definitions to align with SEMANTICS concepts. Basing "Digest" on the selected representation makes it straightforward to apply it to use-cases where the transferred data does require some sort of manipulation to be considered a representation, or conveys a partial representation of a resource eg. Range Requests (see Section 14.2 of SEMANTICS). This document replaces RFC3230 to better align with SEMANTICS and to provide more detailed description of "Digest" usage in request and response cases.  Changes are intended to be semantically compatible with existing implementations but note that negotiation of "Content- MD5" is deprecated deprecate-contentMD5, "Digest" field parameters are obsoleted obsolete-parameters, "md5" and "sha" digest-algorithms are obsoleted broken-algorithms and the "adler32" algorithm is deprecated deprecated-algorithms. The value of "Digest" is calculated on selected representation, which is tied to the value contained in any "Content-Encoding" or "Content- Type" header fields.  Therefore, a given resource may have multiple different digest values. To allow both parties to exchange a Digest of a representation with no content codings (see Section 8.4.1 of SEMANTICS) two more digest- algorithms are added ("id-sha-256" and "id-sha-512"). 1.3. </del> <ins> want-digest defines the Want-Digest request and response header and trailer field, </ins> <del> The goals of this proposal are: </del> <ins> algorithms, broken-algorithms, deprecated-algorithms and deprecate-contentMD5 and describe algorithms and their relation to Digest, </ins> <del> Digest coverage for either the resource's "representation data" or "selected representation data" communicated via HTTP. </del> <ins> acting-on-resources details computing representation digests, </ins> <del> Support for multiple digest-algorithms. </del> <ins> obsolete-parameters obsoletes Digest field parameters, </ins> <del> Negotiation of the use of digests. </del> <ins> sri describes the relationship between Digest and Subresource Integrity, and </ins> <del> The goals do not include: </del> <ins> examples-unsolicited and examples-solicited provide examples of using Digest and Want-Digest. </ins> <del> Digest mechanisms do not cover the full HTTP message nor its semantic, as representation metadata is not included in the checksum. Digest mechanisms cover only representation and selected representation data, and do not protect the integrity of associated representation metadata or other message fields. Digest mechanisms do not support authentication of the source of a digest, message or anything else.  These mechanisms, therefore, are not a sufficient defense against many kinds of malicious attacks. </del> <ins> 1.2. </ins> <del> Digest mechanisms do not provide message privacy. </del> <ins> This document defines the "Digest" request and response header and trailer field.  At a high level the value contains a checksum, computed over "selected representation data" (Section 3.2; SEMANTICS), that the recipient can use to validate integrity. "Digest" supports algorithm agility.  The "Want-Digest" field allows endpoints to express interest in "Digest" and preference of algorithms. </ins> <del> Digest mechanisms do not support authorization or other kinds of access controls. </del> <ins> Basing "Digest" on the selected representation makes it straightforward to apply it to use-cases where the transferred data requires some sort of manipulation to be considered a representation, or conveys a partial representation of a resource, for example Range Requests (see Section 14.2 of SEMANTICS). Historically, the Content-MD5 header field provided an HTTP integrity mechanism but HTTP/1.1 (RFC7231, Appendix B) obsoleted it due to inconsistent handling of partial responses.  RFC3230 defined the concept of "instance" digests and a more flexible integrity scheme to help address issues with Content-MD5.  It first introduced the "Digest" and "Want-Digest" fields.  HTTP terminology has evolved since RFC3230 was published.  The concept of "instance" has been superseded by "selected representation". This document replaces RFC3230.  The "Digest" and "Want-Digest" field definitions are updated to align with the terms and notational conventions in SEMANTICS.  Changes are intended to be semantically compatible with existing implementations but note that negotiation of "Content-MD5" is deprecated deprecate-contentMD5, "Digest" field parameters are obsoleted obsolete-parameters, "md5" and "sha" digest- algorithms are obsoleted broken-algorithms, and the "adler32" algorithm is deprecated deprecated-algorithms. Calculating the value of "Digest" using selected representation means it is tied to the "Content-Encoding" and "Content-Type" header fields.  Therefore, a given resource may have multiple different digest values.  To allow both parties to exchange a Digest of a representation with no content codings (see Section 8.4.1 of SEMANTICS) two more digest-algorithms are added ("id-sha-256" and "id-sha-512"). "Digest" is used for representation integrity.  It does not provide integrity for HTTP messages or fields.  However, it can be combined with other mechanisms that protect representation metadata, such as digital signatures, in order to protect the phases of an HTTP exchange in whole or in part. "Digest" does not define means for authentication, authorization or privacy. </ins> <del> 1.4. </del> <ins> 1.3. </ins> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and