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Cleaner syntax (@method and @path are separate, versus rolling it together as (request-target)), clearly lays out the expectations of which implementation decisions must be made when rolling it into a larger protocol/system (section 1.4), more standardized signature methods (including ed25519), and can be possible to build and make use of well-tested reusable libraries than having to need some niche implementation that only applies to ActivityPub.

In cursory glance, it's just a few syntax changes to 'upgrade' existing implementations. But part of it isn't just RFC9421 itself, but an opportunity to fix the state of HTTP Signatures in ActivityPub within the same effort.