Define the intended implementer

See https://lists.w3.org/Archives/Public/public-trace-context/2020Feb/0004.html

This spec is intended for vendors who operate distributed tracing systems (or operate distributed systems and want to use or connect to distributed tracing systems) and this protocol doesn’t rely on any changes from web browsers, although it’s possible that client-side JavaScript could use these headers as part of a distributed tracing system. The motivation is to encourage interop among several vendors in this space that are using HTTP and Web technologies, although it’s also noted that this could apply beyond HTTP.

Reviewing specs of this kind is a little less common for PING, since we’re most often looking at APIs or protocols that involve web site and web browser behaviors. The WG notes in their README that their intention is to ask for exceptions to CORS limitations on these headers, which would rely on web browsers treating these trace headers differently from other headers when making cross-origin requests, but that isn’t mentioned in this specification.

Start with the linked W3C mailing-list discussion and the Trace Context README to understand the intended implementers and the proposed browser/CORS treatment of the trace headers. Done means the specification clearly states its target implementers and whether browser behavior or exceptions to CORS are in scope.