Some initial thoughts on WSD v0:

As WSD was based on WKD, it would really help if it was similarly structured (and that will also mean the missing IANA considerations will be apparent).

I much prefer when examples are inline with the section that they apply to, instead of as an appendix... it makes it easier to see what's going on, especially with a new spec when things tend to be in flux.

WSD. Does this operate like direct vs advanced mode? It seems to be only direct mode... but then in A.2 the concept of a WSD Service Provider appears (and I'd expect that to be more like advanced mode, which isn't supported).

2.1.1. "To allow the mapping of incoming HTTP requests directly to the file system" ... that is a big assumption (mapping requests to storage in a file system)

2.1.1. what are the limits (minima and maxima) for size (length) of "local-part"?

"Service providers may require additional authentication methods that may necessitate the use of JavaScript" ... if authentication is required, the endpoint should return 401/403 and WWW-Authenticate headers as a clue for what to do

3.1. "newline-separate value" ... all header: value things are newline separated, so that is superfluous.

3.1 why does you need a "timestamp" header? what purpose does it serve? (if you do need it, better named as "wsd-timestamp" than just "timestamp"

3.1 "the upload MUST be aborted and return" => "the upload MUST return" (2 instances)

"WSD servers SHOULD NOT index the WSD well-known directory." there is better phrasing in §5 of WKD for this.

OpenPGP notation in other software

Web Signature Directory v0