"During the discovery step, if the sender discovers the endpoint is localhost or a loopback IP address (127.0.0.0/8), it SHOULD NOT send the Webmention."
I think this should specifically apply to not sending to webmentions to loopbacks, i.e. I suggest a (likely) editorial change to:
"During the discovery step, if the sender discovers the endpoint is localhost or a loopback IP address (127.0.0.0/8), it SHOULD NOT send the Webmention to that endpoint."
Also the implementation report says:
"[ ] The sender avoids sending a Webmention to a loopback address (SHOULD)"
which is consistent with my suggested editorial change.
In https://www.w3.org/TR/2016/PR-webmention-20161101/#avoid-sending-webmentions-to-localhost it says:
"During the discovery step, if the sender discovers the endpoint is localhost or a loopback IP address (127.0.0.0/8), it SHOULD NOT send the Webmention."
I think this should specifically apply to not sending to webmentions to loopbacks, i.e. I suggest a (likely) editorial change to:
"During the discovery step, if the sender discovers the endpoint is localhost or a loopback IP address (127.0.0.0/8), it SHOULD NOT send the Webmention to that endpoint."
Also the implementation report says:
"[ ] The sender avoids sending a Webmention to a loopback address (SHOULD)"
which is consistent with my suggested editorial change.