[JimDabell]

> Based on experience with Pingbacks, the Webmention specification requires the sending site to have a mentioning URL on a publicly available web page. This requirement by itself cuts down significantly on spam as it increases the cost of sending it. (Pingbacks/Trackbacks didn't have this requirement so it was easy to programmatically spew spam in all directions.)

It wasn’t required for spec. conformance, but it was recommended by the spec. and implemented by all the early implementations.

> Upon receiving a request, servers MAY do what they like. However, the following steps are RECOMMENDED:

> The server MAY attempt to fetch the source URI to verify that the source does indeed link to the target.

— https://www.hixie.ch/specs/pingback/pingback#TOC3

> In addition to this, there's no requirement to show the received Webmention, so there's less benefit to some spammers in these cases.

This wasn’t a requirement for Pingback either.