Summary

  • The W3C Webmention Recommendation sends two URLs, source and target, from a page that links to another page. The receiver retrieves the source and verifies that the link exists before using the notice.
  • The receiver may display a verified reply, use it in another way, or moderate it. The standard requires filtering when source content is republished. The W3C publication record dates the Recommendation to 12 January 2017.

A reply needed somewhere to live

Imagine two small sites. One writer publishes a post. A second writer answers it on a separate domain and links back. The reply has a durable address under its author's control, but the original page has no automatic reason to know that the answer exists. A reader who never leaves the first page misses the conversation. A reader who follows the link can find it, provided the author or someone else has placed that link where it can be seen.

Earlier linkback systems tried to carry the news. Pingback 1.0 used an XML-RPC pingback.ping call that named the source and target URLs. The Webmention standard describes itself as a simplified descendant: a form-encoded HTTP POST replaced the XML-RPC payload. That sounds like a small engineering bargain. It mattered because publishing software could send a notice with ordinary web forms and because the receiving side's proof obligation became explicit in the protocol. The link, rather than the sender's assertion about the link, had to survive inspection.

Webmention starts after the sender has published a source page. The sender fetches the target page, discovers its Webmention endpoint through a Link header or an HTML link, then sends source and target to that endpoint. The Recommendation's protocol steps make the two URLs the whole essential message. The source is the address of the page carrying the link. The target is the address being linked to. The receiver can queue the request, fetch the source itself and check for the target link. A successful POST says the notice was accepted for handling; the receiving site's later use of it is another decision.

The receiving page kept its editorial gate

The receiving site gains information it could not reliably infer from its own page alone: another public page now points at this one. That fact can feed a comment, a reply list, a like counter or a private notification. The W3C text lets the receiver choose. It also permits moderation before public display. The sender owns the reply's source page; the target owner controls the surface on which visitors to the target might see it. A cross-site conversation can therefore have two publishers and two addresses without forcing one publisher to cede its page to the other.

Verification has teeth and limits. The receiver must fetch the source if it wants to use the notice, then find an actual reference to the exact target URL. That filters out a notice consisting only of a claimed relationship. A real link still says little about the sender's identity, the quality of the reply or whether it deserves a place on the target page. The standard's security section tells receivers to encode or filter any source content they republish and allows moderation. A link is evidence of a link. The receiving site still makes the publication call.

The source can change after the first notice. Webmention asks senders to resend notices when posts are updated. A deleted source should return 410 Gone and send a new notice, allowing receivers to remove or mark the old interaction. The receiver should handle repeated source-target notices idempotently and update a prior record when verification succeeds again. These rules gave a distributed conversation a way to revise itself. They also assigned work to the receiver: fetch, verify, store, refresh, moderate and sometimes remove a piece of the conversation it did not host.

The IndieWeb community's account traces an early draft to October 2012. The W3C history shows a first public Working Draft in January 2016 and the Recommendation one year later. Those dates mark a community-built linkback idea becoming a web standard. They do not measure how many sites deployed it. The visible historical change is narrower and more useful: a reply could remain at its author's URL while the article it answered gained a standard way to hear about it.