How webmentions are supposed to work

Webmentions came up in a recent conversation at the KrautPress Website Club, and Manuel just wrote about them, so I thought I might write down what webmentions are and how they’re supposed to work. “Supposed to” is the kicker here, because implementations vary considerably from system to system. The basic idea: when you link to a post, your site notifies that post via a webmention. That’s it! Webmention is a W3C Recommendation from 2017 that describes a simple protocol for notifying a URL when another site links to it. (This only works if the linked site has a webmention endpoint, and the receiving site checks that the link really exists before accepting it.) Interestingly, the Webmention spec itself doesn’t define likes or replies. It only says “this page links to that page.” The different types come from a separate layer: microformats2 markup on the linking page. The receiving site reads that markup and decides what kind of mention it received. A W3C Note called Post Type Discovery…

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