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rel=webmention is a way to link to a webmention endpoint.
Webmention is a simple way to notify any URL when you link to it on your site. From the receiver's perspective, it's a way to request notifications when other sites link to it. Webmention is a modern update to Pingback, using only HTTP and x-www-form-urlencoded content rather than XMLRPC requests. Webmention supersedes Pingback.