The W3C Social Web Working Group is calling for implementations of Webmention, which is now a Candidate Recommendation. Webmention provides a mechanism for a website to notify other websites that it has content which links to them and when the source content changes or is deleted. This mechanism is a core building block for a decentralized (social) Web because it allows sites to automatically learn about connected content without any prior setup or agreement. For users, an immediate benefit is cross-site comments. Before being brought to W3C for standardization, Webmention already had twenty-seven independent implementations in the IndieWebCamp community.