XFN

XHTML Friends Network (XFN 1.1, by Tantek Çelik, Matt Mullenweg and Eric Meyer) describes people you link to: friendship (contact, acquaintance, friend), met, professional (co-worker, colleague), plus family, romantic and geographical values. Its lasting value is rel="me" ("this profile is also me"): when your Mastodon 6,501 profile lists your site and the site links back with rel="me", Mastodon shows a checkmark.