Per profile page, you could say.
It works by crosslinking: your mastodon profile page contains a tag like <link rel="me" href="https://mywebpage.example/aboutme"/>. Your aboutme page needs to link back, e.g. <a rel="me" href="https://mastodon.instance/@myusername">Mein Mastodon-Profil</a>.
This crosslinking can be checked automatically, and the instance of the viewer does exactly that.
@anathem but is this only on mastodon.social or across federated nodes?
Can i validate people on my node or do they go through this process?
Everyone needs to do that crosslinking for their own profile. That green checkmark is proof of control: By linking back, one provides proof of control over the linked web page.
There is no admin flag for that, because the green checkmark really is validated by the instance of the _visitor_*. That way, you only have to trust the admin of your own instance to not tamper with the validation code.
*(except for anonymous profile views, then it's done by the instance of the profile owner)
@jmulein Oh, and: the verification can be done by any Mastodon instance for any Mastodon user on any instance. It's definitely not limited to the big flagship mastodon.social.
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