There is a internet standard called rel="me" where two accounts can link to each other with rel="me" as an attribute, and through that, users (or rather clients) can verifiably state that the two accounts are the same person.

At the moment, mbin profiles cannot do this, as the user does not have control over the raw HTML of their profile (and I wouldn't suggest changing that). So instead, I'd suggest adding a new profile field for (at least one) website the profile wants to be associated with, which then gets rendered in the profile page with a rel="me" attribute. In fact, mbin could even try parsing that linked site for the backlink, and display some sort of verified indicator.

Mastodon uses rel="me" for its website verification process, so there's precedent for this on the fediverse.

I also think this would possibly enable or at least assist in user migration - imagine clients that use rel="me" to merge accounts in one interface - perhaps even across different software, not just instances. That's OOS for this FR though.