<link rel="me">
Identity verification without signup: two links pointing at each other.
A rel="me" says: this other address is also me. It is the basis of verification on the IndieWeb and on Mastodon — if your site points at the profile with rel="me" and the profile points back at the site, the link earns the verified badge, no central authority involved.
The proof lives in the reciprocity: anyone can link to any profile, but only the owner can edit both ends. It works in the invisible <link> of the <head> as well as in a visible <a> in the body.
<link rel="me" href="https://mastodon.social/@voce">