If browsers were to support this, there would be some tricky UI/UX questions about how readily the browser provides information to a new website about accounts you have on other sites.

Such a feature might best be specified as an extension to this API:

https://developer.mozilla.org/en-US/docs/Web/API/Credential_...

which although experimental does have some level of browser support.

The web credential API is meant to be an integration point for passwords and AccountChooser in your browser. But I don't know if any other browsers than Chrome plan to implement it.

Don't expose it to the web page, but treat it as a native UI control.

Unless you centralize the list as AccountChooser.com does using Local Storage.