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

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_Management_API https://developer.mozilla.org/en-US/docs/Web/API/Credential_... which although experimental does have some level of browser support.