2022-04-26 UTC
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[tantek] Or for a standards-based example, OAuth. As in, an OAuth based login from one site (Twitter) can't be used with others (GitHub). Clients specifically have to support each and every provider one at a time. (This can segue somewhat into, and here's how IndieAuth solves that with actual interop that allows users to switch which apps they use and/or which IndieAuth provider they use, independently)