IndieWeb
From Organic Design wiki
The indieweb (short for "independent web") is a movement that aims at making the web more independent and decentralised again with a focus on people using their own sites instead of walled gardens. The idea is that by using open standards, sites can all be a part of the same unified social network regardless of what software they use - even static html sites can be a part of the Indieweb.
You can use the SelfAuth PHP library to allow your standalone site to become an Indieweb auth provider. You can test your site's Indieweb auth capability with here.