When logged in to https://openid.stackexchange.com, there is "Use your own URL to log in" link:
Which leads to this:
- What does the URL do?
- How do I use it to log in?
- Why HTML tags?
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<link> tags are for OpenID delegation, which allows you to use a URL owned by you as the endpoint for an OpenID login, but have the actual authentication handled by the Stack Exchange OpenID provider.
Put another way, it lets you use a custom URL without having to set up your own OpenID endpoint server.
The benefit of this is that if you use a third-party OpenID service and it goes away (like MyOpenID did), your identifier on all the sites you log into was your own URL, and you can modify the page it points to to reference a different provider without it impacting the sites you were already using.