Because I "control" the page, and because most people have reason to trust dang/Hacker News to not manipulate my "About Page", it serves as an identity.
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That being said, Hacker News "About" pages probably isn't the best place for a general web identity. But maybe a static-page at "neocities.org", or "Github", etc. etc. serve as an identity in practice.
All we need for "identity" is a webpage that the world reasonably expects us to control.
For an extreme example, my "identity" is simply my page at https://news.ycombinator.com/user?id=dragontamer.
Because I "control" the page, and because most people have reason to trust dang/Hacker News to not manipulate my "About Page", it serves as an identity.
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That being said, Hacker News "About" pages probably isn't the best place for a general web identity. But maybe a static-page at "neocities.org", or "Github", etc. etc. serve as an identity in practice.