Solid appears to use TLS client certificates for authentication. (Granted, they're working on some kind of OAuth integration too.) WebID-TLS appears to have the same severe drawback that HTTP authentication does: the UI is provided by the browser, and so can't be customized, branded, or tweaked in any meaningful way by authentication providers. The TLS certificate dialog my browser presented just now attempting to use the "hello world" live demo also appeared to be extremely intimidating for non-technical users: for example, it presented long strings of hex numbers.

I don't think Solid is going to get any meaningful adoption until the signup and authentication flows resemble those of major current social platforms, which have enjoyed years of usability optimization.