I just tried creating a new account; clicked the “authorize through GitHub” button and then authorized. The result is a page indistinguishable from the original signup page, except that the url has had “loggedin” appended. Clicked several more times before I realized what was happening, and ended up with this url: https://beta.observablehq.com/loggedin/loggedin/loggedin/loggedin/
It wasn’t until I closed the window and went back to beta.observablehq.com that I saw myself properly logged in.
Using macOS 10.13.4 Beta (17E190a); Safari Version 11.1 (13605.1.33.1.2).