Antoine, outright marvellous! The notebooks render beautifully. And already now in production!
Because of this, I consider SHA as the premier choice of citing GitHub repos.
Many thanks!

Antoine, I am really impressed! This already makes a big difference. And there are more than a million notebooks out there on GitHub.

Wow!

There is a complication when a notebook refers to images via a relative link.
For example, the notebook above (it is a real one) refers to an image with by images/dans.png, but when I am in NBviewer looking at the SHA copy of it, that gets translated to the url