I think I broke it by creating the username `index.html`. Sorry, I wasn't expecting it to do anything.
We roped off all of /.well-known/ and it is after all under a dot directory which ought to have been caught by a lot of existing software anyway, it seems like if people have things they want treated this way they ought to live in /.well-known/ too, it's not as though the registration process is onerous, you just need to write a specification for your thing.
Here’s the one for my site: https://memalign.github.io/googlefc60e159eedb4219.html
Betas ftw!