Pages has not updated from <user>.github.io, which contains some modified content, after 5 hours. I thought it might be due to the network problem today, but that was resolved two hours ago.
It’s ok now.
I wasn’t serving JS from an external source and I suspect that error prevented the Pages site from updating. As soon as I served up the JS it was requesting, the site updated to reflect all the changes.
So, it appears you can’t test how a small change looks before fixing up all errors, including innocuous ones like a 404.