At the moment, my control panel looks like this. It’s a bit sad, really. I’m using Firefox on a Mac and have no idea why it’s doing it. I’ve tried different encoding, etc.
When I switch across to Chrome, it just works.
I asked on the Mozilla forum, and eventually learned that I had messed it up. WordPress doesn’t like it when people lay down font preferences manually. So I fixed it. It was simple. As DickVL said:
Make sure that you allow pages to choose their own fonts:
Firefox > Preferences > Content : Fonts & Colors > Advanced: [X] “Allow pages to choose their own fonts, instead of my selections above”