Okay, here is a stripped version of the file. I've retained only one html file with some text, mostly lorem ipsum, but the code is the same and the entire css is there. Any tips would be welcome, I tried the inspector but didn't see what I was supposed to be looking for.
(There's a bunch of files that must have the exact same problem. As it happens, I also just bought an OUP edition of Fanny Burney's Cecilia, this one
https://www.kobo.com/nl/nl/ebook/cec..._j5nCGK&cPos=1, which suffers from the same problem, but probably for different reasons. It's maddening, because other books in that World Classics series don't have those problems - although some have others. It seems it's hard to get it right.)