I seem to remember that they had some very opinionated rules at the beginning regarding allowed spelling and typography. Some of them felt distinctly American to me. I don't know if that's still the case.
It’s en-US typography (flavoured by the Chicago Manual of Style). Spelling is based on the original book, though some modernisations are made. Commits with these always start with [Editorial] for easy later reference, and are typically things like to-day -> today.
Apart from that, they produce nice editions.