For me, this online-book is the most beautiful site on the entire web, at least with the default Valkyrie font. It's a prime example of how excellent content design can be made in black-and-white, and the sorta-recent wave of "this is a website" pages didn't come close to it. White space there is simply magical, every single interval is perfect―I envy the author for the ability to achieve that.
The site, together with other Butterick's online-books, also uses a rather non-traditional styling for headings: they stand out just a bit from the body text by the use of bold face, and that's enough. Every day, I see examples of either loud and heavy headings, or subheadings that are not obvious―on very popular sites―and I keep thinking how I would apply Butterick's trick, alas to no avail.