Earlier quoted context omitted.
It doesn't have the meticulous shine of Tufte's books, but this is a straightfoward CSS guide. It's a starting point, don't you think? Butterick's implementation is more than a CSS guide though, right? I thought his design was to show off the power of Pollen[0], his online-book-publishing system. [0] http://pollenpub.com/
It's a starting point, but a starting point that directs users to fake italic, fake small-caps, mismatched mono text, inconsistent apostrophes, and ugly underlines. (Oh, and sidenotes and other margin content just disappears on a phone.) If you're going to start from a template, it at least shouldn't lead you astray in multiple ways.
There's a lot of stuff you'd want to clean up before you let Mr Tufte see it.