I agree regarding the example document. It's been troubling, because the fact of the matter is that those headings are legitimate, it's just that there's a mismatch between the goals of the description document and the goals of the kinds of documents that Tufte CSS is best suited for. I agree strongly with your idea of making several sample documents to experiment with. See
https://github.com/daveliepmann/tufte-css/issues/26I'll try to look into your points about leading, text size, and so on, but it would help a lot more if you could open a GH issue for them. As I've noted elsewhere in this thread, a real italic font has been added, and I'm considering what to do about small caps. To me it doesn't look so bad as-is.
I don't want Tufte CSS to look like a nicely typeset book, and it would take a lot to convince me to indent the first line of new paragraphs. I find that repulsive on the web.
Your last paragraph is gold. I wrote something similar about the table style earlier today. For me, I think this project is an exercise in making style choices using a rough template as a guiding star. I too worry that others will focus more on the finger than that star it is pointing at, by slapping this on top of existing content and calling it a day.