Lesswrong for both sidebars: the heading based TOC on the left, and the margin notes on the right: https://www.lesswrong.com/posts/bJ2haLkcGeLtTWaD5/welcome-to...

For interactive / code snippets Maxime Heckel: https://blog.maximeheckel.com/posts/the-study-of-shaders-wit...

Honorable mentions Maggie Applebaum https://maggieappleton.com/ai-enlightenment Marek Chotoborski https://zanlib.dev/blog/number-inputs-in-react/

Line width, sane fonts, avoiding clever shit unless very polished, gets you a long way.