right click -> inspect

HTML looks pretty handwritten (lack of crazy css classes and simple structure)

<link rel="stylesheet" href="https://fi-le.net/css/tufte.css">

They are using tufte.css https://edwardtufte.github.io/tufte-css/

Thank you! As the other commenters already figured out, I manually write HTML and use tufte.css with some minor customizations.

Part of it is this tufte.css: https://edwardtufte.github.io/tufte-css/