It's JavaScript for the interactive/reactive parts, but the notebook is implemented in a self-contained HTML file: it handles everything from document structure to editing and display. It's also not dependant on 3rd party software (par browser), and is as durable, portable and easy-to-use as any HTML is.

Thanks for sharing! This was a great read.

There's a lot of sites between pure html and modern css. "Simply" rendering things differently would definitely break a lot of sites. Just imagine if browsers started defaulting to *{box-sizing: border-box;}.

Also: https://www.programmingfonts.org/

Reactive HTML Notebooks