Out of curiosity, why `<notebook>` as an element and not a standard web-component `<observable-notebook>` or something like that if the goal is to go all-in on "normal" web development?

They made the syntax look familiar so that standard editors (and LLM’s) will work, but this is still a custom file format that requires a build step to convert into an HTML page. See my other comment.

The build tool is open source and unlike a Jupyter notebook, it’s easy to edit without a custom editor. So you don’t need their editor, but it will still be nicer to edit with their editor.

self-plug that you might be interested in: I built a thin wrapper around the Observable Runtime that allows you to more easily use it in a pure HTML context: https://maxbo.me/celine/

I think the goal is explicitly not to go all-in on normal web development, to maintain some opportunity to monetize.

D3.js started as/still is just a (collection of) JS libraries. Then Observable came along, and while it is a nice tool for tinkering with D3, it was not at all obvious how to then move your finished Observable D3 viz to a fully self-hosted, regular website (without paying a subscription to use their custom runtime or servers or whatever).

Now I guess they realize they strayed way too far from "the web" and are back-tracking.

I was wondering the same.

FWIW, the HTML custom elements spec does require a hyphen: https://html.spec.whatwg.org/multipage/custom-elements.html#...