But I didn't love the choices for how to implement it here. Dynamic, reactive HTML can be a lot more declarative than this, and Observable is cool, but strays from standard JS.
I started to build a reactive HTML system called Heximal that eventually will have notebook support, but it's declarative, based on HTML templates and custom elements with a expression / reactivity system (based on the TC39 Signals proposal) on top.
https://github.com/elematic/heximal
It's a bit like a mashup of HTMX, Tangle, Curvenote, and Polymer. Or like HTML if were natively reactive.
I think it will lend it self to graphical editing and notebook user cases quite well.