One final question/request (for which I'd happily pay for!) : it would be awesome to have a path away from Observable's infra if desired. Say I _really_ want to host a particular notebook locally: is something like that planned? I know this is not a trivial feature since notebooks can call other notebooks, but I'd love to develop stuff on Observable knowing that should the worst happen and it doesn't exist anymore, I can run it all locally on my webpage.
We’ve been scrambling to get things ready enough for this initial launch, but Mike managed to get the Standard Library open sourced this morning: https://github.com/observablehq/notebook-stdlib
One of the next pieces we'd like to open-source is the Notebook Runtime — All of the JS that you need to take a blank webpage, and host a notebook within it.
After that, there's some file format details to figure out — hopefully a notebook can just be published and consumed as a standard ES Module. But then we'll be most of the way there.