I think one of the main reasons was the lack of dynamic import support until fairly recently [0], being able to import code dynamically without crazy <script> h

I think one of the main reasons was the lack of dynamic import support until fairly recently [0], being able to import code dynamically without crazy <script> hacks is very useful in a notebook. The tools you mentioned all pre-date that I think. Secondly notebooks have to run sandboxed. Most notebooks run the output/code in an iframe but the editor is in the main window (e.g. JSFiddle). The moment you do that you can no longer support notebook style output (cell-output-cell-output) without some serialization step as you can only embed the iframe in one place (so there would be a single output pane). This is also how Project Iodide worked [1]. Starboard's approach is to put the entire editor/runtime in the sandbox which I think is a superior approach for a notebook. It only talks to the outside frame over a thin API (for saving notebooks, refreshing, syncing the content). [0]: https://caniuse.com/es6-module-dynamic-import https://caniuse.com/es6-module-dynamic-import [1]: https://alpha.iodide.io/ https://alpha.iodide.io/