I had trouble getting CodeMirror to work in Observable because it detects amd and tries to load modules with it, using relative paths. I got it working using an iFrame instead, with srcdoc to let me build up the source of it:
It would be neat if it could work without an iFrame (if anyone has any ideas, Iād appreciate it), but I got it working inside an iFrame using postMessage both to update the value of the view and to resize it! And now I have jsx and go in an Observable document.