The spreadsheet course where every lesson is already open

A financial-modeling instructor sells a course. Lesson 6 is a DCF model. The old format: a video, a download link, and a third of students emailing that it won't open on their Mac.

Now lesson 6 is a page with an iframe. The iframe is a display URL: LibreOffice Calc, running in a container, with the lesson's workbook already open at the right tab. Each student gets their own container, cloned from the lesson's snapshot. Break the model completely and restoring the snapshot puts it back in seconds.

Four hundred students, four hundred containers, one flat-rate bare metal server. Clones share disk blocks copy-on-write, and a student who isn't mid-lesson costs nothing.

No install, no file version, no "which Excel do you have." The student doesn't download the exercise file. They're already inside it.