Offline and updates
All the computing happens in your browser. No expression, no imported data, no document goes to a server — that is a property of the architecture, not a marketing promise: the calculation engine itself is downloaded and executed on your machine.
What happens on the first two visits
The first visit loads the application, then the calculation engine (Python, SymPy, NumPy, SciPy) in the background. It is the only visit that needs the network for anything beyond the page.
From the second visit on, everything is cached: the application, the engine, the fonts. The notebook opens and computes without a network.
Checking
Open the site twice, let the engine load, then cut the network (flight mode) and reload the page: the application must open and a calculation must complete. If it does not, the engine had not finished caching on the first visit.
Updates
A new version downloads in the background and applies at the next reload: a version never changes under your fingers in the middle of a calculation. Your documents are saved in your browser and survive the update.
If the engine has to be rebuilt — after an update, or after a calculation is stopped — the document replays its variables on its own and says so. You have nothing to retype.
Erasing
Everything is local: clearing your browser's site data erases the documents along with the cache. Export what you want to keep as .supercalc — see export and sharing.
Try it
Visit the site, wait until the engine is ready (a calculation completes), close the tab. Reopen it: the calculation is immediate. Switch to flight mode and reload: the application opens, the Heat balance template opens and recomputes, with no network.
Where to go next
The detail of what is saved and what is not is in the privacy policy.