Standalone database / AJAX combination

Friend of mine is doing a webdev course and one of the requirements is making a small website which saves and receives data from a data source. Personally I would just use PHP, some random DB and JS.

The requirements unfortunately state the whole thing has to work on its own without any form of special software. Basically just a browser.

What's a good way to tackle this problem? It just needs to be basic and work on its own. They could theoretically dump json in the localStorage I suppose but if anyone has a better idea I would love to hear it