The idea of ”local first” is sort of related: https://www.inkandswitch.com/local-first/

I think tldraw is an application that does this well in the browser. When you open it up you can start working right away, your work is automatically saved in localstorage or similar, but you can always save to a file. And you can also click share and get live collaboration on the document. Saving to a file still works the same.

It’s best of both worlds I think. I get the convenience of a web application with sync/collaboration, and I have control of the files if I want to. The format is plain JSON and if the original application goes away I could put together a renderer so that I can view the file.