You can use the app in the browser but notes are saved only to that browser session. The device-to-device sync and local dictation is desktop only. All notes are saved as markdown files in a folder you choose on the desktop app.
Source: https://github.com/rootstring/set
Releases are available for macOS, Windows (x64), and Linux (x64 only for now): https://writewithset.com?download=true