Chrome offers the most robust support for Progressive Web Apps, and will allow you to download Atticus and use it as an app.
You can also use Atticus in other browsers like Firefox, Brave, or Arc but these browsers don't natively support downloading PWAs. Keep in mind, however, the browser version and the app version have the exact same features, including being able to use Atticus offline - as long as you're logged in. Whether you use the app or browser, you can write and format offline, you just won't be able to import or export.
We do suggest avoiding Edge and Safari as both of these browsers include features that interfere with the way Atticus operates.