what's the difference between the Windows version and the offline version?
The Windows version is a little better in general of you can run it. You can load up any mods, whereas the offline browser version requires mod files to be hosted in some way to be loaded up. Also the Windows version has better performance overall and save files are permanent, whereas in the browser version, save files are tied to browser local storage and can sometimes be deleted automatically or accidentally.
That said, the browser versions do have some good points of their own. For one, you can use browser extensions and features that may help you, like for translation or accessibility. The browser version is also compatible with many more operating systems and devices, is smaller to download, and can even be played on the web without having to download anything.