This works pretty well, actually. Starts up in about a minute, although on Brave browser I did have to disable Shields and uBlock Origin (may have been the latter that was blocking something).
Interestingly enough, I caught a brief glimpse of a Windows OS window during start up, so it appears to be running on a Windows emulator inside the browser (worked perfectly well on my Linux PC).
After I logged in (only needed to the first time) my avatar was where I last left it, and everything looks like the desktop app version does. Mouse control is slightly different as I could not long-click and drag the view around with my mouse, but the arrows etc all worked perfectly. The resolution also looked very sharp.
In truth, this could be easier than using the desktop app, as when you go in a second time, you don’t have to again fill the password in. And of course for new users, no installation first.
See https://nwn.blogs.com/nwn/2025/01/sl-browser-streaming-project-zero.html