This web app is also available packaged as an installable Windows app which runs completely on your computer, independently from this web server here, and doesn't need Internet access. From a security point of view it is of course safer because it removes most of the risks of sending seeds and secret keys to some remote server that might log and steal.
The remaining risk: It's not yet open-source so you can't check yourself yet whether the app does anything fishy. If in any doubt don't use the app for more than educational purposes.
If there is enough interest I can make a local Linux version available.