This signer must run from a local file, not a web server.
Press Ctrl+S (Cmd+S on Mac) to save, then open the saved .html file.
For best security, use an airgapped device (Tails OS, dedicated offline machine).
This signer does not need internet. Browser spellcheckers, translation extensions, and telemetry can leak secrets.
Firefox: Menu > More > Work Offline
Chrome: DevTools (F12) > Network > check Offline
Best: physically unplug and refresh.