One HTML file, 57KB. No build step, no dependencies, no CDN, no backend. Save it to your desktop, turn off your wifi, and it keeps working — your data stays in your own browser because there is nowhere to sync it to. Line items, 22 currencies, tax, discounts, deposits, saved clients, and print-to-PDF at A4. Add #selftest to the URL and it runs 26 assertions on the money and date maths in front of you. Free and unlicensed, source on GitHub.