Thanks for the feedback and link, much appreciated. Since the application is fully client-side, anyone could download the site and run it locally without any issues. Maybe making this more apparent by providing a download link (similar to Bitcoin paper wallet) is a good idea.

I have a GitHub project (https://github.com/cxam/passed.pw) open at the moment to track issues and plan to make this open source soon after fixing any bugs found during the initial trials.

All the libraries used in this project are open source and I have listed these on the about page with the relevant links.