About the App
The app is, somewhat atypically for Monero-related software, written in C#, using ASP.NET Core MVC. A big thank you goes here to the TurtleCoin devs and their cs-turtlecoin library, which made my life building the app much easier. (TurtleCoin is a fork of Monero.)
I plan to open-source the app eventually, but only after some maturing period and clean-up work. When in any doubt consider it being malicious and don't use it.
If you use this web app for more than educational purposes and hand it real Monero addresses, seeds and keys, please be aware that it's server-based: The server will receive all your info. You will then trust me not to abuse that info to spy on you or even steal your coins, and keep this website safe from hackers.
If you are on Windows consider using the installable Windows app.
About the Author
My name is René Brunner. You can find some info about me here on my personal website.
I am into Monero since 2017. I am quite active on the Monero subreddit as rbrunner7. I also contribute to the Monero codebase every now and then; my biggest work by far is the MMS, the Multisig Messaging System. If interested check its manual on GetMonero.org.