Ficdown is completely open source. If you find any bugs or other issues, please report them on the Github repositories linked below. I'm also more than happy to look at pull requests for bug fixes.
The main Ficdown parsing and rendering engine, as well as the command-line tool, are written in C#. The source code is available on Github here:
The in-browser Ficdown player is written in Typescript, it's the same script used to create the playable examples on the Learn page. The source code is available on Github here: