Parsley is primarily built to serve as an easily extensible markdown parser for the Echelon Matrix Client. As a result of the above, it is primarily built to handle a subset of Markdown without having to deal with the security repercussions of complete md rendering.
Non Nix/NixOS users, simply install all the packages used within the flake.nix within the development environment.
For Nix/NixOS users enable direnv and allow the environment to be loaded if you use flakes.