There's also Iodide from Mozilla, which is pretty cool: https://alpha.iodide.io/

GitHub: https://github.com/iodide-project/iodide

Post explaining what it is: https://hacks.mozilla.org/2019/03/iodide-an-experimental-too...

It uses this markdown like file format: https://iodide-project.github.io/docs/jsmd/