Actually, if you go to https://tryappservice.azure.com and create a temporary (free, login required, no registration, etc.) web app, you can edit it in real time with Monaco.

You can do that in Go, NodeJS, Python... Etc.

VSCode was actually based on a web-based version that was previously launched in Azure and called... Monaco https://dzone.com/articles/first-look-visual-studio

Isn't it mostly HTML/JS based already?

It uses Electron as a shell, so "yes"