Love Firenvim. Although I think there are different usecases for a vim-in-a-website and something like firenvim (or emacs-anywhere[1], which is what I'm using nowadays, after I've crossed over to the evil side) - as you can use vim-in-a-website from another machine without any setup, like vscode.dev for vscode, for instance. Self-hosting something like this could mean you could add your dotfiles, or maybe even have the website be able to pull a .vimrc from a publicly available dotfile repository, that would be pretty neat.
[1] https://github.com/zachcurry/emacs-anywhere