Beware: This is very much work-in-progress. Things can and probably will be broken, even on master.
This repo contains a proof-of-concept for an Arduino IDE based on Theia. It's built on top of a version of the arduino-cli that sports a gRPC interface.
The easiest way to try the browser version is using Gitpod: https://gitpod.io/#github.com/typefox/arduino-poc
requires protoc to be in the PATH
and some other prerequisites.
git clone https://github.com/typefox/arduino-poc
cd arduino-poc
yarn
yarn --cwd arduino-ide-electron start