This is very cool! But it's a bit weird to cross-compile the program using Emscripten for the offline case too. That surely makes debugging using an IDE virtually impossible.

Wouldn't it be better to compile it to native C++ and just use the web for the interface? You could embed a simple web server for that purpose.

Interesting idea indeed. The offline version of the simulator grew out of the online version, so that's why it was designed like this. However, I'd rather see a Fast Model integration if we're going to run C++ anyway. Fast Models give an accurate idea of the microcontroller model underneath so should be very useful. Could create some web layer on top of it, although I'm not sure how complex it would be.

Note on the IDE debugging: If you have an IDE plugin that supports the Chrome Debugger Protocol and source maps you could emulate such a thing with f.e. https://github.com/Microsoft/vscode-chrome-debug. You'll miss a wide variety of stuff though; that's why Fast Models would be cool, they integrate with IDEs already.

Here's another approach - https://jumper.io

It supports the nRF52832 and the STM32F411.

No cross compiling involved. It also provides debugging capabilities using the command line interface tool.