If it also uses screen emulation, SPI protocols, IC2, and sensors, it's built from scratch. It also integrates the Arduino CLI for compiling the project and using external libraries.
I created an algorithm for the wiring (currently working on it). I've also integrated more Arduino models and the Raspberry Pi Pico. I plan to integrate ESP32, but I haven't been able to finish the emulator using QEMU as a base.
It's not just an avr8js frontend
If anyone wants to try it quickly, there's a live demo available here: