У вашего броузера проблема в совместимости с HTML5
Arduino UNO/nano + EasyBuilder Pro over RS232/485 via RTU protocol
complete project: https://github.com/dejanra/arduino_modbus_rtu
MODBUS library (written by Jason Vreeland under GNU license):
https://code.google.com/archive/p/arduino-modbus-slave/downloads