Arduino limits

Not supported

Other boards. Only the Arduino Uno is emulated: not the Nano, Mega, ESP32 or RP2040.
Serial input. The serial monitor is output only. Serial.read() has nothing to read.
Extra libraries. Keypad, DHT, NeoPixel and OLED libraries are not available, and you can't upload your own.
Some parts. SPI devices, ultrasonic sensors, steppers, relays and similar parts are not available yet.
C++ exceptions.
More than one programmed board in a circuit.

Speed. The simulation aims for real time, but can run slower on a slower computer or with many parts.
Reloading. If you reload the page, the board restarts from reset, and your code is rebuilt.
Fast signals. The circuit samples each pin every 50 µs, so signals faster than about 10 kHz are approximate.
Live rooms. In a shared room each browser runs its own board, so they can drift apart.
Current dots. The moving current dots on wires are approximate.
EEPROM. It starts blank every time the board starts.

If something doesn't behave like a real Arduino, tell support and say which part and which sketch. See Get help.