Waiting before checking for the next input…
Only press "Start turn now" once your game shows "Waiting...!" — skipping while an animation is still playing can desync the battle.
Connect your GB-Link adapter to begin
Load your Pokémon game
Select the game's ROM (.gb or .gbc) and its .sav together, or just the ROM and add the save afterwards with Load save. Your files stay on this device.
Only this browser has these. Download the saves you want to keep.
Keys: arrows, X or A = A, Z or S = B, Enter = Start, Shift = Select. Gamepads work too.