Rock paper scissors
Rock, paper, scissors, by hand in front of the camera. Best of three, and your throw locks on “shoot”.
Show your hand to the camera. On the count of “rock, paper, scissors, shoot”, make rock (a fist), paper (an open hand) or scissors (two fingers): your throw locks on “shoot”. Two rounds wins the game.
Nothing leaves your browser: the game only reads points on your face or hands, or the microphone’s volume, and never sends a picture or a sound.