For anyone who doesn’t know:
Just out of curiousity, is that a fault or a fair serve?
Not a tennis expert, but I believe it is fair. He was standing in the correct position. He tossed the ball in the air. He hit the ball with the racket exactly once. He hit the ball before the ball hit the ground. The ball landed in the opposite square where it is supposed to land. I don’t believe there are any rules about what you do with your body otherwise.
I think it’s a foot fault. It’s pretty close, but it looks like his foot lands before he hit the ball.