@keyframes pop {
    0% {
      transform: scale(0);
    }
    50% {
      transform: scale(1.2);
    }
    100% {
      transform: scale(1);
    }
  }
  
  .cell.x::before,
  .cell.o::before {
    animation: pop 0.3s ease-out;
  }
  
  .cell.x::before {
    content: "X";
    color: #ff3b30;
  }
  
  .cell.o::before {
    content: "O";
    color: #4cd964;
  }