如何使鼠标指针居中以使其成为十字准线？

<div id="pointer"></div>

#pointer {
  position: fixed;
  top: 0;
  left: 0;
  width: 100vw;
  height: 100vh;
  cursor: crosshair;
}

document.addEventListener('mousemove', function(event) {
  var pointer = document.getElementById('pointer');
  var x = event.clientX;
  var y = event.clientY;
  pointer.style.transform = 'translate(' + x + 'px, ' + y + 'px)';
});