如何使div的网格居中(纯JS和CSS)

<div id="container">
  <div id="grid"></div>
</div>

#container {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

#grid {
  width: 200px;
  height: 200px;
  background-color: #ccc;
}

window.addEventListener('resize', centerGrid);

function centerGrid() {
  var container = document.getElementById('container');
  var grid = document.getElementById('grid');
  var containerWidth = container.offsetWidth;
  var containerHeight = container.offsetHeight;
  var gridWidth = grid.offsetWidth;
  var gridHeight = grid.offsetHeight;
  
  var leftOffset = (containerWidth - gridWidth) / 2;
  var topOffset = (containerHeight - gridHeight) / 2;
  
  grid.style.left = leftOffset + 'px';
  grid.style.top = topOffset + 'px';
}

centerGrid(); // 初始居中