如何在模态图像上居中并保持焦点

.modal {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

.modal {
  max-width: 100%;
  max-height: 100%;
  overflow: auto;
}

var previousFocus = document.activeElement;

function openModal() {
  var modal = document.getElementById('modal');
  modal.style.display = 'block';
  modal.focus();
}

function closeModal() {
  var modal = document.getElementById('modal');
  modal.style.display = 'none';
  previousFocus.focus();
}