JSFiddle - React, Tailwind, and code Playground

HTML

<div id='a'>
  <div id='b'>
    
    <div class='x'>
      <h1>A</h1>
    </div><!--</x>-->
    <div class='x'>
      <h1>B</h1>
    </div><!--</x>-->

    <div class='x'>
      <h1>C</h1>
    </div><!--</x>-->

    <div class='x'>
      <h1>D</h1>
    </div><!--</x>-->
    
  </div><!--</b>-->
</div><!--</a>-->

JavaScript

var divElems = document.getElementsByClassName('x');
var divB = document.getElementById('b');

var output = [];
for(var i =divElems.length - 1; i >= 0;i--){
  output.push(divElems[i]);
}

b.innerHTML = '';
output.forEach(function(eachDiv){
	b.appendChild(eachDiv);
});