How to order (sort) a <li> list with numeric content?

Below should do the trick:

$(function() {
  $('button').click(function() {
    var liContents = [];
    $('ul li').each(function() {
      liContents.push(parseInt($(this).text(), 10));
    });
    liContents.sort(numOrdDesc);
    $('ul li').each(function() {
      $(this).text(liContents.pop());
    });
  });
});

function numOrdDesc(a, b) {
  return (b - a);
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<ul>
  <li>20</li>
  <li>10</li>
  <li>5</li>
  <li>3</li>
  <li>32</li>
  <li>25</li>
  <li>6</li>
  <li>12</li>
  <li>8</li>
</ul>

<button>Sort</button>


var li = $('ul li');
li.sort(function(a, b) {
    if(parseInt($(a).text()) > parseInt($(b).text()))
        return 1;
    else return -1;
});
$('ul').empty().html(li);

There are jQuery plugins to handle this as well. Check out TinySort