JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/jstree/3.2.1/themes/default/style.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jstree/3.2.1/jstree.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.12.1/jquery.min.js"></script>
<div id="treeview">
  <ul>
    <li>
      <a href="#">abcdefghijkl</a>
      <ul>
        <li>
          <a href="#" onclick="goTo('index.php?module=klimaat&amp;pagina=dashboard&amp;id=6',false);">
          Beneden</a>
        </li>
        <li>
          <a href="#" onclick="goTo('index.php?module=klimaat&amp;pagina=dashboard&amp;id=7',false);">
          Boven</a>
        </li>
      </ul>
    </li>
  </ul>
</div>

CSS

a .jstree-icon {
  background-image: url("https://jsfiddle.net/img/logo@2x.png")!important;
  background-repeat:no-repeat!important;
  background-position: 0!important;
  background-size:20px;// This size is just for this specific image
 }

JavaScript

$("#treeview").jstree().on('loaded.jstree', function()
{
    $("#treeview").jstree('open_all');
});