JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li>a</li>
  <li>b</li>
  <li>c</li>
  <li class="sub">d</li>
  <li class="sub">e</li>
  <li class="sub">f</li>
  <li>g</li>
  <li>h</li>
  <li class="sub">i</li>
  <li class="sub">j</li>
  <li class="sub">k</li>
  <li>l</li>
 </ul>

JavaScript

$(document).ready(function() {
    $('li.sub').filter(':not(li.sub + li.sub)').each(function() {
        $(this).nextUntil(':not(li.sub)').andSelf().wrapAll('<li><ul>');
    });
});