JSFiddle - React, Tailwind, and code Playground

HTML

<div id="result"></div>

JavaScript

$(document).ready(function(){
  var xmlstr = '<test><val attr="&lt;popo" /></test>';
  var div = document.createElement("div");
  div.innerHTML = xmlstr;
  var val = div.getElementsByTagName("val")[0];
  val.setAttribute('pilou', '<toto>&');

  $("#result").text(div.innerHTML);
});