JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">
  <div id="z">z</div>
  <div id="x">x</div>
</div>

JavaScript

var z   = document.querySelector("#z"),
    x   = document.querySelector("#x"),
    box = document.querySelector(".box"),
    out = "";
    out = z.outerHTML + x.outerHTML;
    alert(out);
    box.innerHTML = out;