JSFiddle - React, Tailwind, and code Playground

HTML

<div><span>FOO</span></div>

JavaScript

var foo = document.querySelector('div');
var root = new WebKitShadowRoot(foo);
var doc = document.implementation.createHTMLDocument();
var baz = doc.createElement('span');
baz.innerHTML = '<content></content> BAR';
[].forEach.call(baz.childNodes, function(node) {
    root.appendChild(document.importNode(node));
});