JSFiddle - React, Tailwind, and code Playground

HTML

<iframe id="container" src="http://www.google.com"></iframe>
<button onclick="createElement()">Click me!</button>

CSS

#container{
    width: 100%;
    height:300px;
    overflow: hidden;
}

JavaScript

function createElement(){
    var doc = document.getElementById('container');
    
    var ifrDoc = doc.contentWindow || doc.contentDocument;
    if (ifrDoc.document) ifrDoc = ifrDoc.document;
    
    
    var elem = ifrDoc.createElement("div");
    elem.innerHTML = 'Test';
    ifrDoc.body.appendChild(elem);
}