JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="userinput" cols="50" rows="10">
<img src="dummy.png" onerror="alert('owned');" onload="alert('still owned');" />
</textarea>
<br />
<button id="button">Inspect user input without inserting into DOM</button>

JavaScript

var userinput = document.getElementById("userinput"),
	button = document.getElementById("button");

button.addEventListener("click", function () {
	var content = userinput.value,
        
        // Use DOMParser instead of a div from current document.
        // https://developer.mozilla.org/en-US/docs/Web/API/DOMParser
    	parser = new DOMParser(),
    	doc = parser.parseFromString(content, "text/html");
    
    console.log("Before: " + content);
    
    var imgs = doc.getElementsByTagName("img");
    console.log(imgs.length);
    
    for (var i = 0; i < imgs.length; i += 1) {
        imgs[i].src = "dummy2.png";
    }
    console.log("After: " + doc.body.innerHTML);
    
    // Update the HTML string, without triggering any network request from img src or JS code within!
    userinput.value = doc.body.innerHTML;
});