<textarea id="code"></textarea>

<br>
<button onclick="validateHTML(document.getElementById('code').value)" id="submit">Submit</button>

<br>
<div id="message"></div>

textarea{
  width:100%;
  height:200px;
}

function validateHTML(htmlString){
    let parser = new DOMParser();
    let doc = parser.parseFromString(htmlString, "application/xml");
    let errorNode = doc.querySelector('parsererror');
    document.getElementById('message').innerHTML = '';
    if (errorNode) {
        document.getElementById('message').appendChild(errorNode);
    } else {
        document.getElementById('message').innerText = 'Valid HTML!';
    }
}