Render HTML inside a textarea

You can’t really render HTML in a textarea. What you can do is to add a separate div with the contenteditable attribute set to true. And display the textarea content into that div.

You would want to do something like this:

<form>
<textarea id="text" cols="30" rows="10">
Hello this is <strong>just</strong> a <em>text</em>.
</textarea>
</form>

<div id="preview" contenteditable="true"></div>

and then inject the content of the textarea into the div with javascript

const textArea = document.getElementById('text');
const preview = document.getElementById('preview');

preview.innerHTML = textArea.textContent;

Note that this is not instant. For that you would probably need some more Javascript magic.