LGPU48,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
td{
border: 1px solid #FF0000;
}
</style>
</head>
<body>
<textarea id="textarea" rows="20"><table>
<tr>
<td>1</td>
<td>2</td>
</tr>
<tr>
<td>3</td>
<td>добавь сюда цифру</td>
</tr>
</table></textarea>
<div id="div"></div>
<script>
textarea.oninput = () => div.innerHTML = textarea.value;
</script>
</body>
</html>