Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<head>
<style>
table, td {
    border: 1px solid black;
}
</style>
</head>
<body>

<p>Click the buttons to create and delete a thead element for the table.</p>

<table id="myTable">
  <tr>
    <td>cell 1</td>
    <td>cell 2</td>
  </tr>
  <tr>
    <td>cell 3</td>
    <td>cell 4</td>
  </tr>
</table>
<br> 

<button onclick="myCreateFunction()">Create thead</button>
<button onclick="myDeleteFunction()">Delete thead</button>

<script>
function myCreateFunction() {
    var table = document.getElementById("myTable");
    var header = table.createTHead();
    var row = header.insertRow(0);
    var cell = row.insertCell(0);
    cell.innerHTML = "<b>This is a table header</b>";
}

function myDeleteFunction() {
    document.getElementById("myTable").deleteTHead();
}
</script>

</body>
</html>