Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<table id="myTable" border="1">
  <tr>
    <th>Month</th>
    <th>Savings</th>
  </tr>
  <tr>
    <td>January</td>
    <td>$100</td>
  </tr>
  <tr>
    <td>February</td>
    <td>$150</td>
  </tr>
</table>
<br>

<button type="button" onclick="myFunction()">Collapse border</button>

<script>
function myFunction() {
    document.getElementById("myTable").style.borderCollapse = "collapse";
}
</script>

</body>
</html>