Tryit Editor v2.9

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

<h3>A demonstration of how to access a COLGROUP element</h3>

<table>
  <colgroup id="myColgroup" span="2" style="background:red"></colgroup>
  <tr>
    <th>ISBN</th>
    <th>Title</th>
    <th>Price</th>
  </tr>
  <tr>
    <td>3476896</td>
    <td>My first HTML</td>
    <td>$53</td>
  </tr>
  <tr>
    <td>5869207</td>
    <td>My first CSS</td>
    <td>$49</td>
  </tr>
</table>

<p>Click the button to get the number of columns a COLGROUP element should span.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction() {
    var x = document.getElementById("myColgroup").span;
    document.getElementById("demo").innerHTML = x;
}
</script>

</body>
</html>