Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<h3>A demonstration of how to access an OPTGROUP element</h3>

<select size="4">
  <optgroup id="myOptgroup" label="Swedish Cars">
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
    <option value="koenigsegg">Koenigsegg</option>
  </optgroup>
</select>

<p>Click the button to get the label/description of the option group.</p>

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

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

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

</body>
</html>