Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

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

<p>Click the button to set the font size of <mark id="myMark">this marked text</mark> to extra large.</p>

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

<script>
function myFunction() {
    var x = document.getElementById("myMark");
    x.style.fontSize = "x-large";
}
</script>

</body>
</html>