<!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>