<!DOCTYPE html>
<html>
<body>
<h2 class="example">A heading with class="example"</h2>
<p>A paragraph with class="example".</p>
<p>Click the button to add a background color to the first element in the document with class="example".</p>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction() {
document.querySelector(".example").style.backgroundColor = "red";
}
</script>
</body>
</html>