<!DOCTYPE html>
<html>
<body>
<p>This is a p element.</p>
<p>This is also a p element.</p>
<p>Click the button to add a background color to the first p element in the document.</p>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction() {
document.querySelector("p").style.backgroundColor = "red";
}
</script>
</body>
</html>