<!DOCTYPE html>
<html>
<body>
<div class="example">First div element with class="example".</div>
<div class="example">Second div element with class="example".</div>
<p>Click the button to add a yellow background color to the first div element with class="example" (index 0).</p>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction() {
let x = document.getElementsByClassName("example");
x[0].style.backgroundColor = "yellow";
}
</script>
</body>
</html>