<!DOCTYPE html>
<html>
<head>
<style>
div {
border: 1px solid black;
margin: 5px;
}
</style>
</head>
<body>
<div id="myDIV">
<p class="child">A p element with class="child" in a div.</p>
<p class="child">Another p element with class="child" in a div.</p>
<p>A p element in div with a <span class="child">span element with class="child"</span> inside of it.</p>
</div>
<p>Click the button to add a background color to all elements with class="child" inside the div element with id="myDIV".</p>
<button onclick="myFunction()">Try it</button>
<p><strong>Note:</strong> The getElementsByClassName() method is not supported in Internet Explorer 8 and earlier versions.</p>
<script>
function myFunction() {
var x = document.getElementById("myDIV");
var y = x.getElementsByClassName("child");
var i;
for (i = 0; i < y.length; i++) {
y[i].style.backgroundColor = "red";
}
}
</script>
</body>
</html>