<html>
<head>
<title>lalala</title>
<script type="text/javascript" src="jquery-3.1.1.min.js"></script>
</head>
<body>
<span>a1</span>
<span>a2</span>
<span>a3</span>
<script></script>
</body>
<script></script>
<script type="text/javascript">
$('span:nth-last-child(3)').css("color", "red");
</script>
</html>