<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("#btn1").click(function(){
$("p:first").css("background-color","red");
});
$("#btn2").click(function(){
$("p:first-child").css("background-color","yellow");
});
});
</script>
</head>
<body>
<button id="btn1">:first</button>
<button id="btn2">:first-child</button><br><br>
<div style="border:1px solid">
<p>The first paragraph in div.</p>
<p>The last paragraph in the div.</p>
</div><br>
<div style="border:1px solid">
<p>The first paragraph in another div.</p>
<p>The last paragraph in another div.</p>
</div>
</body>
</html>