<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("p:only-of-type").css("background-color","yellow");
});
</script>
</head>
<body>
<div style="border:1px solid;">
<p>The first paragraph.</p>
<p>The last paragraph.</p>
</div><br>
<div style="border:1px solid;">
<p>The only paragraph.</p>
</div><br>
<div style="border:1px solid;">
<span>This is a span element.</span>
<p>The only paragraph.</p>
</div><br>
</body>
</html>