实例分析: :first :first-child 之间在元素选取方面的区别
实例:
<!DOCTYPE html>
<html lang='zh-cn'>
<head>
<title>Insert you title</title>
<meta http-equiv='description' content='this is my page'>
<meta http-equiv='keywords' content='keyword1,keyword2,keyword3'>
<meta http-equiv='Content-Type' content='text/html; charset=utf-8'>
<script type='text/javascript' src='./js/jquery-1.12.1.min.js'></script>
<style type='text/css' >
*{margin:0;padding:0;}
html{font:400 15px/1.2em 'Courier New';color:#666;}
div > *{margin:10px 0;cursor:pointer;}
#demo{750px;margin:75px auto;}
button{padding:5px 15px;border:0;outline:none;margin-top:-5px;border-radius:2px;font:400 12px/1.2em 'Courier New';}
.active{background-color:yellow;transition: .3s;}
.hover{background-color:red;transition: .3s}
p{text-indent:8px;}
</style>
<script type='text/javascript'>
$(function(){
$('#first').click(function(){
$('p:first').toggleClass('hover');
});
$('#firstChild').click(function(){
$('p:first-child').toggleClass('active');
});
});
</script>
</head>
<body>
<div id='demo'>
<div style='border:1px solid #3695BC'>
<p>The first paragraph in div.</p>
<p>The last paragraph in the div.</p>
</div>
<div style='border:1px solid #3695BC'>
<p>The first paragraph in another div.</p>
<p>The last paragraph in another div.</p>
</div>
<button id='first'>:first</button>
<button id='firstChild'>:first-child</button>
</div>
</body>
</html>