是javaScript，而且用html+css也可以实现，不过比较麻烦，而且限制比较大。
你可以看看这段代码
HTML部分：
<!DOCTYPE html>
<head>
<meta http-equiv=”Content-Type” content=”text/html; charset=utf-8″ />
<title>css3折叠式苹果菜单</title>
<link rel=”stylesheet” title=”Default” href=”style.css” type=”text/css” media=”screen”>
</head><body>
<div class=”accordionMenu”>
<h1>Accordion Menu</h1>
<div id=”brand” class=”menuSection”>
<h2><a href=”#brand”>Brand</a></h2>
<p>
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea
commodo consequat. Duis aute irure dolor in reprehenderit in voluptate
velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint
occaecat cupidatat non proident, sunt in culpa qui officia deserunt
mollit anim id est laborum.Lorem ipsum dolor sit amet, consectetur
adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore
magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco
laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor
in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla
pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa
qui officia deserunt mollit anim id est laborum.
</p>
</div>
<div id=”promotion” class=”menuSection”>
<h2><a href=”#promotion”>Promotion</a></h2>
<p>
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea
commodo consequat. Duis aute irure dolor in reprehenderit in voluptate
velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint
occaecat cupidatat non proident, sunt in culpa qui officia deserunt
mollit anim id est laborum.
</p>
</div>
<div id=”event” class=”menuSection”>
<h2><a href=”#event”>Event</a></h2>
<p>
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea
commodo consequat. Duis aute irure dolor in reprehenderit in voluptate
velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint
occaecat cupidatat non proident, sunt in culpa qui officia deserunt
mollit anim id est laborum.
</p>
</div>
</div>
</body>
</html>
=====
css部分
.accordionMenu {
font: 12px Arial, Verdana, sans-serif;
color:#424242;
background: #fff;
padding: 10px;
width: 500px;
margin:0;
}
.accordionMenu h1 {
margin: 0;
font-size: 20px;
text-shadow: 2px 2px 2px #aeaeae;
}
.accordionMenu h2 {
margin:5px 0;
padding:0;
}
.accordionMenu h2 a {
font-size: 13px;
display: block;
font-weight: normal;
color:#424242;
text-shadow: 2px 2px 2px #aeaeae;
text-decoration:none;
margin:0;
padding:10px 10px;
background: #8f8f8f;
background: -moz-linear-gradient( top, #cecece, #8f8f8f); /* FF, Flock */
background: -webkit-gradient(linear, left top, left bottom, from(#cecece), to(#8f8f8f));
-webkit-border-radius: 5px;
-moz-border-radius: 5px;
border-radius: 5px;
}
.accordionMenu :target h2 a,
.accordionMenu h2 a:hover,
.accordionMenu h2 a:active {
background: #2288dd;
background: -moz-linear-gradient( top, #6bb2ff, #2288dd);
background: -webkit-gradient(linear, left top, left bottom, from(#6bb2ff), to(#2288dd));
color:#FFF;
}
.accordionMenu p {
padding:0 10px;
margin:0;
height: 0;
overflow: hidden;
-moz-transition: height 0.5s ease-in;
-webkit-transition: height 0.5s ease-in;
-o-transition: height 0.5s ease-in;
transition: height 0.5s ease-in;
}
.accordionMenu :target p {
overflow: auto;
height:100px;
}