原理：
1、radio 與a 重疊，
2、radio zindex屬性比a大，
3、radio透明
參考 http://www.imooc.com/video/7144
<html>
<head>
<style type="text/css">
.nav {
position: fixed;
bottom: 10px;
width: 100%;
height: 50px;
}
input, a {
position: absolute;
width: 20%;
height: 100%;
}
input {
z-index: 100;
opacity: 0;
}
a {
text-decoration:none;
text-align: center;
background: yellow;
line-height: 50px;
}
.nav_item1, .nav_item1 + a {
left: 0%;
}
.nav_item2, .nav_item2 + a {
left: 20%;
}
.nav_item3, .nav_item3 + a {
left: 40%;
}
.nav_item4, .nav_item4 + a {
left: 60%;
}
.nav input:checked + a {
background-color: #ff0000;
font-size: 26px;
}
</style>
</head>
<body>
<div class="nav">
<input type="radio" name="nav_radio" checked="checked" class="nav_item1">
<a href="#">aa</a>
<input type="radio" name="nav_radio" class="nav_item2">
<a href="#">bb</a>
<input type="radio" name="nav_radio" class="nav_item3">
<a href="#">cc</a>
<input type="radio" name="nav_radio" class="nav_item4">
<a href="#">dd</a>
</div>
</body>
</html><html>
<head>
<style type="text/css">
.nav {
position: relative;
}
.nav-item {
display: inline-block;
width: 20%;
height: 50px;
margin-left: 10px;
position: relative;
}
input {
position: absolute;
top: 0px;
left: 0px;
width: 100%;
z-index: 100;
height: 100%;
opacity: 0;
}
a {
position: absolute;
top: 0px;
width: 100%;
height: 100%;
text-align: center;
background: yellow;
line-height: 50px
}
.nav input:checked + a {
background-color: #ff0000;
font-size: 26px;
}
</style>
</head>
<body>
<div class="nav">
<div class="nav-item">
<input type="radio" name="nav_radio" checked="checked">
<a href="#">aa</a>
</div>
<div class="nav-item">
<input type="radio" name="nav_radio">
<a href="#">aa</a>
</div>
<div class="nav-item">
<input type="radio" name="nav_radio">
<a href="#">aa</a>
</div>
<div class="nav-item">
<input type="radio" name="nav_radio">
<a href="#">aa</a>
</div>
</div>
</body>
</html>html簡單，css就要複雜點，
html 複雜，css就能簡單點，
html 簡單至上，後臺人員只要看html，當然結構簡單更好，