个人答案。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>3.1页面头部制作练习题</title>
<style type="text/css">
li{
float:left;
width:50px;
height:30px;
list-style:none;
text-align:center;
line-height:30px;
}
a{
display:block;
height:30px;
}
li a:link,li a:visited{
text-decoration:none;
color:black;
}
li a:hover,li a:active{
text-decoration:none;
color:white;
background:#BE3948;
}
</style>
</head>
<body>
<h3>课程难度</h3>
<ul>
<li><a href="">全部</a></li>
<li><a href="">初级</a></li>
<li><a href="">中级</a></li>
<li><a href="">高级</a></li>
</ul>
</body>
</html>
