<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>鼠标移入下划线展开</title>
<style type="text/css">
#underline{
width: 200px;
height: 50px;
background: #ddd;
margin: 20px;
position: relative;
}
#underline:after{
content: "";
width: 0;
height: 5px;
background: blue;
position: absolute;
top: 100%;
left: 50%;
transition: all .8s;
}
#underline:hover:after{
left: 0%;
width: 100%;
}
</style>
</head>
<body>
<div id="underline"></div>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>鼠标移入下划线展开</title>
<style type="text/css">
#underline{
/* width: 200px; */
height: 50px;
background: #ddd;
margin: 20px;
position: relative;
}
#underline:after{
content: "";
width: 0;
height: 5px;
background: blue;
position: absolute;
top: 100%;
left: 0%;
transition: all .8s;
}
#underline:hover:after{
left: 0%;
width: 100%;
}
</style>
</head>
<body>
<a href="#"><div id="underline">women</div></a>
</body>
</html>
原文：https://www.cnblogs.com/hudunyu/p/12889073.html