纯CSS实现导航栏下划线跟随的示例代码有哪些？

<ul>
 <li>aaaa</li>
 <li>bbbbbbb</li>
 <li>cccc</li>
 <li>dddd</li>
 <li>eeee</li>
</ul>

ul {
 padding:0;
 margin:0;
 font-size:0;
}
li {
 font-size:20px;
 padding:5px 15px 5px 15px;
 display: inline-block;
 position:relative;
 cursor: pointer;
 transition: all 0.5s;
}
li::before {
 content:'';
 display:block;
 position:absolute;
 width:0%;
 bottom:0;
 left:100%;
 border-bottom: 2px solid #999;
 transition: all 0.5s;
}
li:hover::before {
 left:0;
 width:100%;
}
li:hover ~ li::before {
 left:0;
}

li:hover::before {
 left:0;
 width:100%;
}
li:hover ~ li::before {
 left:0;
}