本篇会放置多种下划线滑动效果，一篇一篇增加，更新中
1、当前 tab 出现下划线的同时，前一个下划线同时消失（出现方向与消失方向保持一致），伴随过渡效果。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>change tab</title>
<style>
ul {
display: flex;
position: absolute;
width: 800px;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
list-style: none;
}
li {
position: relative;
padding: 20px;
color: #000;
line-height: 1;
transition: 0.2s all linear;
cursor: pointer;
}
li::before {
content: "";
position: absolute;
top: 0;
left: 100%;
width: 0;
height: 100%;
border-bottom: 2px solid #f00;
transition: 0.2s all linear;
}
li:active {
background: #000;
color: #fff;
}
.active ~ li::before {
left: 0;
}
.active::before {
width: 100%;
left: 0;
top: 0;
}
.hover::before{
width: 200%;
}
</style>
</head>
<body>
<ul>
<li class="active tab" data-index='0'>Nav Underline</li>
<li class="tab" data-index='1'>不可思议的CSS</li>
<li class="tab" data-index='2'>导航栏</li>
<li class="tab" data-index='3'>光标小下划线跟随</li>
<li class="tab" data-index='4'>PURE CSS</li>
</ul>
</body>
<script>
var lis = document.getElementsByClassName('tab');
for(var i=0; i<lis.length; i++){
lis[i].onclick = function(){
var that = this;
for(var i=0; i<lis.length; i++){
lis[i].classList.remove('active')
this.classList.add('active')
}
}
}
</script>
</html>