自动动画 css javascript 下划线

Question

需要像这样制作 CSS (Javascript) 下划线（来自 codepen 的一些示例）

@import url(http://fonts.googleapis.com/css?family=Quando);
*, *:after, *:before {
  box-sizing: border-box;
  -moz-box-sizing: border-box;
}
* {margin:0;padding:0;border:0 none;position: relative; outline: none;
}
html, body {
  background:  #004050;
  font-family: Quando;
  font-weight: 300;
  width: 100%;
}
h2, h3 {
  background: #0D757D;
  width: 100%;
  min-height: 200px;
  line-height: 200px;
  font-size: 3rem;
  font-weight: normal;
  text-align: center;
  color: rgba(0,0,0,.4);
  margin: 3rem auto 0;
}
.uno {background: #ff5e33;}
.dos.bis {background: #85144B;}
.dos {background: #FADD40;}
h3 {background: #2B5B89;}

h2 > a, h3 > a {
  text-decoration: none;
  color: rgba(0,0,0,.4);
  z-index: 1;
}

h2 > a:before {
  content: "";
  position: absolute;
  width: 100%;
  height: 3px;
  bottom: 0;
  left: 0;
  background: #9CF5A6;
  visibility: hidden;
  border-radius: 5px;
  transform: scaleX(0);
  transition: .25s linear;
}
h2 > a:hover:before,
h2 > a:focus:before {
  visibility: visible;
  transform: scaleX(1);
}
.uno a:before {
  background: rgba(0,0,0,0);
  box-shadow: 0 0 10px 2px #ffdb00;  
}
.dos > a:after {
  content: "";
  position: absolute;
  width: 100%;
  height: 7px;
  border: 1px solid #000;
  bottom: -2px;
  left: 0;
  background: #fff;
  border-radius: 5px;
  opacity: 0;
  transform: scalex(0);
  transition: .5s;
}
.dos.bis > a:after {
  opacity: .05;
  transform: scalex(1);
}
.dos:hover > a:after {
  opacity: .15;
  transform: scalex(1);
}
.dos.bis > a:before {
  background: rgba(0,0,0,0);
  box-shadow: 0 0 10px 2px #FADD40;
}
.dos > a:before {
  background: rgba(0,0,0,0);
  box-shadow: 0 0 10px 2px #FF5E33;
}

h3 > a:before {
  content: "";
  background: #7FDBFF;
  position: absolute;
  width: 100%;
  height: 5px;
  bottom: 0;
  left: 0;
  border-radius: 5px;
  transform: scaleX(0);
  animation: 1.4s forwards no-hover-v linear;
  animation-fill-mode: forwards;
  z-index: -1;
}
h3 > a:hover:before,
h3 > a:focus:before {
  animation: .5s forwards hover-v linear;
  animation-fill-mode: forwards;
}
@keyframes hover-v {
  0% {
      transform: scaleX(0);
      height: 5px;
     }
  45% {   
      transform: scaleX(1.05);
      height: 5px;
     }
  55% {height: 5px;}
  100% {
      transform: scaleX(1.05);
      height: 3.8rem;
     }
}
@keyframes no-hover-v {
  0% {
      transform: scaleX(1.05);
      height: 3.8rem;
     }
  45% {height: 5px;}
  55% {   
      transform: scaleX(1.05);
      height: 5px;
      opacity: 1;
     }
  
  100% {
      transform: scaleX(0);
      height: 5px;
      opacity: .02;
     }
}

p {padding: .5rem;}
p a {color: rgba(255,255,255,.5)}

<h2>
  <a href=''>:hover, please</a>
</h2>

但下划线必须从左到右开始增长，然后以一定的速度从右到左减少。并且下划线必须在没有鼠标悬停链接的情况下自动进行动画处理。

当链接有鼠标悬停时，下划线必须同时向右增长。

请编码如何做到这一点。

Answer 1

使用transform-origin控制scale()的方向，可以通过类触发添加/移除边框。

var a = document.getElementById("a");

setTimeout(function() {
  a.classList.add("under");
  setTimeout(function() {
    a.classList.remove("under");
  }, 2000);
}, 500);

h2 {
  background: #0D757D;
  width: 100%;
  min-height: 200px;
  line-height: 200px;
  font-size: 3rem;
  font-weight: normal;
  text-align: center;
  color: rgba(0,0,0,.4);
  margin: 3rem auto 0;
}

h2 > a {
  text-decoration: none;
  color: rgba(0,0,0,.4);
  z-index: 1;
  position: relative;
}

h2 > a:before {
  content: "";
  position: absolute;
  width: 100%;
  height: 3px;
  bottom: 0;
  left: 0;
  background: #9CF5A6;
  visibility: hidden;
  border-radius: 5px;
  transform: scaleX(0);
  transition: .25s linear;
  transform-origin: 0 0;
}
h2 > a.under:before {
  visibility: visible;
  transform: scaleX(1);
}

<h2>
  <a id="a" href=''>:hover, please</a>
</h2>