CSS3伪类垂直菜单背景滑动动画效果

body{
  margin: 0px;
  padding: 0px;
  background: #e74c3c;
  font-family: 'Lato', sans-serif;
}

nav{
  float: none;
  clear: both;
  width: 30%;
  margin: 10% auto;
  background: #eee;
}

nav ul {
  list-style: none;
  margin: 0px;
  padding: 0px;
}
nav li{
    float: none;
  width: 100%;
}
nav li a{
  display: block;
  width: 100%;
  padding: 20px;
  border-left: 5px solid;
  position: relative;
  z-index: 2;
  text-decoration: none;
  color: #444;
  box-sizing: border-box; 
  -moz-box-sizing: border-box; 
  -webkit-box-sizing: border-box;
}

nav li a:hover{ border-bottom: 0px; color: #fff;}
nav li:first-child a{ border-left: 10px solid #3498db; }
nav li:nth-child(2) a{ border-left: 10px solid #ffd071; }
nav li:nth-child(3) a{ border-left: 10px solid #f0776c; }
nav li:last-child a{ border-left: 10px solid #1abc9c; }

nav li a:after {
  content: "";
  height: 100%;
  left: 0;
  top: 0;
  width: 0px; 
  position: absolute;
  transition: all 0.3s ease 0s;
  -webkit-transition: all 0.3s ease 0s;
  z-index: -1;
}
nav li a:hover:after{ width: 100%; }
nav li:first-child a:after{ background: #3498db; }
nav li:nth-child(2) a:after{ background: #ffd071; }
nav li:nth-child(3) a:after{ background: #f0776c; }
nav li:last-child a:after{ background: #1abc9c; }