bottombordertransition – 从中​​间扩展

color: $color-3399E5;
&::before{
                            content: "";
                             position: absolute;
                             top: 0;
                             left: 0;
                             border-bottom: 2px solid $color-3399E5;
                             padding-top: 5px;
                              0;
                            height: 100%;
                         }
                         &:hover{
                             color: $color-3399E5;
                         }
                         &:hover::before{
                              100%;
                             transition: 0.35s all ease-in 0.1s;
                         }

。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。
body {
  padding: 50px;
}
 
a,a:hover {
  color: #000;
  text-decoration: none;
}
 
li {
  display: inline-block;
  position: relative;
  padding-bottom: 3px;
  margin-right: 10px;
}
li:last-child {
  margin-right: 0;
}
 
li:after {
  content: '';
  display: block;
  margin: auto;
  height: 3px;
   0px;
  background: transparent;
  transition: width .5s ease,background-color .5s ease;
}
li:hover:after {
   100%;
  background: blue;
}
<ul>
  <li><a href="#">HOME</a></li>
  <li><a href="#">PAGE</a></li>
  <li><a href="#">ABOUT US</a></li>
  <li><a href="#">CONTACT US</a></li>
</ul>

　

深入理解CSS过渡transition 
CSS3 transition下不同过渡类型效果展示 >>