a{
font-family:sans-serif;
color:#fff;
text-indent:1rem;
background-color:#ccc;
display:inline-flex;
flex:1 1 20%;
font-size:2rem;
align-self:stretch;
align-items:center;
text-decoration:none;
transition:box-shadow 1s;
}
a+a{
border-left:1px solid red;
}
a:hover{
box-shadow:inset 0 -3px 0 #CC3232;
}
1)a+a选择器,让第一个div元素不会被选中
2)box-shadow元素不影响页面布局,但border元素会影响
3)transition实现css3过渡效果