SOURCE

<div class="input-group">
  <div class="input">
    <input type="input" placeholder="账号" />
    <span class="input-underline"></span>  
  </div>
</div>

.input-group {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}
.input {
  position: relative;
}
.input > input {
  box-sizing: border-box;
  border: none;
  height: 30px;
  background-color:transparent;
  border-bottom: 1px solid #e6e6e6;
  outline: 0;
}
.input span {
  display: inline-block;
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 1px;
  border-bottom: 1px solid #31b794;
  transition: all 300ms linear;
  transform: scale(0);
}
.input input:focus + span {
  transform: scale(1);
}