css 模拟material文本框

<div class="form-item">
   <input required id="username" type="text">
   <span class="bar"></span>
    <label for="username">用户名</label>
</div>

.form-item {
  margin: 100px auto;
  position: relative;
  width: 300px;
  text-align: center;
}
input {
  width: 100%;
  padding: 10px 0;
  font-size: 16px;
  border: none;
  border-bottom: 1px solid #ccc;
  background: transparent;
  outline: none;
  transition: border-color 0.3s ease;
}
label {
  position: absolute;
  top: 50%;
  left: 0;
  transform: translateY(-50%);
  pointer-events: none;
  transition: all 0.3s ease;
  color: #999;
}
.bar {
  position: absolute;
  bottom: -1px;
  height: 2px;
  width: 0;
  left: 50%;
  transform: translateX(-50%);
  background: #0a53be;
  transition:  0.4s ease;
}
input:focus ~ .bar{
  width: 100%;
}
input:focus ~ label{
  color: #0a53be;
  transform: translateY(-40px);
  font-size: 18px;
}
input:valid ~ label{
  color: #0a53be;
  transform: translateY(-40px);
  font-size: 18px;
}