css实现Material文本框

 * {
    margin: 0;
    padding: 0;
  }
 
  html,
  body {
    width: 100%;
    height: 100%;
    position: relative;
  }
 
  .form-item {
    width: 300px;
    height: 40px;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    border-bottom: 1px solid #898989;
  }
 
  #userName {
    width: 100%;
    height: 100%;
    border: none;
    outline: none;
  }
 
  .bar {
    display: block;
    margin: 0 auto;
    width: 0;
    height: 2px;
    background-color: #5264ae;
    transition: 0.4s ease;
  }
 
  label {
    font-size: 26px;
    position: absolute;
    top: -7px;
    left: 0;
    transition: 0.4s ease;
  }
 
  .form-item input:focus~.bar {
    width: 100%;
  }
 
  .form-item input:valid~label {
    color: #5264ae;
    transform: translateY(-30px);
  }
 
  .form-item input:focus~label {
    color: #5264ae;
    transform: translateY(-30px);
  }

    <div class="form-item">
        <input type="text" required id="userName">
        <span class="bar"></span>
        <label for="userName"> User Name</label>
    </div>