input placeholder 文字提示样式

<div class="form-item">
  <input type="text" placeholder="请输入用户名">
  <span>中文、6~13个字符span>
div>

<br>

<div class="form-item2">
  <input type="text" placeholder="请输入用户名">
  <span>中文、6~13个字符span>
div>
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<style lang="less">
 .form-item {
      position: relative;
      display: inline-block;
      padding: 15px 0;
      font-size: 12px;

      input:placeholder-shown + span {
          opacity: 0;
      }
      input:focus + span {
          position: absolute;
          right: 0;
          top: 0;
          opacity: 1;
          transition: opacity .2s;
      }
  }


  .form-item2 {
      position: relative;
      display: inline-block;
      font-size: 12px;

      input {
          padding-top: 15px;

          &::placeholder {
              position: absolute;
              left: 0;
              top: 50%;
              transform: translateY(-50%);
          }

          & + span {
              position: absolute;
              left: 0;
              top: 50%;
              transform: translateY(-50%);
              visibility: hidden;
          }
      }

      input:focus {
          &::placeholder {
              visibility: hidden;
          }

          & + span {
              top: 0;
              transform: translateY(0);
              transition: all .2s;
              visibility: visible;
          }
      }
  }
style>

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