SOURCE

<div class="box">
    <div class="filter-box"></div>
    <div class="content-box">
        <div class="txt">这是一行字</div>
    </div>
</div>

.box{
  position: fixed;
  background: #f1f1f1;
  height: 100vh;
  width: 100vw;
}  
.filter-box{
        position: absolute;
        left: 0;
        top: 0;
        z-index: 2;
        width: 100%;
        height: 100%;
        pointer-events: none;
    }
    .content-box{
        position: absolute;
        left: 0;
        top: 0;
        right: 0;
        bottom: 0;
        z-index: 1;
        margin: auto;
        width: 500px;
        height: 500px;
      background:yellow;
    }
    .filter-box::before,
    .filter-box::after,
    .content-box::before,
    .content-box::after{
        position: absolute;
        content: '';
        opacity: 0.5;
        background-color: #fff;
        z-index: 2;
        pointer-events: none;
    }
.txt{position:relative;top: -10px;}