6.背景模糊

<div class="box" style="position: relative;">
   <div class="box bg-blur"></div>
   <div class="msg">mesggg</div>
</div>
  .bg-blur {
    width: 100%;
    height: 240px;
    background-repeat:no-repeat;
    background-size:cover;
    -webkit-filter:blur(3px);
    -moz-filter:blur(3px);
    -o-filter:blur(3px);
    -ms-filter:blur(3px);
    filter:blur(3px);
    position:absolute;
    left:0;
    top:0;
    z-index: -20;
}