css怎么设置背景图片的透明度

div {

  width: 200px;

  height: 200px;

  display: block;

  position: relative;

}

div::after {

  content: "";

  background: url(image.jpg);

  opacity: 0.5;

  top: 0;

  left: 0;

  bottom: 0;

  right: 0;

  position: absolute;

  z-index: -1;  

}