SOURCE

<div class="box">
      <img class="img" src="https://gimg2.baidu.com/image_search/src=http%3A%2F%2Fwww.08lr.cn%2Fuploads%2Fallimg%2F170513%2F1-1F513164126.jpg&refer=http%3A%2F%2Fwww.08lr.cn&app=2002&size=f9999,10000&q=a80&n=0&g=0n&fmt=jpeg?sec=1614342078&t=573647a01fca7ff9b16298550fa909ce" />
</div>

.box{
    position: absolute;
    width: 250px;
    height:300px;
    background:#0000ff;
    /*有下面一行时，层叠上下文元素是box容器，
    没有这一行时层叠上下文元素是根元素*/
    z-index: 0;
}

.img{
    position: relative;
    width:200px;
    margin-left: 150px;
    z-index: -1;
}