CSS img 宽度固定,高度自适应

<div class="up">
      <img class="bg" src="bg.png"/>
  </div>

.up{
  width: 100%;
  height: 100%;
  position: relative;
  display: flex;
  /*justify-content: center;*/
}
.bg{
  position:absolute;
  top:0;
  left:0;
  right:0;
  width:100%;
  margin:auto;
}