css 图片等宽等高

html

<div class="autoimg">
    <img src="xxx.jpg" />
</div>

css

.autoimg {
  position: relative;
  width: 100%;
  height: 0;
  padding-bottom: 100%;
  overflow: hidden;
}
.autoimg img {
    position:absolute;
    top: 0;
    left: 0;
    width: 100%;
}