SOURCE

// 父元素高度坍塌，因为box元素设置float后会脱离文档流，同时clear
// 会环绕浮动元素

<div class="box-wrapper">
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="clear">
    clear
  </div>
</div>

.box-wrapper {
  border: 5px solid red;
}
.box-wrapper .box {
  float: left; 
  width: 100px; 
  height: 100px; 
  margin: 20px; 
  background-color: green;
}