清除浮动

<div class="content clear">
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
</div>

方法一: bfc

.clear {
  overflow: hidden;
}

方法二: 伪元素

.clear::after {
  content: "";
  display: block;
  clear: both;
}