Edit in JSFiddle

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

<div class="box">
  <div class="float">
    float
  </div>
</div>

.box {
  width: 100px;
  border: solid 1px #ccc;
  margin-bottom: 10px;
}
.float {
  float: left;
  width: 80px;
  height: 50px;
  background-color: #ff0000;
}
.box-clear:after {
  content: '';
  display: block;
  clear: both;
}