SOURCE

<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;
}
.clear:after{
  display:block;
  content:"";
  clear:both;
  border:2px solid blue;
}