<div class="main">
  <div class="left">left</div>
  <div class="right">right</div>
</div>
.main {
  padding: 10px;
  width: 1000px;
  background: #aaa;
}

.left {
  float: left;
  width: 600px;
  height: 200px;
  background: #6FB0CC;
}

.right {
  float: left;
  width: 300px;
  height: 200px;
  background: #ABD16C;
}

/*これを追加*/
.main:after {
  content: '';
  display: table;
  clear: both;
}
Rerun