<!DOCTYPE html>
<style>
.container {
  border: solid 2px;
  display: flow-root;
  width: 300px;
}

.child {
  background: green;
  border: solid 2px;
  box-sizing: border-box;
  float: left;
  height: 100px;
}
</style>

<div class="container">
  <div class="child" style="width: 100px;"></div>
  <div class="child" style="width: 200px;"></div>
</div>
