<!DOCTYPE html>
<style>
.container {
  width: 150px;
  border: solid 2px;
}

.left {
  float: left;
  background: green;
  width: 100px;
  height: 100px;
}

.right {
  float: right;
  background: green;
  width: 100px;
  height: 100px;
}
</style>

<div class="container">
  <div class="left"></div>
  <div class="right"></div>
</div>
