<!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>