<style>
.left-box {
  background: red;
  float: left;
}
.right-box {
  background: blue;
  float: right;
}
</style>
<div class='left-box'>왼쪽</div>
<div class='right-box'>오른쪽</div>

<style>
.left-box {
  background: red;
  float: left;
}
.right-box {
  background: yellow;
  float: right;
}
</style>
<div class='left-box'>왼쪽1</div>
<div class='left-box'>왼쪽2</div>
<div class='right-box'>오른쪽2</div>
<div class='right-box'>오른쪽1</div>

<style>
.left-box {
  position: absolute;
  top: 0; left: 0;
  background: red;
}
.right-box {
  position: absolute;
  top: 0; right: 0;
  background: blue;
}
</style>
<div class='left-box'>왼쪽</div>
<div class='right-box'>오른쪽</div>