Edit in JSFiddle

<div class="content-area">
<main>
  <h3>My blog post</h3>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec viverra nec nulla vitae mollis.</p>
  <p>etc.</p>
</main>

<aside>
  <h3>About the author</h3>
  <p>Frontend Designer from Bordeaux, currently working for Improbable in sunny London.</p>
</aside>
</div>
.content-area{
  width: 100%;
  height: 300px;
}
.content-area main{
  width:50%;
  float:right;
}
.content-area aside{
  width:50%;
  float:left;
 
}