A CodePen by Anonymous.

index.html

<div id="main">
  <div id="sidebar">
    <div id="right"></div>
    
    <div id="left"></div>
  </div>
</div>

style.css

html, body {
  height: 100%; 
}

#main {
  height: 100%;
  background-color: #f0f0f0;
  position: relative;
}

#sidebar {
  position: relative;
  background-color: #111;
  margin-left: 200px;
  height: 100%;
}

#right, #left {
  float: right; 
}

#right {
  width: 300px;
  background-color: #ff0000;
  height: 100%;
  position: relative;
}

#left {
  background-color: #fff000;
  margin: 20px;
  height: 40px;
  width: 100%;
  right: 300px;
}