<div class="parent">
  <div class="child main">Main</div>
  <div class="child side">Side</div>
</div>
* {
  margin: 0;
  padding: 0;
}
body {
  min-width: 500px;
  margin: 2em;
}



.parent {
  display: flex;
}
.child {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 200px;
  color: #fff;
  font-size: 1.4rem;
}
.main {
  flex: 1;
  background: #3498db;
}
.side {
  width: 200px;
  background: #e74c3c;
}

External CSS

  1. https://fonts.googleapis.com/css?family=Open+Sans:400,300

External JavaScript

This Pen doesn't use any external JavaScript resources.