<div class="header">Header</div>
<div class="content">Content</div>
html, body {
  height: 100%;
}
body {
  display: flex;
  flex-direction: column;
}
.header {
  background-color: yellow;
}
.content {
  background: lightgreen; 
  flex-grow: 1;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.