Take advantage of available viewport space by giving the last element the remaining available space in current viewport, even when resizing the window.

<div class="container">
  <div>Div 1</div>
  <div>Div 2</div>
  <div>Div 3</div>
</div>

html,
body {
  height: 100%;
  margin: 0;
}

.container {
  height: 100%;
  display: flex;
  flex-direction: column;
}

.container > div:last-child {
  background-color: tomato;
  flex: 1;
}

Explanation

1. height: 100% set the height of container as viewport height.
2. display: flex enables flexbox.
3. flex-direction: column set the direction of flex items’ order from top to down.
4. flex-grow: 1 the flexbox will apply remaining available space of container to last child element.

• The parent must have a viewport height. flex-grow: 1 could be applied to the first or second element, which will have all available space.

Browser support

⚠️ Needs prefixes for full support.

• https://caniuse.com/#feat=flexbox