Responsive design: centralize a full-screen square in any screen size

Question

The problem is simple:

• The main part of my page is a square that should be shown at the center of the screen in all screen sizes and orientations
• The square should use the screen efficiently and be as big as possible without the need to scroll
• No scrollbars, no fixed sizing, no overflow-hidden, no Javascript.
• Flexbox is encouraged.

This is how the page should look like in a landscape or portrait browser:

Here is a CodePen as a starting point.

<div class="body">
  <div class="square">
    Square
  </div>
</div>

Answer 1

Here's my attempt to achieve the end goal.

The key point is to use vmin viewport percentage length for both width and height properties of the square box:

Example Here

.body, .square {
  display: flex;
  align-items: center;
  justify-content: center;
}

.body {
  min-height: 100vh;
}

.square {
  width: 100vmin;
  height: 100vmin;
}

(vendor prefixes omitted for brevity, check the "Compiled View" in the demo).