Maintain aspect ratio according to width and height

It is possible to fit and center a square div in the viewport and always maintain it's aspect ratio according to width and height?

Requirements :

• only CSS
• the size of the square must adapt to the smallest dimension (width or height) of viewport regardless of the orientation (landscape or portrait) of the viewport.
• the square must be centered horizontaly and verticaly in the viewport

Example:

The aspect-ratio property (2022)

To maintain the aspect ratio of a div according to width and height, you can use the aspect-ratio property (MDN reference).
This allows you to maintain any aspect ratio according to the viewport size or to the size of the parent element.

Maintaining aspect-ratio according to the viewport size (width and height) :

.ar-1-1 {
  aspect-ratio: 1 / 1;
  background: orange;
}

.ar-1-19 {
  aspect-ratio: 16 / 9;
  background: pink;
}

div {
  max-width: 100vw;
  max-height: 100vh;
  margin-bottom: 5vh;
}


/** For the demo **/

body {
  margin: 0;
}

<div class="ar-1-1">Aspect ratio 1:1</div>
<div class="ar-1-19">Aspect ratio 1:19</div>

Maintaining aspect ratyio according to the parent elements size (width and height) :

section {
  width: 50%;
  margin: 0 auto 5vh;
  height:250px;
}
.ar-1-1 {
  aspect-ratio: 1 / 1;
  background: orange;
}

.ar-1-19 {
  aspect-ratio: 16 / 9;
  background: pink;
}

div {
  max-width: 100%;
  max-height: 100%;
}


/** For the demo **/

body {
  margin: 0;
}

<section>
<div class="ar-1-1">Aspect ratio 1:1</div>
</section>
<section>
<div class="ar-1-19">Aspect ratio 1:19</div>
</section>

Previous answer with vmin units

This still works fine but it doesn't allow to maintain the aspect ratio according to a parent element size.

To maintain the aspect ratio of a div according to width and height in the viewport, you can use one HTML tag with:

1. vmin units for the sizing :

   vmin 1/100th of the minimum value between the height and the width of the viewport.
   (source : MDN)

2. position: absolute and margin: auto; for the centering

DEMO (resize both window height and width to see it in action)

Features :

• keeps it's aspect ratio according to width and height
• stays centered in viewport horizontaly and verticaly
• never overflows the viewport

Browser support :

vmin units are supported by IE10+ (canIuse) for IE9 support, you need to use a fallback with vm units instead of vmin like this :

width: 100vm; /* <-- for IE9 */
height: 100vm; /* <-- for IE9 */
width: 100vmin; 
height: 100vmin;

Full code:

body {
  margin:0; /* To prevent scrollbars */
}

div{
  /* Aspect ratio */
  height:100vm; width:100vm; /* IE9 fallback */
  width: 100vmin; 
  height: 100vmin;
  /*Centering */
  position: absolute;
  top:0;bottom:0;
  left:0;right:0;
  margin: auto;
  /* styling */
  background: gold;
}

<div>whatever content you wish</div>