A PostCSS plugin to fix an element's dimensions to an aspect ratio.

Explanation

The plugin provides three new properties and one new value type:

The aspect-ratio property makes the height of this element relative to its width, height will be dynamic based on the ratio. aspect-ratio has two aliases you can use instead:
- ratio
- aspect
An aspect-ratio property that includes a value expressed as 'NUM:NUM' (eg. '4:3') will automatically be converted to a percentage (3/4 * 100 = 75%). You must wrap the value in single ' or double " quotes.

The effect is achieved using the quirky behaviour of CSS percentage padding; any element with a percentage value for its padding property will use the width of its container to calculate that percentage. Therefore this plugin requires a specific HTML structure to work. The element you wish to constrain with an aspect ratio and a single inner element that will hold its contents.

<div class="aspect-box">
  <div class="aspect-box__content">
    <!-- Any content you like, very useful for video and image elements. -->
  </div>
</div>

Install

npm install postcss-aspect-ratio --save

Example 1

A simple example using the custom ratio value '16:9'.

/* Input. */
.aspect-box {
  position: relative;
  background: lime;
  aspect-ratio: '16:9';
}
/* Output. */
.aspect-box {
  position: relative;
  background: lime;
  box-sizing: border-box;
}
.aspect-box > * /* This targets .aspect-box__content */ {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  box-sizing: border-box;
}
.aspect-box:before /* This pseudo element uses the padding trick to set the height. */ {
  position: relative;
  display: block;
  content: "";
  padding-top: 56.25%;
  box-sizing: border-box;
}

Example 2

A more complex example using the ratio value calc('4:3' - 20px).

/* Input. */
.aspect-box {
  position: relative;
  background: lime;
  aspect-ratio: calc('4:3' - 20px);
}
/* Output. */
.aspect-box {
  position: relative;
  background: lime;
  box-sizing: border-box;
}
.aspect-box > * {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  box-sizing: border-box;
}
.aspect-box:before {
  position: relative;
  display: block;
  content: "";
  padding-top: calc(75% - 20px);
  box-sizing: border-box;
}