Simple implementation of fixed aspect ratio in pure CSS

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CSS Aspect Ratio
This package provides a simple implementation of fixed aspect ratio in pure CSS. It requires support for CSS Custom Properties.
Modular implementation approach adapted from https://sgom.es/posts/2017-02-17-css-custom-properties-as-your-api/
CSS-only aspect ratio trick adapted from https://alistapart.com/article/creating-intrinsic-ratios-for-video

Rationale / background

A common problem is how to avoid content shifting down as images load. This is usually solved by adding the height and width of an image in HTML, so that the browser can allocate enough space for the content ahead of time.
<img src="kitten.jpg" height="1024" width="768" alt="A cute kitten">

However, if you want to adjust the dimensions for the image in CSS (say, by applying a max-width to the image), you lose the ability to maintain the aspect ratio. Instead, the width and height get applied separately, and the image gets deformed.
This package allows you to specify the width and have the height calculated automatically, ensuring that the aspect ratio is maintained even if the image is resized.
It's also generic enough to be applied to any block, not just images.


npm i --save-dev css-aspect-ratio



In Sass, first make sure node_modules is in your includePaths. Then:
@import "css-aspect-ratio/css-aspect-ratio";


In Less, install less-plugin-npm-import and do:
@import (inline) "npm://css-aspect-ratio/css-aspect-ratio.css";

Plain CSS

In plain CSS, make sure you use a bundler or build tool to copy dependencies. To import:
@import /path/to/css-aspect-ratio.css;
You can also use unpkg as a CDN:
@import https://unpkg.com/css-aspect-ratio@1/css-aspect-ratio.css;


<div class="aspect-ratio"
 style="width: 768px; --aspect-ratio-w: 4; --aspect-ratio-h: 3;">

  <img src="kitten.jpg" alt="A cute kitten">

aspect-ratio takes two custom properties:
  • --aspect-ratio-w defines the width portion of the aspect ratio, e.g. 16 in 16:9.
  • --aspect-ratio-h defines the height portion of the aspect ratio, e.g. 9 in 16:9.

Progressive enhancement

CSS Aspect Ratio works as progressive enhancement, leaving the resizing entirely to the browser if custom properties are not supported. This means that for things like images, aspect ratio will be preserved, but vertical space will not be allocated ahead of time, resulting in content shifting down during load.