Bram.us

Old vs. New. Image by @una.

Intro

Back in May 2020 I was very delighted to read that the first Working Draft of the CSS Box Sizing Module Level 4 got published, as it featured an addition to CSS that I’ve been wanting for a long time now: native support for aspect ratio boxes through the new aspect-ratio CSS property.

With Chromium 89 (current Canary) and Firefox 85 (current Nightly) already supporting aspect-ratio unflagged, it’s time to start playing with this new addition and start thinking about dropping all those nasty hacks to mimic aspect ratios in CSS. Let’s take a look …

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Welcome aspect-ratio

In short, the aspect-ratio property allows you to define a preferred aspect ratio on elements:

.box {
  width: 20vw;
  aspect-ratio: 16 / 9;
}

[CodePen Demo]

In the example above the .box will have a preferred aspect ratio of 16:9. Since its width is set to 20vw, the resulting height will be 20vw / 16 * 9 = 11.25vw. Easy, right?

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Allowed values for aspect-ratio

The value as set in the example above for aspect-ratio is of the <ratio> type:

• It typically consists of two numbers separated by a /. The first parameter targets the width and the second one the height.
• It’s also allowed to pass in just a single number. In that case the second number will be we considered to be 1. E.g. a <ratio> of 2 will translate to 2 / 1.
• Passing in a 0 for either of the numbers is not allowed.
• The spaces around the / are not required, so 2/1 is also a valid <ratio> value.

Another allowed value for the aspect-ratio property — which also is the default — is auto. This indicates that the box has no preferred aspect ratio and should size itself as it normally would.

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The fine print

Aspect of what?

Depending upon which of width or height you set, the box dimensions will be calculated against that.

.box {
  width: 20vw;
  aspect-ratio: 16 / 9; /* Dimensions will be calculated against the width,
                           yielding a height of 11.25vw (20vw / 16 * 9) */
}

.box {
  height: 20vw;
  aspect-ratio: 16 / 9; /* Dimensions will be calculated against the height,
                           yielding a width of 35.55vw (20vw / 9 * 16) */
}

aspect-ratio+width+height = 🚫

Setting an aspect-ratio won’t have effect on elements that have both a CSS width and CSS height set to a value other than auto. Only one of width or height can be explicitly set, and the other should remain set to auto.

.box {
  width: 20vw;
  height: 20vw;
  aspect-ratio: 16 / 9; /* won't have any effect! */
}

aspect-ratio + percentage based width/height

In case one of the width and height should be set to a percentage based value such as 100%, the targeted box will take a look at the direct parent element’s dimensions to define its value upon that.

.parent {
  height: 100px;
}
.parent .box {
  height: 100%;
  aspect-ratio: 1 / 1; /* .box will be 100px by 100px */
}

There’s some more edge cases here too, but let’s not get too deep into the spec 😉

aspect-ratio sets a preferred aspect ratio

Setting an aspect-ratio will tell the browser that this is a preferred aspect ratio. Should the content of the box be larger, then the box will simply grow.

div {
  aspect-ratio: 1/1;
  /* 'width' and 'height' both default to 'auto' */
}
+----------+  +----------+  +----------+
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~ |
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~ |
| ~~~~~~~  |  | ~~~~~~~~ |  | ~~~~~~~~ |
|          |  | ~~~      |  | ~~~~~~~~ |
+----------+  +----------+  | ~~~~~~~~ |
                            | ~~~~~~   |
                            +----------+

To maintain the aspect-ratio, you can set overflow to auto so that a scrollbar will be shown should the contents be larger:

div {
  overflow: auto;
  aspect-ratio: 1/1;
}
+----------+  +----------+  +----------+
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~^|
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~ |
| ~~~~~~~  |  | ~~~~~~~~ |  | ~~~~~~~~ |
|          |  | ~~~      |  | ~~~~~~~~v|
+----------+  +----------+  +----------+

What also works, is setting min-height

Overriding the min-height property also maintains the 1:1 aspect ratio, but will result in content overflowing the box if it is not otherwise handled.

div {
  aspect-ratio: 1/1;
  min-height: 0;
}
+----------+  +----------+  +----------+
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~ |
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~ |
| ~~~~~~~  |  | ~~~~~~~~ |  | ~~~~~~~~ |
|          |  | ~~~      |  | ~~~~~~~~ |
+----------+  +----------+  +-~~~~~~~~-+
                              ~~~~~~    

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Demos

Using aspect-ratio with a fallback for older browsers

Thanks to the powerful @supports it’s possible to add a fallback for browsers that don’t support aspect-ratio. In the demo below (based upon this demo by Una) a fallback using the padding-top hack is applied:

[CodePen Demo]

Using aspect-ratio with CSS Variables

By introducing a CSS Variable CSS Custom Property it’s possible to make your code more generic and extract away a .aspect-ratio class.

[CodePen Demo]

To use it, add apply the .aspect-ratio on the element you want, and pass in a --aspect-ratio CSS Custom Property:

<div
  class="aspect-ratio"
  style="--aspect-ratio: 16/9;"
>I am an aspect ratio box</div>

The code is written so that it will use the value for --aspect-ratio in both the fallback and the modern version.

Automatically setting aspect-ratio on iframes and the like

When you embed an iframe you most likely set its width and height HTML attribute.

<iframe
  src="https://www.youtube.com/embed/e7BkmF8CJpQ"
  width="560"
  height="315"></iframe>

It’s possible to use the values of these attributes to automatically set the aspect-ratio.

iframe[width][height] {
  aspect-ratio: attr(width) / attr(height);
}

Heck, you could even target [width][height] if you’d want!

[CodePen Demo]

<iframe
  src="https://www.youtube.com/embed/e7BkmF8CJpQ"
  width="560"
  height="315"
  style="--aspect-ratio: 560 / 315"
></iframe>

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In Closing

After 8 years of wanting this feature to land in CSS (ref) I’m very happy to see this addition make it into the spec. It’s still a Working Draft right now, but that doesn’t stop me from being excited about it already. I hope you are too 🙂