Responsive CSS border radius with the Fab Four technique

Written by Kiril Peyanski ✏️

Browser support for various CSS features has gone a long way in the last couple of years. While vertically centering a div is something we execute fairly easily this way, there are still a couple of crucial features we are missing.

You are probably already familiar with media-queries, which apply styling based on the browser viewport or the screen resolution, but sometimes we might need container-queries, which are still only available under feature flags for some browsers.

Today we will implement the Fab Four technique to apply a border-radius to an element depending on its container dimension, rather than the screen width.

Author’s note: I have recently stumbled upon this technique on Twitter where Frank Yan (a Facebook engineer) further explained its implication. In the thread it was suggested that this piece of code should not be “human-written,” so I’ve got you covered by the end of this blog post.

What does this technique do?

The Fab Four technique can be implemented through the usage of various CSS functions like min, max, calc, and clamp to calculate if a specific CSS rule should be applied, while comparing it to the dimension of the container element instead of the device viewport.

Let's take a look at the following demo with a couple of card components wrapped in a resizable container:

See the Pen Untitled by Kiril Peyanski (@kspeyanski) on CodePen.

The specific requirement we want to implement here is to remove the border-radius of the cards whenever their container is less than 400px wide.

Author’s note: The CSS equivalent to debugging with console.log is border: 1px solid red, and you should not feel bad about using it!

Taking a deeper look at our code, we can observe the following border-radius calculation:

.dynamic-card {
  border-radius: max(0px, min(16px, (100% - 400px + 1px) * 9999)) / 16px;
}

It might look strange at first sight, but we will go over each calculation and explain it further. We want to identify the following variables in the code:

• Desired border-radius
• Container width breakpoint

.dynamic-card {
  border-radius: max(0px, min(DESIRED_BORDER_RADIUS, (100% - WIDTH_BREAKPOINT + 1px) * 9999)) / DESIRED_BORDER_RADIUS;
}

In order to deep-dive into the above CSS rule, we would have to split it into three different calculations:

• The min calculation
• The max calculation
• The division by pixels

The min calculation

  min(DESIRED_BORDER_RADIUS, (100% - WIDTH_BREAKPOINT + 1px) * 9999)

What is happening here? Well, first we have to think of what the 100% is doing. Setting width: 100% to a CSS element will (in most cases) expand the element to its full container width, meaning that when the 100% is used in a min function, it will always return the width of our container.

This means that if our container is 480px wide and we've defined a WIDTH_BREAKPOINT of 400px, the calculation would be:

  /* 480px - 400px + 1px = 81px * 9999 = a really big POSITIVE number */
  min(DESIRED_BORDER_RADIUS, (480px - 400px + 1px) * 9999)

The * 9999 part of the calculation is just to be sure we're always either way above the max value or way below the min value. Otherwise, we might end up with something in between.

Take a look at the alternative scenario where the container width is less than our predefined breakpoint:

  /* 320px - 400px + 1px = -79px * 9999 = a really big NEGATIVE number */
  min(DESIRED_BORDER_RADIUS, (320px - 400px + 1px) * 9999)

Since we have a min function, we have the following result from the calculation:

• When the container width is above the breakpoint, the final result would be DESIRED_BORDER_RADIUS (or 16px in our example)
• When the container width is below the breakpoint, we have a negative number (or -789,921 in our example)

The max calculation This is the next function that would be called in our complex set of calculations:

  max(0px, RESULT_FROM_MIN)

We already know that we would receive either a positive border radius or a negative number. By applying the result from the min calculation, we get the following:

• When the container width is above the breakpoint: RESULT_FROM_MIN (or 16px in our case)
• When the container width is below the breakpoint: 0px

The / DESIRED_BORDER_RADIUS calculation

  border-radius: RESULT_FROM_MAX / DESIRED_BORDER_RADIUS;

This is not a division operator, but an extended syntax to apply border-radius to an element. The code above will be evaluated (notice the missing / character):

  border-top-left-radius:     RESULT_FROM_MAX DESIRED_BORDER_RADIUS;
  border-top-right-radius:    RESULT_FROM_MAX DESIRED_BORDER_RADIUS;
  border-bottom-right-radius: RESULT_FROM_MAX DESIRED_BORDER_RADIUS;
  border-bottom-left-radius:  RESULT_FROM_MAX DESIRED_BORDER_RADIUS;

This is important, as the CSS parsing engine is not particularly happy with how we try to define the border radius with the min and max functions, so we use this neat little trick.

We can use the clamp function as an alternative, but we avoid doing so since it's not supported in Safari 12.

Comparison to media-queries

This technique relies on our CSS parsing engine to account for the 100% in the calculation. This percentage would evaluate to the width of our container as opposed to the screen width, which we can use in media-queries.

The following code would work in most scenarios, but would fail when we have a resizable container as in the example above:

.dynamic-card {
    border-radius: 0;
}
@media screen and (min-width: 400px) {
  .dynamic-card {
    border-radius: 16px;
  }
}

Reversing the logic

The Fab Four technique could be applied in multiple scenarios depending on the application requirements. Sometimes a specific rule should be applied whenever the container width is less than a specific breakpoint. In such cases, we can invert the logic:

From:

(100% - WIDTH_BREAKPOINT + 1px) * 9999))

To:

(WIDTH_BREAKPOINT - 1px - 100%) * 9999))

This would apply the desired value only when the width is below the breakpoint.

Utilities

Let's face the fact that this calculation is not straightforward and hard to read, even when you are familiar with the technique. Luckily for us, we can extract SASS mixins, or utility functions for various CSS-in-JS libraries (like styled-components).

Native CSS with CSS variables

  .dynamic-card {
    --border-radius: 16px;
    --breakpoint: 400px;
  }
  .dynamic-border {
    border-radius: max(0px, min(var(--border-radius), (100% - var(--breakpoint) + 1px) * 9999)) / var(--border-radius);
  }

SASS mixin

The following SASS mixin helps us implement the Fab Four technique in SASS based projects:

@mixin dynamic-border-radius($value, $breakpoint) {
  & {
    border-radius: #{"max(0px, min(#{$value}, 100% - #{$breakpoint} + 1px) * 9999) / #{$value}"};
  }
}
.dynamic-card {
  @include dynamic-border-radius(16px, 400px);
}

CSS-in-JS

import styled, { css } from 'styled-components';
const dynamicBorderRadius = (value, breakpoint) => css`
  border-radius: max(0px, min(${value}px, 100% - ${breakpoint}px + 1px) * 9999) /
    ${value}px;
`;
const Card = styled('div')`
    ${dynamicBorderRadius(16, 400)};
`;

Conclusion

With the web moving forward, we will be seeing more features like container-queries implemented in most web browsers. Until this day comes, we can use techniques like the Fab Four to implement specific application requirements, which don’t always look straightforward.

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