Sometimes I want to set the value of a CSS property to that of a different property, even if I don’t know what that value is, and even if it changes later. Unfortunately though, that’s not possible (at least, there isn’t a CSS function that specifically does that).

In my opinion, it’d be super useful to have something like this (for interpolation, maybe you’d throw calc-size() in there as well):

/* Totally hypothetical */
button {
  border-radius: compute(height, self);
  border-radius: compute(height, inherit);
  border-radius: compute(height, #this);
}

In 2021, Lea Verou explained why, despite being proposed numerous times, implementing such a general-purpose CSS function like this isn’t feasible. Having said that, I do remain hopeful, because things are always evolving and the CSSWG process isn’t always linear.

In the meantime, even though there isn’t a CSS function that enables us to get the value of a different property, you might be able to achieve your outcome using a different method, and those methods are what we’re going to look at today.

The fool-proof CSS custom properties method

We can easily get the value of a different CSS property using custom properties, but we’d need to know what the value is in order to declare the custom property to begin with. This isn’t ideal, but it does enable us to achieve some outcomes.

Let’s jump back to the example from the intro where we try to set the border-radius based on the height, only this time we know what the height is and we store it as a CSS custom property for reusability, and so we’re able to achieve our outcome:

button {
  --button-height: 3rem;
  height: var(--button-height);
  border-radius: calc(var(--button-height) * 0.3);
}

We can even place that --button-height custom property higher up in the CSS cascade to make it available to more containment contexts.

:root {
  /* Declare here to use anywhere */
  --button-height: 3rem;

  header {
    --header-padding: 1rem;
    padding: var(--header-padding);
  
    /* Height is unknown (but we can calculate it) */
    --header-height: calc(var(--button-height) + (var(--header-padding) * 2));
  
    /* Which means we can calculate this, too */
    border-radius: calc(var(--header-height) * 0.3);
  
    button {
      /* As well as these, of course */
      height: var(--button-height);
      border-radius: calc(var(--button-height) * 0.3);

      /* Oh, what the heck */
      padding-inline: calc(var(--button-height) * 0.5);
    }
  }
}

I guess when my math teacher said that I’d need algebra one day. She wasn’t lying!

The unsupported inherit() CSS function method

The inherit() CSS function, which isn’t currently supported by any web browser, will enable us to get the value of a parent’s property. Think: the inherit keyword, except that we can get the value of any parent property and even modify it using value functions such as calc(). The latest draft of the CSS Values and Units Module Level 5 spec defines how this’d work for custom properties, which wouldn’t really enable us to do anything that we can’t already do (as demonstrated in the previous example), but the hope is that it’d work for all CSS properties further down the line so that we wouldn’t need to use custom properties (which is just a tad longer):

header {
  height: 3rem;

  button {
    height: 100%;

    /* Get height of parent but use it here */
    border-radius: calc(inherit(height) * 0.3);
    padding-inline: calc(inherit(height) * 0.5);
  }
}

There is one difference between this and the custom properties approach, though. This method depends on the fixed height of the parent, whereas with the custom properties method either the parent or the child can have the fixed height.

This means that inherit() wouldn’t interpolate values. For example, an auto value that computes to 3rem would still be inherited as auto, which might compute to something else when inherit()-ed., Sometimes that’d be fine, but other times it’d be an issue. Personally, I’m hoping that interpolation becomes a possibility at some point, making it far more useful than the custom properties method.

Until then, there are some other (mostly property-specific) options.

The aspect-ratio CSS property

Using the aspect-ratio CSS property, we can set the height relative to the width, and vice-versa. For example:

div {
  width: 30rem;

  /* height will be half of the width */
  aspect-ratio: 2 / 1;

  /* Same thing */
  aspect-ratio: 3 / 1.5;

  /* Same thing */
  aspect-ratio: 10 / 5;

  /* width and height will be the same */
  aspect-ratio: 1 / 1;
}

Technically we don’t “get” the width or the height, but we do get to set one based on the other, which is the important thing (and since it’s a ratio, you don’t need to know the actual value — or unit — of either).

The currentColor CSS keyword

The currentColor CSS keyword resolves to the computed value of the color property. Its data type is <color>, so we can use it in place of any <color> on any property on the same element. For example, if we set the color to red (or something that resolves to red), or if the color is computed as red via inheritance, we could then declare border-color: currentColor to make the border red too:

body {
  /* We can set color here (and let it be inherited) */
  color: red;

  button {
    /* Or set it here */
    color: red;

    /* And then use currentColor here */
    border-color: currentColor;
    border: 0.0625rem solid currentColor;
    background: hsl(from currentColor h s 90);
  }
}

This enables us to reuse the color without having to set up custom properties, and of course if the value of color changes, currentColor will automatically update to match it.

While this isn’t the same thing as being able to get the color of literally anything, it’s still pretty useful. Actually, if something akin to compute(background-color) just isn’t possible, I’d be happy with more CSS keywords like currentColor.

In fact, currentBackgroundColor/currentBackground has already been proposed. Using currentBackgroundColor for example, we could set the border color to be slightly darker than the background color (border-color: hsl(from currentBackgroundColor h s calc(l - 30))), or mix the background color with another color and then use that as the border color (border-color: color-mix(currentBackgroundColor, black, 30)).

But why stop there? Why not currentWidth, currentHeight, and so on?

The from-font CSS keyword

The from-font CSS keyword is exclusive to the text-decoration-thickness property, which can be used to set the thickness of underlines. If you’ve ever hated the fact that underlines are always 1px regardless of the font-size and font-weight, then text-decoration-thickness can fix that.

The from-font keyword doesn’t generate a value though — it’s optionally provided by the font maker and embedded into the font file, so you might not like the value that they provide, if they provide one at all. If they don’t, auto will be used as a fallback, which web browsers resolve to 1px. This is fine if you aren’t picky, but it’s nonetheless unreliable (and obviously quite niche).

We can, however, specify a percentage value instead, which will ensure that the thickness is relative to the font-size. So, if text-decoration-thickness: from-font just isn’t cutting it, then we have that as a backup (something between 8% and 12% should do it).

Don’t underestimate CSS units

You probably already know about vw and vh units (viewport width and viewport height units). These represent a percentage of the viewport’s width and height respectively, so 1vw for example would be 1% of the viewport’s width. These units can be useful by themselves or within a calc() function, and used within any property that accepts a <length> unit.

However, there are plenty of other, lesser-known units that can be useful in a similar way:

• 1ex: equal to the computed x-height
• 1cap: equal to the computed cap height
• 1ch: equal to the computed width of the 0 glyph
• 1lh: equal to the computed line-height (as long as you’re not trimming or adding to its content box, for example using text-box or padding, respectively, lh units could be used to determine the height of a box that has a fixed number of lines)

And again, you can use them, their logical variants (e.g., vi and vb), and their root variants (e.g., rex and rcap) within any property that accepts a <length> unit.

In addition, if you’re using container size queries, you’re also free to use the following container query units within the containment contexts:

• 1cqw: equal to 1% of the container’s computed width
• 1cqh: equal to 1% of the container’s computed height
• 1cqi: equal to 1% of the container’s computed inline size
• 1cqb: equal to 1% of the container’s computed block size
• 1cqmin: equal to 1cqi or 1cqb, whichever is smallest
• 1cqmax: equal to 1cqi or 1cqb, whichever is largest

That inherit() example from earlier, you know, the one that isn’t currently supported by any web browser? Here’s the same thing but with container size queries:

header {
  height: 3rem;
  container: header / size;

  @container header (width) {
    button {
      height: 100%;
      border-radius: calc(100cqh * 0.3);
      padding-inline: calc(100cqh * 0.5);
    }
  }
}

Or, since we’re talking about a container and its direct child, we can use the following shorter version that doesn’t create and query a named container (we don’t need to query the container anyway, since all we’re doing is stealing its units!):

header {
  height: 3rem;
  container-type: size;

  button {
    height: 100%;
    border-radius: calc(100cqh * 0.3);
    padding-inline: calc(100cqh * 0.5);
  }
}

However, keep in mind that inherit() would enable us to inherit anything, whereas container size queries only enable us to inherit sizes. Also, container size queries don’t work with inline containers (that’s why this version of the container is horizontally stretched), so they can’t solve every problem anyway.

In a nutshell

I’m just going to throw compute() out there again, because I think it’d be a really great way to get the values of other CSS properties:

button {
  /* self could be the default */
  border-radius: compute(height, self);
  /* inherit could work like inherit() */
  border-radius: compute(height, inherit);
  /* Nice to have, but not as important */
  border-radius: compute(height, #this);
}

But if it’s just not possible, I really like the idea of introducing more currentColor-like keywords. With the exception of keywords like from-font where the font maker provides the value (or not, sigh), keywords such as currentWidth and currentHeight would be incredibly useful. They’d make CSS easier to read, and we wouldn’t have to create as many custom properties.

In the meantime though, custom properties, aspect-ratio, and certain CSS units can help us in the right circumstances, not to mention that we’ll be getting inherit() in the future. These are heavily geared towards getting widths and heights, which is fine because that’s undoubtedly the biggest problem here, but hopefully there are more CSS features on the horizon that allow values to be used in more places.

On Inheriting and Sharing Property Values originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.