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CSS Variables + calc() + rgb() = Enforcing High Contrast Colors

As you may know, the recent updates and additions to CSS are extremely powerful. From Flexbox to Grid, and — what we’re concerned about here — Custom Properties (aka CSS variables), all of which make robust and dynamic layouts and interfaces easier than ever while opening up many other possibilities we used to only dream of.

The other day, I was thinking that there must be a way to use Custom Properties to color an element's background while maintaining a contrast with the foreground color that is high enough (using either white or black) to pass WCAG AA accessibility standards.

It’s astonishingly efficient to do this in JavaScript with a few lines of code:

var rgb = [255, 0, 0];

function setForegroundColor() {
  var sum = Math.round(((parseInt(rgb[0]) * 299) + (parseInt(rgb[1]) * 587) + (parseInt(rgb[2]) * 114)) / 1000);
  return (sum > 128) ? 'black' : 'white';
}

This takes the red, green and blue (RGB) values of an element’s background color, multiplies them by some special numbers (299, 587, and 144, respectively), adds them together, then divides the total by 1,000. When that sum is greater than 128, it will return black; otherwise, we’ll get white. Not too bad.

The only problem is, when it comes to recreating this in CSS, we don't have access to a native if statement to evaluate the sum. So,how can we replicate this in CSS without one?

Luckily, like HTML, CSS can be very forgiving. If we pass a value greater than 255 into the RGB function, it will get capped at 255. Same goes for numbers lower than 0. Even negative integers will get capped at 0. So, instead of testing whether our sum is greater or less than 128, we subtract 128 from our sum, giving us either a positive or negative integer. Then, if we multiply it by a large negative value (e.g. -1,000), we end up with either very large positive or negative values that we can then pass into the RGB function. Like I said earlier, this will get capped to the browser’s desired values.

Here is an example using CSS variables:

:root {
  --red: 28;
  --green: 150;
  --blue: 130;

  --accessible-color: calc(
    (
      (
        (
          (var(--red) * 299) +
          (var(--green) * 587) +
          (var(--blue) * 114)
        ) / 1000
      ) - 128
    ) * -1000
  );
}

.button {
  color:
    rgb(
      var(--accessible-color),
      var(--accessible-color),
      var(--accessible-color)
    );
  background-color:
    rgb(
      var(--red),
      var(--green),
      var(--blue)
    );
}

If my math is correct (and it's very possible that it's not) we get a total of 16,758, which is much greater than 255. Pass this total into the rgb() function for all three values, and the browser will set the text color to white.

Throw in a few range sliders to adjust the color values, and there you have it: a dynamic UI element that can swap text color based on its background-color while maintaining a passing grade with WCAG AA.

See the Pen
CSS Only Accessible Button
by Josh Bader (@joshbader)
on CodePen.

Putting this concept to practical use

Below is a Pen showing how this technique can be used to theme a user interface. I have duplicated and moved the --accessible-color variable into the specific CSS rules that require it, and to help ensure backgrounds remain accessible based on their foregrounds, I have multiplied the --accessible-color variable by -1 in several places. The colors can be changed by using the controls located at the bottom-right. Click the cog/gear icon to access them.

See the Pen
CSS Variable Accessible UI
by Josh Bader (@joshbader)
on CodePen.

There are other ways to do this

A little while back, Facundo Corradini explained how to do something very similar in this post. He uses a slightly different calculation in combination with the hsl function. He also goes into detail about some of the issues he was having while coming up with the concept:

Some hues get really problematic (particularly yellows and cyans), as they are displayed way brighter than others (e.g. reds and blues) despite having the same lightness value. In consequence, some colors are treated as dark and given white text despite being extremely bright.

What in the name of CSS is going on?

He goes on to mention that Edge wasn’t capping his large numbers, and during my testing, I noticed that sometimes it was working and other times it was not. If anyone can pinpoint why this might be, feel free to share in the comments.

Further, Ana Tudor explains how using filter + mix-blend-mode can help contrast text against more complex backgrounds. And, when I say complex, I mean complex. She even goes so far as to demonstrate how text color can change as pieces of the background color change — pretty awesome!

Also, Robin Rendle explains how to use mix-blend-mode along with pseudo elements to automatically reverse text colors based on their background-color.

So, count this as yet another approach to throw into the mix. It’s incredibly awesome that Custom Properties open up these sorts of possibilities for us while allowing us to solve the same problem in a variety of ways.

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