Dynamically Adjusting Text Color for Readability: Introducing the `contrast-color()` Function

Have you ever struggled to choose the right text color for a website because the background color changes dynamically? Picking a color that provides sufficient contrast across a range of backgrounds can be tricky. Fortunately, CSS is evolving to make this easier! This article explores the contrast-color() function, a handy CSS feature currently available in Safari Technology Preview, that automatically selects either black or white text based on the background color, ensuring optimal readability. Let's dive in!

What is the `contrast-color()` Function?

The contrast-color() function in CSS doesn't calculate contrast ratios like you might initially think. Instead, it directly resolves to either black or white, depending on which color provides the best contrast against the specified background color. Think of it as a smart switch: you give it a color, and it gives you back either black or white, whichever is more readable.

Important Note: As of this writing, contrast-color() is primarily available in Safari Technology Preview. Browser support is evolving, so be sure to check compatibility before using it in production.

How Does it Work?

The syntax is straightforward:

color: contrast-color( <color> );

Where <color> is any valid CSS color value – a named color, a hex code, an rgb() value, an hsl() value, etc. The browser then analyzes the provided color and chooses either black or white for the text color.

Let's illustrate with some examples. Imagine we have a div with a dynamically changing background color using JavaScript.

<!DOCTYPE html>
<html>
<head>
<title>Contrast Color Example</title>
<style>
  .container {
    width: 200px;
    height: 100px;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 20px;
  }

  .dynamic-text {
    color: contrast-color(var(--background-color)); /* Use a CSS variable */
  }
</style>
</head>
<body>

<div class="container" id="myContainer">
  <span class="dynamic-text">Hello!</span>
</div>

<button onclick="changeBackgroundColor()">Change Background</button>

<script>
function changeBackgroundColor() {
  const container = document.getElementById("myContainer");
  const colors = ["#f00", "#0f0", "#00f", "#ff0", "#f0f", "#0ff"]; // Some example colors
  const randomColor = colors[Math.floor(Math.random() * colors.length)];

  // Set the background color using a CSS variable
  container.style.setProperty('--background-color', randomColor);
  container.style.backgroundColor = randomColor;
}
</script>

</body>
</html>

In this example:

We have a div element with the class container.
The dynamic-text class applies the contrast-color() function, using a CSS variable --background-color.
JavaScript is used to dynamically change the background color of the container element. We set the background color directly on the element and set the CSS variable. This is important because contrast-color() uses the value of the CSS variable.
The contrast-color() function automatically sets the text color to either black or white, depending on the background color.

In Safari Technology Preview, clicking the "Change Background" button will cycle through different background colors, and the text color will intelligently switch between black and white to maintain readability.

Technical Deep Dive: How Does the Browser Choose?

While the exact algorithm isn't explicitly defined, browsers likely use a perceptual luminance calculation to determine which color provides better contrast. Perceptual luminance takes into account how humans perceive brightness differently across the color spectrum. Green, for instance, is perceived as brighter than blue, even if they have the same numerical luminance value.

A simplified explanation is that the browser probably calculates the relative luminance of the provided color. If the luminance is above a certain threshold, black is chosen; otherwise, white is chosen.

Practical Implications and Use Cases

The contrast-color() function is incredibly useful in several scenarios:

Dynamic Themes: Websites that allow users to switch between light and dark themes can use contrast-color() to automatically adjust text colors, ensuring readability regardless of the chosen theme.
Dynamically Generated Content: If you're creating web applications that generate content with varying background colors (e.g., data visualizations, dashboards), contrast-color() can ensure that text remains legible.
Accessibility: While not a complete accessibility solution, it's a helpful tool for improving readability and ensuring sufficient contrast for users with visual impairments. It's always best practice to test with accessibility tools and guidelines to ensure compliance.
UI/UX Design: Provides a simple and efficient method for designing user interfaces with dynamic color schemes.

Limitations and Considerations

Browser Support: As mentioned earlier, contrast-color() is currently only supported in Safari Technology Preview. Check browser compatibility tables before using it in production.
Limited Color Choices: The function only chooses between black and white. If you need more granular control over text color, you'll need to rely on other techniques, such as custom contrast ratio calculations.
Not a Replacement for Accessibility Testing: While helpful, contrast-color() doesn't guarantee full accessibility. Always test your website with accessibility tools and guidelines to ensure compliance with WCAG standards.

Conclusion

The contrast-color() function offers a simple yet powerful way to dynamically adjust text color for optimal readability. As browser support expands, it will become an invaluable tool for web developers creating dynamic and accessible web applications. While it has its limitations, its ease of use and potential for improving user experience make it a feature worth keeping an eye on. Remember to test its functionality in supported browsers and consider its limitations before implementing it in production.

Inspired by an article from https://css-tricks.com/exploring-the-css-contrast-color-function-a-second-time/

What is the contrast-color() Function?