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2026-04-187 min read

The var() Function (Web Development)

Learn The var() Function (Web Development) step by step with clear examples and exercises.

Why This Matters

Understanding the var() function in CSS is crucial for any web developer looking to streamline their workflow and write more maintainable code. By using CSS variables, you can store and reuse values throughout your stylesheets, making it easier to maintain consistency across your project and make global changes without having to search through multiple selectors for the same property.

Prerequisites

Before diving into the var() function, you should have a basic understanding of:

  1. HTML (HyperText Markup Language)
  2. CSS (Cascading Style Sheets)
  3. Selectors and properties in CSS
  4. The box model in CSS
  5. Flexbox and Grid layouts
  6. Basic JavaScript for creating dynamic content using CSS variables

Note:

If you are new to web development, it is recommended that you familiarize yourself with these prerequisites before proceeding with this lesson.

Core Concept

The var() function in CSS allows you to define custom properties, also known as CSS variables, which can be reused throughout your stylesheets. This makes it easier to maintain consistency across your project, as well as making it simpler to make global changes without having to search through multiple selectors for the same property.

Defining Custom Properties (CSS Variables)

To define a custom property, you use the -- double-dash prefix followed by the name of the variable and an initial value:

:root {
--main-color: #333;
}

In this example, we've defined a CSS variable named --main-color, with an initial value of #333. The :root selector targets the root element (``) of our document.

Using Custom Properties (CSS Variables)

To use a custom property in your styles, you simply refer to it by its name inside the var() function:

body {
background-color: var(--main-color);
}

In this example, we've used the var() function to set the background-color of the ` element to the value of our --main-color` variable.

Nesting and Inheritance

Custom properties can also be nested within other custom properties, allowing for more complex relationships between variables:

:root {
--main-color: #333;
--secondary-color: var(--main-color) * 0.5;
}

body {
background-color: var(--secondary-color);
}

In this example, we've defined a second custom property named --secondary-color, which is calculated as half the value of our --main-color variable. This demonstrates how custom properties can be used to create relationships between variables and make your styles more modular.

Inheritance and Default Values

Custom properties can also inherit values from their parent elements, just like regular CSS properties:

:root {
--main-color: #333;
}

body {
--header-color: var(--main-color);
}

header {
background-color: var(--header-color);
}

In this example, we've defined a custom property named --header-color, which inherits the value of our --main-color variable. We then use that variable to set the background color of our ``.

Fallback Values

If a custom property is not defined or inheritance fails, the var() function will fall back to a specified default value:

:root {
--main-color: #333;
}

body {
background-color: var(--main-color, #fff);
}

In this example, we've provided a fallback value of #fff for our --main-color variable. If the variable is not defined or inheritance fails, the background-color of the ` will default to white (#fff`).

Worked Example

Let's create a simple web page with a custom color scheme using CSS variables:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>CSS Variables Example</title>
<style>
:root {
--main-color: #333;
--secondary-color: var(--main-color) * 0.5;
--header-height: 100px;
}

body {
background-color: var(--main-color);
color: var(--secondary-color);
}

header {
height: var(--header-height);
background-color: var(--main-color);
}
</style>
</head>
<body>
<header>
<h1>CSS Variables Example</h1>
</header>
<main>
<p>Welcome to the CSS Variables Example!</p>
<p>The page has a custom color scheme using CSS variables.</p>
</main>
</body>
</html>

In this example, we've defined three custom properties: --main-color, --secondary-color, and --header-height. We then use these variables to style the page, including setting the background color of the `, the color of the text, and the height and background color of the `.

Common Mistakes

  1. Forgetting the double-dash prefix: Custom properties must be prefixed with two dashes (--) to be recognized as variables.
  2. Not defining custom properties at the root level: To ensure that your custom properties are globally accessible, define them at the :root level.
  3. Incorrect inheritance syntax: Make sure you use the correct syntax for inheriting values from parent elements (e.g., var(--parent-property)).
  4. Not providing fallback values: If a custom property is not defined or inheritance fails, provide a fallback value to avoid unexpected results.
  5. Overusing CSS variables: While CSS variables can make your styles more maintainable, be careful not to overuse them and create unnecessary complexity in your code.

Common Mistakes (Continued)

  1. Using CSS variables with non-supporting browsers: Older versions of Internet Explorer do not support CSS variables. To ensure compatibility, use polyfills or fallback styles for these browsers.
  2. Forgetting to update variable values: When making changes to the design, remember to update the corresponding CSS variables to maintain consistency across your project.
  3. Not using meaningful names for custom properties: Use descriptive names for your custom properties to make your code easier to understand and maintain.
  4. Not testing cross-browser compatibility: Test your web page in multiple browsers to ensure that your CSS variables are working correctly and consistently across all platforms.
  5. Not considering accessibility: Be mindful of color contrast and other accessibility considerations when using CSS variables to create a custom color scheme for your web page.

Practice Questions

  1. Define a custom property named --link-color with an initial value of #00f. Use this variable to style the links on a web page.
  2. Create a CSS reset using CSS variables to ensure consistent styling across different browsers and devices.
  3. Write a CSS stylesheet that allows users to change the theme (light or dark) of a web page using custom properties.
  4. Implement a responsive design using CSS variables, where the layout adjusts based on the screen size.
  5. Create a dynamic navigation menu using JavaScript and CSS variables to allow for easy color changes based on user preference.
  6. Write a function that generates random colors using CSS variables for use in a web page's background or other design elements.
  7. Use CSS variables to create a customizable typography system, allowing users to adjust font sizes, line heights, and colors easily.
  8. Implement a dark mode toggle switch using JavaScript and CSS variables to allow users to switch between light and dark themes on your web page.
  9. Create a CSS variable-based design system that includes predefined color schemes, typography settings, and layout grids for rapid development of consistent web pages.
  10. Write a script that generates a random gradient background using CSS variables for use in a web page's header or other design elements.

FAQ

  1. Can I use CSS variables in HTML? No, CSS variables are not directly usable in HTML. However, you can create dynamic content by combining JavaScript and CSS variables.
  2. Do all browsers support CSS variables? Most modern browsers support CSS variables, but older versions of Internet Explorer do not. You can use polyfills to ensure compatibility across different browsers.
  3. Can I nest custom properties inside other custom properties? Yes, you can nest custom properties, allowing for more complex relationships between variables.
  4. What happens if a custom property is not defined or inheritance fails? If a custom property is not defined or inheritance fails, the var() function will fall back to a specified default value (if provided) or an empty string (if no default value is provided).
  5. Can I use CSS variables with media queries? Yes, you can use CSS variables with media queries to create responsive designs that adjust based on different screen sizes and devices.
  6. Are there any limitations to the number of custom properties I can define? There are no hard limits to the number of custom properties you can define, but be mindful of creating unnecessary complexity in your code by overusing CSS variables.
  7. How do I ensure that my web page is accessible when using CSS variables for a custom color scheme? Be mindful of color contrast and other accessibility considerations when using CSS variables to create a custom color scheme for your web page. You can use tools like the Web Content Accessibility Guidelines (WCAG) to help ensure your web page is accessible.
  8. Can I use CSS variables with third-party libraries or frameworks? Yes, you can use CSS variables with third-party libraries and frameworks, but be mindful of any potential conflicts or limitations when doing so.
  9. Is it possible to animate CSS properties using CSS variables? Yes, you can animate CSS properties that are based on CSS variables by changing the variable values over time using keyframes or JavaScript.
  10. How do I test my web page for compatibility with different browsers when using CSS variables? You can use browser testing tools like BrowserStack, Sauce Labs, or CrossBrowserTesting to ensure your web page works correctly across multiple browsers and devices.
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