CSS @when: The Future of Conditional Styling

The world of web development is constantly evolving, and CSS is no exception. One of the most exciting and promising recent additions to the CSS landscape is the @when rule. This powerful feature introduces a new way to apply styles conditionally, going beyond the limitations of traditional media queries and opening up possibilities for more dynamic and responsive designs.

What is CSS @when?

The @when rule allows you to apply CSS styles based on specific conditions. It acts as a more versatile and expressive alternative to media queries, container queries, and even JavaScript-based solutions for conditional styling. Think of it as an "if-then" statement for your CSS.

The fundamental syntax of the @when rule is as follows:

            @when (condition) {
  /* Styles to apply when the condition is true */
}

            

              
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The condition can be a variety of logical expressions, making the @when rule incredibly flexible. We'll explore some specific examples later in this article.

Why Use @when? Benefits and Advantages

The @when rule offers several key advantages over existing methods for conditional styling:

• Increased Flexibility: Unlike media queries, which are primarily based on viewport size, @when allows you to base styles on a wide range of factors, including custom properties (CSS variables), element attributes, and potentially even the presence or absence of specific HTML elements.
• Improved Readability and Maintainability: By encapsulating conditional logic directly within your CSS, @when makes your code more self-documenting and easier to understand. This reduces the need for complex JavaScript solutions and improves the overall maintainability of your stylesheets.
• Enhanced Reusability: You can define complex conditional styles within @when rules and reuse them throughout your website or application. This promotes code reuse and reduces redundancy.
• Potentially Better Performance: In some cases, using @when can lead to performance improvements compared to JavaScript-based solutions, as the browser can handle the conditional logic natively.
• Future-Proofing Your Code: As CSS continues to evolve, @when provides a solid foundation for building more dynamic and responsive user interfaces.

Understanding the Condition Syntax

The real power of @when lies in its ability to handle a wide variety of conditions. Here are some examples:

1. Checking Custom Properties (CSS Variables)

You can use custom properties to store state information and then use @when to apply styles based on the value of those properties. This is particularly useful for creating themes or allowing users to customize the appearance of your website.

            :root {
  --theme-color: #007bff; /* Default theme color */
  --font-size: 16px;
}

@when (--theme-color = #007bff) {
  body {
    background-color: #f0f8ff; /* Apply a light background when the theme color is blue */
  }
}

@when (--font-size > 18px) {
  p {
    line-height: 1.6;
  }
}

            

              
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In this example, the first @when rule checks if the --theme-color custom property is equal to #007bff. If it is, it applies a light background color to the body element. The second @when rule checks if the --font-size is greater than 18px, and if it is, it adjusts the line height of paragraph elements.

2. Evaluating Boolean Conditions

@when can also handle boolean conditions, allowing you to apply styles based on whether a particular condition is true or false. This can be particularly useful for toggling styles based on user preferences or application state.

            :root {
  --dark-mode: false;
}

@when (--dark-mode = true) {
  body {
    background-color: #333;
    color: #fff;
  }
}

            

              
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In this example, the @when rule checks if the --dark-mode custom property is set to true. If it is, it applies a dark background color and light text color to the body element, effectively enabling a dark mode.

3. Combining Conditions with Logical Operators

For more complex scenarios, you can combine multiple conditions using logical operators such as and, or, and not. This allows you to create highly specific and nuanced conditional styles.

            :root {
  --is-mobile: false;
  --is-logged-in: true;
}

@when (--is-mobile = true and --is-logged-in = true) {
  /* Styles to apply only on mobile devices and when the user is logged in */
  .mobile-menu {
    display: block;
  }
}

            

              
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In this example, the @when rule checks if both --is-mobile and --is-logged-in are set to true. If both conditions are met, it displays a mobile menu element.

4. Checking Element Attributes

While the exact syntax may evolve, the proposed specifications allow checking element attributes directly in the condition. This could be incredibly useful for styling elements based on their attributes, potentially replacing the need for JavaScript-based attribute selectors in many cases.

            @when (element.hasAttribute('data-active')) {
  [data-active] {
    border: 2px solid blue;
  }
}

@when (element.getAttribute('data-theme') = 'dark') {
  body {
    background-color: black;
    color: white;
  }
}

            

              
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The first example checks if an element has the attribute `data-active`. The second example checks if the attribute `data-theme` is equal to 'dark'.

5. Style Queries and Container Queries (Potential Integration)

While still under development, there is potential for @when to integrate with style queries and container queries. This would allow you to apply styles based on the styles applied to a parent container or the size of a container, further enhancing the responsiveness and adaptability of your designs. This is particularly interesting because the current container query syntax is rather verbose; `@when` may offer a more concise alternative.

Practical Examples and Use Cases

Let's explore some practical examples of how you can use the @when rule in real-world scenarios:

1. Dynamic Theme Switching

As shown earlier, you can easily implement dynamic theme switching using custom properties and the @when rule. This allows users to customize the appearance of your website based on their preferences.

2. Adaptive Navigation Menus

You can use @when to create navigation menus that adapt to different screen sizes or device orientations. For example, you could display a full navigation menu on desktop screens and a hamburger menu on mobile devices.

3. Form Validation

You can use @when to style form elements based on their validation status. For example, you could highlight invalid input fields with a red border.

4. Content Visibility

@when can be used to show or hide content based on various conditions. You could show a specific promotional banner only to users who haven't already seen it, based on a cookie value or a user setting.

5. Internationalization (i18n)

Although not its primary purpose, @when could be used in conjunction with custom properties to adapt styling based on the user's locale. For example, you might adjust font sizes or spacing to better accommodate different character sets.

For instance, assuming you have a custom property `--locale` that stores the user's locale:

            :root {
  --locale: 'en-US'; /* Default locale */
}

@when (--locale = 'ar') { /* Arabic */
  body {
    direction: rtl; /* Right-to-left layout */
    font-family: 'Arial', sans-serif; /* Example font */
  }
}

@when (--locale = 'zh-CN') { /* Simplified Chinese */
  body {
    font-family: 'Microsoft YaHei', sans-serif; /* Example font */
    font-size: 14px; /* Adjust font size if needed */
  }
}

            

              
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This example adjusts the text direction and font based on whether the locale is Arabic (`ar`) or Simplified Chinese (`zh-CN`). While this isn't a direct replacement for proper i18n techniques (like serving different content), it demonstrates how @when can contribute to a more localized experience.

Current Status and Browser Support

As of late 2024, the @when rule is still a relatively new feature and is not yet fully supported by all major browsers. It is currently behind experimental flags in some browsers like Chrome and Edge.

You can track the progress of @when and its browser support on websites like Can I use and the official CSS specifications.

Important Note: Because the feature is experimental, the exact syntax and behavior of the @when rule may change before it is finalized and fully supported by all browsers. It's essential to stay up-to-date with the latest developments and adjust your code accordingly.

Polyfills and Workarounds

While waiting for full browser support, you can use polyfills or JavaScript-based workarounds to emulate the behavior of the @when rule. These solutions typically involve using JavaScript to evaluate the conditions and apply the appropriate styles.

However, keep in mind that polyfills and workarounds can have performance implications and may not perfectly replicate the behavior of the native @when rule. It's important to carefully consider the trade-offs before using them in production environments.

Best Practices for Using @when

Here are some best practices to keep in mind when using the @when rule:

• Use Clear and Concise Conditions: Make sure your conditions are easy to understand and avoid overly complex expressions.
• Document Your Code: Add comments to explain the purpose of your @when rules, especially if they involve complex logic.
• Test Thoroughly: Test your code across different browsers and devices to ensure that it works as expected.
• Consider Performance: Avoid using excessively complex @when rules that could negatively impact performance.
• Progressive Enhancement: Use @when as a progressive enhancement. Your core styling should work even without it, and the @when rule should only add extra functionality.

The Future of CSS Conditional Styling

The @when rule represents a significant step forward in the evolution of CSS conditional styling. It offers a more flexible, readable, and maintainable alternative to traditional media queries and JavaScript-based solutions.

As browser support for @when grows, it is likely to become an increasingly important tool for web developers. By embracing this new feature, you can build more dynamic, responsive, and user-friendly web applications.

Alternatives and Considerations

While @when offers compelling advantages, it's important to consider existing alternatives and scenarios where they might be more appropriate:

• Media Queries: For simple viewport-based adjustments, media queries remain a straightforward and well-supported option.
• Container Queries: When styling components based on the size of their containers, container queries (once fully supported) will be a powerful choice. However, @when may offer a more concise syntax for some container query scenarios.
• JavaScript: For highly complex conditional logic or when interacting with external APIs, JavaScript might still be necessary. However, aim to move logic into CSS where possible for better performance and maintainability.
• CSS Feature Queries (@supports): Use @supports to detect if a particular CSS feature is supported by the browser. This is crucial for providing fallback styles when @when is not available.

Real-World Example: Component-Based Styling with @when

Let's imagine a scenario where you have a reusable "card" component on your website. You want to style the card differently based on whether it's featured or not, and whether the user is logged in. We'll use custom properties to manage these states.

            /* Basic card styling */
.card {
  border: 1px solid #ccc;
  padding: 15px;
  margin-bottom: 20px;
}

/* Featured card styling */
@when (element.hasAttribute('data-featured')) {
  .card[data-featured] {
    border-color: #007bff;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.1);
  }

  .card[data-featured] .card-title {
    font-weight: bold;
    color: #007bff;
  }
}

/* User-specific styling (requires JavaScript to set --logged-in) */
@when (--logged-in = true) {
  .card {
    /* Additional styling for logged-in users */
    background-color: #f9f9f9;
  }
}

/* Combining conditions */
@when (element.hasAttribute('data-featured') and --logged-in = true) {
  .card[data-featured] {
    /* Even more specific styling */
    background-color: #e9ecef;
    transform: translateY(-5px); /* Subtle lift effect */
  }
}

            

              
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In this example:

• The first @when block styles the card when it has the data-featured attribute.
• The second @when block provides a user-specific style for logged-in users (assuming you have a JavaScript mechanism to set the `--logged-in` custom property).
• The final @when block combines both conditions, applying even more specific styling when both conditions are met.

This example showcases the flexibility of @when in handling complex conditional styling scenarios within a component-based architecture.

Conclusion

The CSS @when rule is a game-changing feature that empowers web developers to create more dynamic, responsive, and maintainable stylesheets. As browser support matures, @when is poised to become an indispensable tool for building modern web applications. By embracing this powerful new feature, you can unlock new possibilities for conditional styling and deliver exceptional user experiences.

Stay tuned for future updates and developments as the @when rule continues to evolve and shape the future of CSS!

Further Reading and Resources

• CSS Conditional Rules Module Level 3 (Official Specification - check for updates!)
• Can I use... (Browser support tracking)
• Mozilla Developer Network (MDN) (CSS Documentation)