CSS @when Rule: Mastering Conditional Style Application

The world of CSS is constantly evolving, offering developers more powerful and flexible ways to style web pages. One such feature gaining traction is the @when rule, also known as the CSS Conditional Rules Module Level 1. This rule allows you to apply CSS styles conditionally, based on specific conditions being met. It’s a powerful tool for responsive design, feature detection, and creating more robust and adaptable stylesheets.

What is the CSS @when Rule?

The @when rule is a conditional at-rule in CSS that allows you to define styles that are only applied if certain conditions are true. Think of it as an if statement for your CSS. Unlike media queries, which primarily focus on viewport characteristics (screen size, orientation, etc.), @when provides a more general and extensible way to handle conditional styling. It expands upon existing conditional at-rules like @supports and @media.

Key Advantages of Using @when

• Improved Code Readability: By encapsulating conditional logic within @when blocks, you make your CSS easier to understand and maintain. The intent behind specific style applications becomes clearer.
• Enhanced Flexibility: @when can handle more complex conditions than traditional media queries, particularly when combined with feature queries and JavaScript-driven logic (using CSS Custom Properties).
• Simplified Feature Detection: @when integrates seamlessly with @supports, allowing you to apply styles only when specific browser features are available. This is crucial for progressive enhancement.
• More Semantic Styling: @when allows you to style elements based on their state or context, leading to more semantic and maintainable CSS. For example, styling elements based on data attributes or custom properties set by JavaScript.

Syntax of the @when Rule

            @when <condition> {
  /* CSS rules to apply when the condition is true */
}
            

              
                Copy
              
            
          

The <condition> can be any valid boolean expression that evaluates to true or false. This expression often involves:

• Media Queries: Conditions based on viewport characteristics (e.g., screen width, device orientation).
• Feature Queries (@supports): Conditions based on browser support for specific CSS features.
• Boolean Algebra: Combining multiple conditions using logical operators (and, or, not).

Practical Examples of @when in Action

1. Responsive Design with @when and Media Queries

The most common use case for @when is responsive design, where you adjust styles based on screen size. While media queries can achieve this on their own, @when provides a more structured and readable approach, especially when dealing with complex conditions.

            @when (min-width: 768px) and (max-width: 1023px) {
  body {
    font-size: 18px;
    line-height: 1.6;
  }
  .container {
    width: 720px;
    margin: 0 auto;
  }
}
            

              
                Copy
              
            
          

In this example, the styles within the @when block are applied only when the screen width is between 768px and 1023px (typical tablet size). This provides a clear and concise way to define styles for specific viewport ranges.

Internationalization Note: Responsive design is critical for a global audience. Consider varying screen sizes across different regions. For example, mobile usage is higher in some countries, making mobile-first design even more crucial.

2. Feature Detection with @when and @supports

@when can be combined with @supports to apply styles only when a specific CSS feature is supported by the browser. This allows you to progressively enhance your website, providing a better experience for users with modern browsers while still maintaining compatibility with older ones.

            @when supports(display: grid) {
  .container {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    grid-gap: 20px;
  }
}

@when not supports(display: grid) {
  .container {
    /* Fallback styles for browsers that don't support grid */
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
  }
  .item {
    width: calc(50% - 10px); /* Adjust width for older browsers */
  }
}
            

              
                Copy
              
            
          

Here, we use @supports to check if the browser supports CSS Grid Layout. If it does, we apply grid-based styles to the .container. If not, we provide fallback styles using flexbox to ensure a similar layout is achieved in older browsers.

Global Accessibility Note: Feature detection is important for accessibility. Older browsers may lack support for newer ARIA attributes or semantic HTML5 elements. Provide appropriate fallbacks to ensure content remains accessible.

3. Combining Media Queries and Feature Queries

The real power of @when comes from its ability to combine media queries and feature queries to create more complex and nuanced conditional styling rules.

            @when (min-width: 768px) and supports(backdrop-filter: blur(10px)) {
  .modal {
    background-color: rgba(255, 255, 255, 0.8);
    backdrop-filter: blur(10px);
  }
}
            

              
                Copy
              
            
          

In this example, the .modal element will only have a blurred backdrop when the screen width is at least 768px and the browser supports the backdrop-filter property. This allows you to create visually appealing effects on modern browsers while avoiding potential performance issues or rendering glitches on older ones.

4. Styling Based on Custom Properties (CSS Variables)

@when can also be used in conjunction with CSS Custom Properties (also known as CSS variables) to create dynamic and state-driven styling. You can use JavaScript to update the value of a custom property, and then use @when to apply different styles based on that value.

First, define a custom property:

            :root {
  --theme-color: #007bff; /* Default theme color */
  --is-dark-mode: false;
}

            

              
                Copy
              
            
          

Then, use @when to apply styles based on the value of the custom property:

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

              
                Copy
              
            
          

Finally, use JavaScript to toggle the value of the --is-dark-mode custom property:

            document.getElementById('darkModeToggle').addEventListener('click', function() {
  document.documentElement.style.setProperty('--is-dark-mode', this.checked);
});
            

              
                Copy
              
            
          

This allows users to switch between light and dark themes, with the CSS dynamically updating based on the custom property value. Note that direct comparison of CSS variables in `@when` might not be universally supported across browsers. Instead, you might need to use a workaround with a media query checking for a non-zero value:

            @when ( --is-dark-mode > 0 ) { ... }
            

              
                Copy
              
            
          

However, ensure the custom property has a numeric value for this to function correctly.

Accessibility Note: Providing alternative themes (e.g., dark mode) is crucial for accessibility. Users with visual impairments may benefit from high-contrast themes. Ensure that your theme switch is accessible via keyboard and screen readers.

5. Styling Based on Data Attributes

You can also use @when with data attributes to style elements based on their data values. This can be useful for creating dynamic interfaces where elements change appearance based on user interaction or data updates.

For example, let's say you have a list of tasks, and each task has a data-status attribute that indicates its status (e.g., "todo", "in-progress", "completed"). You can use @when to style each task differently based on its status.

            [data-status="todo"] {
  /* Default styles for todo tasks */
  color: #333;
}

@when attribute(data-status string equals "in-progress") {
  [data-status="in-progress"] {
    color: orange;
    font-style: italic;
  }
}

@when attribute(data-status string equals "completed") {
  [data-status="completed"] {
    color: green;
    text-decoration: line-through;
  }
}
            

              
                Copy
              
            
          

Note: support for the attribute() test condition might be limited or not fully implemented across all browsers currently. Always test thoroughly.

Browser Compatibility and Polyfills

As of late 2024, browser support for the @when rule is still evolving. While many modern browsers support the core functionality, some older browsers may not. Therefore, it's crucial to check compatibility tables and use appropriate fallbacks or polyfills where necessary.

Always consult resources like Can I use... to check the current browser support status of @when and related features.

Best Practices for Using @when

• Keep Conditions Simple: Avoid overly complex conditions within @when blocks. Break down complex logic into smaller, more manageable chunks.
• Provide Fallbacks: Always provide fallback styles for browsers that don't support the features you're using in your @when rules. This ensures a consistent experience across different browsers.
• Test Thoroughly: Test your CSS in a variety of browsers and devices to ensure that your @when rules are working as expected.
• Use Meaningful Comments: Add comments to your CSS to explain the purpose of each @when rule and the conditions it's based on. This will make your code easier to understand and maintain.
• Consider Performance: Avoid using @when rules excessively, as they can potentially impact performance. Optimize your CSS to minimize the number of rules that need to be evaluated.

Conclusion

The @when rule is a powerful addition to the CSS toolbox, offering developers a more flexible and expressive way to apply styles conditionally. By combining it with media queries, feature queries, and CSS Custom Properties, you can create more robust, adaptable, and maintainable stylesheets. While browser support is still evolving, @when is a feature worth exploring and incorporating into your modern web development workflow.

As the web continues to evolve, mastering features like @when will be essential for creating engaging, accessible, and performant experiences for users around the world. Embrace the power of conditional styling and unlock new possibilities in your CSS development.