CSS @when: Mastering Conditional Styling for Modern Web Design

In the ever-evolving landscape of web development, the ability to create dynamic and responsive user interfaces is paramount. CSS, the cornerstone of visual presentation, continues to introduce powerful features that empower developers to build more intelligent and adaptive websites. One such groundbreaking feature is the @when at-rule, which allows for conditional styling, enabling us to apply CSS rules only when specific conditions are met. This opens up a world of possibilities for creating truly responsive and context-aware designs.

What is CSS @when?

The @when at-rule is a powerful addition to the CSS specification that works in conjunction with the @media or @supports rules. It acts as a conditional block, meaning that the CSS declarations within its scope will only be applied if the specified condition evaluates to true. Essentially, it provides a more granular and expressive way to control when certain styles are active, moving beyond the traditional block-level conditioning of @media queries alone.

Think of it as a highly refined `if` statement for your CSS. Instead of applying an entire set of styles based on a broad condition, @when lets you target specific declarations within a rule, making your stylesheets more efficient and maintainable.

The Synergy: @when with @media and @supports

The true power of @when is realized when it's used in combination with existing conditional at-rules:

1. @when with @media Queries

This is arguably the most common and impactful use case for @when. Traditionally, you might wrap entire CSS rules within a @media query. With @when, you can now conditionally apply specific declarations within a rule based on media query conditions.

Example: Responsive Typography

Let's say you want to adjust the font size of a paragraph, but only when the viewport is wider than 768 pixels. Without @when, you might do this:

            .my-paragraph {
  font-size: 16px;
}

@media (min-width: 768px) {
  .my-paragraph {
    font-size: 18px;
  }
}

            

              
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Now, using @when, you can achieve the same result more concisely and with greater control:

            .my-paragraph {
  font-size: 16px;
  @when (min-width: 768px) {
    font-size: 18px;
  }
}

            

              
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In this example:

• The base font-size of 16px is always applied.
• The font-size of 18px is applied only when the viewport width is 768 pixels or more.

This approach is incredibly useful for making granular adjustments to specific properties based on screen size, orientation, or other media features, without duplicating entire rule sets.

Global Example: Adapting UI Elements for Different Devices

Consider a global e-commerce platform. A product card might display a compact view on mobile devices but a more detailed view on larger screens. Using @when with @media, you can elegantly handle these transitions:

            .product-card {
  padding: 10px;
  text-align: center;
  @when (min-width: 600px) {
    padding: 20px;
    text-align: left;
  }
  @when (min-width: 1024px) {
    padding: 30px;
    display: flex;
    align-items: center;
  }
}

.product-image {
  width: 100%;
  height: 150px;
  @when (min-width: 600px) {
    width: 200px;
    height: 200px;
  }
  @when (min-width: 1024px) {
    width: 250px;
    height: 250px;
    margin-right: 20px;
  }
}

            

              
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This allows the .product-card and its internal elements like .product-image to adapt their styles progressively as the viewport size increases, providing a tailored experience across a wide range of devices globally.

2. @when with @supports Queries

The @supports at-rule allows you to check if a browser supports a specific CSS property-value pair. By combining it with @when, you can conditionally apply styles only when a particular browser feature is available.

Example: Using a New CSS Feature

Imagine you want to use the experimental backdrop-filter property. Not all browsers or older versions support this. You can use @when with @supports to apply it gracefully:

            .modal-background {
  background-color: rgba(0, 0, 0, 0.5);
  @when supports (backdrop-filter: blur(10px)) {
    backdrop-filter: blur(10px);
  }
}

            

              
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In this case:

• The background-color is always applied as a fallback.
• The backdrop-filter is applied only when the browser supports the backdrop-filter: blur(10px) declaration.

This is crucial for progressive enhancement, ensuring that your design is functional and visually appealing even in environments that don't support the latest CSS features.

Global Example: Progressive Enhancement for Animations

Consider a website featuring subtle animations. Some advanced animations might rely on newer CSS properties like animation-composition or specific easing functions. You can use @when and @supports to provide a fallback or a simpler animation for browsers that don't support these advanced properties.

            .animated-element {
  transform: translateX(0);
  transition: transform 0.5s ease-in-out;

  @when supports (animation-composition: replace) {
    /* More advanced animation properties or sequences */
    animation: slideIn 1s forwards;
    animation-composition: replace;
    animation-timing-function: cubic-bezier(0.25, 0.1, 0.25, 1);
  }
}

@keyframes slideIn {
  from { transform: translateX(-100%); }
  to { transform: translateX(0); }
}

            

              
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Here, browsers supporting animation-composition: replace will get a more sophisticated animation sequence, while others will fall back to the simpler transition property, ensuring a consistent, albeit varied, experience for users worldwide.

3. Combining @when with Multiple Conditions

You can also chain multiple conditions within a single @when rule, creating even more specific styling logic. This is done using logical operators like and, or, and not.

Example: Complex Responsive Logic

Let's imagine a scenario where a sidebar should only be hidden on smaller screens, but only if a specific user preference setting (hypothetically indicated by a class on the body) is not active.

            .sidebar {
  display: block;
  width: 250px;

  /* Hide sidebar on small screens AND not in preference mode */
  @when (max-width: 767px) and not (.no-sidebar-on-mobile) {
    display: none;
  }

  /* Show sidebar on larger screens OR if preference mode is active on small screens */
  @when (min-width: 768px) or (.sidebar-on-mobile) {
    display: block;
  }
}

            

              
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This level of conditional styling allows for highly intricate UI behaviors tailored to specific contexts and user interactions.

Syntax and Best Practices

The basic syntax for @when is straightforward:

            selector {
  property: value;
  @when (condition) {
    property: value;
  }
}

            

              
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When combining multiple conditions, the syntax becomes:

            selector {
  property: value;
  @when (condition1) and (condition2) {
    property: value;
  }
  @when (condition1) or (condition2) {
    property: value;
  }
  @when not (condition) {
    property: value;
  }
}

            

              
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Key Best Practices:

• Prioritize Readability: While @when can make styles more concise, avoid overly complex nested conditions that might become difficult to decipher. Break down complex logic into separate rules if necessary.
• Progressive Enhancement: Always provide a graceful fallback for browsers or environments that don't support the features targeted by your @when rules, especially when used with @supports.
• Performance: While @when itself is generally efficient, be mindful of overly complex conditional logic that might impact parsing performance, though this is rarely an issue with typical usage.
• Browser Support: Keep an eye on browser support for @when and its companion at-rules. As of its introduction, adoption is growing, but it's essential to test across your target browsers. Use tools like Can I Use to check the latest compatibility information.
• Global Reach: When designing for a global audience, leverage @when with @media to cater to the vast array of device sizes and resolutions prevalent worldwide. Consider different network conditions as well; you might use prefers-reduced-motion media queries within @when to disable animations for users who have opted out.
• Maintainability: Use @when to keep related styles together. If a property's value changes based on a condition, it's often more maintainable to have both the default and conditional values within the same rule block, rather than scattering them across different @media queries.

Browser Support and Future Outlook

The @when at-rule is a relatively new addition to the CSS landscape. As of its initial widespread adoption, it is supported in modern browsers like Chrome, Firefox, Safari, and Edge. However, it's crucial to always check the latest browser compatibility data for specific versions and features.

The W3C CSS Working Group continues to refine and expand CSS capabilities. Features like @when, along with other conditional rules and nesting, signal a move towards more programmatic and expressive styling capabilities in CSS. This trend is vital for building complex, adaptive, and user-friendly web experiences that cater to a diverse global user base.

As web design continues to embrace adaptability and personalization, @when will become an indispensable tool in the developer's arsenal. Its ability to fine-tune styles based on a wide array of conditions, from device characteristics to browser capabilities, empowers us to create more sophisticated and context-aware interfaces.

Conclusion

CSS @when is a powerful and elegant feature that significantly enhances our ability to write conditional styles. By leveraging its synergy with @media and @supports, developers can create more responsive, adaptive, and robust web designs. Whether you're adjusting typography for different screen sizes, conditionally applying advanced CSS features, or building complex interactive UIs, @when provides the precision and flexibility needed to meet the demands of modern web development. Embracing this feature will undoubtedly lead to more sophisticated and user-centric digital experiences for a global audience.

Start experimenting with @when in your projects today to build smarter, more adaptable, and future-proof websites.