CSS is always evolving, and new proposed rules provide extra features that you should be aware of. This article will explain two new expansions, @when and @else, so that you can be prepared.

Using CSS when/else rules

If you have wondered why CSS holds such importance in web development, the answer becomes clear once you understand its role in shaping the visual aspects of websites. As we all know, CSS (Cascading Style Sheets) plays a significant role in developing good-looking web pages as they control the layout, colors, fonts, and various design elements. However, what if there was a way to go beyond the traditional approach and apply styles conditionally based on specific criteria? In this article, we will explore the proposed CSS @when/@else rules, which can enable developers to achieve precisely that.

It’s important to note that these rules are still in the proposal stage and are not part of the standard CSS implemented in browsers yet. Therefore, they cannot be used in production environments at this time. Developers can look forward to these features being adopted in the future, which will enhance the power and flexibility of CSS for conditional styling. Please see this W3C module for more information.

That being said, let’s dive in to understand what the @when/@else rules are.

Understanding CSS @when/@else rules

With CSS, @when/@else rules provide a mechanism for applying styles selectively based on specific conditions. In simpler terms, they allow developers to define a set of styles to be applied when a condition is met and an alternative set of styles when it is not. This approach differs from traditional CSS, where styles are applied universally.

Below is a table that distinguishes the @when/@else rules from the traditional CSS.

In traditional CSS, styles are universally applied to elements that match a selector.

For example:

.nav {
  display: flex;
  justify-content: space-between;
  background-color: #333;
  color: #fff;
  padding: 1rem;
}

.nav-item {
  margin-right: 1rem;
}

/* Responsive styles */
@media (max-width: 768px) {
  .nav {
    flex-direction: column;
  }
  
  .nav-item {
    margin-right: 0;
    margin-bottom: 0.5rem;
  }
}

This traditional CSS approach applies the main styles for the .nav and .nav-item universally, while the responsive styles inside the @media target specific screen widths to modify the layout and spacing for smaller screens.

Here’s an example using the @when/@else rule syntax:

/* Using @when and @else */
@when media(max-width: 768px) and supports(display: flex) {
  .nav {
    flex-direction: column;
  }
  
  .nav-item {
    margin-right: 0;
    margin-bottom: 0.5rem;
  }
}

@else {
  .nav {
    display: flex;
    justify-content: space-between;
    background-color: #333;
    color: #fff;
    padding: 1rem;
  }
  
  .nav-item {
    margin-right: 1rem;
  }
}

As you can see, with the @when rule, we check if the screen width is less than or equal to 768px and if the browser supports Flexbox. If both conditions are true, the first block of styles is applied (vertical layout). Otherwise, the @else block provides the traditional horizontal layout.

NOTE: The @when and @else rules simplify conditional styling, making your CSS more concise and easier to manage.

Benefits of using the @when/@else rules

Now that we have a glimpse of CSS’s @when/@else rules let’s explore some of its benefits.

Generalization with @when

The @when rule generalizes conditionals, allowing you to combine different queries (such as @media and @support) into a single statement. You can wrap multiple conditions into one block instead of writing separate conditional rules for specific tasks.

For example, suppose you want to test whether a media screen is below 769px and whether the browser supports both grid and flex display features. With @when, you can create a generalized rule like this:

@when media(max-width: 769px) and supports(display: grid) and supports(display: flex) {
  .grid {
    grid-template-columns: 1fr;
  }
  
  .flex {
    flex-direction: row;
  }
}

In the above code, the @when rule is followed by the media query condition (max-width: 769px) and the supports conditions for display: grid and display: flex. Inside the rule, the styles for the .grid and .flex classes are specified accordingly.

Mutually exclusive rules with @else

The @else statement allows you to write mutually exclusive rules. If the conditions in the @when block are evaluated as false, the CSS engine ignores that style block and falls back to default styles. For instance, you can create styles that apply only when certain conditions are met; if not, the fallback styles take effect.

Improved readability

Using comparison operators in @when and @else makes media queries easier to read and understand. It simplifies complex logic by combining multiple conditions into concise statements.

Support extensions

The Level 4 CSS update includes extensions to support rules, allowing testing for supported font technologies and other features.

Implementing @when/@else rules in CSS

To implement the @when/@else rules, we can use a combination of CSS selectors and properties.

Here’s a step-by-step guide on how to implement the @when/@else rules in CSS:

• Check browser support: Before you start, verify if your target browser supports the @when/@else rules. You can check resources like “Can I use” for the most current information.
• Write the base CSS: Define the base CSS styles that will apply regardless of conditions.
• Define your conditions: Determine the conditions under which you want to apply different styles. These can be based on media queries, feature support, or other environmental conditions.
• Use @when to apply conditional styles: Start with the @when rule followed by your condition in parentheses. Inside the curly braces {}, write the CSS rules that should apply when the condition is true.
• Use @else for alternative styles: Follow the @when block with an @else block. Inside the @else curly braces, write the CSS rules that should apply when the @when condition is not met.
• Test your styles: After implementing your conditional rules, test them across browsers and devices to ensure they work as expected.

Here’s an example to illustrate the process:

/* Base CSS */
.container {
  padding: 20px;
}

/* Conditional CSS */
@when media(max-width: 600px) {
  .container {
    padding: 10px; /* Smaller padding for narrow screens */
  }
}

@else {
  .container {
    padding: 20px; /* Default padding */
  }
}

In this example, the .container will have a padding of 10px if the viewport width is less than 600px. Otherwise, it will default to a padding of 20px.

Now, let’s take a look at common use cases.👇

• Responsive design: Adjusting layout and font sizes based on viewport dimensions. For example, switching from a multi-column layout to a single-column layout on mobile devices.
• Feature detection: Applying styles only if certain CSS features are supported by the browser. For example, using grid layout styles only if the browser supports CSS Grid.
• Environment adaptation: Changing styles based on user preferences or environmental conditions like dark mode. For example, a dark color scheme can be applied if the user has set their system to dark mode.
• State-based styling: Modifying styles based on the state of an element, such as hover or focus. For example, changing the background color of a button when it is hovered over.
• Fallbacks for older browsers: Providing alternative styles for browsers that do not support modern CSS properties. For example, using flexbox as a fallback for browsers that do not support grid layout.

Here’s an example that combines responsive design and feature detection:

/* Base styles */
.container {
  display: flex;
  flex-wrap: wrap;
}

/* Conditional styles */
@when media(max-width: 600px) and supports(display: grid) {
  .container {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(150px, 1fr));
  }
}

@else {
  .container {
    display: flex;
    flex-direction: column;
  }
}

In this example, if the viewport is less than 600px wide and the browser supports CSS Grid, the .container will use a grid layout with flexible columns. If not, it will default to a flex column layout.

Advanced techniques for CSS @when/@else rules

Advanced Techniques for CSS @when/@else rules offer more control over styles based on different conditions like complex media queries, feature detection, or custom environmental factors. This section talks about advanced approaches and tips that help optimize the usage of @when/@else rules.

Let’s go!

Combining with CSS preprocessors

CSS preprocessors like Sass or LESS can enhance the @when/@else rules by allowing you to use variables, functions, and other programming concepts within your CSS.

Here’s an example using Sass:

$breakpoint: 600px;

@mixin responsive($property, $value) {
  @when media(max-width: $breakpoint) {
    #{$property}: $value;
  }
  @else {
    #{$property}: adjust-value($value);
  }
}

.container {
  @include responsive(padding, 20px);
}

In the code above, the responsive mixin applies different padding values based on the viewport width, using a variable for the breakpoint.

To learn more about Sass, read this.

Dynamic styling based on conditions

Dynamic styling can be achieved by setting CSS variables or classes that change styles based on certain conditions. For instance, JavaScript can be used to toggle a class based on user interaction or environmental changes.

Here’s an example(JavaScript code):

window.addEventListener('resize', () => {
  if (window.innerWidth < 600) {
    document.body.classList.add('mobile');
  } else {
    document.body.classList.remove('mobile');
  }
});

CSS code:

body.mobile .container {
  /* Styles for mobile */
}

@else {
  .container {
    /* Default styles */
  }
}

The JavaScript listens for window resize events and adds or removes a ‘mobile’ class to the body, which could trigger different styles based on the @when/@else rules.

Responsive design using @when/@else rules

Responsive design aims to make web applications look good on all devices. With the @when/@else rules, you could write more readable media queries.

For example:

@when media(max-width: 768px) {
  .column {
    width: 100%;
  }
}

@else {
  .column {
    width: 50%;
  }
}

This example changes the column width based on the viewport size, making it responsive to different device screens.

Best practices for @when/@else rules

When working with CSS, especially with the proposed @when/@else rules, it’s important to follow best practices to write efficient and maintainable code. Here are some tips and debugging strategies:

• Plan your CSS: Before writing your CSS, plan the styles that will be needed, the different layouts, and specific overrides. Avoid too much overriding.
• Use flexible units: For maximum flexibility, use relative units like ems, rems, percentages, or viewport units.
• Keep it consistent: Whether you’re working alone or with a team, consistency in naming conventions, color descriptions, and formatting is key.
• Comment your CSS: Comments can help you and others understand the structure and purpose of your CSS. Use them to separate logical sections.

Debugging @when/@else rules

When you come across issues with the functionality or appearance of the @when/@else rules, it’s important to use debugging techniques to identify and fix the underlying problems.

Here are some debugging techniques:

• Browser DevTools: Modern browsers come with DevTools that allow you to inspect elements and see which CSS rules are being applied. Use this to troubleshoot unexpected behaviors.
• Isolation: If a particular set of rules isn’t working as expected, try isolating them in a separate stylesheet or HTML document to see if other styles are interfering.
• Process of elimination: Temporarily comment out other CSS rules to see if they are causing the issue.

Conclusion

In our discussion, we explored the concept of @when/@else rules in CSS, which are part of a proposed extension to CSS conditionals. We covered their syntax and potential use cases and provided a step-by-step guide on how they might be implemented. We also discussed advanced techniques, such as combining these rules with CSS preprocessors and using them for responsive design. Although these rules offer promising benefits for readability, maintainability, and flexibility in styling, it’s important to note that they are not yet part of the standard CSS implementation and thus are not supported in current browsers.

For further reading and learning, you can check the following resources:

• MDN Web Docs provides a comprehensive guide on CSS conditional rules, including the @when and @else rules which are planned but not yet implemented.
• W3cubDocs offers support tables for CSS @when/@else conditional rules, detailing the syntax and usage.
• LogRocket Blog gives a first look at extending CSS @when/@else chains, including practical uses within style sheets.
• Can I use provides information on the browser support and syntax for CSS @when/@else conditional rules.

Thank you for reading :).

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