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An Introduction to CSS Nesting

CSS Nesting provides a way to structure your CSS selectors in a hierarchy, making your stylesheets more readable and maintainable. The feature, traditionally available only through preprocessors like Sass and Less, is now becoming more accessible in native CSS, with some features already implemented and others in draft proposal stages.

Historical Context on CSS Nesting

CSS preprocessors like Sass and Less have long offered nesting capabilities, allowing for a more organized and intuitive way to write styles that reflect the hierarchical structure of HTML. However, native CSS has lacked this feature, leading to the exploration of bringing nesting capabilities directly to CSS. The development of native CSS Nesting is a step towards reducing reliance on preprocessors, aiming to bring the benefits of nesting while keeping stylesheets clean and semantic.

The Basic Syntax of CSS Nesting

As we already mentioned, this feature allows you to structure your CSS selectors hierarchically, enhancing readability and maintainability. Traditionally available through preprocessors like Sass and Less, the feature is transitioning to native CSS. Below is a simplified example:

nav {
  ul {
    margin: 0;
    padding: 0;
    list-style: none;
  }

  li { display: inline; margin-right: 5px; }

  a { text-decoration: none; }
}

In this snippet:

  • The ul, li, and a selectors are nested within the nav selector, reflecting the HTML structure.

In addition, the & symbol, crucial in certain scenarios like pseudo-classes, represents the parent selector:

nav {
  &.active { background-color: #f0f0f0; }
  &:hover { text-decoration: underline; }
}

Here:

  • &.active targets nav elements with a class of active, while &:hover targets them on hover.

Nested at-rules like @layer are also introduced:

@layer components {
  .button {
    color: #fff;
    &.is-primary { background-color: #007BFF; }
  }
}
  • The @layer at-rule groups stylistic features, which can potentially improve organization in larger projects.

Furthermore, at-rules like @media, @supports, and @container can be nested, offering advanced styling capabilities.

Specificity in CSS Nesting

Specificity in CSS Nesting is determined by the highest specificity in the associated selector list, similar to the :is() function. This ensures that the correct styles are applied even in complex nested scenarios.

#x, y {
  & z { color: green; }
}
.example z { color: yellow; }

In this example, we're using arbitrary element names x, y, and z to demonstrate the concept. The & z selector has a specificity of 1-0-0 due to the #x id selector, overriding the .example z selector with a specificity of 0-1-0, and thus, color: green; is applied. Understanding specificity is essential for effectively using this feature, especially in complex stylesheets.

Advancements and Considerations in CSS Nesting

A notable advantage of this feature is direct browser parsing, which improves styling efficiency by reducing pre-compilation steps associated with preprocessors.

Other advantages include:

  • Organization: Nesting groups style rules for related elements, mirroring the HTML structure.

  • Reduced File Size: Minimizes the need to repeat selectors, thereby reducing file size.

  • Ease of Refactoring: Grouped styles allow for easier modification and deletion.

Relaxed Parsing and the & Selector

The evolution includes relaxed parsing, allowing type selectors to be nested without requiring the & selector, leading to a more intuitive syntax. However, the & selector remains valuable, especially in scenarios like compound selectors or pseudo-classes, to maintain the correct relationship between selectors. For instance, in compound selectors, the & selector is crucial to avoid unintended whitespace interference that could break the selector.

Cascading and Specificity

Concerns around cascading and specificity continue with nested @supports rules potentially affecting the order of applied styles. The specificity within nested selectors follows the highest specificity rule, akin to the :is() function, ensuring accurate style application even in complex nested scenarios.

Combinators and Concatenation

CSS combinators like the sibling (+) can be used with or without the & selector. Unlike preprocessors, the feature doesn't support concatenation to create new classes, a limitation when adapting methodologies like BEM.

Browser Compatibility and Support

Browser compatibility is evolving with Firefox leading in support for the latest specification. You can follow the "Can I Use" compatibility status to stay updated.

The feature is an ongoing development in the CSS ecosystem, striving to bring the intuitive organizational benefits of preprocessors to native CSS. Our overview covers the essential aspects, but the feature is still under active discussion and refinement. For those interested in diving deeper into the technical details, the resources below provide comprehensive insights.

Useful Resources

W3C's Editor's Draft of CSS Nesting Module

W3C's Working Draft of CSS Nesting Module