Une introduction à l’imbrication CSS native —

L’imbrication est l’une des principales raisons d’utiliser un préprocesseur CSS tel que Toupet. La fonctionnalité est maintenant arrivée dans le CSS standard du navigateur avec une syntaxe similaire. Pouvez-vous supprimer la dépendance du préprocesseur de votre système de construction ?

L’imbrication CSS permet de gagner du temps de frappe et peut faciliter la lecture et la maintenance de la syntaxe. Jusqu’à présent, vous deviez taper des chemins de sélecteur complets comme ceci :

.parent1 .child1,
.parent2 .child1 {
  color: red;
}

.parent1 .child2,
.parent2 .child2 {
  color: green;
}

.parent1 .child2:hover,
.parent2 .child2:hover {
  color: blue;
}

Maintenant vous pouvez nid sélecteurs enfants à l’intérieur de leur parent, comme ceci :

.parent1, .parent2 {

  .child1 {
    color: red;
  }

  .child2 {
    color: green;

    &:hover {
      color: blue;
    }
  }

}

Vous pouvez imbriquer les sélecteurs aussi profondément que vous le souhaitez, mais méfiez-vous d’aller au-delà de deux ou trois niveaux. Il n’y a pas de limite technique à la profondeur d’imbrication, mais cela peut rendre le code plus difficile à lire et le CSS résultant peut devenir inutilement verbeux.

Jusqu’en avril 2023, aucun navigateur ne comprenait la syntaxe CSS imbriquée. Vous aviez besoin d’une étape de génération avec un préprocesseur CSS tel que Sass, Less ou PostCSS pour transformer le code imbriqué en syntaxe de sélecteur complet standard. L’imbrication est maintenant arrivée dans Chrome 112+ et Safari 16.5+, avec la prise en charge de Firefox plus tard en 2023 (la version 115 l’a disponible derrière un drapeau de fonctionnalité).

Pouvez-vous abandonner votre préprocesseur en faveur de l’imbrication native ? Comme d’habitude… ça dépend. La syntaxe d’imbrication native a évolué au cours des deux dernières années. Il est superficiellement similaire à Sass – qui plaira à la plupart des développeurs Web – mais ne vous attendez pas à ce que tout le code SCSS fonctionne directement comme vous le souhaitez.

Règles d’imbrication CSS natives

Vous pouvez imbriquer n’importe quel sélecteur dans un autre, mais il devoir commencer par un symbole tel que &, . (pour un HTML class), # (pour un HTML id), @ (pour une requête média), :, ::, *, +, ~, >ou [. In other words, it cannot be a direct reference to an HTML element. This code is invalid and the <p> selector is not parsed:

.parent1 {

  
  p {
    color: blue;
  }

}

The easiest way to fix this is to use an ampersand (&), which references the current selector in an identical way to Sass:

.parent1 {

  
  & p {
    color: blue;
  }

}

Alternatively, you could use one of these:

• > p — but this would style direct children of .parent1 only

• :is(p) — but :is() uses the specificity of the most specific selector

• :where(p) — but :where() has zero specificity

They would all work in this simple example, but you could encounter specificity issues later with more complex stylesheets.

The & also allows you to target pseudo-elements and pseudo-classes on the parent selector. For example:

p.my-element {

  &::after {}

  &:hover {}

  &:target {}

}

If you don’t use &, you’ll be targeting all child elements of the selector and not p.my-element itself. (The same would occur in Sass.)

Note that you can use an & anywhere in the selector. For example:

.child1 {

  .parent3 & {
    color: red;
  }

}

This translates to the following non-nested syntax:

.parent3 .child1 { color: red; }

You can even use multiple & symbols in a selector:

ul {

  & li & {
    color: blue;
  }

}

This would target nested <ul> elements (ul li ul), but I’d recommend against using this if you want to keep your sanity!

Finally, you can nest media queries. The following code applies a cyan color to paragraph elements — unless the browser width is at least 800px, when they become purple:

p {

  color: cyan;

  @media (min-width: 800px) {
    color: purple;
  }

}

Native CSS Nesting Gotchas

Native nesting wraps parent selectors in :is(), and this can lead to differences with Sass output.

Consider the following nested code:

.parent1, #parent2 {
  .child1 {}
}

This effectively becomes the following when it’s parsed in a browser:

:is(.parent1, #parent2) .child1 {}

A .child1 element inside .parent1 has a specificity of 101, because :is() uses the specificity of its most specific selector — in this case, the #parent2 ID.

Sass compiles the same code to this:

.parent1 .child1,
#parent2 .child1 {
}

In this case, a .child1 element inside .parent1 has a specificity of 002, because it matches the two classes (#parent2 is ignored). Its selector is less specific than the native option and has a greater chance of being overridden in the cascade.

You may also encounter a subtler issue. Consider this:

.parent .child {

  .grandparent & {}

}

The native CSS equivalent is:

.grandparent :is(.parent .child) {}

This matches the following mis-ordered HTML elements:

<div class="parent">
  <div class="grandparent">
    <div class="child">MATCH</div>
  </div>
</div>

MATCH becomes styled because the CSS parser does the following:

1. It finds all elements with a class of child which also have an ancestor of parent — at any point up the DOM hierarchy.

2. Having found the element containing MATCH, the parser checks whether it has an ancestor of grandparent — again, at any point up the DOM hierarchy. It finds one and styles the element accordingly.

This is not the case in Sass, which compiles to this:

.grandparent .parent .child {}

The HTML above is not styled, because the element classes don’t follow a strict grandparent, parent, and child order.

Finally, Sass uses string replacement, so declarations such as the following are valid and match any element with an outer-space class:

.outer {
  &-space { color: black; }
}

Native CSS ignores the &-space selector.

Do You Still Require a CSS Preprocessor?

In the short term, your existing CSS preprocessor remains essential. Native nesting is not supported in Firefox or Chrome/Safari-based browsers, which have not received updates in a few months.

The Sass development team has announced they will support native CSS nesting in .css files and output the code as is. They will continue to compile nested SCSS code as before to avoid breaking existing codebases, but will start to emit :is() selectors when global browser support reaches 98%.

I suspect preprocessors such as PostCSS plugins will expand nested code for now but remove the feature as browser support becomes more widespread.

Of course, there are other good reasons to use a preprocessor — such as bundling partials into a single file and minifying code. But if nesting is the only feature you require, you could certainly consider native CSS for smaller projects.

Summary

CSS nesting is one of the most useful and practical preprocessor features. The browser vendors worked hard to create a native CSS version which is similar enough to please web developers. There are subtle differences, though, and you may encounter unusual specificity issues with (overly) complex selectors, but few codebases will require a radical overhaul.

Native nesting may make you reconsider your need for a CSS preprocessor, but they continue to offer other benefits. Sass and similar tools remain an essential part of most developer toolkits.

To delve more into native CSS nesting, check out the W3C CSS Nesting Specification.