@layer

/* statement at-rules */
@layer layer-name;
@layer layer-name, layer-name, layer-name;

/* block at-rules */
@layer {rules}
@layer layer-name {rules}

where:

layer-name

Is the name of the cascade layer.

rules

Is the set of CSS rules in the cascade layer.

Rules within a cascade layer cascade together, giving more control over the cascade to web developers. Styles that are not defined in a layer always override styles declared in named and anonymous layers.

The following diagram shows layer priorities where layers are declared in 1, 2, ..., N order.

As noted in the above diagram, important declarations, declarations with the !important flag, have priority over normal declarations, or regular declarations without the !important flag. The order of precedence among important rules is the inverse of normal rules. Transitions have the greatest precedence. Next in order of highest to lowest priority are the important user agent declarations, important user declarations, and important author declarations; in that order. Users can specify styles using browser preferences, operating system preferences, or browser extensions. Their important declarations take precedence over author, or web developer written, important declarations.

Within author styles, all important declarations within CSS layers take precedence over any important declarations declared outside of a layer, while all normal declarations within CSS layers have a lower priority than declarations declared outside of a layer. The declaration order matters. The first declared layer gets the lowest priority and the last declared layer gets the highest priority. However, the priority is reversed when the !important flag is used.

The @layer at-rule is used to create a cascade layer in one of three ways.

The first way is to use a @layer block at-rule to create a named cascade layer with the CSS rules for that layer inside, like so:

@layer utilities {
  .padding-sm {
    padding: 0.5rem;
  }

  .padding-lg {
    padding: 0.8rem;
  }
}

The second way is to use a @layer statement at-rule to create one or more comma-separated named cascade layers without assigning any styles. This can be a single layer, as shown below:

@layer utilities;

Multiple layers can be defined at once, as shown below:

@layer theme, layout, utilities;

This is useful because the initial order in which layers are declared indicates which layer has precedence. As with declarations, the last layer to be listed will win if declarations are found in multiple layers. Therefore, with the preceding example, if a competing rule was found in theme and utilities, the one in utilities would win and be applied.

A rule in utilities would be applied even if it has lower specificity than the rule in theme. This is because once the layer order has been established, specificity and order of appearance are ignored. This enables using simpler CSS selectors because you do not have to ensure that a selector will have high enough specificity to override competing rules; all you need to ensure is that it appears in a later layer.

The third way is to create an unnamed layer using a @layer block at-rule without including a layer name. For example:

@layer {
  p {
    margin-block: 1rem;
  }
}

This creates an anonymous cascade layer. This layer functions in the same way as named layers; however, rules cannot be assigned to it later. The order of precedence for anonymous layers is the order in which layers are declared, named or not, and lower than the styles declared outside of a layer.

Another way to create a cascade layer is by using @import. In this case, the rules would be in the imported stylesheet. Remember that the @import at-rule must precede all other types of rules, except @charset and @layer rules.

@import "theme.css" layer(utilities);

Layers may be nested. For example:

@layer framework {
  @layer layout {
  }
}

To append rules to the layout layer inside framework, join the two names with a ..

@layer framework.layout {
  p {
    margin-block: 1rem;
  }
}

@layer = 
  @layer <layer-name>? { <rule-list> }  |
  @layer <layer-name># ;                

<layer-name> = 
  <ident> [ '.' <ident> ]*  

In the following example, two CSS rules are created. One for the <p> element outside of any layer and one inside a layer named type for .box p.

Without layers, the selector .box p would have the highest specificity, and therefore, the text Hello, world! will display in green. As the type layer comes before the anonymous layer created to hold non-layer content, the text will be purple.

Also notice the order. Even though we declare the non-layered style first, it's still applied after the layer styles.

HTML

<div class="box">
  <p>Hello, world!</p>
</div>

CSS

p {
  color: rebeccapurple;
}

@layer type {
  .box p {
    font-weight: bold;
    font-size: 1.3em;
    color: green;
  }
}

Result

In the following example, two layers are created with no rules applied, then CSS rules are applied to the two layers. The base layer defines a color, border, font-size, and padding. The special layer defines a different color. As special comes last when the layers were defined, the color it provides is used and the text is displayed using rebeccapurple. All of the other rules from base still apply.

HTML

<div class="item">
  I am displayed in <code>color: rebeccapurple</code> because the
  <code>special</code> layer comes after the <code>base</code> layer. My green
  border, font-size, and padding come from the <code>base</code> layer.
</div>

CSS

@layer base, special;

@layer special {
  .item {
    color: rebeccapurple;
  }
}

@layer base {
  .item {
    color: green;
    border: 5px solid green;
    font-size: 1.3em;
    padding: 0.5em;
  }
}

Result

• @import
• CSSLayerBlockRule
• CSSLayerStatementRule
• !important
• revert-layer
• Introducing the CSS cascade
• Cascade, specificity, and inheritance
• Cascade layers
• The future of CSS: Cascade layers on bram.us (2021)