CSS Cascading and Inheritance Level 6

1. Introduction and Missing Sections

This is a diff spec over CSS Cascading and Inheritance Level 5. It is currently an Exploratory Working Draft: if you are implementing anything, please use Level 5 as a reference. We will merge the Level 5 text into this draft once it reaches CR.

2. Cascading

The cascade takes an unordered list of declared values for a given property on a given element, sorts them by their declaration’s precedence as determined below, and outputs a single cascaded value.

2.1. Cascade Sorting Order

The cascade sorts declarations according to the following criteria, in descending order of precedence:

Origin and Importance

The origin of a declaration is based on where it comes from and its importance is whether or not it is declared with !important (see below). The precedence of the various origins is, in descending order:

1. Transition declarations [css-transitions-1]
2. Important user agent declarations
3. Important user declarations
4. Important author declarations
5. Animation declarations [css-animations-1]
6. Normal author declarations
7. Normal user declarations
8. Normal user agent declarations

Declarations from origins earlier in this list win over declarations from later origins.

Context

A document language can provide for blending declarations sourced from different encapsulation contexts, such as the nested tree contexts of shadow trees in the [DOM].

When comparing two declarations that are sourced from different encapsulation contexts, then for normal rules the declaration from the outer context wins, and for important rules the declaration from the inner context wins. For this purpose, [DOM] tree contexts are considered to be nested in shadow-including tree order.

Note: This effectively means that normal declarations belonging to an encapsulation context can set defaults that are easily overridden by the outer context, while important declarations belonging to an encapsulation context can enforce requirements that cannot be overridden by the outer context.

The Style Attribute

Separately for normal and important declarations, declarations that are attached directly to an element (such as the contents of a style attribute) rather than indirectly mapped by means of a style rule selector take precedence over declarations the same importance that are mapped via style rule.

Layers

Declarations within each origin and context can be explicitly assigned to a cascade layer. For the purpose of this step, any declaration not assigned to an explicit layer is added to an implicit final layer.

Cascade layers (like declarations) are sorted by order of appearance, see § 2.4.1 Layer Ordering. When comparing declarations that belong to different layers, then for normal rules the declaration whose cascade layer is latest in the layer order wins, and for important rules the declaration whose cascade layer is earliest wins.

Note: This follows the same logic used for precedence of normal and important origins, thus the !important flag maintains the same “override” purpose in both settings.

Specificity

The Selectors module [SELECT] describes how to compute the specificity of a selector. Each declaration has the same specificity as the style rule it appears in. The declaration with the highest specificity wins.

Scope Proximity

When comparing declarations that appear in style rules with different scoping roots, then the declaration with the fewest generational or sibling-element hops between the scoping root and the scoped style rule subject wins. For this purpose, style rules without a scoping root are considered to have infinite proximity hops.

Order of Appearance

The last declaration in document order wins. For this purpose:

• Style sheets are ordered as in final CSS style sheets.
• Declarations from imported style sheets are ordered as if their style sheets were substituted in place of the @import rule.
• Declarations from style sheets independently linked by the originating document are treated as if they were concatenated in linking order, as determined by the host document language.
• Declarations from style attributes are ordered according to the document order of the element the style attribute appears on, and are all placed after any style sheets. [CSSSTYLEATTR]

The output of the cascade is a (potentially empty) sorted list of declared values for each property on each element.

2.2. Cascading Origins

CSS Cascading 5 § 6.2 Cascading Origins

cascade origin

2.3. Important Declarations: the !important annotation

CSS Cascading 5 § 6.3 Important Declarations: the !important annotation

important normal

2.4. Cascade Layers

CSS Cascading 5 § 6.4 Cascade Layers

2.4.1. Layer Ordering

CSS Cascading 5 § 6.4.3 Layer Ordering

2.5. Scoping Styles: the @scope rule

A scope is a subtree or fragment of a document, which can be used by selectors for more targeted matching. A scope is formed by determining:

• The scoping root node, which acts as the upper bound of the scope, and optionally:

• The scoping limit elements, which act as the lower bounds.

An element is in scope if:

• It is an inclusive descendant of the scoping root, and

• It is not an inclusive descendant of a scoping limit.

Note: In contrast to Shadow Encapsulation, which describes a persistent one-to-one relationship in the DOM between a shadow host and its nested shadow tree, multiple overlapping scopes can be defined in relation to the same elements.

Scoped styles are described in CSS using the @scope block at-rule, which declares a scoping root and optional scoping limits associated with a set of style rules.

@scope (.light-scheme) {
  /* Only match links inside a light-scheme */
  a { color: darkmagenta; }
}
@scope (.dark-scheme) {
  /* Only match links inside a dark-scheme */
  a { color: plum; }
}
@scope (.media-object) {
  /* Only match author images inside a media-object */
  .author-image { border-radius: 50%; }
}

@scope (.media-object) to (.content > *) {
  img { border-radius: 50%; }
  .content { padding: 1em; }
}

Should scoping limits be added to the definition of scoped selectors?

2.5.1. Effects of @scope

The @scope at-rule has three primary effects on the style rules it contains:

• The style rules in an @scope <rule-list> are scoped style rules.

• The :scope selector is defined to match the @scope rule’s scoping root, including the featureless shadow host when that host is the scoping root. The & selector is defined to represent the selector representing the scoping root (the <scope-start> selector), or else :scope if no selector was specified.

• The cascade prioritizes declarations with a more proximate scoping root, regardless of specificity or order of appearance by applying scope proximity between the scoping root and the subject of each scoped style rule.

Note: Unlike Nesting, selectors within an @scope rule do not acquire the specificity of any parent selector(s) in the @scope prelude.

@scope (#hero) {
  img { border-radius: 50%; }
}
:where(#hero) img { border-radius: 50%; }

<section data-scope="main-component">
  <p data-scope="main-component">...<p>
  <!-- sub-component root is in both scopes -->
  <section data-scope="main-component sub-component">
    <!-- children are only in the inner scope -->
    <p data-scope="sub-component">...<p>
  </section>
</section>

p[data-scope~='main-component'] { color: red; }
p[data-scope~='sub-component'] { color: blue; }
/* both sections are part of the outer scope */
section[data-scope~='main-component'] { background: snow; }
/* the inner section is also part of the inner scope */
section[data-scope~='sub-component'] { color: ghostwhite; }

<section data-scope="main-component">
  <p>...<p>
  <section data-scope="sub-component">
    <p>...<p>
  </section>
</section>

@scope ([data-scope='main-component']) to ([data-scope]) {
  p { color: red; }
  /* only the outer section is part of the outer scope */
  section { background: snow; }
}
@scope ([data-scope='sub-component']) to ([data-scope]) {
  p { color: blue; }
  /* the inner section is only part of the inner scope */
  section { color: ghostwhite; }
}

@scope ([data-scope='main-component']) to ([data-scope] > *) {
  p { color: red; }
  /* both sections are part of the outer scope */
  section { background: snow; }
}

2.5.2. Syntax of @scope

The syntax of the @scope rule is:

@scope [(<scope-start>)]? [to (<scope-end>)]? {
  <rule-list>
}

where:

• <scope-start> is a <selector-list> selector used to identify the scoping root(s).

• <scope-end> is a <selector-list> selector used to identify any scoping limits.

• the <rule-list> represents the scoped style rules.

Pseudo-elements cannot be scoping roots or scoping limits; they are invalid both within <scope-start> and <scope-end>.

2.5.3. Scoped Style Rules

Scoped style rules differ from non-scoped rules in the following ways:

• Their selectors can only match elements that are in scope. (This only applies to the subject; the rest of the selector can match unrestricted.)

• They accept a <relative-selector-list> as their prelude (rather than just a <selector-list>). Such relative selectors are relative to :scope.

• Any selector in the <relative-selector-list> that does not start with a combinator but does contain the nesting selector or the :scope selector, is interpreted as a non-relative selector (but the subject must still be in scope to match).

@scope (#my-component) {
  p { color: green; }
  :scope p { color: green; }
}

@scope (#my-component) {
  > p { color: green; }
  :scope > p { color: green; }
}

@scope (#my-component) {
  :scope { border: thin solid; }
  & { border: thin solid; }
  main :scope p { color: green; }
  main & p { color: green; }
}

2.5.4. Identifying Scoping Roots and Limits

A @scope rule produces one or more scopes as follows:

Finding the scoping root(s)

For each element matched by <scope-start>, create a scope using that element as the scoping root. If no <scope-start> is specified, the scoping root is the parent element of the owner node of the stylesheet where the @scope rule is defined. (If no such element exists and the containing node tree is a shadow tree, then the scoping root is the shadow host. Otherwise, the scoping root is the root of the containing node tree.) Any :scope or & selectors in <scope-start> are interpreted as defined for its outer context.

Finding any scoping limits

For each scope created by a scoping root, its scoping limits are set to all elements that are in scope and that match <scope-end>, interpreting :scope and & exactly as in scoped style rules.

<div>
  <style>
    @scope {
      p { color: red; }
    }
  </style>
  <p>this is red</p>
</div>
<p>not red</p>

<div id="foo">
  <style>
    @scope (#foo) {
      p { color: red; }
    }
  </style>
  <p>this is red</p>
</div>
<p>not red</p>

/* .content is only a limit when it is a direct child of the :scope */
@scope (.media-object) to (:scope > .content) { ... }

/* .content is only a limit when the :scope is inside .sidebar */
@scope (.media-object) to (.sidebar :scope .content) { ... }

2.5.5. Scope Nesting

@scope rules can be nested. In this case, just as with the nested style rules, the prelude selectors of the inner @scope (those defining its scope) are scoped by the selectors of the outer one.

Note: The resulting scope for further nested scoped style rules is practically constrained by both the outer and inner @scope rules, but the scoping root is defined by the innermost @scope. Since scope proximity is measured between a scoped style rule subject and scoping root, only the innermost @scope matters for determining scope proximity of nested @scope rules.

Should the scope proximity calculation be impacted by nesting scopes? [Issue #10795]

@scope (.parent-scope) {
  @scope (:scope > .child-scope) to (:scope .limit) {
    :scope .content {
      color: red;
    }
  }
}

@scope (.parent-scope > .child-scope) to (.parent-scope > .child-scope .limit) {
  .parent-scope > .child-scope .content {
    color: red;
  }
}

Global name-defining at-rules such as @keyframes or @font-face or @layer that are defined inside @scope are valid, but are not scoped or otherwise affected by the enclosing @scope rule. However, any style rules contained by such rules (e.g. within @layer) are scoped.

2.6. Precedence of Non-CSS Presentational Hints

CSS Cascading 5 § 6.4 Cascade Layers

3. CSSOM

3.1. The CSSScopeRule interface

The CSSScopeRule interface represents the @scope rule:

[Exposed=Window]
interface CSSScopeRule : CSSGroupingRule {
  readonly attribute CSSOMString? start;
  readonly attribute CSSOMString? end;
};

start of type CSSOMString

The start attribute returns the result of serializing the <scope-start> of the rule (without the enclosing parentheses), or null if there is no <scope-start>.

end of type CSSOMString

The end attribute returns the result of serializing the <scope-end> of the rule (without the enclosing parentheses), or null if there is no <scope-end>.

4. Changes

This appendix is informative.

4.1. Changes since the 21 March 2023 Working Draft

Significant changes since the 21 March 2023 Working Draft include:

• The :scope selector can match the featureless shadow host when that host is the scoping root element. (Issue 9025)

• <scope-start> and <scope-end> selectors are unforgiving. (Issue 10042)

• A @scope rule without <scope-start> scopes to the shadow host instead of the shadow root. (Issue 9178)

• Clarified that scope proximity is a single measurement of the steps between a single scoping root and scoped style rule subject (Issue 10795 has been opened to discuss this futher).

• Removed strong scope proximity. (Issue 6790)

• Removed the scoped descendant combinator (deferred). (Issue 8628)

• Added the CSSScopeRule interface. (Issue 8626)

4.2. Changes since the 21 December 2021 First Public Working Draft

Significant changes since the 21 December 2021 First Public Working Draft include:

• Clarified @scope effects on nested :scope and & selectors. (Issue 8377)

• Removed @scope prelude from specificity calculation. (Issue 8500)

• Specified how name-defining at-rules behave in @scope. (Issue 6895)

• Added implicit scopes by making <scope-start> optional. (Issue 6606)

• Disallowed pseudo-elements in the @scope prelude. (Issue 7382)

• Removed selector scoping notation. (Issue 7709)

• Scoping limit elements are excluded from the resulting scope. (Issue 6577)

4.3. Additions Since Level 5

The following features have been added since Level 5:

• The definition of a scope, as described by a combination of <scope-start> and <scope-end> selectors.

• The in-scope (:in()) pseudo-class for selecting with lower-boundaries

• The @scope rule for creating scoped stylesheets

• The definition of scope proximity in the cascade

4.4. Additions Since Level 4

The following features have been added since Level 4:

• Added cascade layers to the cascade sort criteria (and defined style attributes as a distinct step of the cascade sort criteria so that they interact appropriately).

• Introduced the @layer rule for defining cascade layers.

• Added layer/layer() option to @import definition.

• Introduced the revert-layer keyword for rolling back values to previous layers.

4.5. Additions Since Level 3

The following features have been added since Level 3:

• Introduced revert keyword, for rolling back the cascade.

• Introduced supports() syntax for supports-conditional @import rules.

• Added encapsulation context to the cascade sort criteria to accommodate Shadow DOM. [DOM]

• Defined the property two aliasing mechanisms CSS uses to support legacy syntaxes. See CSS Cascading 4 § 3.1 Property Aliasing.

4.6. Additions Since Level 2

The following features have been added since Level 2:

• The all shorthand
• The initial keyword
• The unset keyword
• Incorporation of animations and transitions into the cascade.

Acknowledgments

David Baron, Tantek Çelik, Keith Grant, Giuseppe Gurgone, Theresa O’Connor, Florian Rivoal, Noam Rosenthal, Simon Sapin, Jen Simmons, Nicole Sullivan, Lea Verou, and Boris Zbarsky contributed to this specification.

5. Privacy Considerations

• User preferences and UA defaults expressed via application of style rules are exposed by the cascade process, and can be inferred from the computed styles they apply to a document.

6. Security Considerations

• The cascade process does not distinguish between same-origin and cross-origin stylesheets, enabling the content of cross-origin stylesheets to be inferred from the computed styles they apply to a document.

• The @import rule does not apply the CORS protocol to loading cross-origin stylesheets, instead allowing them to be freely imported and applied.

• The @import rule assumes that resources without Content-Type metadata (or any same-origin file if the host document is in quirks mode) are text/css, potentially allowing arbitrary files to be imported into the page and interpreted as CSS, potentially allowing sensitive data to be inferred from the computed styles they apply to a document.