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.

Strong Scoping Proximity

If two declarations both have elements selected by scoped descendant relationships applying strong scoping proximity, then the declaration with the fewest generational hops between the ancestor/descendant element pair wins.

If multiple such pairs are represented, their strong scoping proximity weights are compared from innermost scoping relationship to outermost scoping relationship (with any missing pairs weighted as infinity).

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.

Weak Scoping Proximity

If two declarations both have elements selected by scoped descendant relationships applying weak scoping proximity, then the declaration with the fewest generational hops between the ancestor/descendant element pair wins.

If multiple such pairs are represented, their weak scoping proximity weights are compared from innermost scoping relationship to outermost scoping relationship (with any missing pairs weighted as infinity).

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]

Does scope proximity belong above or below specificity in the cascade? [Issue #6790]

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 <stylesheet> are scoped style rules.

• The :scope selector is defined to match the @scope rule’s 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 source order by applying weak scoping proximity between the scoping root and the subject of each scoped style rule.

Should @scope use strong or weak scoping proximity? Strong scoping proximity causes declarations to be weighted more strongly by scope proximity than by their selector’s specificity. Weak scoping proximity causes declarations of the same specificity to be weighted by proximity to their scoping root before falling back to source ordering, but declarations of higher specificity win over more tightly-scoped declarations. The Working Group currently leans towards weak proximity, and recommends that as a starting point for prototypes. [Issue #6790]

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>)]? {
  <stylesheet>
}

where:

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

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

• the <stylesheet> 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, then 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 selectors of the inner @scope (including those defining its scope) are scoped by the selectors of the outer one.

@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. Scoped Descendant Combinator

The scoped descendant combinator describes a descendant relationship between two elements. A selector of the form A >> B represents an element B that is an arbitrary descendant of some ancestor element A.

This combinator differs from the descendant combinator in that it applies weak scoping proximity to the relationship between A and B. It does not change the :scope element.

Should the scoped descendant combinator use strong or weak scoping proximity? Should it even exist? It’s defined here to work the way many people expected the regular descendant combinator to work...

.light-scheme >> a { color: darkmagenta; }
.dark-scheme >> a { color: plum; }

.light-scheme >> a:focus { color: teal; }

2.7. 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 must return a value as follows:

The @scope rule has an associated <scope-start>

The result of serializing that <scope-start>.

Otherwise

An empty string.

end of type CSSOMString

The end attribute must return a value as follows:

The @scope rule has an associated <scope-end>

The result of serializing that <scope-end>.

Otherwise

An empty string.

4. Changes

This appendix is informative.

4.1. 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 the CSSScopeRule interface.

• 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)

4.2. 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

4.3. 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.

4.4. 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.

4.5. 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.