CSS Scoping Module Level 1

Abstract

This specification defines various scoping/encapsulation mechanisms for CSS, including scoped styles and the @scope rule, Shadow DOM selectors, and page/region-based styling. CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

Table of Contents

1 Introduction

...

2 Scoped Styles

Scoped style rules apply only within a subtree of a document, rather than matching against the entire document. Scoping has two primary effects:

• The selector of the scoped style rule is restricted to match only elements within scope. See Scoped Selectors in [SELECTORS4].
• The cascade prioritizes scoped rules over unscoped ones, regardless of specificity. See Cascading by Scope in [CSS3CASCADE].

2.1 Scoping Mechanisms

Style rules can be scoped using constructs defined in the document language or using the @scope rule in CSS.

2.1.1 Document Markup for Scoping

Document languages may define a mechanism for a stylesheet to be scoped to some element in the document. For example, in HTML, a style element with a scoped attribute defines a stylesheet that is scoped to the style element’s parent element. [HTML]

2.1.2 CSS Syntax for Scoping: the @scope rule

The @scope at-rule allows authors to create scoped style rules using CSS syntax. The syntax of the @scope rule is:

@scope <selector> {
  <stylesheet>
}

where the elements matched by the <selector> are scoping roots for the style rules in <stylesheet>, and selectors of style rules scoped by @scope are scope-contained to their scoping root.

This rule makes it very easy for authors to create scoped style sheets, which could affect the optimization strategies for implementing scoped styles.

If multiple elements match the <selector>, the <stylesheet> is effectively duplicated and scoped independently to each one. Authors should avoid using overly-generic selectors as it can have confusing interactions with the cascade.

@scope div {
  span {
    color: blue;
  }
}
@scope section {
  span {
    color: orange;
  }
}

<div>
  <section>
    <div>
      <span>text</span>
    </div>
  </section>
</div>

@scope rules can be nested. In this case, just as with the nested style rules, the selector of an outer @scope scope-contains the selector of the inner one.

The specificity of selectors inside the @scope rule is calculated locally: the selector specifying the scoping element is ignored. However, because scoped styles override non-scoped styles, style rules inside the @scope will override rules outside of it.

  @scope aside {
    p { color: green; }
  }
  aside#sidebar p { color: red; }

If multiple @scope rules apply to an element, should they be cascaded by specificity?

2.2 Querying the Scoping Context

2.2.1 Selecting the Scoping Root: :scope pseudo-class

In a scoped stylesheet, the :scope pseudo-class, defined in [SELECTORS4], matches the scoping root.

2.2.2 Selecting Outside the Scope: :scope-context() pseudo-class

:scope-context(<selector>) div {...}
scope(<selector>) div {...}
\scope <selector>\ div {...}
<selector> \scope\ div {...}

3 Shadow Encapsulation

The Shadow DOM spec augments the DOM with several new concepts, several of which are relevant to CSS.

A shadow tree is a document fragment that can be attached to any element in the DOM. The root of the shadow tree is a shadow root, a non-element node which is associated with a shadow host. An element can have any number of shadow trees, which are ordered by creation time. The most recently-created shadow tree on an element is the active shadow tree for that element.

An element with a shadow tree is a shadow host. It is the host element for its shadow trees.

The descendants of a shadow host must not generate boxes in the formatting tree. Instead, the contents of the active shadow tree generate boxes as if they were the contents of the element instead.

In several instances in shadow DOM, elements don’t have element parents (instead, they may have a shadow root as parent, or something else). An element without a parent, or whose parent is not an element, is called a top-level element.

While the children of a shadow host do not generate boxes normally, they can be explicitly pulled into a shadow tree and forced to render normally. This is done by assigning the elements to a distribution list. An element with a distribution list is an insertion point.

This specification does not define how to assign elements to a distribution list, instead leaving that to the Shadow DOM spec. At the time this spec is written, however, only content elements in a shadow tree can have distribution lists.

An insertion point must not generate any boxes. Instead, the elements in its distribution list generate boxes as normal, as if they all replaced the insertion point in-place. (Akin to the behavior of display-box: contents.)

3.1 Shadow DOM Selection Model

Elements in the element tree additionally have zero or more shadow trees and zero or one distribution lists.

Note: The "descendants" of an element are based on the children of the element, which does not include the shadow trees or distribution lists of the element.

When a selector is matched against a shadow tree, the initial selector match list is the shadow host, followed by all the top-level elements of the shadow tree and their descendants, ordered by a pre-order traversal.

3.1.1 Host Elements in a Shadow Tree

A shadow host is outside of the shadow trees it hosts, but it is sometimes useful to be able to style it from inside the shadow tree context.

For the purpose of Selectors, a host element also appears in each of its shadow trees, with the contents of the shadow tree treated as its children. If an element has multiple shadow trees, it appears in each shadow tree’s context independently; each shadow tree sees itself as the contents of the host element, not the other shadow trees.

The host element is not selectable by any means except for the :host and :host-context() pseudo-classes. That is, in this context the shadow host has no tagname, ID, classes, or attributes, and the only additional information is has is that the :host pseudo-class matches it. In particular, the host element isn’t matched by the * selector either.

3.2 Shadow DOM Selectors

Shadow DOM defines a few new selectors to help select elements in useful way related to Shadow DOM.

This section is still under discussion. Feedback and advice on intuitive syntax for the following functionality would be appreciated.

3.2.1 Selecting Into the Light: the :host, :host(), and :host-context() pseudo-classes

The :host pseudo-class, when evaluated in the context of a shadow tree, matches the shadow tree’s host element. In any other context, it matches nothing.

The :host() function pseudo-class has the syntax:

:host( <compound-selector> )

When evaluated in the context of a shadow tree, it matches the shadow tree’s host element if the host element, in its normal context, matches the selector argument. In any other context, it matches nothing.

  <x-foo class="foo">
    <"shadow tree">
      <div class="foo">...</div>
    </>
  </x-foo>

Ordinary, selectors within a shadow tree can’t see elements outside the shadow tree at all. Sometimes, however, it’s useful to select an ancestor that lies somewhere outside the shadow tree, above it in the document.

The :host-context() functional pseudo-class tests whether there is an ancestor, outside the shadow tree, which matches a particular selector. Its syntax is:

:host-context( <compound-selector> )

When evaluated in the context of a shadow tree, the :host-context() pseudo-class matches the host element, if the host element or one of its ancestors matches the provided <compound-selector>. For the purpose of this pseudo-class, the "ancestor" of an element is:

if the element is distributed to a distribution list

the content element it is ultimately distributed to.

if the element is a top-most element in a shadow tree

the host element

otherwise

the element’s parent, if it has one.

Note: This means that the selector pierces through shadow boundaries on the way up, looking for elements that match its argument, until it reaches the document root.

3.2.2 Selecting Into the Dark: the ::shadow pseudo-element

If an element has at least one shadow tree, the ::shadow pseudo-element matches the shadow roots themselves. In HTML, the shadow root is represented by ShadowRoot objects.

The ::shadow pseudo-element must not generate boxes, unless specified otherwise in another specification. However, for the purpose of Selectors, the ::shadow pseudo-element is considered to be the root of the shadow tree, with the top-level elements in the shadow tree the direct children of the ::shadow pseudo-element.

  <x-foo>
    <"shadow tree">
      <div>
        <span id="not-top">...</span>
      </div>
      <span id="top">...</span>
    </>
  </x-foo>

3.2.3 Selecting Shadow-Projected Content: the ::content pseudo-element

The ::content pseudo-element matches the distribution list itself, on elements that have one.

::content is a confusingly general name for something that is specific to the projected content of a shadow tree.

The ::content pseudo-element must not generate boxes, unless specified otherwise in another specification. However, for the purpose of Selectors, the ::content pseudo-element is considered to be the parent of the elements in the distribution list.

  <x-foo>
    <div id="one" class="foo">...</div>
    <div id="two">...</div>
    <div id="three" class="foo">
      <div id="four">...</div>
    </div>
    <"shadow tree">
      <div id="five">...</div>
      <div id="six">...</div>
      <content select=".foo"></content>
    </"shadow tree">
  </x-foo>

3.2.4 Selecting Through Shadows: the /deep/ combinator

When a /deep/ combinator is encountered in a selector, replace every element in the selector match list with every element reachable from the original element by traversing any number of child lists or shadow trees.

  <x-foo>
    <"shadow tree">
      <div>
        <span id="not-top">...</span>
      </div>
      <span id="top">...</span>
      <x-bar>
        <"shadow tree">
          <span id="nested">...</span>
        </>
      </x-bar>
    </>
  </x-foo>

This is basically a super-descendant combinator. If the descendant combinator had a real glyph, it would potentially be interesting to just double it. Maybe we can give the descendant combinator a pseudonym of >>, as it itself is a super-child combinator? Then /deep/ could be spelled >>>

3.3 Shadow Cascading & Inheritance

3.3.1 Cascading

To address the desired cascading behavior of rules targetting elements in shadow roots, this specification extends the cascade order defined in the Cascade specification. [CSS3CASCADE]

An additional cascade criteria must be added, between Origin and Scope, called Shadow Tree.

• When comparing two declarations, if one of them is in a shadow tree and the other is in a document that contains that shadow tree, then for normal rules the declaration from the outer document wins, and for important rules the declaration from the shadow tree wins.

  Note: This is the opposite of how scoped styles work.

• When comparing two declarations, if both are in shadow trees with the same host element, then for normal rules the declaration from the shadow tree that was created most recently wins, and for important rules the declaration from the shadow tree that was created less recently wins.

When calculating Order of Appearance, the tree of trees, defined by the Shadow DOM specification, is used to calculate ordering.

3.3.2 Inheritance

The top-level elements of a shadow tree inherit from their host element.

The elements in a distribution list inherit from the parent of the content element they are ultimately distributed to, rather than from their normal parent.

4 Fragmented Styling

Fragmented content can be styled differently based on which line, column, page, region, etc. it appears in. This is done by using an appropriate fragment pseudo-element, which allows targetting individual fragments of an element rather than the entire element.

In our example, the designer wants to make text flowing into #region1 dark blue and bold. This design can be expressed as shown below.

#region1::region p {
  color: #0C3D5F;
  font-weight: bold;
}

The ::region pseudo-element is followed by a p relative selector in this example. The color and font-weight declarations will apply to any fragments of paragraphs that are displayed in #region1. The following figure shows how the rendering changes if we apply this styling specific to #region1. Note how less text fits into this box now that the font-weight is bold instead of normal.

Note: This feature is an extension of ::first-line styling.

4.1 Region-based Styling: the ::region pseudo-element

A ::region pseudo-element represents a relationship between a selector that matches a CSS Region, and a relative selector that matches some named flow content. This allows style declarations to be applied to fragments of named flow content flowing into particular regions.

<region selector>::region <content selector>  {
    ... CSS styling declarations ...
}

When the ::region pseudo-element is appended to a selector that matches one or more CSS Regions, this creates a 'flow fragment' selector. The flow fragment selector specifies which range of elements in the flow can be matched by the relative selector. The relative selector can match elements in the range(s) (see [DOM]) of the named flow that are displayed fully or partially in the selected region(s).

Elements that are fully or partially in the flow fragment range may match the relative selector. However, the style declarations only apply to the fragment of the element that is displayed in the corresponding region(s).

Only a limited list of properties apply to a ::region pseudo-element:

Either this list should be all functionally inheritable properties, or all properties. Why is it a seemingly-arbitrary subset of all properties, including box properties?

1. font properties
2. color property
3. opacity property
4. background property
5. word-spacing
6. letter-spacing
7. text-decoration
8. text-transform
9. line-height
10. alignment and justification properties
11. border properties
12. rounded corner properties
13. border images properties
14. margin properties
15. padding properties
16. text-shadow
17. box-shadow
18. box-decoration-break
19. width

In the following example, the named flow “article-flow” flows into “region-1” and “region-2”.

<style>
  #div-1 {
    flow-into: article-flow;
  }

  #region-1, #region-2 {
    flow-from: article-flow;
  }

  /* region styling */
  #region-1::region p  {
    margin-right: 5em;
  }
</style>

<body>
  <div id="div-1">
      <p id="p-1">...</p>
      <p id="p-2">...</p>
  </div>
  <div id="region-1"></div>
  <div id="region-2"></div>
</body>

•  div div-1
•  paragraph p-1
•  paragraph p-2
•  range of flow that fits into region-1
•  range of flow that fits into region-2

The region styling applies to flow content that fits in region-1. The relative selector matches p-1 and p-2 because these paragraphs flow into region-1. Only the fragment of p-2 that flows into region-1 is styled with the pseudo-element.

All of the selectors in a ::region pseudo-element contribute to its specificity. So the specificity of the ::region pseudo-element in the example above would combine the id selector’s specificity with the specificity of the type selector, resulting in a specificity of 101.

Selectors that match a given element or element fragment (as described above), participate in the CSS Cascading order as defined in [CSS21].

We’ll need some way to query the styles of a fragment in a particular region. getComputedStyle() isn’t enough, because an element can exist in multiple regions, for example, with each fragment receiving different styles.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words "for example" or are set apart from the normative text with class="example", like this:

Informative notes begin with the word "Note" and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

References

Normative References

Informative References

Index

Property index

Issues Index

:scope-context(<selector>) div {...}
scope(<selector>) div {...}
\scope <selector>\ div {...}
<selector> \scope\ div {...}