CSS Scoping Module Level 1

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-scoping/
Latest published version:
https://www.w3.org/TR/css-scoping-1/
Previous Versions:
Test Suite:
http://test.csswg.org/suites/css-scoping-1_dev/nightly-unstable/
Issue Tracking:
CSSWG Issues Repository
Inline In Spec
Editor:
Tab Atkins Jr. (Google)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This specification defines scoping/encapsulation mechanisms for CSS, focusing on the Shadow DOM scoping mechanism.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-scoping” in the title, like this: “[css-scoping] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 15 September 2020 W3C Process Document.

1. Introduction

...

2. Default Styles for Custom Elements

This section is experimental, and is under active discussion. Do not implement without consulting the CSSWG.

When defining custom elements, one often wants to set up "default" styles for them, akin to the user-agent styles that apply to built-in elements. This is, unfortunately, hard to do in vanilla CSS, due to issues of scoping and specificity—the element in question might be used in shadow trees, and thus is unreachable by any selector targeting it in the outermost document; and selectors, even low-specificity ones like simple type selectors, can accidentally override author-level styles meant to target the element.

To aid in this, this section defines a way to create a stylesheet of "default element styles" for a given element. This stylesheet applies across the entire document, in all shadow trees, and the rules in it apply at the user-agent origin, so author-level rules automatically win.

Windows gain a private slot [[defaultElementStylesMap]] which is a map of local names to stylesheets.

These stylesheets must apply to every document in the window. They must be interpreted as user agent stylesheets.

Note: This implies, in particular, that they apply to all shadow trees in every document, and that the declarations in them are from the user-agent origin.

For the purpose of the cascade, these stylesheets are ordered after the user agent’s own stylesheets; their relative ordering doesn’t matter as it is not observable.

Within these stylesheets, complex selectors must be treated as invalid. Every compound selector must be treated as containing an additional type selector that selects elements with the local name that the stylesheet is keyed with.

Do we need to restrict the at-rules that can be used in these sheets? For example, do we allow an @font-face? I’m going to leave it as allowed unless/until I hear complaints.

This specification does not define how to add to, remove from, or generally manipulate the [[defaultElementStylesMap]]. It is expected that other specifications, such as [DOM], will define ways to do so.

3. Shadow Encapsulation

3.1. Informative Explanation of Shadow DOM

The following is a non-normative explanation of several concepts normatively defined in the DOM Standard [DOM], to aid in understanding what this spec defines without having to fully grok the DOM Standard.

In addition to the qualities of an element tree defined in Selectors Level 4 § data-model, the DOM Standard adds several new concepts related to shadow trees, several of which are relevant to CSS.

An element can host a shadow tree, which is a special kind of document fragment with a shadow root (a non-element node) at its root. Children of the shadow root are ordinary elements and other nodes. The element hosting the shadow tree is its host, or shadow host.

The elements in a shadow tree are not descendants of the shadow host in general (including for the purposes of Selectors like the descendant combinator). However, the shadow tree, when it exists, is used in the construction of the flattened element tree, which CSS uses for all purposes after Selectors (including inheritance and box construction).

Loosely, the shadow tree is treated as the shadow host’s contents instead of its normal light tree contents. However, some of its light tree children can be "pulled into" the shadow tree by assigning them to slots. This causes them to be treated as children of the slot for CSS purposes. The slots can then be assigned to slots in deeper shadow trees; luckily, slots themselves don’t generate boxes by default, so you don’t get an unpredictable cascade of slot wrapper elements disrupting your CSS.

If nothing is explicitly assigned to a slot, the slot’s own children are instead assigned to it, as a sort of "default" contents.

3.2. Shadow DOM and Selectors

3.2.1. Matching Selectors Against Shadow Trees

When a selector is matched against a shadow tree, the selector match list is initially the shadow host, followed by all children of the shadow tree’s shadow root and their descendants, ordered by a pre-order traversal.

Rewrite this against the newer call forms of the matching algorithms.

Note: Remember that the descendants of an element are based on the light tree children of the element, which does not include the shadow trees of the element.

When a selector is matched against a tree, its tree context is the root of the root elements passed to the algorithm. If the tree context is a shadow root, that selector is being matched in the context of a shadow tree.

For example, any selector in a stylesheet embedded in or linked from an element in a shadow tree is in the context of a shadow tree. So is the argument to querySelector() when called from a shadow root.

Declarations inherit the tree context of the selector that was matched to apply them.

3.2.2. Selecting Shadow Hosts from within a Shadow Tree

A shadow host is outside of the shadow tree it hosts, and so would ordinarily be untargettable by any selectors evaluated in the context of the shadow tree (as selectors are limited to a single tree), but it is sometimes useful to be able to style it from inside the shadow tree context.

For the purpose of Selectors, a shadow host also appears in its shadow tree, with the contents of the shadow tree treated as its children. (In other words, the shadow host is treated as replacing the shadow root node.)

When considered within its own shadow trees, the shadow host is featureless. Only the :host, :host(), and :host-context() pseudo-classes are allowed to match it.

Why is the shadow host so weird?

The shadow host lives outside the shadow tree, and its markup is in control of the page author, not the component author.

It would not be very good if a component used a particular class name internally in a shadow tree stylesheet, and the page author using the component accidentally also used the same class name and put it on the shadow host. Such a situation would result in accidental styling that is impossible for the component author to predict, and confusing for the page author to debug.

However, there are still some reasonable use-cases for letting a stylesheet in a shadow tree style its shadow host. (For example, the component might want to be laid out as a flexbox, requiring the shadow host to be set to display: flex.) So, to allow this situation but prevent accidental styling, the shadow host appears but is completely featureless and unselectable except through :host and its related functional forms, which make it very explicit when you’re trying to match against markup provided by the page author.

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

:host-context()

In only one current engine.

FirefoxNoneSafariNoneChrome54+
Opera41+Edge79+
Edge (Legacy)NoneIENone
Firefox for AndroidNoneiOS SafariNoneChrome for Android54+Android WebView54+Samsung Internet6.0+Opera Mobile41+

:host

In all current engines.

Firefox63+Safari10+Chrome54+
Opera41+Edge79+
Edge (Legacy)NoneIENone
Firefox for Android63+iOS Safari10+Chrome for Android54+Android WebView54+Samsung Internet6.0+Opera Mobile41+

:host()

In all current engines.

Firefox63+Safari10+Chrome54+
Opera41+Edge79+
Edge (Legacy)NoneIENone
Firefox for Android63+iOS Safari10+Chrome for Android54+Android WebView54+Samsung Internet6.0+Opera Mobile41+

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

The specificity of :host is that of a pseudo-class. The specificity of :host() is that of a pseudo-class, plus the specificity of its argument.

Note: This is different from the specificity of similar pseudo-classes, like :is() or :not(), which only take the specificity of their argument. This is because :host is affirmatively selecting an element all by itself, like a "normal" pseudo-class; it takes a selector argument for syntactic reasons (we can’t say that :host.foo matches but .foo doesn’t), but is otherwise identical to just using :host followed by a selector.

For example, say you had a component with a shadow tree like the following:
<x-foo class="foo">
  <"shadow tree">
    <div class="foo">...</div>
  </>
</x-foo>

For a stylesheet within the shadow tree:

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.

For example, a group of components can define a handful of color themes they know how to respond to. Page authors could opt into a particular theme by adding a specific class to the components, or higher up 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 shadow host, if the shadow host or one of its shadow-including ancestors matches the provided <compound-selector>. In any other context, it matches nothing.

The specificity of :host-context() is that of a pseudo-class, plus the specificity of its argument.

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.

3.2.4. Selecting Slot-Assigned Content: the ::slotted() pseudo-element

::slotted

In all current engines.

Firefox63+Safari10+Chrome50+
Opera37+Edge79+
Edge (Legacy)NoneIENone
Firefox for Android63+iOS Safari10+Chrome for Android50+Android WebView50+Samsung Internet5.0+Opera Mobile37+

The ::slotted() pseudo-element represents the elements assigned, after flattening, to a slot. This pseudo-element only exists on slots.

The ::slotted() pseudo-element is an alias for other elements in the tree, and does not generate any boxes itself.

The grammar of the ::slotted() pseudo-element is:

::slotted( <compound-selector> )

The ::slotted() pseudo-element represents the elements that are:

The ::slotted() pseudo-element can be followed by a tree-abiding pseudo-element, like ::slotted()::before, representing the appropriate pseudo-element of the elements represented by the ::slotted() pseudo-element.

The specificity of ::slotted() is that of a pseudo-element, plus the specificity of its argument.

For example, say you had a component with both children and a shadow tree, like the following:
<x-foo>
  <div id="one" slot="foo" class="foo">...</div>
  <div id="two" slot="foo">...</div>
  <div id="three" class="foo">
    <div id="four" slot="foo">...</div>
  </div>
  <"shadow tree">
    <div id="five">...</div>
    <div id="six">...</div>
    <slot name="foo"></slot>
  </"shadow tree">
</x-foo>

For a stylesheet within the shadow tree, a selector like ::slotted(*) selects #one and #two only, as they’re the elements assigned to the sole slot element. It will not select #three (no slot attribute) nor #four (only direct children of a shadow host can be assigned to a slot).

A selector like ::slotted(.foo), on the other hand, will only select #one, as it matches .foo, but #two doesn’t.

Note: Note that a selector like ::slotted(*) is equivalent to *::slotted(*), where the * selects many more elements than just the slot element. However, since only the slot elements are slots, they’re the only elements with a ::slotted() pseudo-element as well.

Note: ::slotted() can only represent the elements assigned to the slot. Slots can also be assigned text nodes, which can’t be selected by ::slotted(). The only way to style assigned text nodes is by styling the slot and relying on inheritance.

3.3. Shadow Trees and the Cascade

See CSS Cascading 4 § 6.1 Cascade Sorting Order.

3.4. Flattening the DOM into an Element Tree

While Selectors operates on the DOM tree as the host language presents it, with separate trees that are unreachable via the standard parent/child relationship, the rest of CSS needs a single unified tree structure to work with. This is called the flattened element tree (or flat tree), and is constructed as follows:

  1. Let pending nodes be a list of DOM nodes with associated parents, initially containing just the document’s root element with no associated parent.

  2. Repeatedly execute the following substeps until pending nodes is empty:

    1. Pop the first element from pending nodes, and assign it to pending node.

    2. Insert pending node into the flat tree as a child of its associated parent. (If it has no associated parent, it’s the document root—just insert it into the flat tree as its root.)

    3. Perform one of the following, whichever is the first that matches:

      pending node is a shadow host
      Append the child nodes of the shadow root of the shadow tree it hosts to pending nodes, with pending node as their associated parent.
      pending node is a slot
      Find slottables for pending node, and append them to pending nodes, with pending node as their associated parent.

      If no slottables were found for pending node, instead append its children to pending nodes, with pending node as their associated parent.

      Otherwise,
      Append the child nodes of pending node’s light tree to pending nodes, with pending node as their associated parent.

Note: In other words, the flat tree is the top-level DOM tree, but shadow hosts are filled with their shadow tree children instead of their light tree children (and this proceeds recursively if the shadow tree contains any shadow hosts), and slots get filled with the nodes that are assigned to them (and this proceeds recursively if the slots are themselves assigned to a slot in a deeper shadow tree).

A non-obvious result of this is that elements assigned to a slot inherit from that slot, not their light-tree parent or any deeper slots their slot gets assigned to. This means that text nodes are styled by the shadow tree of their parent, with nobody else capable of intervening in any way. Do we want an additional pseudo-element for targeting those text nodes so they can be styled at all slot-assignment levels, like normal elements can be? This implies it needs to work for text nodes in the light tree before they’re assigned downwards, so this can’t just be a ::slotted() variant. Luckily, this is a long-standing request!

3.4.1. Slots and Slotted Elements in a Shadow Tree

Slots must act as if they were assigned display: contents via a rule in the UA origin. This must be possible to override via display, so they do generate boxes if desired.

Note: A non-obvious result of assigning elements to slots is that they inherit from the slot they’re assigned to. Their original light tree parent, and any deeper slots that their slot gets assigned to, don’t affect inheritance.

3.5. Name-Defining Constructs and Inheritance

Shadow trees are meant to be an encapsulation boundary, allowing independent authors to share code without accidentally polluting each other’s namespaces. For example, element IDs, which are generally meant to be unique within a document, can be validly used multiple times as long as each use is in a different shadow tree.

Similarly, several at-rules in CSS, such as @keyframes or @font-face, define a name that later at-rules or properties can refer to them by. Like IDs, these names are globally exposed and unique within a document; also like IDs, this restriction is now loosened to being unique within a given shadow tree.

However, property inheritance can carry values from one tree to another, which complicates referencing the correct definition of a given name. Done naively, this can produce surprising and confusing results for authors. This section defines a set of concepts to use in defining and referencing "global" names in a way that respects encapsulation and doesn’t give surprising results.

If an at-rule or property defines a name that other CSS constructs can refer to it by, such as a @font-face font-family name or an @keyframes name, it must be defined as a tree-scoped name. Tree-scoped names are "global" within a particular node tree; unless otherwise specified, they’re associated with the root of the element hosting the stylesheet that the at-rule or property is defined in.

Properties or descriptors that reference a "global" name, such as the font-family or animation-name properties, must define their value as a tree-scoped reference. Tree-scoped references implicitly capture a node tree root along with their specified value: unless otherwise specified, the root of the element hosting the stylesheet that the property or descriptor is defined in. This root reference stays with the tree-scoped reference as it is inherited.

Whenever a tree-scoped reference is dereferenced to find the CSS construct it is referencing, first search only the tree-scoped names associated with the same root as the tree-scoped reference must be searched. If no relevant tree-scoped name is found, and the root is a shadow root, then repeat this search in the root's host's node tree. (In other words, tree-scoped names "inherit" into descendant shadow trees, so long as they don’t define the same name themselves.)

TODO: Fix all the at-rules that define global names, and the properties that reference them, to use these concepts.

For example, given the following document (using the imaginary <::shadow> markup to indicate an element’s shadow tree):

<p class=outer>Here’s some text in the outer document’s "foo" font.
<style>
  @font-face {
    font-family: foo;
    src: url(https://example.com/outer.woff);
  }
  body { font-family: foo; }
  my-component::part(text) { font-family: foo; }
</style>

<my-component>
  <::shadow>
    <p class=inner-default>I’m inheriting the outer document’s font-family.
    <p class=inner-styled>And I’m explicitly styled to be in the component’s "foo" font.
    <p class=part-styled part=text>
      I’m explicitly styled by the outer document,
      and get the outer document’s "foo" font.
    <style>
      @font-face {
        font-family: foo;
        src: url(https://example.com/inner.woff);
      }
      .inner-styled { font-family: foo; }
    </style>
  </::shadow>
</my-component>

The .outer element references the outer @font-face, using the "outer.woff" file.

The .inner-default element inherits the font-family: foo value from the outer document, using the same tree-scoped reference as .outer, and thus also uses the "outer.woff" font file.

The .inner-style element, on the other hand, receives a font-family: foo from the stylesheet inside the shadow, and thus its tree-scoped reference refers to the shadow’s @font-family, and it uses the "inner.woff" file.

The .part-styled element also receives its style from the outer document, tho by being directly set rather than by inheritance. Thus, its tree-scoped reference also refer’s to the outer document, and it uses the "outer.woff" file.

Here is a more complex example, showing three levels of trees, and illustrating precisely how tree-scoped names and tree-scoped references inherit.
<style>
  @font-face {
    font-family: foo;
    src: url(https://example.com/outer.woff);
  }
  body { font-family: foo; }
</style>

<child-component>
  <::shadow>
    <style>
      @font-face {
        font-family: foo;
        src: url(https://example.com/inner.woff);
      }
    </style>

    <grandchild-component>
      <::shadow>
        <p class=inner-default>
          I’m inheriting the outer document’s "foo" font.
        </p>
        <p class=inner-search>
          And I can’t find a local "foo" font,
          so I’m searching further up the tree,
          and find the shadow’s "foo" font.
        </p>
        <style>
        .inner-search { font-family: foo; }
        </style>
      </::shadow>
    </grandchild-component>
  </::shadow>
</child-component>

Here, just as in the previous example, .inner-default is inheriting the font-family: foo declared in the outer document, and so it ends up referencing the outer document’s @font-face, and is rendered with the "outer.woff" file.

On the other hand, .inner-search recieves its style from a stylesheet in <grandchild-component>’s shadow tree, so it attempts to find a @font-face defining a foo font in that tree. There is no such @font-face, so it starts walking up the shadow trees, finding an appropriate @font-face in <child-component>, so it’s rendered with the "inner.woff" file.

3.5.1. Serialized Tree-Scoped References

If a tree-scoped reference is serialized, it serializes only its value; the associated root is lost.

This implies that `el.style.foo = getComputedStyle(el).foo;` is not necessarily a no-op, like it typically was before shadow trees existed.

For example, given the following document (using the imaginary <::shadow> markup to indicate an element’s shadow tree):

<p class=outer>Here’s some text in the outer document’s "foo" font.
<style>
  @font-face {
    font-family: foo;
    src: url(foo.woff);
  }
  body { font-family: foo; }
</style>

<my-component>
  <::shadow>
    <p class=inner-default>I’m inheriting the outer document’s font-family.
    <p class=inner-styled>And I’m explicitly styled to be in the component’s "foo" font.
    <style>
      @font-face {
        font-family: foo;
        src: url(https://example.com/foo.woff);
      }
      .inner-styled { font-family: foo; }
    </style>
    <script>
      const innerDefault = document.querySelector('.inner-default');
      const innerStyled = document.querySelector('.inner-styled');
      const defaultFont = getComputedStyle(innerDefault).fontFamily;
      const styledFont = getComputedStyle(innerStyled).fontFamily;

      console.log(defaultFont == styledFont); // true!
    </script>
  </::shadow>
</my-component>

The .outer element is styled with the outer document’s "foo" @font-face. The .inner-default element inherits font-family from the outer document, meaning it inherits a tree-scoped reference referencing that outer document, and so it’s in the same font as .outer.

Meanwhile, .inner-styled is explicitly styled from inside the shadow root, so it receives a fresh tree-scoped reference referencing its shadow tree, and it is instead styled the shadow’s own "foo" @font-face.

Despite that, the script running inside the component sees the two elements as having the same value for font-family, because the root-reference part of a tree-scoped reference is not preserved by serialization. If it were to set innerDefault.style.fontFamily = defaultFont; (thus setting the font-family property of the element’s attribute stylesheet, which lives in the shadow tree), the .inner-default element would suddenly switch to the same font as .inner-styled!

Note: The [css-typed-om-1] is expected to reflect the root reference of a tree-scoped reference in its reification rules for values, allowing authors to tell what node tree the reference is taking its values from, and allowing values to be transported across node trees without changing their meaning.

4. Changes

The following significant changes were made since the 3 April 2014 Working Draft.

5. Privacy and Security Considerations

This specification introduces Shadow DOM and some shadow-piercing capabilities, but this does not introduce any privacy or security issues—shadow DOM, as currently specified, is intentionally not a privacy/security boundary (and the parts of the UA that use shadow DOM and do have a privacy/security boundary implicitly rely on protections not yet specified, which protect them from the things defined in this specification).

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:

This is an example of an informative example.

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.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Tests

Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.


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.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ANIMATIONS-1]
Dean Jackson; et al. CSS Animations Level 1. 11 October 2018. WD. URL: https://www.w3.org/TR/css-animations-1/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 29 August 2021. WD. URL: https://www.w3.org/TR/css-cascade-5/
[CSS-CASCADE-6]
CSS Cascading and Inheritance Level 6 URL: https://drafts.csswg.org/css-cascade-6/
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 1 June 2021. WD. URL: https://www.w3.org/TR/css-color-4/
[CSS-COLOR-5]
Chris Lilley; et al. CSS Color Module Level 5. 22 July 2021. WD. URL: https://www.w3.org/TR/css-color-5/
[CSS-COUNTER-STYLES-3]
Tab Atkins Jr.. CSS Counter Styles Level 3. 27 July 2021. CR. URL: https://www.w3.org/TR/css-counter-styles-3/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 3 September 2021. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-FONTS-4]
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 4. 29 July 2021. WD. URL: https://www.w3.org/TR/css-fonts-4/
[CSS-FONTS-5]
Myles Maxfield; Chris Lilley. CSS Fonts Module Level 5. 29 July 2021. WD. URL: https://www.w3.org/TR/css-fonts-5/
[CSS-LISTS-3]
Elika Etemad; Tab Atkins Jr.. CSS Lists and Counters Module Level 3. 17 November 2020. WD. URL: https://www.w3.org/TR/css-lists-3/
[CSS-PSEUDO-4]
Daniel Glazman; Elika Etemad; Alan Stearns. CSS Pseudo-Elements Module Level 4. 31 December 2020. WD. URL: https://www.w3.org/TR/css-pseudo-4/
[CSS-SYNTAX-3]
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 16 July 2019. CR. URL: https://www.w3.org/TR/css-syntax-3/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[SELECTORS-4]
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 21 November 2018. WD. URL: https://www.w3.org/TR/selectors-4/

Informative References

[CSS-TYPED-OM-1]
Shane Stephens; Tab Atkins Jr.; Naina Raisinghani. CSS Typed OM Level 1. 10 April 2018. WD. URL: https://www.w3.org/TR/css-typed-om-1/
[SELECTORS-3]
Tantek Çelik; et al. Selectors Level 3. 6 November 2018. REC. URL: https://www.w3.org/TR/selectors-3/

Issues Index

Do we need to restrict the at-rules that can be used in these sheets? For example, do we allow an @font-face? I’m going to leave it as allowed unless/until I hear complaints.
Rewrite this against the newer call forms of the matching algorithms.
A non-obvious result of this is that elements assigned to a slot inherit from that slot, not their light-tree parent or any deeper slots their slot gets assigned to. This means that text nodes are styled by the shadow tree of their parent, with nobody else capable of intervening in any way. Do we want an additional pseudo-element for targeting those text nodes so they can be styled at all slot-assignment levels, like normal elements can be? This implies it needs to work for text nodes in the light tree before they’re assigned downwards, so this can’t just be a ::slotted() variant. Luckily, this is a long-standing request!
TODO: Fix all the at-rules that define global names, and the properties that reference them, to use these concepts.