我看电子游戏作文600字:CSS Shadow Parts

W3C First Public Working Draft,

This version:
//www.i2d1.net/TR/2018/WD-css-shadow-parts-1-20181115/
Latest published version:
//www.i2d1.net/TR/css-shadow-parts-1/
Editor's Draft:
//drafts.csswg.org/css-shadow-parts/
Issue Tracking:
Inline In Spec
GitHub Issues
Editors:
Tab Atkins-Bittner (Google)
(Google)

Abstract

This specification defines the ::part() pseudo-element on shadow hosts, allowing shadow hosts to selectively expose chosen elements from their shadow tree to the outside page for styling purposes.

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

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at //www.i2d1.net/TR/.

This document is a First Public Working Draft.

Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-shadow-parts” in the title, preferably like this: “[css-shadow-parts] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

1. Introduction

Shadow DOM allows authors to separate their page into "components", subtrees of markup whose details are only relevant to the component itself, not the outside page. This reduces the chance of a style meant for one part of the page accidentally over-applying and making a different part of the page look wrong. However, this styling barrier also makes it harder for a page to interact with its components when it actually wants to do so.

This specification defines the ::part() pseudo-element, which allows an author to style specific, purposely exposed elements in a shadow tree from the outside page’s context. In combination with custom properties, which let the outside page pass particular values (such as theme colors) into the component for it to do with as it will, these pseudo-elements allow components and the outside page to interact in safe, powerful ways, maintaining encapsulation without surrendering all control.

1.1. Motivation

For custom elements to be fully useful and as capable as built-in elements it should be possible for parts of them to be styled from outside. Exactly what can be styled from outside should be controlled by the element author. Also, it should be possible for a custom element to present a stable "API" for styling. That is, the selector used to style a part of a custom element should not expose or require knowledge of the internal details of the element. The custom element author should be able to change the internal details of the element while leaving the selectors untouched.

The previous proposed method for styling inside the shadow tree, the >>> combinator, turned out to be too powerful for its own good; it exposed too much of a component’s internal structure to scrutiny, defeating some of the encapsulation benefits that using Shadow DOM brings. For this, and other performance-related reasons, the >>> combinator was eventually removed from the live profile.

This left us with using custom properties as the only way to style into a shadow tree: the component would advertise that it uses certain custom properties to style its internals, and the outer page could then set those properties as it wished on the shadow host, letting inheritance push the values down to where they were needed. This works very well for many simple theming use-cases.

However, there are some cases where this falls down. If a component wishes to allow arbitrary styling of something in its shadow tree, the only way to do so is to define hundreds of custom properties (one per CSS property they wish to allow control of), which is obviously ridiculous for both usability and performance reasons. The situation is compounded if authors wish to style the component differently based on pseudo-classes like :hover; the component needs to duplicate the custom properties used for each pseudo-class (and each combination, like :hover:focus, resulting in a combinatorial explosion). This makes the usability and performance problems even worse.

We introduce ::part() to handle this case much more elegantly and performantly. Rather than bundling everything into custom property names, the functionality lives in selectors and style rule syntax, like it’s meant to. This is far more usable for both component authors and component users, should have much better performance, and allows for better encapsulation/API surface.

It’s important to note that ::part() offers absolutely zero new theoretical power. It is not a rehash of the >>> combinator, it is simply a more convenient and consistent syntax for something authors can already do with custom properties. By separating out the explicitly "published" parts of an element (the shadow part map from the sub-parts that it merely happens to contain, it also helps with encapsulation, as authors can use ::part() without fear of accidental over-styling.

2. Exposing a Shadow Element:

Elements in a shadow tree may be exported for styling by stylesheets outside the tree using the part and exportparts attributes.

Each element has a part name list which is an ordered set of tokens.

Each element has a part name map which is an ordered map, with keys that are strings (part names to expose to selectors outside this element) and values that are ordered sets of strings (part names that are selectable inside this element).

Each shadow root can be thought of as having a part element map with keys that are strings and values that are ordered sets of elements.

The part element map is described only as part of the algorithm for calculating style in this spec. It is not exposed via the DOM, as calculating it may be expensive and exposing it could allow access to elements inside closed shadow roots.

Part element maps are affected by the addition and removal of elements and changes to the part name lists and part name maps of elements in the DOM.

To calculate the part element map of a shadow root, outerRoot:

  1. For each element, el within outerRoot
    1. For each name in el’s part name list, add el to outerRoot’s part element map under the key name.
    2. If el is a shadow host itself then let innerRoot be its shadow root:
      1. Calculate innerRoot’s part element map.
      2. For each key, outerName, in el’s part name map and for each token innerName under that key look up innerName in innerRoot’s shadow part element map to get a (possibly empty) set of elements and add these elements to outerRoot’s part element map under outerName

There is no need for the part element map values to be ordered, can we drop that?

2.1. Naming a Shadow Element: the part attribute

Any element in a shadow tree can have a part attribute. This is used to expose the element outside of the shadow tree.

The part attribute is parsed as a space-separated list of tokens representing the part names of this element.

Note: It’s okay to give a part multiple names. The "part name" should be considered similar to a class, not an id or tagname.

<style>
  c-e::part(textspan) { color: red; }
</style>

<template id="c-e-template">
  <span part="textspan">This text will be red</span>
</template>
<c-e></c-e>
<script>
  // Add template as custom element c-e
  ...
</script>

2.2. Forwarding a Shadow Element: the exportparts attribute

Any element in a shadow tree can have a exportparts attribute. If the element is a shadow host, this is used to allow styling of parts from hosts inside the shadow tree by rules outside this the shadow tree (as if they were elements in the same tree as the host, named by a part attribute).

The exportparts attribute is parsed as a comma-separated list of part mappings. Each part mapping is one of:

innerIdent : outerIdent

this adds ?[ outerIdent → innerIdent ]? to el’s part name map.

ident

Is shorthand for ident : ident.

anything else

Ignored for error-recovery / future compatibility.

Note: It’s okay to map a sub-part to several names.

Decide whether to allow "ident1 : ident2 ident3 ..." as shorthand for "ident1 : ident2, ident1 : ident3, ...". <https://github.com/w3c/csswg-drafts/issues/2411>

Decide whether to allow wild-card forwarding, e.g exportparts="button-* buttons". Consider excluding sub-parts that have been explicitly forwarded. Consider a mechanism to exclude sub-parts without forwarding them. <https://github.com/w3c/csswg-drafts/issues/2411>

<style>
  c-e::part(textspan) { color: red; }
</style>

<template id="c-e-outer-template">
  <c-e-inner exportparts="innerspan textspan"></c-e-inner>
</template>

<template id="c-e-inner-template">
  <span part="innerspan">
    This text will be red because the containing shadow
    host forwards innerspan to the document as "textspan"
    and the document style matches it.
  </span>
  <span part="textspan">
    This text will not be red because textspan in the document style
    cannot match against the part inside the inner custom element
    if it is not forwarded.
  </span>
</template>

<c-e></c-e>
<script>
  // Add template as custom elements c-e-inner, c-e-outer
    ...
</script>

3. Selecting a Shadow Element: the ::part() pseudo-element

The ::part() pseudo-element allows you to select elements that have been exposed via a part attribute. The syntax is:

::part() = ::part( <ident> )

The ::part() pseudo-element only matches anything when the originating element is a shadow host. If the originating element’s shadow root’s part element map contains the specified <ident>, ::part() matches the element or elements keyed to that <ident>. Otherwise, it matches nothing.

For example, if you have a custom button that contains a "label" element that is exposed for styling (via part="label"), you can select it with #the-button::part(label).

The shadow-part pseudo-elements can take additional pseudo-classes after them, such as x-button::part(label):hover, but never match the structural pseudo-classes or any other pseudo-classes that match based on tree information rather than local element information.

The shadow-part pseudo-elements also can take additional pseudo-elements after them, such as x-button::part(label)::before, but never match additional shadow-part pseudo-elements.

For example, x-panel::part(confirm-button)::part(label) never matches anything. This is because doing so would expose more structural information than is intended.

If the <x-panel>’s internal confirm button had used something like part="label => confirm-label" to forward the button’s internal parts up into the panel’s own part element map, then a selector like x-panel::part(confirm-label) would select just the one button’s label, ignoring any other labels.

4. Extensions to the Element Interface

partial interface Element {
  [SameObject, PutForwards=value] readonly attribute DOMTokenList part;
};

The part attribute’s getter must return a DOMTokenList object whose associated element is the context object and whose associated attribute’s local name is part. The token set of this particular DOMTokenList object are also known as the element’s parts.

Define this as a superglobal in the DOM spec. Issue(w3c/csswg-drafts#2414): Define IDL for structured setting and getting of `exportparts`. <https://github.com/w3c/csswg-drafts/issues/2414>

5. Microsyntaxes for parsing

5.1. Rules for parsing part mappings

A valid part mapping is a pair of tokens separated by a U+003A COLON charater and any number of space characters before or after the U+003A COLON The tokens must not contain U+003A COLON or U+002C COMMA characters.

The rules for parsing a part mapping are as follows:

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Collect a sequence of code points that are space characters

  4. Collect a sequence of code points that are not space characters or U+003A COLON characters, and let first token be the result.

  5. If first token is empty then return error.

  6. Collect a sequence of code points that are space characters.

  7. If the end of the input has been reached, return the pair first token/first token

  8. If character at position is not a U+003A COLON character, return error.

  9. Consume the U+003A COLON character.

  10. Collect a sequence of code points that are space characters.

  11. Collect a sequence of code points that are not space characters or U+003A COLON characters. and let second token be the result.

  12. If second token is empty then return error.

  13. Collect a sequence of code points that are space characters.

  14. If position is past the end of input then return error.

  15. Return the pair first token/second token.

5.2. Rules for parsing a list of part mappings

A valid list of part mappings is a number of valid part mappings separated by a U+002C COMMA charater and any number of space characters before or after the U+002C COMMA

The rules for parsing a list of part mappings are as follow:

  1. Let input be the string being parsed.

  2. Split the string input on commas. Let unparsed mappings be the resulting list of strings.

  3. Let mappings be an initially empty list of pairs of tokens. This list will be the result of this algorithm.

  4. For each string unparsed mapping in unparsed mappings, run the following substeps:

    1. If unparsed mapping is empty or contains only space characters, continue to the next iteration of the loop.

    2. Let mapping be the result of parsing unparsed mapping using the rules for parsing part mappings.

    3. If mapping is an error then continue to the next iteration of the loop. This allows clients to skip over new syntax that is not understood.

    4. Append mapping to mappings.

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.

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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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 //www.i2d1.net/Style/CSS/Test/. Questions should be directed to the [email protected] mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-SCOPING-1]
Tab Atkins Jr.; Elika Etemad. CSS Scoping Module Level 1. 3 April 2014. WD. URL: //www.i2d1.net/TR/css-scoping-1/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 14 August 2018. WD. URL: //www.i2d1.net/TR/css-values-4/
[CSS-VARIABLES-1]
Tab Atkins Jr.. CSS Custom Properties for Cascading Variables Module Level 1. 3 December 2015. CR. URL: //www.i2d1.net/TR/css-variables-1/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[SELECTORS-4]
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 2 February 2018. WD. URL: //www.i2d1.net/TR/selectors-4/
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. 15 December 2016. ED. URL: https://heycam.github.io/webidl/

IDL Index

partial interface Element {
  [SameObject, PutForwards=value] readonly attribute DOMTokenList part;
};

Issues Index

There is no need for the part element map values to be ordered, can we drop that? ?
Decide whether to allow "ident1 : ident2 ident3 ..." as shorthand for "ident1 : ident2, ident1 : ident3, ...". <https://github.com/w3c/csswg-drafts/issues/2411> ?
Decide whether to allow wild-card forwarding, e.g exportparts="button-* buttons". Consider excluding sub-parts that have been explicitly forwarded. Consider a mechanism to exclude sub-parts without forwarding them. <https://github.com/w3c/csswg-drafts/issues/2411> ?
Define this as a superglobal in the DOM spec. Issue(w3c/csswg-drafts#2414): Define IDL for structured setting and getting of `exportparts`. <https://github.com/w3c/csswg-drafts/issues/2414> ?