csswg-drafts/css3-hierarchies/Overview.src.html at 850c79aeb9d8b99aae5e09f34722097eaeabf23f · xfq/csswg-drafts · GitHub

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<!DOCTYPE html>
<html lang="en"><head profile="http://www.w3.org/2006/03/hcard">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

	<title>CSS Hierarchies Module Level 3</title>
	<link rel="stylesheet" type="text/css" href="../default.css">
	<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS]">
	<style type="text/css">
		/* delete this block when you've cleared out all the .replaceme elements */
		.replaceme {
			border: solid red;
			padding: 0 0.5em;
			margin: 0 0.1em;
			background: yellow;
			color: green;
		}
		.replaceme:before {
			 content: "FIXME(";
			 color: black;
		}
		.replaceme:after {
			 content: ")";
			 color: black;
		}
	</style>
	<style>
	pre.idl { border:solid thin; background:#eee; color:#000; padding:0.5em }
	pre.idl :link, pre.idl :visited { color:inherit; background:transparent }
	</style>
</head>

<body><div class="head">
<!--logo-->

<h1>CSS Hierarchies Module Level 3</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>

	<dl>
		<dt>This version:
		<!--<dd><a href="[VERSION]">[VERSION]</a>-->
		<dd><a href="//dev.w3.org/csswg/css3-hierarchies/">http://dev.w3.org/csswg/css3-hierarchies/</a>

		<dt>Latest version:
		<!--<dd><a href="[LATEST]">[LATEST]</a>-->
		<dd><a href="//www.w3.org/TR/css3-hierarchies/">http://www.w3.org/TR/css3-hierarchies/</a>

		<dt>Editor's draft:
		<dd><a href="//dev.w3.org/csswg/css3-hierarchies/">http://dev.w3.org/csswg/css3-hierarchies/</a>
	<!--
		<dt>Previous version:
		<dd><a href="http://www.w3.org/PreviousVersionURI">
			http://www.w3.org/PreviousVersionURI</a>
	-->
		<dt>Editors:
		<dd class="vcard"><span class="fn">Shane Stephens</span>,
			<span class="org">Google, Inc.</span>,
			<span class="email">shans@google.com</span>
		<dd class="vcard">
				<a href="http://xanthir.com/contact/" rel=url>
					<span class=fn>Tab Atkins Jr.</span>,
				</a>
				<span class=org>Google, Inc.</span>
	</dl>

<!--copyright-->

<hr title="Separator for header">
</div>

<h2 class="no-num no-toc" id="abstract">Abstract</h2>

	<p>CSS is a language for describing the rendering of structured documents
	(such as HTML and XML) on screen, on paper,  in speech, etc. This module
	contains the features of CSS level&nbsp;3 relating to the hierarchical nesting of style rules.
	It includes and extends the functionality of CSS level&nbsp;2 [[!CSS21]],
	which builds on CSS level&nbsp;1 [[CSS1]].
	The main extension compared to level&nbsp;2 is the ability to nest a style rule within another rule,
	allowing greater modularisation and readibility of CSS documents.

<h2 class="no-num no-toc" id="status">Status of this document</h2>

<p><strong>This document is purely an Editor's Draft.  It has not yet been adopted by the Working Group, and should not be considered to be part of CSS.</strong></p>

<p>The following features are at risk: …

<h2 class="no-num no-toc" id="contents">Table of contents</h2>

<!--toc-->

<h2 id="intro">
Introduction</h2>

	<p><em>This section is not normative.</em>

	<p>CSS beyond level 2 is a set of modules, divided up to allow the specifications to develop incrementally, along with their implementations. This specification is one of those modules.

	<p>This module describes support for nesting a style rule within another style rule, allowing the inner rule's selector to reference the elements matched by the outer rule.  This feature allows related styles to be aggregated into a single structure within the CSS document, improving readability and maintainability.

<h3 id="placement">
Module Interactions</h3>

	<p>This module introduces new parser rules that extend the [[!CSS21]] parser model. This module introduces new parser rules that extend the [[CSS4SELECTORS]] module.</p>

<h3 id="values">
Values</h3>

	<p>This specification does not define any new properties or values.

<h3 id="motivation">
Motivation</h3>

	<p>CSS Rules for even moderately complicated web pages include lots of duplication for the purpose of styling related content. For example, here is a portion of the CSS markup for one version of the [[CSS3COLOR]] module:

	<div class='example'>
		<pre>
table.colortable td {text-align:center }
table.colortable td.c { text-transform:uppercase }
table.colortable td:first-child, table.colortable td:first-child+td { border:1px solid black }
table.colortable th {text-align:center; background:black; color:white }

table.tprofile th.title {background:gray; color:white}
table.tprofile th { width:29%;padding:2px }
table.tprofile td { width:71%;padding:2px }

table.hslexample { background: #808080; padding:1em; margin:0; float:left; }
table.hslexample td,table.hslexample th { font-size:smaller;width:3em }</pre>
	</div>

	<p>Hierarchies allow the grouping of related style rules, like this:

	<div class='example'>
		<pre>
table {
	&.colortable {
		& td {
			text-align:center
			&.c { text-transform:uppercase }
			&:first-child, &:first-child + td { border:1px solid black }
		}
		& th {text-align:center; background:black; color:white }

	&.tprofile {
		& th {
			width:29%;padding:2px;
			&.title {background:gray; color:white}
		}
		& td { width:71%;padding:2px }
	}

	&.hslexample {
		background: #808080; padding:1em; margin:0; float:left;
		& td, & th { font-size:smaller;width:3em }
	}
}</pre>
	</div>

	<p>Besides removing duplication, the grouping of related rules improves the readability and maintainability of the resulting CSS.

<h2 id="nesting-selector">
The &amp; Selector</h2>

	<p>This specification provides a mechanism that allows for the nesting of style rules within other style rules. A nested style rule can used anywhere that a declaration can be used.</p>

	<p>In order to accomplish nesting, this specification defines a new simple selector called the nesting selector, represented in selectors by the '&amp;' character. All complex selectors in the selector lists of nested rule sets must start with the nesting selector. The nesting selector represents the elements matched by the parent rule set's selector list.</p>

	<p class='issue'>Introducing the ''&amp;'' character will cause issues with CSS embedded directly in XML, as it's the first character used in CSS syntax that either requires escaping or using CDATA.  Do we need to change this?</p>

	<div class='example'>
		<p>The following example using Hierarchies:

		<pre>
div {
	& .keyword {color: red;}
	&:hover {background-color: rgb(200, 255, 255);}
}</pre>

		<p>...produces the same results as the following CSS:

		<pre>
div .keyword {color:red;}

div:hover {background-color: rgb(200, 255, 255);}</pre>
	</div>

	<div class='example'>
		<p>The following example using Hierarchies:

		<pre>
div, p {
	& .keyword, & .constant {color: red;}
}</pre>

		<p>...produces the same results as the following CSS:

		<pre>
div .keyword, div .constant, p .keyword, p .constant {
	color:red;
}</pre>
	</div>

	<p>Multiple style rules can be embedded within a style rule. Style rules can be embedded arbitrarily deeply. Embedded style rules and properties can both appear in a single declaration block, and in any order.</p>

	<div class='example'>
		<p>The following example using Hierarchies:

		<pre>
div, p {
	& .keyword {color: green;}
	font-size: 10px;
	& .constant {
		color: red;
		&:hover:after { content: " [" attr(value) "]";}
		background-color: green;
	}
}</pre>

		<p>...produces the same results as the following CSS:

		<pre>
div, p {font-size: 10px;}
div .keyword, p .keyword {color: green;}
div .constant, p .constant {color: red; background-color: green;}
div .constant:hover:after, p .constant:hover:after {content: " [" attr(value) "]";}</pre>
	</div>

	<p class='note'>Note: Though it's unlikely that stylesheets authored with Hierarchies will be useful in legacy user agents, as the nested sections will be ignored by the error-handling rules, authors can minimize the damage caused by error-recovery by always putting properties before nested rules.</p>

<h2 id="Grammar">
Grammar modifications</h2>

	<p>This specification alters the CSS Core Grammar and the Selectors grammar.</p>

<h3 id="css-2-grammar">
Core CSS Grammar</h3>

	<p>The following modifications are required to the [[!CSS21]] grammar as defined in Section 4.1.1.</p>

	<p>The ruleset rule is changed to:</p>

	<pre>ruleset : selectors_group? ruleset-body</pre>

	<p>The following rules are added:</p>

	<pre>
ruleset-body           : '{' S* ruleitem? [ ';' S* ruleitem? ]* '}' S*
ruleitem               : declaration | nested-ruleset
nested-ruleset         : nested-selectors-group ruleset-body</pre>

<h3 id="selectors4-grammar">
Selectors 4 Grammar</h3>

	<p>The following modifications are required to the [[!CSS4SELECTORS]] grammar as defined in Section 16.1.</p>

	<p>The following rules are added:</p>

	<pre>
nested-selectors-group   : nested-selector [ COMMA S* nested-selector ]
nested-selector          : nested-compound-selector [ combinator compound-selector ]*
nested-compound-selector : NEST [ HASH | class | attrib | pseudo | negation ]*</pre>

<h3 id="selectors4-lexical-scanner">
Selectors 4 Lexical Scanner</h3>

	<p>The following modifications are required to the [[!CSS4SELECTORS]] lexical scanner as defined in Section 16.2.</p>

	<p>Immediately below:</p>

	<pre>"-->"            return CDC;</pre>

	<p>the following rule is inserted:</p>

	<pre>"&"              return NEST;</pre>

<h2 id="cssom">
CSS Object Model Modifications</h2>

	<p>The following attribute is required to be added to the CSSStyleRule object defined in Section 6.4.3 of [[!CSSOM]]:</p>

	<pre class='idl'>
interface CSSStyleRule : CSSRule {
           attribute DOMString selectorText;
  <ins>readonly attribute DOMString expandedSelectorText;</ins>
  <ins>readonly attribute CSSRuleList cssRules;</ins>
  readonly attribute CSSStyleDeclaration style;
};</pre>

	<p>The <code>cssRules</code> attribute must return a <code>CSSRuleList</code> object for the list of CSS rules specified within the style rule.</p>

	<p>The <code>expandedSelectorText</code> attribute must return a <code>DOMString</code> object containing the result of replacing the ''&amp;'' selector in the <code>selectorText</code> attribute with the <code>expandedSelectorText</code> attribute from the parent rule.  If the rule has no parent rule, the <code>expandedSelectorText</code> attribute must return a <code>DOMString</code> object containing the same text as the <code>selectorText</code> attribute.</p>

	<p class='issue'>The OM here is meant to reflect the OM for ''@media'' rules.  In particular, the lack of a link from a rule to its parent matches rules nested in ''@media''.  Should we add such a property to both of these?</p>

<h2 id="conformance">
Conformance</h2>

<h3 id="conventions">
Document Conventions</h3>

	<p>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.

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

	<p>Examples in this specification are introduced with the words “for example”
	or are set apart from the normative text with <code>class="example"</code>,
	like this:

	<div class="example">
		<p>This is an example of an informative example.</p>
	</div>

	<p>Informative notes begin with the word “Note” and are set apart from the
	normative text with <code>class="note"</code>, like this:

	<p class="note">Note, this is an informative note.</p>

<h3 id="conformance-classes">
Conformance Classes</h3>

	<p>Conformance to <var class="replaceme">CSS TEMPLATE Module</var>
	is defined for three conformance classes:
	<dl>
		<dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
			</dt><dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
			style sheet</a>.
		</dd><dt><dfn>renderer</dfn></dt>
			<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
			that interprets the semantics of a style sheet and renders
			documents that use them.
		</dd><dt><dfn id="authoring-tool">authoring tool</dfn></dt>
			<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
			that writes a style sheet.
	</dd></dl>

	<p>A style sheet is conformant to <var class="replaceme">CSS TEMPLATE Module</var>
	if all of its declarations that use properties defined in this module
	have values that are valid according to the generic CSS grammar and the
	individual grammars of each property as given in this module.

	<p>A renderer is conformant to <var class="replaceme">CSS TEMPLATE Module</var>
	if, in addition to interpreting the style sheet as defined by the
	appropriate specifications, it supports all the features defined
	by <var class="replaceme">CSS TEMPLATE Module</var> 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.)

	<p>An authoring tool is conformant to <var class="replaceme">CSS TEMPLATE Module</var>
	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.

<h3 id="partial">
Partial Implementations</h3>

	<p>So that authors can exploit the forward-compatible parsing rules to
	assign fallback values, CSS renderers <strong>must</strong>
	treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
	as appropriate</a>) any at-rules, properties, property values, keywords,
	and other syntactic constructs for which they have no usable level of
	support. In particular, user agents <strong>must not</strong> 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.</p>

<h3 id="experimental">
Experimental Implementations</h3>

	<p>To avoid clashes with future CSS features, the CSS2.1 specification
	reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
	syntax</a> for proprietary and experimental extensions to CSS.

	<p>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.
	</p>

<h3 id="testing">Non-Experimental Implementations</h3>

	<p>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.

	<p>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.

	<p>Further information on submitting testcases and implementation reports
	can be found from on the CSS Working Group's website at
	<a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
	Questions should be directed to the
	<a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
	mailing list.

<h3 id="cr-exit-criteria">
CR Exit Criteria</h3>

	<p class="issue">[Change or remove the following CR exit criteria if
	the spec is not a module, but, e.g., a Note or a profile. This text was <a href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>

	<p>
	For this specification to be advanced to Proposed Recommendation,
	there must be at least two independent, interoperable implementations
	of each feature. Each feature may be implemented by a different set of
	products, there is no requirement that all features be implemented by
	a single product. For the purposes of this criterion, we define the
	following terms:

	<dl>
		<dt>independent </dt><dd>each implementation must be developed by a
		different party and cannot share, reuse, or derive from code
		used by another qualifying implementation. Sections of code that
		have no bearing on the implementation of this specification are
		exempt from this requirement.

		</dd><dt>interoperable </dt><dd>passing the respective test case(s) in the
		official CSS test suite, or, if the implementation is not a Web
		browser, an equivalent test. Every relevant test in the test
		suite should have an equivalent test created if such a user
		agent (UA) is to be used to claim interoperability. In addition
		if such a UA is to be used to claim interoperability, then there
		must one or more additional UAs which can also pass those
		equivalent tests in the same way for the purpose of
		interoperability. The equivalent tests must be made publicly
		available for the purposes of peer review.

		</dd><dt>implementation </dt><dd>a user agent which:

		<ol class="inline">
			<li>implements the specification.

			</li><li>is available to the general public. The implementation may
			be a shipping product or other publicly available version
			(i.e., beta version, preview release, or “nightly build”).
			Non-shipping product releases must have implemented the
			feature(s) for a period of at least one month in order to
			demonstrate stability.

			</li><li>is not experimental (i.e., a version specifically designed
			to pass the test suite and is not intended for normal usage
			going forward).
		</li></ol>
	</dd></dl>

	<p>The specification will remain Candidate Recommendation for at least
	six months.

<h2 class="no-num" id="acknowledgments">Acknowledgments</h2>

	<p><var class="replaceme">[acknowledgments]</var></p>

<h2 class="no-num" id="references">References</h2>


<h3 class="no-num" id="normative-references">Normative references</h3>
<!--normative-->

<h3 class="no-num" id="other-references">Other references</h3>
<!--informative-->

<h2 class="no-num" id="index">Index</h2>
<!--index-->


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