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<!doctype html><html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<title>Text Justification</title>
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<meta content="exploring" name="csswg-work-status">
<meta content="UD" name="w3c-status">
<meta content="This Note serves as a clearinghouse to further information on worldwide conventions for text justification: the process of stretching text to fill a line." name="abstract">
<link href="../default.css" rel="stylesheet" type="text/css">
<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
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<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="http://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2016/logos/W3C" width="72"> </a> </p>
<h1>Text Justification</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Unofficial Proposal Draft, <time class="dt-updated" datetime="2016-08-18">18 August 2016</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:
<dd><a class="u-url" href="https://drafts.csswg.org/css-text-3/text-justify-i18n">https://drafts.csswg.org/css-text-3/text-justify-i18n</a>
<dt>Latest published version:
<dd><a href="https://www.w3.org/TR/text-justify-i18n/">https://www.w3.org/TR/text-justify-i18n/</a>
<dt class="editor">Editor:
<dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://fantasai.inkedblade.net/contact">Elika J. Etemad / fantasai</a> (<span class="p-org org">Invited Expert</span>)
<dt>Issue Tracking:
<dd><span><a href="https://github.com/w3c/csswg-drafts/labels/text-justify-1">GitHub Issues</a></span>
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2016 <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="http://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>). W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply. </p>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<div class="p-summary" data-fill-with="abstract">
<p>This Note serves as a clearinghouse to further information on worldwide conventions for text justification: the process of stretching text to fill a line.</p>
<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.
</div>
<h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
<div data-fill-with="status">
<p></p>
</div>
<div data-fill-with="at-risk"></div>
<nav data-fill-w
77FB
ith="table-of-contents" id="toc">
<h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
<ol class="toc" role="directory">
<li><a href="#intro"><span class="secno">1</span> <span class="content"> Introduction</span></a>
<li>
<a href="#references"><span class="secno">2</span> <span class="content">References</span></a>
<ol class="toc">
<li><a href="#zh"><span class="secno">2.1</span> <span class="content"> Chinese Writing System (Han Ideographs)</span></a>
<li><a href="#ja"><span class="secno">2.2</span> <span class="content"> Japanese Writing System</span></a>
<li><a href="#ko"><span class="secno">2.3</span> <span class="content"> Korean Writing System</span></a>
<li><a href="#latn"><span class="secno">2.4</span> <span class="content"> Latin (Roman) Writing System</span></a>
<li><a href="#ethiopic"><span class="secno">2.5</span> <span class="content"> Ethiopic Writing System</span></a>
<li><a href="#arabic"><span class="secno">2.6</span> <span class="content"> Arabic Writing System (and Other Cursive Systems)</span></a>
<li><a href="#tibetan"><span class="secno">2.7</span> <span class="content"> Tibetan Writing System</span></a>
<li><a href="#southeast"><span class="secno">2.8</span> <span class="content"> Southeast Asian Writing Systems</span></a>
<li><a href="#other"><span class="secno">2.9</span> <span class="content"> Other Writing Systems</span></a>
</ol>
<li>
<a href="#guide"><span class="secno">3</span> <span class="content"> Guidance for Authors and Implementers</span></a>
<ol class="toc">
<li><a href="#lang"><span class="secno">3.1</span> <span class="content"> Tagging Content By Writing System</span></a>
<li><a href="#unknown"><span class="secno">3.2</span> <span class="content"> Justifying Untagged Content</span></a>
</ol>
<li><a href="#acknowledgements"><span class="secno">4</span> <span class="content"> Acknowledgements</span></a>
<li>
<a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
<ol class="toc">
<li><a href="#document-conventions"><span class="secno"></span> <span class="content"> Document conventions</span></a>
<li><a href="#conform-classes"><span class="secno"></span> <span class="content"> Conformance classes</span></a>
<li>
<a href="#conform-responsible"><span class="secno"></span> <span class="content"> Requirements for Responsible Implementation of CSS</span></a>
<ol class="toc">
<li><a href="#conform-partial"><span class="secno"></span> <span class="content"> Partial Implementations</span></a>
<li><a href="#conform-future-proofing"><span class="secno"></span> <span class="content"> Implementations of Unstable and Proprietary Features</span></a>
<li><a href="#conform-testing"><span class="secno"></span> <span class="content"> Implementations of CR-level Features</span></a>
</ol>
</ol>
<li>
<a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
<ol class="toc">
<li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
</ol>
<li>
<a href="#references0"><span class="secno"></span> <span class="content">References</span></a>
<ol class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
</ol>
</ol>
</nav>
<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content"> Introduction</span><a class="self-link" href="#intro"></a></h2>
<p>Since the amount of content on a line tends to vary,
even if minutely, from line to line within a paragraph,
typographers have come up with various methods
for effective <dfn data-dfn-type="dfn" data-lt="full justification | full text justification" data-noexport="" id="full-justification">full justification<a class="self-link" href="#full-justification"></a></dfn>—<wbr>causing the text to completely fill the text—<wbr>in order to create visual alignment on both edges of a paragraph.</p>
<p>Typographic conventions for full text justification depend on the writing system,
the content language,
and the calligraphic style of the text.
Results also tend to vary based on the capabilities of the layout engine
and a given typographer’s preferences for weighing
its various detrimental effects on typographic color and readability.</p>
<p>This document collects together references for further information
on the typographic conventions for full justification
as they apply to the various writing systems around the world,
together with some guidance for implementers handling unpredictable Web content.
(General information and technical requirements for CSS
are described under the <a href="https://dev.w3.org/csswg/css-text-3/#justification">Justification</a> section of <a data-link-type="biblio" href="#biblio-css3text">[CSS3TEXT]</a>.)</p>
<strong class="advisement"> Additional information and references are hereby solicited;
please send any suggestions for additions, clarifications, corrections, and other improvements
to the <a href="https://www.w3.org/International/">W3C Internationalization Working Group</a> at <a href="mailto:www-international@w3.org">www-international@w3.org</a>.</strong>
<p class="note" role="note">Note: Information on which languages use which writing systems
is maintained in the Unicode CLDR.</p>
<h2 class="heading settled" data-level="2" id="references"><span class="secno">2. </span><span class="content">References</span><a class="self-link" href="#references"></a></h2>
<h3 class="heading settled" data-level="2.1" id="zh"><span class="secno">2.1. </span><span class="content"> Chinese Writing System (Han Ideographs)</span><a class="self-link" href="#zh"></a></h3>
<p>Historically, Chinese was written as Han ideographs, with no punctuation.
Under this system, justification was automatic,
as the characters fit perfectly into a square grid.
However, the introduction of punctuation in recent centuries,
plus the increase in mixed-script text
(such as the inclusion of European numbers and/or words, phrases, names, and trademarks)
has created a need for adjustments within a line.</p>
<p>Chinese notably does not use word spaces,
so these do not provide a justification opportunity within the lines;
thus justification techniques focus on adjustments to spacing around punctuation,
script-change boundaries,
and inter-character spacing.</p>
<ul>
<li><a href="https://www.w3.org/TR/clreq/#line_composition_rules_for_punctuation_marks">Chinese Layout Requirements: Line Composition Rules for Punctuation Marks</a>
<li><a href="https://www.w3.org/TR/clreq/#chinese_and_western_mixed_text_composition">Chinese Layout Requirements: Composition of Chinese and Western Mixed Texts</a>
</ul>
<h3 class="heading settled" data-level="2.2" id="ja"><span class="secno">2.2. </span><span class="content"> Japanese Writing System</span><a class="self-link" href="#ja"></a></h3>
<p>Like Chinese, Japanese was historically written in Han ideographs;
however it has sinc
93CD
e developed its own phonetic scripts
Hiragana and Katakana (collectively, Kana).
While pure kana texts do exist,
particularly in children’s literature,
Han ideographs (Kanji, in Japanese) continue to be an integral part of normal Japanese text,
and are interspersed with kana within a sentence.</p>
<p>Like Chinese,
embraced European-inspired punctuation, numerals, and other foreign snippets
that don’t conform to the standard full-width character grid.
The Japanese writing system also does not use word spaces,
and similarly focuses on adjustments to spacing around punctuation,
script-change boundaries,
and inter-character spacing,
with a notable preference for compression of intra-glyph spacing
over expansion between glyphs.</p>
<ul>
<li><a href="https://www.w3.org/TR/jlreq/#line_adjustment">Japanese Layout Requirements: Line Adjustment</a>
<li><a href="https://www.w3.org/TR/jlreq/#opportunities_for_intercharacter_space_reduction_during_line_adjustment">Japanese Layout Requirements: Opportunities for Inter-character Space Reduction during Line Adjustment</a>
<li><a href="https://www.w3.org/TR/jlreq/#opportunities_for_intercharacter_space_expansion_during_line_adjustment">Japanese Layout Requirements: Opportunities for Inter-character Space Expansion during Line Adjustment</a>
</ul>
<h3 class="heading settled" data-level="2.3" id="ko"><span class="secno">2.3. </span><span class="content"> Korean Writing System</span><a class="self-link" href="#ko"></a></h3>
<p>Like Japanese, Korean was historically written in pure Han ideographs,
and has since developed its own phonetic script, Hangul.
Also like Japanese, it has adopted punctuation and numerals.
However, unlike Japanese, Korean has also adopted word spaces,
and tends towards narrow (Western-style, rather than full-width) punctuation.
This allows it to use inter-word justification:
as in English publications, this method stretches the spaces between words
in order to fill the line.</p>
<p>While Han ideographs (Hanja, in Korean) were kept as part of the writing system,
they have become increasingly scarce over time
such that many documents are written in pure Hangul,
and some only use Hanja as inline annotations for disambiguation among homophones
rather than as part of the main text.
However, Hanja and Hangul together remain important components of Korean writing.</p>
<ul>
<li><a href="https://www.w3.org/TR/klreq/#paraadjust">Hangul Layout Requirements: Paragraph Adjustment</a>
<li><a href="https://www.w3.org/TR/klreq/#line-adjust">Hangul Layout Requirements: Line Adjustment Process</a>
</ul>
<h3 class="heading settled" data-level="2.4" id="latn"><span class="secno">2.4. </span><span class="content"> Latin (Roman) Writing System</span><a class="self-link" href="#latn"></a></h3>
<p>Quite possibly the writing system familiar to more people than any other,
the Latin writing system derives from the Roman alphabet,
including a few additional characters and diacritic marks
to accommodate languages such as Icelandic and modern Vietnamese.
Thanks to the Europeans in the Age of Exploration,
their missionaries,
and the Western-dominated global scholastic culture of the modern age,
most languages in the world have one or more Latin transcriptions,
even those that do not use it as their primary writing system.</p>
<p>The Latin alphabet is a phonetic system with disjoint letterforms,
and typically uses spaces between words.
This allows it to use inter-word justification,
although it can and sometimes does increase the spacing between individual letters as well.
Since it is frequently adopted into other writing systems,
it can sometimes adopt characteristics of that system;
for example, some styles of Japanese typesetting
treat Latin letters the same as Japanese characters
for the purpose of line-breaking and justification.</p>
<ul>
<li><a href="https://www.w3.org/TR/dpub-latinreq/#justification">Latin Layout Requirements: Justification</a>
</ul>
<h3 class="heading settled" data-level="2.5" id="ethiopic"><span class="secno">2.5. </span><span class="content"> Ethiopic Writing System</span><a class="self-link" href="#ethiopic"></a></h3>
<p>Like Latin, the Ethiopic writing system uses an alphabet of disjoint letters
and uses punctuation to indicate the break between words.
Unlike Latin, Ethiopic traditionally uses a visible word separator—<wbr>the Ethiopic Word Space U+1361 “፡”—<wbr>although modern documents sometimes use a regular space U+0020 “ ” instead.
Justification strategies are as for Latin:
increasing the space at the word separator,
and/or distributing space between letters.</p>
<ul>
<li><a href="https://w3c.github.io/elreq/#ethiopic_justification">Ethiopic Layout Requirements: Justification </a>
</ul>
<a href="https://w3c.github.io/elreq/#ethiopic_justification"> <h3 class="heading settled" data-level="2.6" id="arabic"><span class="secno">2.6. </span><span class="content"> Arabic Writing System (and Other Cursive Systems)</span><a class="self-link" href="#arabic"></a></h3> <p>Arabic is a cursive script,
meaning its letters are typically joined together within a word.
This creates additional challenges,
as the usual method for stretching out text—<wbr>inserting spaces between glyphs—<wbr>does not work.</p> <p>Since Arabic uses spaces between words,
one method for justification is inter-word justification—<wbr>stretching out the spaces within the line to fill it.
However, most styles of Arabic writing prefer calligraphic elongation or compression,
distorting the shapes and connections between letters
in order to fill the line while preserving its typographic color.
This is often called “kashida”, meaning “stretched”.
A simplistic variant of this technique inserts elongation marks
(sometimes represented with U+0640 “ـ” TATWEEL)
at appropriate points in the text.</p> </a>
<ul>
<a href="https://w3c.github.io/elreq/#ethiopic_justification"> </a>
<li><a href="https://w3c.github.io/elreq/#ethiopic_justification"></a><a href="https://www.tug.org/tugboat/tb27-2/tb87benatia.pdf">Arabic Text Justification</a>
<li><a href="https://quod.lib.umich.edu/j/jep/3336451.0013.105/--justify-just-or-just-justify?rgn=main;view=fulltext">Justify Just or Just Justify (Arabic text justification)</a>
<li><a href="https://rishida.net/blog/?p=1059">Typography questions for HTML & CSS: Arabic justification</a>
<li><a href="https://www.cle.org.pk/Publication/papers/2004/rule-based-expert-system.pdf">Rule-based expert system for Urdu nastaleeq justification</a>
<li><a href="https://www.unicode.org/L2/L2015/15148-ethiopic-wordspace.pdf">Proposal to Reclassify Ethiopic Wordspace as a Space Separator (Zs) Symbol</a>
</ul>
<p>Syriac and Mongolian
have properties similar to Arabic,
and in the absence of additional information should be given
similar treatment for justification.</p>
<h3 class="heading settled" data-level="2.7" id="tibetan"><span class="secno">2.7. </span><span class="content"> Tibetan Writing System</span><a class="self-link" href="#tibetan"></a></h3>
<p>Tibetan is a Brahmic writing system related to Indic scripts like Devanagari and Gujarati;
however, unlike these systems, it does not use Western-style punctuation
nor spaces between words,
and instead uses the Tibetan Tsheg Mark U+0F0B “་”
between syllables
and its own punctuation marks such as the Tibetan Shad U+0F0D “།” and Tibetan Nyis Shad U+ 0F0E “༎”,
which indicate the end of longer segments.</p>
<p>Justification techniques used in Tibetan include stretching the space after a shad, minutely increasing the spaces after tsheg marks,
and simply filling the remaining space on a line with tsheg marks.</p>
<ul>
<li><a href="https://r12a.github.io/scripts/tibetan#justification">Tibetan Script Notes</a>
<li><a href="https://www.chinaw3c.org/layout-workshop-report.html#talk5">Tibetan Script Requirements (.ppt)</a>
</ul>
<h3 class="heading settled" data-level="2.8" id="southeast"><span class="secno">2.8. </span><span class="content"> Southeast Asian Writing Systems</span><a class="self-link" href="#southeast"></a></h3>
<p>In Southeast Asian systems such as Thai and Lao,
letters are merged together into “clusters”.
There are no spaces between words
(lines must be broken by dictionary),
but spaces serve to separate larger units of text.</p>
<p>Techniques for justification include stretching spaces on the line
(if it happens to have any)
and interspersing extra space between clusters.</p>
<p>Scripts in this category include
Khmer, Myanmar, Lao, and Thai.</p>
<h3 class="heading settled" data-level="2.9" id="other"><span class="secno">2.9. </span><span class="content"> Other Writing Systems</span><a class="self-link" href="#other"></a></h3>
<p>Most (but not all) writing systems not mentioned here
have discrete letters, like Latin,
and in the absence of more specific information
may be assumed to justify in a similar manner.</p>
<p class="note" role="note">Note: Readers who wish to provide such “more specific information”
are invited (and strongly encouraged)
to contact the <a href="mailto:www-internation@w3.org">W3C Internationalization Working Group</a> so that this document may be updated.</p>
<h2 class="heading settled" data-level="3" id="guide"><span class="secno">3. </span><span class="content"> Guidance for Authors and Implementers</span><a class="self-link" href="#guide"></a></h2>
<h3 class="heading settled" data-level="3.1" id="lang"><span class="secno">3.1. </span><span class="content"> Tagging Content By Writing System</span><a class="self-link" href="#lang"></a></h3>
<p>While most languages have a preferred writing system,
many can be transcribed into a different system.
As a common example, most languages have a Latin transcription,
and can thus be written in the Latin writing system.
In these cases the document typically adopts the typographic conventions of the Latin writing system:
for example Japanese “romaji” and Chinese Pinyin use word spaces and justify accordingly.
As another example, historical ideographic Korean
(<code>ko-Hant</code>)
does not use word spaces,
and should therefore be justified as for Chinese.</p>
<p>Authors can indicate the use of the Latin writing system
with the <code>-Latn</code> language subtag,
e.g. <code>ja-Latn</code> for Japanese romaji.
Other subtags exist for other writing systems,
see ????.
Some common/historical examples follow:</p>
<div class="example" id="example-95b4e286">
<a class="self-link" href="#example-95b4e286"></a>
<dl>
<dt><code>zh-Latn</code>
<dd>Chinese, written in Latin transcription
<dt><code>ko-Hant</code>
<dd>Korean, written in Hanja (Chinese ideographic characters)
<dt><code>??-Arab</code>
<dd>Turkish, written in Arabic script.
<dt><code>??-???</code>
<dd>Mongolian, written in Cyrillic
<dt><code>??-???</code>
<dd>Mongolian, written in traditional Mongolian script.
</dl>
</div>
<p>UAs should assume the most common writing system for a given language
when choosing a justification strategy,
but must not assume that writing system
if the author has explicitly indicated a different one.</p>
<h3 class="heading settled" data-level="3.2" id="unknown"><span class="secno">3.2. </span><span class="content"> Justifying Untagged Content</span><a class="self-link" href="#unknown"></a></h3>
<p>Web browsers frequently have to deal with untagged, potentially mixed-script content.
The following are some guidelines for designing a strategy to deal with such content.</p>
<ul>
<li>Since Chinese and Japanese do not use spaces to provide justification opportunities,
CJK content (Han, Hiragana, Katakana, and Hangul)
should be allowed to accept inter-character spacing.
<li>Since Japanese content prefers compression,
CJK fullwidth punctuation characters, if present on a line,
should be compressed at a higher priority (if possible) than expanding spaces
or letter-spacing.
<li>Since Korean prefers expanding spaces to expanding between characters,
spaces should be expanded at a higher priority (if possible)
than letter-spacing.
</ul>
<strong class="advisement"> Authors should use (correct) language tags
in order to get the best possible typographic behavior.
For example, if Japanese text is tagged as Japanese,
the UA knows to preferentially compress the space rather than expand it.</strong>
<h2 class="heading settled" data-level="4" id="acknowledgements"><span class="secno">4. </span><span class="content"> Acknowledgements</span><a class="self-link" href="#acknowledgements"></a></h2>
<p>This document was compiled with guidance from:
the W3C <a href="http://www.w3.org/International/">Internationalization</a> and <a href="http://www.w3.org/Style/CSS/">CSS</a> Working Groups,
and the W3C <a href="https://www.w3.org/2007/02/japanese-layout/">Japanese</a>, <a href="https://www.w3.org/International/groups/chinese-layout/">Chinese</a>, <a href="http://w3c.github.io/klreq/">Korean</a>,
and <a href="https://www.w3.org/International/groups/ethiopic-layout/">Ethiopic</a> Language Task Forces.</p>
</main>
<h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
<h3 class="heading settled" id="document-conventions"><span class="content"> Document conventions</span><a class="self-link" href="#document-conventions"></a></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>
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </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: </p>
<div class="example" id="example-52448c84">
<a class="self-link" href="#example-52448c84"></a>
<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>
<p class="note" role="note">Note, this is an informative note. </p>
<p>Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <code><strong class="advisement"></code>, like
this: <strong class="advisement"> UAs MUST provide an accessible alternative. </strong> </p>
<h3 class="heading settled" id="conform-classes"><span class="content"> Conformance classes</span><a class="self-link" href="#conform-classes"></a></h3>
<p>Conformance to this specification
is defined for three conformance classes: </p>
<dl>
<dt>style sheet
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS
style sheet</a>.
<dt>renderer
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that interprets the semantics of a style sheet and renders
documents that use them.
<dt>authoring tool
<dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that writes a style sheet.
</dl>
<p>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. </p>
<p>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.) </p>
<p>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. </p>
<h3 class="heading settled" id="conform-responsible"><span class="content"> Requirements for Responsible Implementation of CSS</span><a class="self-link" href="#conform-responsible"></a></h3>
<p>The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future. </p>
<h4 class="heading settled" id="conform-partial"><span class="content"> Partial Implementations</span><a class="self-link" href="#conform-partial"></a></h4>
<p>So that authors can exploit the forward-compatible parsing rules to assign fallback values, <strong>CSS renderers <em>must</em> treat as invalid
(and <a href="http://www.w3.org/TR/CSS2/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</strong>.
In particular, user agents <em>must not</em> 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. </p>
<h4 class="heading settled" id="conform-future-proofing"><span class="content"> Implementations of Unstable and Proprietary Features</span><a class="self-link" href="#conform-future-proofing"></a></h4>
<p>To avoid clashes with future stable CSS features,
the CSSWG recommends <a href="http://www.w3.org/TR/CSS/#future-proofing">following best practices</a> for the implementation of <a href="http://www.w3.org/TR/CSS/#unstable">unstable</a> features and <a href="http://www.w3.org/TR/CSS/#proprietary-extension">proprietary extensions</a> to CSS. </p>
<h4 class="heading settled" id="conform-testing"><span class="content"> Implementations of CR-level Features</span><a class="self-link" href="#conform-testing"></a></h4>
<p>Once a specification reaches the Candidate Recommendation stage,
implementers should release an <a data-link-type="dfn" href="http://www.w3.org/TR/CSS/#vendor-prefix">unprefixed</a> 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. </p>
<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>
<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.
<script src="https://www.w3.org/scripts/TR/2016/fixup.js"></script>
</p>
<h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
<h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
<ul class="index">
<li><a href="#full-justification">full justification</a><span>, in §1</span>
<li><a href="#full-justification">full text justification</a><span>, in §1</span>
</ul>
<h2 class="no-num no-ref heading settled" id="references0"><span class="content">References</span><a class="self-link" href="#references0"></a></h2>
<h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
<dl>
<dt id="biblio-css3text">[CSS3TEXT]
<dd>Elika Etemad; Koji Ishii. <a href="http://dev.w3.org/csswg/css-text-3/">CSS Text Module Level 3</a>. 10 October 2013. LCWD. URL: <a href="http://dev.w3.org/csswg/css-text-3/">http://dev.w3.org/csswg/css-text-3/</a>
<dt id="biblio-rfc2119">[RFC2119]
<dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
</dl>