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CSS Text Module Level 3

W3C Working Draft,

This version:
http://www.fyrkh.live/TR/2019/WD-css-text-3-20191113/
Latest published version:
http://www.fyrkh.live/TR/css-text-3/
Editor's Draft:
https://drafts.csswg.org/css-text-3/
Previous Versions:
Test Suite:
http://test.csswg.org/suites/css3-text/nightly-unstable/
Issue Tracking:
Tracker
Inline In Spec
GitHub Issues
Editors:
Elika J. Etemad / fantasai (Invited Expert)
(Invited Expert)
Florian Rivoal (Invited Expert)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This CSS module defines properties for text manipulation and specifies their processing model. It covers line breaking, justification and alignment, white space handling, and text transformation.

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 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 http://www.fyrkh.live/TR/.

Publication as a 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-text” in the title, preferably like this: “[css-text] …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 March 2019 W3C Process Document.

This publication partially addresses the issues in the disposition of comments since the October 2013 Last Call Working Draft, and, while a marked improvement over the previous draft, is not considered to be entirely up-to-date at the time of publication. A completed dispostion of comments and corresponding draft will be published once the issues are fully addressed and reviewed by the CSSWG and Internationalization WG.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

1. Introduction

This module describes the typesetting controls of CSS; that is, the features of CSS that control the translation of source text to formatted, line-wrapped text. Various CSS properties provide control over case transformation, white space collapsing, text wrapping, line breaking rules and hyphenation, alignment and justification, spacing, and indentation.

Note: Font selection is covered in CSS Fonts Level 3 [CSS-FONTS-3].

Features for decorating text, such as underlines, emphasis marks, and shadows, (previously part of this module) are covered in CSS Text Decoration Level 3 [CSS-TEXT-DECOR-3].

Bidirectional and vertical text are addressed in CSS Writing Modes Level 3 [CSS-WRITING-MODES-3].

Further information about the typesetting requirements of various languages and writing systems around the world can be found in the Internationalization Working Group’s Typography Index. [TYPOGRAPHY]

1.1. Module Interactions

This module, together with [CSS-TEXT-DECOR-3], replaces and extends the text-level features defined in [CSS2] chapter 16.

In addition to the terms defined below, other terminology and concepts used in this specification are defined in [CSS2] and [CSS-WRITING-MODES-3].

1.2. Values

This specification follows the CSS property definition conventions from [CSS2]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords keywords as their property value. For readability they have not been repeated explicitly.

1.3. Languages and Typesetting

Authors should language-tag their content accurately for the best typographic behavior.

The content language of an element is the (human) language the element is declared to be in, according to the rules of the document language. For example, the rules for determining the content language of an HTML are defined in [HTML], and the rules for determining the content language of an XML element use are defined in [XML10]. Note that it is possible for the content language of an element to be unknown—e.g. untagged content, or content in a document language that does not have a language-tagging facility is considered to have an unknown content language.

Note: Authors can tag content using the global lang attribute in HTML, the universal xml:lang attribute in XML, and the HTTP Content-Language header for content served over HTTP.

The content language an element is declared to be in also identifies the specific written form of that language used in that element, known as the content writing system.

Note: Depending on the document language's facilities for identifying the content language, information about the writing system may only be carried implicitly. That is typically the case with the [BCP47] language tag used in [HTML], although it can optionally indicate the writing system explicitly using a script subtag.

Language and writing system conventions can affect line breaking, hyphenation, justification, glyph selection, and many other typographic effects. In CSS, language-specific typographic tailorings are only applied when the content language is known (declared). Therefore, higher quality typography requires authors to communicate to the UA the correct linguistic context of the text in the document.

More information about language tags and their interpretation, particularly the use of script tags for atypical language + writing-system combinations, can be found in Appendix F. Tagging Content by Writing System.

1.4. Characters and Letters

The basic unit of typesetting is the character. However, because writing systems are not always as simple as the basic English alphabet, what a character actually is depends on the context in which the term is used. For example, in Hangul (the Korean writing system), each square representation of a syllable (e.g. ?=Han) can be considered a character. However, the square symbol is really composed of multiple letters each representing a phoneme (e.g. ?=h, ?=a, ?=n) and these also could each be considered a character.

A basic unit of computer text encoding, for any given encoding, is also called a character, and depending on the encoding, a single encoding character might correspond to the entire pre-composed syllabic character (e.g. ?), to the individual phonemic character (e.g. ?), or to smaller units such as a base letterform (e.g. ?) and any combining marks that vary it (e.g. extra strokes that represent aspiration).

In turn, a single encoding character can be represented in the data stream as one or more bytes; and in programming environments one byte is sometimes also called a character.

Therefore the term character is fairly ambiguous where technical precision is required.

For text layout, we will refer to the typographic character unit as the basic unit of text. Even within the realm of text layout, the relevant character unit depends on the operation. For example, line-breaking and letter-spacing will segment a sequence of Thai characters that include U+0E33 THAI CHARACTER SARA AM differently; or the behaviour of a conjunct consonant in a script such as Devanagari may depend on the font in use. So the typographic character represents a unit of the writing system— such as a Latin alphabetic letter (including its diacritics), Hangul syllable, Chinese ideographic character, Myanmar syllable cluster— that is indivisible with respect to a particular typographic operation (line-breaking, first-letter effects, tracking, justification, vertical arrangement, etc.).

Unicode Standard Annex #29: Text Segmentation defines a unit called the grapheme cluster which approximates the typographic character. A UA must use the extended grapheme cluster (not legacy grapheme cluster), as defined in [UAX29], as the basis for its typographic character unit. However, the UA should tailor the definitions as required by typographic tradition since the default rules are not always appropriate or ideal—and is expected to tailor them differently depending on the operation as needed.

Note: The rules for such tailorings are out of scope for CSS.

The following are some examples of typographic character unit tailorings required by standard typesetting practice:
  • In some scripts such as Myanmar or Devanagari, the typographic character unit for both justification and line-breaking is an entire syllable, which can include more than one [UAX29] grapheme cluster.

  • In other scripts such as Thai or Lao, even though for line-breaking the typographic character matches Unicode’s default grapheme clusters, for letter-spacing the relevant unit is less than a [UAX29] grapheme cluster, and may require decomposition or other substitutions before spacing can be inserted.

    For instance, to properly letter-space the Thai word ?? (U+0E04 + U+0E33), the U+0E33 needs to be decomposed into U+0E4D + U+0E32, and then the extra letter-space inserted before the U+0E32: ?? ?.

    A slightly more complex example is ??? (U+0E19 + U+0E49 + U+0E33). In this case, normal Thai shaping will first decompose the U+0E33 into U+0E4D + U+0E32 and then swap the U+0E4D with the U+0E49, giving U+0E19 + U+0E4D + U+0E49 + U+0E32. As before the extra letter-space is then inserted before the U+0E32: ??? ?.

  • Vertical typesetting [CSS-WRITING-MODES-3] can also require tailoring. For example, when typesetting upright text, Tibetan tsek and shad marks are kept with the preceding grapheme cluster, rather than treated as an independent typographic character unit.

A typographic letter unit or letter for the purpose of this specification is a typographic character unit belonging to one of the Letter or Number general categories in Unicode. [UAX44] See Character Properties for how to determine the Unicode properties of a typographic character unit.

The rendering characteristics of a typographic character unit divided by an element boundary is undefined. Ideally each component should be rendered according to the formatting requirements of its respective element’s properties while maintaining correct shaping and positioning of the typographic character unit as a whole. However, depending on the nature of the formatting differences between its parts and the capabilities of the font technology in use, this is not always possible. Therefore such a typographic character unit may be rendered as belonging to either side of the boundary, or as some approximation of belonging to both. Authors are forewarned that dividing grapheme clusters by element boundaries may give inconsistent or undesired results.

1.5. Text Processing

CSS is built on [UNICODE]. UAs that support Unicode must adhere to all normative requirements of the Unicode Core Standard, except where explicitly overridden by CSS. UAs that use a different encoding are not explicitly supported by the CSS specifications; they are, however, expected to fulfill the same text handling requirements by assuming an appropriate mapping between that encoding and Unicode.

For the purpose of determining adjacency for text processing (such as white space processing, text transformation, line-breaking, etc.), and thus in general within this specification, intervening inline box boundaries and out-of-flow elements must be ignored. With respect to text shaping, however, see §?7.3 Shaping Across Element Boundaries.

2. Transforming Text

2.1. Case Transforms: the text-transform property

Name: text-transform
Value: none | [capitalize | uppercase | lowercase ] || full-width || full-size-kana
Initial: none
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property transforms text for styling purposes. It has no effect on the underlying content, and must not affect the content of a plain text copy & paste operation.

Values have the following meanings:

none
No effects.
capitalize
Puts the first typographic letter unit of each word, if lowercase, in titlecase; other characters are unaffected.
uppercase
Puts all letters in uppercase.
lowercase
Puts all letters in lowercase.
full-width
Puts all typographic character units in fullwidth form. If a character does not have a corresponding fullwidth form, it is left as is. This value is typically used to typeset Latin letters and digits as if they were ideographic characters.
full-size-kana
Converts all small Kana characters to the equivalent full-size Kana. This value is typically used for ruby annotation text, where authors may want all small Kana to be drawn as large Kana to compensate for legibility issues at the small font sizes typically used in ruby.

For capitalize, what constitutes a “word“ is UA-dependent; [UAX29] is suggested (but not required) for determining such word boundaries. Authors should not expect capitalize to follow language-specific titlecasing conventions (such as skipping articles in English). Out-of-flow elements and inline element boundaries must not introduce a text-transform word boundary and must be ignored when determining such word boundaries.

The following example converts the ASCII characters used in abbreviations in Japanese text to their fullwidth variants so that they lay out and line break like ideographs:

abbr:lang(ja) { text-transform: full-width; }

Note: As defined in Text Processing Order of Operations, transforming text affects line-breaking and other formatting operations.

The UA must use the full case mappings for Unicode characters, including any conditional casing rules, as defined in Default Case Algorithm section of The Unicode Standard [UNICODE]. If (and only if) the content language of the element is, according to the rules of the document language, known, then any appropriate language-specific rules must be applied as well. These minimally include, but are not limited to, the language-specific rules in Unicode’s SpecialCasing.txt.

For example, in Turkish there are two “i”s, one with a dot—“?” and “i”— and one without—“I” and “?”. Thus the usual case mappings between “I” and “i” are replaced with a different set of mappings to their respective undotted/dotted counterparts, which do not exist in English. This mapping must only take effect if the content language is Turkish written in its modern Latin-based writing system (or another Turkic language that uses Turkish casing rules); in other languages, the usual mapping of “I” and “i” is required. This rule is thus conditionally defined in Unicode’s SpecialCasing.txt file.

The definition of fullwidth and halfwidth forms can be found on the Unicode consortium web site at [UAX11]. The mapping to fullwidth form is defined by taking code points with the <wide> or the <narrow> tag in their Decomposition_Mapping in [UAX44]. For the <narrow> tag, the mapping is from the code point to the decomposition (minus <narrow> tag), and for the <wide> tag, the mapping is from the decomposition (minus the <wide> tag) back to the original code point.

The mappings for small Kana to full-size Kana are defined in Appendix G. Small Kana Mappings.

When multiple values are specified and therefore multiple transformations need to be applied, they are applied in the following order:

  1. capitalize, upercase, and lowercase
  2. full-width
  3. full-size-kana

Text transformation happens after §?4.1.1 Phase I: Collapsing and Transformation but before §?4.1.3 Phase II: Trimming and Positioning. This means that full-width only transforms U+0020 spaces to U+3000 within preserved white space.

Note: A future level of CSS may introduce the ability to create custom mapping tables for less common text transforms, such as by an @text-transform rule similar to @counter-style from [CSS-COUNTER-STYLES-3].

3. White Space and Wrapping: the white-space property

Name: white-space
Value: normal | pre | nowrap | pre-wrap | break-spaces | pre-line
Initial: normal
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property specifies two things:

Values have the following meanings, which must be interpreted according to the White Space Processing and Line Breaking rules:

normal
This value directs user agents to collapse sequences of white space into a single character (or in some cases, no character). Lines may wrap at allowed soft wrap opportunities, as determined by the line-breaking rules in effect, in order to minimize inline-axis overflow.
pre
This value prevents user agents from collapsing sequences of white space. Segment breaks such as line feeds are preserved as forced line breaks. Lines only break at forced line breaks; content that does not fit within the block container overflows it.
nowrap
Like normal, this value collapses white space; but like pre, it does not allow wrapping.
pre-wrap
Like pre, this value preserves white space; but like normal, it allows wrapping.
break-spaces
The behavior is identical to that of pre-wrap, except that:
  • Any sequence of preserved white space always takes up space, including at the end of the line.

  • A line breaking opportunity exists after every preserved white space character, including between white space characters.

Such preserved white space characters take up space and do not hang, and thus affect the box’s intrinsic sizes (min-content size and max-content size).

Note: This value does not guarantee that there will never be any overflow due to white space: for example, if the line length is so short that even a single white space character does not fit, overflow is unavoidable.

pre-line
Like normal, this value collapses consecutive white space characters and allows wrapping, but preserves segment breaks in the source as forced line breaks.

White space that was not removed or collapsed due to white space processing is called preserved white space.

Note: In some cases, preserved white space and other space separators can hang when at the end of the line; this can affect whether they are measured for intrinsic sizing.

The following informative table summarizes the behavior of various white-space values:

New Lines Spaces and Tabs Text Wrapping End-of-line spaces
normal Collapse Collapse Wrap Remove
pre Preserve Preserve No wrap Preserve
nowrap Collapse Collapse No wrap Remove
pre-wrap Preserve Preserve Wrap Hang
break-spaces Preserve Preserve Wrap Wrap
pre-line Preserve Collapse Wrap Remove

See White Space Processing Rules for details on how white space collapses. An informative summary of collapsing (normal and nowrap) is presented below:

See Line Breaking for details on wrapping behavior.

4. White Space Processing Details

The source text of a document often contains formatting that is not relevant to the final rendering: for example, breaking the source into segments (lines) for ease of editing or adding white space characters such as tabs and spaces to indent the source code. CSS white space processing allows the author to control interpretation of such formatting: to preserve or collapse it away when rendering the document. White space processing in CSS interprets white space characters only for rendering: it has no effect on the underlying document data.

White space processing in CSS is controlled with the white-space property.

CSS does not define document segmentation rules. Segments can be separated by a particular newline sequence (such as a line feed or CRLF pair), or delimited by some other mechanism, such as the SGML RECORD-START and RECORD-END tokens. For CSS processing, each document language–defined segment break and each line feed (U+000A) in the text is treated as a segment break, which is then interpreted for rendering as specified by the white-space property.

Note: A document parser might not only normalize any segment breaks, but also collapse other space characters or otherwise process white space according to markup rules. Because CSS processing occurs after the parsing stage, it is not possible to restore these characters for styling. Therefore, some of the behavior specified below can be affected by these limitations and may be user agent dependent.

Note: Anonymous blocks consisting entirely of collapsible white space are removed from the rendering tree. Thus any such white space surrounding a block-level element is collapsed away. See [CSS2] section 9.2.2.1

Control characters (Unicode category Cc)—other than tabs (U+0009), line feeds (U+000A), and sequences that form a segment breakmust be rendered as a visible glyph which the UA must synthethize if the glyphs found in the font are not visible, and must be otherwise treated as any other character of the Other Symbols (So) general category and Common script. The UA may use a glyph provided by a font specifically for the control character, substitute the glyphs provided for the corresponding symbol in the Control Pictures block, generate a visual representation of its code point value, or use some other method to provide an appropriate visible glyph. As required by [UNICODE], unsupported Default_ignorable characters must be ignored for text rendering.

4.1. The White Space Processing Rules

Except where specified otherwise, White space processing in CSS affects only the document white space characters: spaces (U+0020), tabs (U+0009), and segment breaks.

Note: The set of characters considered document white space (part of the document content) and that considered syntactic white space (part of the CSS syntax) are not necessarily identical. However, since both include spaces (U+0020), tabs (U+0009), and line feeds (U+000A) most authors won’t notice any differences.

Besides Space (U+0020) and No-Break Space (U+00A0), Unicode [UNICODE] defines a number of additional space separator characters. In this specification all characters in the Unicode Zs category (See [UAX44]) except Space (U+0020) and No-Break Space (U+00A0) are collectively referred to as other space separators.

4.1.1. Phase I: Collapsing and Transformation

For each inline (including anonymous inlines; see [CSS2] section 9.2.2.1) within an inline formatting context, white space characters are handled as follows, ignoring bidi formatting characters (characters with the Bidi_Control property [UAX9]) as if they were not there:

Then, the entire block is rendered. Inlines are laid out, taking bidi reordering into account, and wrapping as specified by the white-space property.

The following example illustrates the interaction of white-space collapsing and bidirectionality. Consider the following markup fragment, taking special note of spaces (with varied backgrounds and borders for emphasis and identification):

<ltr>A?<rtl>?B?</rtl>?C</ltr>

where the <ltr> element represents a left-to-right embedding and the <rtl> element represents a right-to-left embedding. If the white-space property is set to normal, the white-space processing model will result in the following:

  • The space before the B (?) will collapse with the space after the A (?).
  • The space before the C (?) will collapse with the space after the B (?).

This will leave two spaces, one after the A in the left-to-right embedding level, and one after the B in the right-to-left embedding level. The text will then be ordered according to the Unicode bidirectional algorithm, with the end result being:

A??BC

Note that there will be two spaces between A and B, and none between B and C. This is best avoided by putting spaces outside the element instead of just inside the opening and closing tags and, where practical, by relying on implicit bidirectionality instead of explicit embedding levels.

4.1.2. Segment Break Transformation Rules

When white-space is pre, pre-wrap, break-spaces, or pre-line, segment breaks are not collapsible and are instead transformed into a preserved line feed (U+000A).

For other values of white-space, segment breaks are collapsible. Any collapsible segment break immediately following another collapsible segment break is removed. Then any remaining segment break is either transformed into a space (U+0020) or removed depending on the context before and after the break:

For this purpose, Emoji (Unicode property Emoji) with an East Asian Width property of Wide or Neutral are treated as having an East Asian Width property of Ambiguous.

Note: The white space processing rules have already removed any tabs and spaces after the segment break before these checks take place.

The purpose of the segment break transformation rules (and white space collapsing in general) is to “unbreak” text that has been broken into segments to make the document source code easier to work with. In languages that use word separators, such as English and Korean, “unbreaking” a line requires joining the two lines with a space.
Here is an English paragraph
that is broken into multiple lines
in the source code so that it can
more easily read in a text editor.

Here is an English paragraph that is broken into multiple lines in the source code so that it can be more easily read in a text editor.

Eliminating a line break in English requires maintaining a space in its place.

In languages that have no word separators, such as Chinese, “unbreaking” a line requires joining the two lines with no intervening space.

這個段落是呢么長,
在一行寫不行。最好
用三行寫。

這個段落是呢么長,在一行不行。最好用三行寫。

Eliminating a line break in Chinese requires eliminating any intervening white space.

The segment break transformation rules thus use adjacent context to either transform the segment break into a space or eliminate it entirely.

4.1.3. Phase II: Trimming and Positioning

As each line is laid out,

  1. A sequence of collapsible spaces at the beginning of a line is removed.
  2. If the tab size is zero, preserved tabs are not rendered. Otherwise, each preserved tab is rendered as a horizontal shift that lines up the start edge of the next glyph with the next tab stop. If this distance is less than 0.5ch, then the subsequent tab stop is used instead. Tab stops occur at points that are multiples of the tab size from the block’s starting content edge. The tab size is given by the tab-size property.

    Note: See [UAX9] for rules on how U+0009 tabulation interacts with bidi.

  3. A sequence at the end of a line of collapsible spaces is removed.

    In the case of bidirectional text, any sequence of collapsible spaces located at the end of the line prior to bidi reordering [CSS-WRITING-MODES-3] is also removed, and bidi reordering is applied on the remaining content of the line.

  4. If there remains any sequence of white space, and/or other space separators, at the end of a line (after bidi reordering [CSS-WRITING-MODES-3]):
This example shows that conditionally hanging white space at the end of lines with forced breaks provides symmetry with the start of the line. An underline is added to help visualize the spaces.
p {
  white-space: pre-wrap;
  width: 5ch;
  border: solid 1px;
  font-family: monospace;
  text-align: center;
}
<p> 0 </p>

The sample above would be rendered as follows:

0

Since the final space is before a forced line break and does not overflow, it does not hang, and centering works as expected.

This example illustrates the difference between hanging spaces at the end of lines without forced breaks, and conditionally hanging them at the end of lines with forced breaks. An underline is added to help visualize the spaces.
p {
  white-space: pre-wrap;
  width: 3ch;
  border: solid 1px;
  font-family: monospace;
}
<p> 0 0 0 0 </p>

The sample above would be rendered as follows:

0
0 0
0

If p { text-align: right; } was added, the result would be as follows:

0
0 0
0

As the preserved spaces at the end of lines without a forced break must hang, they are not considered when placing the rest of the line during text alignment. When aligning towards the end, this means any such spaces will overflow, and will not prevent the rest of the line’s content from being flush with the edge of the line. On the other hand, preserved spaces at the end of a line with a forced break conditionally hang. Since the space at the end of the last line would not overflow in this example, it does not hang and therefore is considered during text alignment.

In the following example, there is not enough room on any line to fit the end-of-line spaces, so they hang on all lines: the one on the line without a forced break because it must, as well as the one on the line with a forced break, because it conditionally hangs and overflows. An underline is added to help visualize the spaces.
p {
  white-space: pre-wrap;
  width: 3ch;
  border: solid 1px;
  font-family: monospace;
}
<p>0 0 0 0 </p>
0 0
0 0

The last line is not wrapped before the last 0 because characters that conditionally hang are not considered when measuring the line’s contents for fit.

4.2. Tab Character Size: the tab-size property

Name: tab-size
Value: <number> | <length>
Initial: 8
Applies to: block containers
Inherited: yes
Percentages: n/a
Computed value: the specified number or absolute length
Canonical order: n/a
Animation type: by computed value type

This property determines the tab size used to render preserved tab characters (U+0009). A <number> represents the measure as a multiple of the space character’s advance width (U+0020) including its associated letter-spacing and word-spacing. Negative values are not allowed.

5. Line Breaking and Word Boundaries

When inline-level content is laid out into lines, it is broken across line boxes. Such a break is called a line break. When a line is broken due to explicit line-breaking controls (such as a preserved newline character), or due to the start or end of a block, it is a forced line break. When a line is broken due to content wrapping (i.e. when the UA creates unforced line breaks in order to fit the content within the measure), it is a soft wrap break. The process of breaking inline-level content into lines is called line breaking.

Wrapping is only performed at an allowed break point, called a soft wrap opportunity. When wrapping is enabled (see white-space), the UA must minimize the amount of content overflowing a line by wrapping the line at a soft wrap opportunity, if one exists.

In most writing systems, in the absence of hyphenation a soft wrap opportunity occurs only at word boundaries. Many such systems use spaces or punctuation to explicitly separate words, and soft wrap opportunities can be identified by these characters. Scripts such as Thai, Lao, and Khmer, however, do not use spaces or punctuation to separate words. Although the zero width space (U+200B) can be used as an explicit word delimiter in these scripts, this practice is not common. As a result, a lexical resource is needed to correctly identify soft wrap opportunities in such texts.

In some other writing systems, soft wrap opportunities are based on orthographic syllable boundaries, not word boundaries. Some of these systems, such as Javanese and Balinese, are similar to Thai and Lao in that they require analysis of the text to find breaking opportunities. In others such as Chinese (as well as Japanese, Yi, and sometimes also Korean), each syllable tends to correspond to a single typographic letter unit, and thus line breaking conventions allow the line to break anywhere except between certain character combinations. Additionally the level of strictness in these restrictions varies with the typesetting style.

While CSS does not fully define where soft wrap opportunities occur, some controls are provided to distinguish common variations:

Note: Further information on line breaking conventions can be found in [JLREQ] and [JIS4051] for Japanese, [CLREQ] and [ZHMARK] for Chinese, and in [UAX14] for all scripts in Unicode. See also the Internationalization Working Group’s Typography Index [TYPOGRAPHY] which includes more information on additional languages.

5.1. Line Breaking Details

When determining line breaks:

5.2. Breaking Rules for Letters: the word-break property

Name: word-break
Value: normal | keep-all | break-all | break-word
Initial: normal
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property specifies soft wrap opportunities between letters, i.e. where it is “normal” and permissible to break lines of text. Specifically it controls whether a soft wrap opportunity generally exists between adjacent typographic letter units (and/or non-letter typographic character units belonging to the NU, AL, AI, or ID Unicode line breaking classes [UAX14]). It does not affect rules governing the soft wrap opportunities created by white space (as well as by other space separators) and around punctuation. (See line-break for controls affecting punctuation and small kana.)

For example, in some styles of CJK typesetting, English words are allowed to break between any two letters, rather than only at spaces or hyphenation points; this can be enabled with word-break:break-all.

A snippet of Japanese text with English in it. The word 'caption' is broken into 'capt' and 'ion' across two lines.

An example of English text embedded in Japanese being broken at an arbitrary point in the word.

As another example, Korean has two styles of line-breaking: between any two Korean syllables (word-break: normal) or, like English, mainly at spaces (word-break: keep-all).

? ?? ???? ??? ? ? ? ?? ?
?? “??” ?? “??” ??? ??.
? ?? ???? ??? ? ? ? ??
??? “??” ?? “??” ??? ??.

Note: To enable additional break opportunities only in the case of overflow, see overflow-wrap.

Values have the following meanings:

normal
Words break according to their customary rules, as described above. Korean, which commonly exhibits two different behaviors, allows breaks between any two consecutive Hangul/Hanja.
break-all
Breaking is allowed within “words”: specifically, in addition to soft wrap opportunities allowed for normal, any typographic character units resolving to the NU (“numeric”), AL (“alphabetic”), or SA (“Southeast Asian”) line breaking classes [UAX14] are instead treated as ID (“ideographic characters”) for the purpose of line-breaking. Hyphenation is not applied. This option is used mostly in a context where the text consists predominantly of CJK characters with only short non-CJK excerpts, and it is desired that the text be better distributed on each line.

Note: This value does not affect whether there are soft wrap opportunities around punctuation characters. To allow breaks anywhere, see line-break: anywhere.

keep-all
Breaking is forbidden within “words”: implicit soft wrap opportunities between typographic letter units (or other typographic character units belonging to the NU, AL, AI, or ID Unicode line breaking classes [UAX14]) are suppressed, i.e. breaks are prohibited between pairs of such characters (regardless of line-break settings other than anywhere) except where opportunities exist due to dictionary-based breaking. Otherwise this option is equivalent to normal. In this style, sequences of CJK characters do not break.

Note: This is the other common behavior for Korean (which uses spaces between words), and is also useful for mixed-script text where CJK snippets are mixed into another language that uses spaces for separation.

Symbols that line-break the same way as letters of a particular category are affected the same way as those letters.

Here’s a mixed-script sample text:

這是一些漢字 and some Latin ? ??? ?? ???? ??????????????????????????

The break-points are determined as follows (indicated by ‘·’):

word-break: normal
這·是·一·些·漢·字·and·some·Latin·?·???·??·????·???·????????·????????·???????
word-break: break-all
這·是·一·些·漢·字·a·n·d·s·o·m·e·L·a·t·i·n·?·?·?·?·?·?·?·?·?·?·?·?·?·??·?·?·??·?·?·?·?·?·?·??·?·?·?·?·?·?·?·?·?
word-break: keep-all
這是一些漢字·and·some·Latin·?·???·??·????·???·????????·????????·???????

Japanese is usually typeset allowing line breaks within words. However, it is sometimes preferred to suppress these wrapping opportunities and to only allow wrapping at the end of certain sentence fragments. This is most commonly done in very short pieces of text, such as headings and table or figure captions.

This can be achieved by marking the allowed wrapping points with wbr or U+200B ZERO WIDTH SPACE, and suppressing the other ones using word-break: keep-all.

For instance, the following markup can produce either of the renderings below, depending on the value of the word-break property:

<h1>窓ぎわの<wbr>トットちゃん</h1>
h1 { word-break: normal } h1 { word-break: keep-all }
Expected rendering
窓ぎわのトットちゃ
ん
窓ぎわの
トットちゃん
Result in your browser 窓ぎわのトットちゃん 窓ぎわのトットちゃん

When shaping scripts such as Arabic are allowed to break within words due to break-all the characters must still be shaped as if the word were not broken (see §?5.6 Shaping Across Intra-word Breaks).

For compatibility with legacy content, the word-break property also supports a deprecated break-word keyword. When specified, this has the same effect as word-break: normal and overflow-wrap: anywhere, regardless of the actual value of the overflow-wrap property.

5.3. Line Breaking Strictness: the line-break property

Name: line-break
Value: auto | loose | normal | strict | anywhere
Initial: auto
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property specifies the strictness of line-breaking rules applied within an element: especially how wrapping interacts with punctuation and symbols. Values have the following meanings:

auto
The UA determines the set of line-breaking restrictions to use, and it may vary the restrictions based on the length of the line; e.g., use a less restrictive set of line-break rules for short lines.
loose
Breaks text using the least restrictive set of line-breaking rules. Typically used for short lines, such as in newspapers.
normal
Breaks text using the most common set of line-breaking rules.
strict
Breaks text using the most stringent set of line-breaking rules.
anywhere
There is a soft wrap opportunity around every typographic character unit, including around any punctuation character or preserved white spaces, or in the middle of words, disregarding any prohibition against line breaks, even those introduced by characters with the GL, WJ, or ZWJ character class (see [UAX14]) or mandated by the word-break property. The different wrapping opportunities must not be prioritized. Hyphenation is not applied.

Note: This value triggers the line breaking rules typically seen in terminals.

Note: anywhere only allows preserved white spaces at the end of the line to be wrapped to the next line when white-space is set to break-spaces, because in other cases:

When it does have an effect on preserved white space, with white-space: break-spaces, it allows breaking before the first space of a sequence, which break-spaces on its own does not.

CSS distinguishes between four levels of strictness in the rules for text wrapping. The precise set of rules in effect for each of loose, normal, and strict is up to the UA and should follow language conventions. However, this specification does require that:

Note: The requirements listed above only create distinctions in CJK text. In an implementation that matches only the rules above, and no additional rules, line-break would only affect CJK code points unless the writing system is tagged as Chinese or Japanese. Future levels may add additional specific rules for other writing systems and languages as their requirements become known.

As UAs can add additional distinctions between strict/normal/loose modes, these values can exhibit differences in other writing systems as well. For example, a UA with sufficiently-advanced Thai language processing ability could choose to map different levels of strictness in Thai line-breaking to these keywords, e.g. disallowing breaks within compound words in strict mode (e.g. breaking ??????????????????????? as ????????·????????·???????) while allowing more breaks in loose (????????·???·?????·????·???).

Note: The CSSWG recognizes that in a future edition of the specification finer control over line breaking may be necessary to satisfy high-end publishing requirements.

5.4. Hyphenation: the hyphens property

Hyphenation is the controlled splitting of words where they usually would not be allowed to break to improve the layout of paragraphs, typically splitting words at syllabic or morphemic boundaries, and visually indicating the split (usually by inserting a hyphen, U+2010). In some cases, hyphenation may also alter the spelling of a word. Regardless, hyphenation is a rendering effect only: it must have no effect on the underlying document content or on text selection or searching.

Hyphenation occurs when the line breaks at a valid hyphenation opportunity, which is a type of soft wrap opportunity that exists within a word where hyphenation is allowed. In CSS hyphenation opportunities are controlled with the hyphens property. CSS Text Level 3 does not define the exact rules for hyphenation; however UAs are strongly encouraged to optimize their choice of break points and to chose language-appropriate hyphenation points.

Note: The soft wrap opportunity introduced by the U+002D HYPHEN-MINUS character or the U+2010 HYPHEN character is not a hyphenation opportunity, as no visual indication of the split is created when wrapping: these characters are visible whether the line is wrapped at that point or not.

Hyphenation opportunities are considered when calculating min-content intrinsic sizes.

Name: hyphens
Value: none | manual | auto
Initial: manual
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property controls whether hyphenation is allowed to create more soft wrap opportunities within a line of text. Values have the following meanings:

none
Words are not hyphenated, even if characters inside the word explicitly define hyphenation opportunities.

Note: This does not suppress the existing soft wrap opportunities introduced by always visible characters such as U+002D HYPHEN-MINUS or U+2010 HYPHEN.

manual
Words are only hyphenated where there are characters inside the word that explicitly suggest hyphenation opportunities.

In Unicode, U+00AD is a conditional "soft hyphen" and U+2010 is an unconditional hyphen. Unicode Standard Annex #14 describes the role of soft hyphens in Unicode line breaking. [UAX14] In HTML, &shy; represents the soft hyphen character, which suggests a hyphenation opportunity.

ex&shy;ample
auto
Words may be broken at hyphenation opportunities determined automatically by a language-appropriate hyphenation resource in addition to those indicated explicitly by a conditional hyphen. Automatic hyphenation opportunities within a word must be ignored if the word contains a conditional hyphen (&shy; or U+00AD), in favor of the conditional hyphen(s). However, if, even after breaking at such opportunities, a portion of that word is is still too long to fit on one line, an automatic hyphenation opportunity may be used.

Note: In some languages (such as English but not German), it may appropriate to avoid having hyphenation opportunities in mixed case words, as those may indicate proper nouns. This type of heuristic is however not mandated by this specification, as is up to the User Agent and its language-specific hyphenation resource.

Correct automatic hyphenation requires a hyphenation resource appropriate to the language of the text being broken. The UA must therefore only automatically hyphenate text for which the content language is known and for which it has an appropriate hyphenation resource.

For the purpose of the hyphens property, what constitutes a “word” is UA-dependent. However, inline element boundaries and out-of-flow elements must be ignored when determining word boundaries.

Authors should correctly tag their content’s language (e.g. using the HTML lang attribute or the HTTP Content-Language header) in order to obtain correct automatic hyphenation.

The character or characters visually shown due to hyphenation at a hyphenation opportunity created by a conditional hyphen character (&shy; or U+00AD) are inserted in-place, and styled according to any property that applies to the conditional hyphen character.

ex<span style="color:red">&shy;</span>ample

When the markup above is hyphenated, it is rendered as ex-
ample

When shaping scripts such as Arabic are allowed to break within words due to hyphenation, the characters must still be shaped as if the word were not broken (see §?5.6 Shaping Across Intra-word Breaks).

For example, if the Uyghur word “??????” were hyphenated, it would appear as [isolated DAL + isolated ALEF + initial MEEM + medial YEH + hyphen + line-break + final DAL + isolated ALEF MAKSURA] not as [isolated DAL + isolated ALEF + initial MEEM + final YEH + hyphen + line-break + isolated DAL + isolated ALEF MAKSURA]

5.5. Overflow Wrapping: the overflow-wrap/word-wrap property

Name: overflow-wrap, word-wrap
Value: normal | break-word | anywhere
Initial: normal
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property specifies whether the UA may break at otherwise disallowed points within a line to prevent overflow, when an otherwise-unbreakable string is too long to fit within the line box. It only has an effect when white-space allows wrapping. Possible values:

normal
Lines may break only at allowed break points. However, the restrictions introduced by word-break: keep-all may be relaxed to match word-break: normal if there are no otherwise-acceptable break points in the line.
anywhere
An otherwise unbreakable sequence of characters may be broken at an arbitrary point if there are no otherwise-acceptable break points in the line. Shaping characters are still shaped as if the word were not broken, and grapheme clusters must stay together as one unit. No hyphenation character is inserted at the break point. Soft wrap opportunities introduced by anywhere are considered when calculating min-content intrinsic sizes.
break-word
As for anywhere except that soft wrap opportunities introduced by break-word are not considered when calculating min-content intrinsic sizes.

For legacy reasons, UAs must treat word-wrap as an legacy name alias of the overflow-wrap property.

5.6. Shaping Across Intra-word Breaks

When shaping scripts such as Arabic wrap at unforced soft wrap opportunities within words (such as when breaking due to word-break: break-all, line-break: anywhere, overflow-wrap: break-word, overflow-wrap: anywhere, or when hyphenating) the characters must still be shaped (their joining forms chosen) as if the word were still whole.

For example, if the word “?????” is broken between the “?” and “?”, the “?” still takes its initial form (“?”), and the “?” its medial form (“?”)—forming as in “??? | ??”, not as in “??? | ??”.

6. Alignment and Justification

Alignment and justification controls how inline content is distributed within a line box.

6.1. Text Alignment: the text-align shorthand

Name: text-align
Value: start | end | left | right | center | justify | match-parent | justify-all
Initial: start
Applies to: block containers
Inherited: yes
Percentages: see individual properties
Computed value: see individual properties
Animation type: discrete
Canonical order: n/a

This shorthand property sets the text-align-all and text-align-last properties and describes how the inline-level content of a block is aligned along the inline axis if the content does not completely fill the line box. Values other than justify-all or match-parent are assigned to text-align-all and reset text-align-last to auto.

Values have the following meanings:

start
Inline-level content is aligned to the start edge of the line box.
end
Inline-level content is aligned to the end edge of the line box.
left
Inline-level content is aligned to the line left edge of the line box. (In vertical writing modes, this will be either the physical top or bottom, depending on text-orientation.) [CSS-WRITING-MODES-3]
right
Inline-level content is aligned to the line right edge of the line box. (In vertical writing modes, this will be either the physical top or bottom, depending on text-orientation.) [CSS-WRITING-MODES-3]
center
Inline-level content is centered within the line box.
justify
Text is justified according to the method specified by the text-justify property, in order to exactly fill the line box. Unless otherwise specified by text-align-last, the last line before a forced break or the end of the block is start-aligned.
justify-all
Sets both text-align-all and text-align-last to justify, forcing the last line to justify as well.
match-parent
This value behaves the same as inherit (computes to its parent’s computed value) except that an inherited value of start or end is interpreted against the parent’s (or the initial containing block’s, if there is no parent) direction value and results in a computed value of either left or right. When specified on the text-align shorthand, sets both text-align-all and text-align-last to match-parent.

A block of text is a stack of line boxes. This property specifies how the inline-level boxes within each line box align with respect to the start and end sides of the line box. Alignment is not with respect to the viewport or containing block.

In the case of justify, the UA may stretch or shrink any inline boxes by adjusting their text. (See text-justify.) If an element’s white space is not collapsible, then the UA is not required to adjust its text for the purpose of justification and may instead treat the text as having no justification opportunities. If the UA chooses to adjust the text, then it must ensure that tab stops continue to line up as required by the white space processing rules.

If (after justification, if any) the inline contents of a line box are too long to fit within it, then the contents are start-aligned: any content that doesn’t fit overflows the line box’s end edge.

See Bidirectionality and line boxes for details on how to determine the start and end edges of a line box.

6.2. Default Text Alignment: the text-align-all property

Name: text-align-all
Value: start | end | left | right | center | justify | match-parent
Initial: start
Applies to: block containers
Inherited: yes
Percentages: n/a
Computed value: keyword as specified, except for match-parent which computes as defined above
Canonical order: n/a
Animation type: discrete

This longhand of the text-align shorthand property specifies the inline alignment of all lines of inline content in the block container, except for last lines overridden by a non-auto value of text-align-last. See text-align for a full description of values.

Authors should use the text-align shorthand instead of this property.

6.3. Last Line Alignment: the text-align-last property

Name: text-align-last
Value: auto | start | end | left | right | center | justify | match-parent
Initial: auto
Applies to: block containers
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property describes how the last line of a block or a line right before a forced line break is aligned.

If auto is specified, content on the affected line is aligned per text-align-all unless text-align-all is set to justify, in which case it is start-aligned. All other values are interpreted as described for text-align.

6.4. Justification Method: the text-justify property

Name: text-justify
Value: auto | none | inter-word | inter-character
Initial: auto
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete

This property selects the justification method used when a line’s alignment is set to justify (see text-align). The property applies to inlines, but is inherited from block containers to the root inline box containing their inline-level contents. It takes the following values:

auto
The UA determines the justification algorithm to follow, based on a balance between performance and adequate presentation quality. Since justification rules vary by writing system and language, UAs should, where possible, use a justification algorithm appropriate to the text.

For example, the UA could use by default a justification method that is a simple universal compromise for all writing systems—such as primarily expanding word separators and between CJK typographic letter units along with secondarily expanding between Southeast Asian typographic letter units. Then, in cases where the content language of the paragraph is known, it could choose a more language-tailored justification behavior e.g. following [JLREQ] for Japanese, using cursive elongation for Arabic, using inter-word for German, etc.

Two lines of calligraphic Arabic end together due to a mix of compressed and swash forms.

An example of cursively-justified Arabic text, rendered by Tasmeem. Like English, Arabic can be justified by adjusting the spacing between words, but in most styles it can also be justified by calligraphically elongating or compressing the letterforms themselves. In this example, the upper text is extended to fill the line by the use of elongated (kashida) forms and swash forms, while the bottom line is compressed slightly by using a stacked combination for the characters between ? and ?. By employing traditional calligraphic techniques, a typesetter can justify the line while preserving flow and color, providing a very high quality justification effect. However, this is by its nature a very script-specific effect.

Extra space is partly to spaces and partly among CJK and Thai letters.

Mixed-script text with text-justify: auto: this interpretation uses a universal-compromise justification method, expanding at spaces as well as between CJK and Southeast Asian letters. This effectively uses inter-word + inter-ideograph spacing for lines that have word-separators and/or CJK characters and falls back to inter-cluster behavior for lines that don’t or for which the space stretches too far.

none
Justification is disabled: there are no justification opportunities within the text.

No extra space is insesrted.

Mixed-script text with text-justify: none

Note: This value is intended for use in user stylesheets to improve readability or for accessibility purposes.

inter-word
Justification adjusts spacing at word separators only (effectively varying the used word-spacing on the line). This behavior is typical for languages that separate words using spaces, like English or Korean.

Extra space is equally distributed mainly to spaces.

Mixed-script text with text-justify: inter-word

inter-character
Justification adjusts spacing between each pair of adjacent typographic character units (effectively varying the used letter-spacing on the line). This value is sometimes used in East Asian systems such as Japanese.

Extra space is equally distributed at points between spaces and letters of all writing systems.

Mixed-script text with text-justify: inter-character

For legacy reasons, UAs must also support the alternate keyword distribute with the exact same meaning and behavior.

Since optimal justification is language-sensitive, authors should correctly language-tag their content for the best results.

Note: The guidelines in this level of CSS do not describe a complete justification algorithm. They are merely a minimum set of requirements that a complete algorithm should meet. Limiting the set of requirements gives UAs some latitude in choosing a justification algorithm that meets their needs and desired balance of quality, speed, and complexity.

6.4.1. Expanding and Compressing Text

When justifying text, the user agent takes the remaining space between the ends of a line’s contents and the edges of its line box, and distributes that space throughout its contents so that the contents exactly fill the line box. The user agent may alternatively distribute negative space, putting more content on the line than would otherwise fit under normal spacing conditions.

A justification opportunity is a point where the justification algorithm may alter spacing within the text. A justification opportunity can be provided by a single typographic character unit (such as a word separator), or by the juxtaposition of two typographic character units. As with controls for soft wrap opportunities, whether a typographic character unit provides a justification opportunity is controlled by the text-justify value of its parent; similarly, whether a justification opportunity exists between two consecutive typographic character units is determined by the text-justify value of their nearest common ancestor.

Space distributed by justification is in addition to the spacing defined by the letter-spacing or word-spacing properties. When such additional space is distributed to a word separator justification opportunity, it is applied under the same rules as for word-spacing. Similarly, when space is distributed to an justification opportunity between two typographic character units, it is applied under the same rules as for letter-spacing.

A justification algorithm may divide justification opportunities into different priority levels. All justification opportunities within a given level are expanded or compressed at the same priority, regardless of which typographic character units created that opportunity. For example, if justification opportunities between two Han characters and between two Latin letters are defined to be at the same level (as they are in the inter-character justification style), they are not treated differently because they originate from different typographic character units. It is not defined in this level whether or how other factors (such as font size, letter-spacing, glyph shape, position within the line, etc.) may influence the distribution of space to justification opportunities within the line.

The UA may enable or break optional ligatures or use other font features such as alternate glyphs or glyph compression to help justify the text under any method. This behavior is not controlled by this level of CSS. However, UAs must not break required ligatures or otherwise disable features required to correctly shape complex scripts.

If a justification opportunity exists within a line, and text alignment specifies full justification (justify) for that line, it must be justified.

6.4.2. Handling Symbols and Punctuation

When determining justification opportunities, a typographic character unit from the Unicode Symbols (S*) and Punctuation (P*) classes is generally treated the same as a typographic letter unit of the same script (or, if the character’s script property is Common, then as a typographic letter unit of the dominant script).

However, by typographic tradition there may be additional rules controlling the justification of symbols and punctuation. Therefore, the UA may reassign specific characters or introduce additional levels of prioritization to handle justification opportunities involving symbols and punctuation.

For example, there are traditionally no justification opportunities between consecutive U+2014 Em Dash ‘—’, U+2015 Horizontal Bar ‘―’, U+2026 Horizontal Ellipsis ‘…’, or U+2025 Two Dot Leader ‘‥’ characters [JLREQ]; thus a UA might assign these characters to a “never” prioritization level. As another example, certain fullwidth punctuation characters (such as U+301A Left White Square Bracket ‘?’) are considered to contain a justification opportunity in Japanese. The UA might therefore assign these characters to a higher prioritization level than the opportunities between ideographic characters.

6.4.3. Unexpandable Text

If the inline contents of a line cannot be stretched to the full width of the line box, then they must be aligned as specified by the text-align-last property. (If text-align-last is justify, then they must be aligned as for center.)

6.4.4. Cursive Scripts

Justification must not introduce gaps between the joined typographic letter units of cursive scripts such as Arabic. If it is able, the UA may translate space distributed to justification opportunities within a run of such typographic letter units into some form of cursive elongation for that run. It otherwise must assume that no justification opportunity exists between any pair of typographic letter units in cursive script (regardless of whether they join).

The following are examples of unacceptable justification:

Adding gaps between every pair of Arabic letters

Adding gaps between every pair of unjoined Arabic letters

Some font designs allow for the use of the tatweel character for justification. A UA that performs tatweel-based justification must properly handle the rules for its use. Note that correct insertion of tatweel characters depends on context, including the letter-combinations involved, location within the word, and location of the word within the line.

6.4.5. Minimum Requirements for auto Justification

For auto justification, this specification does not define what all of the justification opportunities are, how they are prioritized, or when and how multiple levels of justification opportunities interact. However, it does require that

Further information on text justification can be found in (or submitted to) “Approaches to Full Justification”, which indexes by writing system and language, and is maintained by the W3C Internationalization Working Group. [JUSTIFY]

7. Spacing

CSS offers control over text spacing via the word-spacing and letter-spacing properties, which specify additional space around word separators or between typographic character units, respectively.

7.1. Word Spacing: the word-spacing property

Name: word-spacing
Value: normal | <length>
Initial: normal
Applies to: inline boxes
Inherited: yes
Percentages: N/A
Computed value: an absolute length
Canonical order: n/a
Animation type: by computed value type

This property specifies additional spacing between “words”. Missing values are assumed to be word-spacing:normal. Values are interpreted as defined below:

normal
No additional spacing is applied. Computes to zero.
<length>
Specifies extra spacing in addition to the intrinsic inter-word spacing defined by the font.

Additional spacing is applied to each word separator left in the text after the white space processing rules have been applied, and should be applied half on each side of the character unless otherwise dictated by typographic tradition. Values may be negative, but there may be implementation-dependent limits.

Word-separator characters are typographic character units whose purpose and general usage is to separate words. In [UNICODE] this includes the space (U+0020), the no-break space (U+00A0), the Ethiopic word space (U+1361), the Aegean word separators (U+10100,U+10101), the Ugaritic word divider (U+1039F), and the Phoenician Word Separator (U+1091F).

If there are no word-separator characters, or if a word-separating character has a zero advance width (such as the zero width space U+200B) then the user agent must not create an additional spacing between words.

General punctuation (from U+2000 to U+206F) and fixed-width spaces (such as U+3000 and U+2000 through U+200A) are not considered word-separator characters.

7.2. Tracking: the letter-spacing property

Name: letter-spacing
Value: normal | <length>
Initial: normal
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: an absolute length
Canonical order: n/a
Animation type: by computed value type

This property specifies additional spacing (commonly called tracking) between adjacent typographic character units. Letter-spacing is applied after bidi reordering [CSS-WRITING-MODES-3] and is in addition to kerning [CSS-FONTS-3] and word-spacing. Depending on the justification rules in effect, user agents may further increase or decrease the space between typographic character units in order to justify text.

Values have the following meanings:

normal
No additional spacing is applied. Computes to zero.
<length>
Specifies additional spacing between typographic character units. Values may be negative, but there may be implementation-dependent limits.

For legacy reasons, a computed letter-spacing of zero yields a resolved value (getComputedStyle() return value) of normal.

For the purpose of letter-spacing, each consecutive run of atomic inlines (such as images and inline blocks) is treated as a single typographic character unit.

Letter-spacing must not be applied at the beginning or at the end of a line.

Because letter-spacing is not applied at the beginning or end of a line, text always fits flush with the edge of the block.

p    { letter-spacing: 1em; }

<p>abc</p>

a b c

a b c

UAs therefore must not append letter spacing to the right or trailing edge of a line:

a b c 

Letter spacing between two typographic character units effectively “belongs” to the innermost element that contains the two typographic character units: the total letter spacing between two adjacent typographic character units (after bidi reordering) is specified by and rendered within the innermost element that contains the boundary between the two typographic character units.

A given value of letter-spacing only affects the spacing between characters completely contained within the element for which it is specified:

p    { letter-spacing: 1em; }
span { letter-spacing: 2em; }

<p>a<span>bb</span>c</p>

a b  b c

This also means that applying letter-spacing to an element containing only a single character has no effect on the rendered result:

p    { letter-spacing: 1em; }
span { letter-spacing: 2em; }

<p>a<span>b</span>c</p>

a b c

An inline box only includes letter spacing between characters completely contained within that element:

p    { letter-spacing: 1em; }

<p>a<span>bb</span>c</p>

a b b c

It is incorrect to include the letter spacing on the right or trailing edge of the element:

a b b c

Letter spacing is inserted after RTL reordering, so the letter spacing applied to the inner span below has no effect, since after reordering the "c" doesn’t end up next to "?":

p    { letter-spacing: 1em; }
span { letter-spacing: 2em; }

<!-- abc followed by Hebrew letters alef (?), bet (?) and gimel (?) -->
<!-- Reordering will display these in reverse order. -->
<p>ab<span>c?</span>??</p>

a b c ? ? ?

Letter spacing ignores invisible zero-width formatting characters (such as those from the Unicode Cf category). Spacing must be added as if those characters did not exist in the document.

For example, letter-spacing applied to A&#x200B;B is identical to AB, regardless of where any element boundaries might fall.

When the effective spacing between two characters is not zero (due to either justification or a non-zero value of letter-spacing), user agents should not apply optional ligatures, i.e. those that are not defined as required for fundamentally correct glyph shaping. However, ligatures and other font features specified via the low-level font-feature-settings property take precedence over this rule. See CSS Fonts 3 §7.2 Feature precedence.

For example, if the word “filial” is letter-spaced, an “fi” ligature should not be used as it will prevent even spacing of the text.

filial vs ?lial

Note: In OpenType, required ligatures are expected to be associated to the rlig feature. All other ligatures are therefore considered optional. In some cases, however, UA or platform heuristics apply additional ligatures in order to handle broken fonts; this specification does not define or override such exceptional handling.

7.2.1. Cursive Scripts

If it is able, the UA may apply letter spacing to cursive scripts by translating the total extra space to be distributed to a run of such letters into some form of cursive elongation (or compression, for negative tracking values) for that run that results in an equivalent total expansion (or compression) of the run. Otherwise, if the UA cannot expand text from a cursive script without breaking its cursive connections, it must not apply spacing between any pair of that script’s typographic letter units at all (effectively treating each word as a single typographic letter unit for the purpose of letter-spacing). Both cases will result in an effective spacing of zero between such letters; however the former will preserve the sense of stretching out the text.

Below are some appropriate and inappropriate examples of spacing out Arabic text.

Original text
BAD Even distribution of space between each letter. Notice this breaks cursive joins!
OK Distributing ∑letter-spacing by typographically-appropriate cursive elongation. The resulting text is as long as the previous evenly-spaced example.
OK Suppressing letter-spacing between Arabic letters. Notice letter-spacing is nonetheless applied to non-Arabic characters (like spaces).
BAD Applying letter-spacing only between non-joined letters. This distorts typographic color and obfuscates word boundaries.
Note: Proper cursive elongation or compression of a text can vary depending on the script, typeface, language, location within a word, location within a line, implementation complexity, font capabilities, and calligraphic preferences, and may not be possible in certain cases at all. It may involve the use of shortening ligatures, swash variants, contextual forms, elongation glyphs such as U+0640 ARABIC TATWEEL, or other microtypography. It is outside the scope of CSS to define rules for these effects. Authors should avoid applying letter-spacing to cursive scripts unless they are prepared to accept non-interoperable results.

7.3. Shaping Across Element Boundaries

Text shaping must be broken at inline box boundaries when any of the following are true for any box whose boundary separates the two typographic character units:

Text shaping must not be broken across inline box boundaries when there is no effective change in formatting, or if the only formatting changes do not affect the glyphs (as in applying text decoration).

Text shaping should not be broken across inline box boundaries otherwise, if it is reasonable and possible for that case given the limitations of the font technology.

An example of reasonable and possible shaping across boundaries is Arabic shaping: in many systems this is performed by the font engine, allowing the font to provide variant glyphs with potentially very sophisticated contextual shaping. It’s not generally possible to rely on this system across a font change unless the font engine has an API to provide context, but it is straightforward and therefore quite reasonable for an engine to work around this limitation by, for example, using the zero-width-joiner (U+200D) or zero-width-non-joiner (U+200C) as appropriate to solicit the correct choice of initial/medial/final/isolated glyph.

An example of possible but not reasonable shaping across boundaries is handling a font that is sensitive to 20 characters of context on either side to choose its glyphs: passing all the text before and after the string in question, even through multiple inline boundaries with formatting changes, is complicated. The UA could handle such cases, but is not required to, as they are not typical or fundamentally required by any modern writing system.

An example of impossible shaping accross boundaries is a change in font weight partway through the word “and” in a font where a ligature would replace all three letters of the word “and” with an ampersand glyph (“&”).

8. Edge Effects

Edge effects control the indentation of lines with respect to other lines in the block (text-indent) and how content is measured at the start and end edges of a line (hanging-punctuation).

8.1. First Line Indentation: the text-indent property

Name: text-indent
Value: [ <length-percentage> ] && hanging? && each-line?
Initial: 0
Applies to: block containers
Inherited: yes
Percentages: refers to block container’s own inline-axis inner size
Computed value: computed <length-percentage> value, plus any specified keywords
Canonical order: per grammar
Animation type: by computed value type

This property specifies the indentation applied to lines of inline content in a block. The indent is treated as a margin applied to the start edge of the line box.

Unless otherwise specified by the each-line and/or hanging keywords, only lines that are the first formatted line [CSS2] of an element are affected. For example, the first line of an anonymous block box is only affected if it is the first child of its parent element.

Values have the following meanings:

<length>
Gives the amount of the indent as an absolute length.
<percentage>
Gives the amount of the indent as a percentage of the block container’s own logical width

Percentages must be treated as 0 for the purpose of calculating intrinsic size contributions, but are always resolved normally when performing layout.

Note: This can lead to the element overflowing. It is not recommended to use percentage indents and intrinsic sizing toghether.

each-line
Indentation affects the first line of each block container and each line after a forced line break (but not lines after a soft wrap break).
hanging
Inverts which lines are affected.

If text-align is start and text-indent is 5em in left-to-right text with no floats present, then first line of text will start 5em into the block:

     Since CSS1 it has been possible to
indent the first line of a block element
5em by setting the 'text-indent' property?
to '5em'.

If we add the hanging keyword, then the first line will start flush, but other lines will be indented 5em:

In CSS3 we can instead indent all other
     lines of the block element by 5em
     by setting the 'text-indent' property
     to 'hanging 5em'.

Since the text-indent property only affects the “first formatted line”, a line after a forced break will not be indented.

   For example, in the middle of
this paragraph is an equation,
which is centered:
             x + y = z
The first line after the equation
is flush (else it would look like
we started a new paragraph).

However, sometimes (as in poetry or code), it is appropriate to indent each line that happens to be long enough to wrap. In the following example, text-indent is given a value of 3em hanging each-line, giving the third line of the poem a hanging indent where it soft-wraps at the block’s right boundary:

In a short line of text
There need be no wrapping,
But when we go on and on and on??
   and on,
Sometimes a soft break
Can help us stay on the page.

Note: Since the text-indent property inherits, when specified on a block element, it will affect descendant inline-block elements. For this reason, it is often wise to specify 'text-indent: 0' on elements that are specified 'display: inline-block'.

8.2. Hanging Glyphs

When a glyph at the start or end edge of a line hangs, it is not considered when measuring the line’s contents for fit, alignment, or justification. Depending on the line’s alignment/justification, this can result in the mark being placed outside the line box. The hanging glyph is also not taken into account when computing intrinsic sizes (min-content size and max-content size), and any sizes derived thereof. (The interaction of this measurement and kerning is currently UA-defined; the CSSWG welcomes advice on this point.)

A hanging glyph is still enclosed inside its parent inline box, is still counted as part of the scrollable overflow region [CSS-OVERFLOW-3], and still participates in text justification: its character advance is just not measured when determining how much content fits on the line, how much the line’s contents need to be expanded or compressed for justification, or how to position the content within the line box for text alignment. Effectively, the hanging glyph character advance is re-interpreted as an additional negative margin on the affected edge of its parent inline box; the line is otherwise laid out as usual.

In some cases, a glyph at the end of a line can conditionally hang: it hangs only if it does not otherwise fit in the line prior to justification. It is not considered when measuring the line’s contents for fit; however, any part of it that does not fit is considered to hang. Glyphs that conditionally hang are not taken into account when computing min-content sizes and any sizes derived thereof, but they are taken into account for max-content sizes and any sizes derived thereof.

Non-zero inline-axis borders or padding between a hangable glyph and the edge of the line prevent the glyph from hanging. For example, a period at the end of an inline box with end padding does not hang at the end edge of a line.

Multiple adjacent glyphs can hang together, however there may be limits on how many are allowed to hang (e.g. at most one punctuation character may hang at each edge of the line).

8.2.1. Hanging Punctuation: the hanging-punctuation property

Name: hanging-punctuation
Value: none | [ first || [ force-end | allow-end ] || last ]
Initial: none
Applies to: inline boxes
Inherited: yes
Percentages: n/a
Computed value: specified keyword(s)
Canonical order: per grammar
Animation type: discrete

This property determines whether a punctuation mark, if one is present, hangs and may be placed outside the line box (or in the indent) at the start or at the end of a line of text.

Note: If there is not sufficient padding on the block container, hanging-punctuation can trigger overflow.

Values have the following meanings:

none
No punctuation character is made to hang.
first
An opening bracket or quote at the start of the first formatted line of an element hangs. This applies to all characters in the Unicode categories Ps, Pf, Pi plus the ASCII quote marks “'” U+0027 and “"” U+0022.
last
A closing bracket or quote at the end of the last formatted line of an element hangs. This applies to all characters in the Unicode categories Pe, Pf, Pi plus the ASCII quote marks “'” U+0027 and “"” U+0022.
force-end
A stop or comma at the end of a line hangs.
allow-end
A stop or comma at the end of a line conditionally hangs.

At most one punctuation character may hang at each edge of the line.

Stops and commas allowed to hang include:

U+002C , COMMA
U+002E . FULL STOP
U+060C ? ARABIC COMMA
U+06D4 ? ARABIC FULL STOP
U+3001 IDEOGRAPHIC COMMA
U+3002 IDEOGRAPHIC FULL STOP
U+FF0C FULLWIDTH COMMA
U+FF0E FULLWIDTH FULL STOP
U+FE50 SMALL COMMA
U+FE51 SMALL IDEOGRAPHIC COMMA
U+FE52 SMALL FULL STOP
U+FF61 ? HALFWIDTH IDEOGRAPHIC FULL STOP
U+FF64 ? HALFWIDTH IDEOGRAPHIC COMMA

The UA may include other characters as appropriate.

Note: The CSS Working Group would appreciate if UAs including other characters would inform the working group of such additions.

The allow-end and force-end are two variations of hanging punctuation used in East Asia.

hanging-punctuation: allow-end
p {
  text-align: justify;
  hanging-punctuation: allow-end;
}
hanging-punctuation: force-end
p {
  text-align: justify;
  hanging-punctuation: force-end;
}

The punctuation at the end of the first line for allow-end does not hang, because it fits without hanging. However, if force-end is used, it is forced to hang. The justification measures the line without the hanging punctuation. Therefore when the line is expanded, the punctuation is pushed outside the line.

8.3. Bidirectionality and Line Boxes

The start and end edges of a line box are determined by the inline base direction of the line box. In most cases, this is given by its containing block’s computed direction.

However if its containing block has unicode-bidi: plaintext [CSS-WRITING-MODES-3], the line box’s inline base direction must be determined by the inline base direction of the bidi paragraph to which it belongs: that is, the bidi paragraph for which the line box holds content. An empty line box (i.e. one that contains no atomic inlines or characters other than the line-breaking character, if any), takes its inline base direction from the preceding line box (if any), or, if this is the first line box in the containing block, then from the direction property of the containing block.

In the following example, assuming the <block> is a preformatted block (display: block; white-space: pre) inheriting text-align: start, every other line is right-aligned:

<block style="unicode-bidi: plaintext">
  Latin
  ?·???
  Latin
  ?·???
  Latin
  ?·???
</block>

Note: The inline base direction determined here applies to the line box itself, and not to its contents. It affects text-align-all, text-align-last, text-indent, and hanging-punctuation, i.e. the position and alignment of its contents with respect to its edges. It does not affect the formatting or ordering of its content.

In the following example:

<para style="display: block; direction: rtl; unicode-bidi:plaintext">
<quote style="unicode-bidi:plaintext">????!</quote>", he said.
</para>

The result should be a left-aligned line looking like this:

"!????", he said.

The line is left-aligned (despite the containing block having direction: rtl) because the containing block (the <para>) has unicode-bidi:plaintext, and the line box belongs to a bidi paragraph that is LTR. This is because that paragraph’s first character with a strong direction is the LTR "h" from "he". The RTL "????!" does precede the "he", but it sits in its own bidi-isolated paragraph that is not immediately contained by the <para>, and is thus irrelevant to the line box’s alignment. From from the standpoint of the bidi paragraph immediately contained by the <para> containing block, the <quote>’s bidi-isolated paragraph inside it is, by definition, just a neutral U+FFFC character, so the immediately-contained paragraph becomes LTR by virtue of the "he" following it.

<fieldset style="direction: rtl">
<textarea style="unicode-bidi:plaintext">

Hello!

</textarea>
</fieldset>

As expected, the "Hello!" should be displayed LTR (i.e. with the exclamation mark on the right end, despite the <textarea>’s direction:rtl) and left-aligned. This makes the empty line following it left-aligned as well, which means that the caret on that line should appear at its left edge. The first empty line, on the other hand, should be right-aligned, due to the RTL direction of its containing paragraph, the <textarea>.

Appendix A: Text Processing Order of Operations

The following list defines the order of text operations. (Implementations are not bound to this order as long as the resulting layout is the same.)

  1. white space processing part I (pre-wrapping)
  2. text transformation
  3. text combination
  4. text orientation [CSS-WRITING-MODES-3]
  5. text wrapping while applying per line:
  6. justification (which may affect glyph selection and/or text wrapping, looping back into that step)
  7. text alignment

Appendix B: Conversion to Plaintext

This appendix is normative for the purpose of plaintext copy-paste operations.

When a CSS-rendered document is converted to a plaintext format, it is expected that:

Appendix C: Default UA Stylesheet

This appendix is informative, and is to help UA developers to implement a default stylesheet for HTML, but UA developers are free to ignore or modify as appropriate.

/* make list items and option elements align together */
li, option { text-align: match-parent; }

Appendix D: Scripts and Spacing

This appendix is normative.

Typographic behavior varies somewhat by language, but varies drastically by writing system. This appendix categorizes some common scripts in Unicode 6.0 according to their justification and spacing behavior. Category descriptions are descriptive, not prescriptive; the determining factor is the prioritization of justification opportunities.

block scripts
CJK and by extension all Wide characters (see [UAX11].) The following Unicode scripts are included: Bopomofo, Han, Hangul, Hiragana, Katakana, and Yi. Characters of the East Asian Width property Wide and Fullwidth are also included, but Ambiguous characters are included only if the writing system is Chinese, Korean, or Japanese.
clustered scripts
Clustered scripts have discrete units and break only at word boundaries, but do not use visible word separators. They prioritize stretching spaces, but comfortably admit inter-character spacing for justification. The clustered scripts include, but are not limited to, the following Unicode scripts: Khmer, Lao, Myanmar, New Tai Lue, Tai Le, Tai Tham, Tai Viet, Thai
cursive scripts
Cursive scripts do not admit gaps between their letters for either justification or letter-spacing. The following Unicode scripts are included: Arabic, Mandaic, Mongolian, N’Ko, Phags Pa, Syriac

User agents should update this list as they update their Unicode support to handle as-yet-unencoded cursive scripts in future versions of Unicode, and are encouraged to ask the CSSWG to update this spec accordingly.

Should block and cluster scripts be merged? They have different tolerances for space-justification vs inter-character justification, but both admit both.

Appendix E. Characters and Properties

Unicode defines four code point-level properties that are referenced in CSS typesetting:

East Asian width property
Defined in [UAX11] and given as the East_Asian_Width property in the Unicode Character Database [UAX44].
general category
Defined in [UAX44] and given as the General_Category property in the Unicode Character Database [UAX44].
script property
Defined in [UAX24] and given as the Script property in the Unicode Character Database [UAX44]. (UAs must include any ScriptExtensions.txt assignments in this mapping.)
Vertical Orientation
Defined in [UTR50] as the Vertical_Orientation property and given in the UTR50 data file.

Unicode defines properties for individual code points, but sometimes it is necessary to determine the properties of a typographic character unit. For the purposes of CSS Text, the properties of a typographic character unit are given by the base character of its first grapheme cluster—except in two cases:

Appendix F. Tagging Content by Writing System

This appendix is normative.

While most languages have a preferred writing system, many can also be transcribed into a different writing system. As a common example, most languages have at least one 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 Latin letters and word spaces, and follow Latin line-breaking and justification practices accordingly. As another example, historical ideographic Korean (ko-Hani) does not use word spaces, and should therefore be typeset as for Chinese.

In [HTML] or any other document language using [BCP47] to identify the content language, authors can indicate the use of an atypical writing system with script subtags. For example, to indicate use of the Latin writing system for languages which don’t natively use it, the -Latn script subtag can be added, e.g. ja-Latn for Japanese romaji. Other subtags exist for other writing systems: see [BCP47], [ISO15924], and the ISO15924 script tag registry. Some common/historical examples follow:

zh-Latn
Chinese, written in Latin transcription.
ko-Hani
Korean, written in Hanja (Chinese ideographic characters).
tr-Arab
Turkish, written in Arabic script.
mn-Cyrl
Mongolian, written in Cyrillic.
mn-Mong
Mongolian, written in traditional Mongolian script.

UAs should assume the most common writing system of the specified content language when choosing typographic behaviors such as line-breaking or justification strategies, but must not assume that writing system if the author has explicitly indicated a different one. If the UA has no language-specific knowledge of a particular language and writing system combination, it must use the typographic conventions of the specified writing system (assuming the conventions of a different language if necessary), not the conventions of that language in a different writing system, which would be inappropriate to the writing system used in this case.

The full correspondence between languages and their most common writing system is out of scope for this document. However, User Agents must assume at least the following:

More advice on language tagging can be found in the Internationalization Working Group’s “Language tags in HTML and XML” and “Choosing a Language Tag”.

Appendix G. Small Kana Mappings

Small Kana Map to Full-size Kana
Small Full-size
ぁ U+3041 あ U+3042
ぃ U+3043 い U+3044
ぅ U+3045 う U+3046
ぇ U+3047 え U+3048
ぉ U+3049 お U+304A
? U+3095 か U+304B
? U+3096 け U+3051
っ U+3063 つ U+3064
ゃ U+3083 や U+3084
ゅ U+3085 ゆ U+3086
ょ U+3087 よ U+3088
ゎ U+308E わ U+308F
ァ U+30A1 ア U+30A2
ィ U+30A3 イ U+30A4
ゥ U+30A5 ウ U+30A6
ェ U+30A7 エ U+30A8
ォ U+30A9 オ U+30AA
ヵ U+30F5 カ U+30AB
? U+31F0 ク U+30AF
ヶ U+30F6 ケ U+30B1
? U+31F1 シ U+30B7
? U+31F2 ス U+30B9
ッ U+30C3 ツ U+30C4
? U+31F3 ト U+30C8
? U+31F4 ヌ U+30CC
? U+31F5 ハ U+30CF
? U+31F6 ヒ U+30D2
? U+31F7 フ U+30D5
? U+31F8 ヘ U+30D8
? U+31F9 ホ U+30DB
? U+31FA ム U+30E0
ャ U+30E3 ヤ U+30E4
ュ U+30E5 ユ U+30E6
ョ U+30E7 ヨ U+30E8
? U+31FB ラ U+30E9
? U+31FC リ U+30EA
? U+31FD ル U+30EB
? U+31FE レ U+30EC
? U+31FF ロ U+30ED
ヮ U+30EE ワ U+30EF
? U+FF67 ? U+FF71
? U+FF68 ? U+FF72
? U+FF69 ? U+FF73
? U+FF6A ? U+FF74
? U+FF6B ? U+FF75
? U+FF6F ? U+FF82
? U+FF6C ? U+FF94
? U+FF6D ? U+FF95
? U+FF6E ? U+FF96

Privacy and Security Considerations

This specification introduces no new security considerations.

This specification leaks the user’s installed hyphenation and line-breaking dictionaries.

Acknowledgements

This specification would not have been possible without the help from: Ayman Aldahleh, Bert Bos, Tantek ?elik, James Clark, Emilio Cobos álvarez, Stephen Deach, John Daggett, Martin Dürst, Laurie Anna Edlund, Ben Errez, Yaniv Feinberg, Arye Gittelman, Ian Hickson, Martin Heijdra, Richard Ishida, Masayasu Ishikawa, Michael Jochimsen, Eric LeVine, Ambrose Li, H?kon Wium Lie, Chris Lilley, Ken Lunde, Myles Maxfield, Nat McCully, IM Mincheol, Shinyu Murakami, Paul Nelson, Chris Pratley, Xidorn Quan, Marcin Sawicki, Arnold Schrijver, Rahul Sonnad, Alan Stearns, Michel Suignard, Takao Suzuki, Frank Tang, Chris Thrasher, Etan Wexler, Chris Wilson, Masafumi Yabe and Steve Zilles.

Changes

Changes from the 12 December 2018 Working Draft

Changes from the 6 December 2018 Working Draft

Changes between 6 December 2018 and 12 December 2018 consist of some minor cleanup in the line breaking section and deferring <percentage> values of word-spacing to Level 4 with the expectation that they will be redefined (see discussion in issue 2165).

Changes from the October 2013 CSS3 Text LCWD

THIS CHANGES LIST IS WAY INCOMPLETE PLEASE SEE Disposition of Comments.

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 http://www.fyrkh.live/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-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 17 October 2017. CR. URL: http://www.fyrkh.live/TR/css-backgrounds-3/
[CSS-BOX-3]
Elika Etemad. CSS Box Model Module Level 3. 18 December 2018. WD. URL: http://www.fyrkh.live/TR/css-box-3/
[CSS-CASCADE-4]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 28 August 2018. CR. URL: http://www.fyrkh.live/TR/css-cascade-4/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 11 July 2019. CR. URL: http://www.fyrkh.live/TR/css-display-3/
[CSS-FONTS-3]
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 3. 20 September 2018. REC. URL: http://www.fyrkh.live/TR/css-fonts-3/
[CSS-FONTS-4]
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 4. 20 September 2018. WD. URL: http://www.fyrkh.live/TR/css-fonts-4/
[CSS-INLINE-3]
Dave Cramer; Elika Etemad; Steve Zilles. CSS Inline Layout Module Level 3. 8 August 2018. WD. URL: http://www.fyrkh.live/TR/css-inline-3/
[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 31 July 2018. WD. URL: http://www.fyrkh.live/TR/css-overflow-3/
[CSS-POSITION-3]
Rossen Atanassov; Arron Eicholz. CSS Positioned Layout Module Level 3. 17 May 2016. WD. URL: http://www.fyrkh.live/TR/css-position-3/
[CSS-RUBY-1]
Elika Etemad; Koji Ishii. CSS Ruby Layout Module Level 1. 5 August 2014. WD. URL: http://www.fyrkh.live/TR/css-ruby-1/
[CSS-SIZING-3]
Tab Atkins Jr.; Elika Etemad. CSS Intrinsic & Extrinsic Sizing Module Level 3. 22 May 2019. WD. URL: http://www.fyrkh.live/TR/css-sizing-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: http://www.fyrkh.live/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 31 January 2019. WD. URL: http://www.fyrkh.live/TR/css-values-4/
[CSS-WRITING-MODES-3]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 24 October 2019. PR. URL: http://www.fyrkh.live/TR/css-writing-modes-3/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 30 July 2019. CR. URL: http://www.fyrkh.live/TR/css-writing-modes-4/
[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: http://www.fyrkh.live/TR/CSS2/
[CSSOM-1]
Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). 17 March 2016. WD. URL: http://www.fyrkh.live/TR/cssom-1/
[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. 21 November 2018. WD. URL: http://www.fyrkh.live/TR/selectors-4/
[UAX11]
Ken Lunde 小林劍??. East Asian Width. 25 January 2019. Unicode Standard Annex #11. URL: https://www.unicode.org/reports/tr11/tr11-36.html
[UAX14]
Andy Heninger. Unicode Line Breaking Algorithm. 15 February 2019. Unicode Standard Annex #14. URL: https://www.unicode.org/reports/tr14/tr14-43.html
[UAX24]
Ken Whistler. Unicode Script Property. 6 February 2019. Unicode Standard Annex #24. URL: https://www.unicode.org/reports/tr24/tr24-29.html
[UAX29]
Mark Davis. Unicode Text Segmentation. 15 February 2019. Unicode Standard Annex #29. URL: https://www.unicode.org/reports/tr29/tr29-35.html
[UAX44]
Ken Whistler; Lauren?iu Iancu. Unicode Character Database. 27 February 2019. Unicode Standard Annex #44. URL: https://www.unicode.org/reports/tr44/tr44-24.html
[UAX9]
Mark Davis; Aharon Lanin; Andrew Glass. Unicode Bidirectional Algorithm. 4 February 2019. Unicode Standard Annex #9. URL: https://www.unicode.org/reports/tr9/tr9-41.html
[UNICODE]
The Unicode Standard. URL: https://www.unicode.org/versions/latest/
[UTR50]
Koji Ishii 石井宏治; Ken Lunde 小林劍??. Unicode Vertical Text Layout. 4 February 2019. Unicode Standard Annex #50. URL: https://www.unicode.org/reports/tr50/tr50-22.html

Informative References

[BCP47]
A. Phillips; M. Davis. Tags for Identifying Languages. September 2009. IETF Best Current Practice. URL: https://tools.ietf.org/html/bcp47
[CLREQ]
Bobby Tung; et al. Requirements for Chinese Text Layout 中文排版需求. 28 May 2019. WD. URL: http://www.fyrkh.live/TR/clreq/
[CSS-COUNTER-STYLES-3]
Tab Atkins Jr.. CSS Counter Styles Level 3. 14 December 2017. CR. URL: http://www.fyrkh.live/TR/css-counter-styles-3/
[CSS-TEXT-DECOR-3]
Elika Etemad; Koji Ishii. CSS Text Decoration Module Level 3. 13 August 2019. CR. URL: http://www.fyrkh.live/TR/css-text-decor-3/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[ISO15924]
Code for the representation of names of scripts. International Organization for Standardization. 1998. ISO 15924:1998. Draft International Standard
[JIS4051]
Formatting rules for Japanese documents (『日本語文書の組版方法』). Japanese Standards Association. 2004. JIS X 4051:2004. In Japanese
[JLREQ]
Yasuhiro Anan; et al. Requirements for Japanese Text Layout. 3 April 2012. NOTE. URL: http://www.fyrkh.live/TR/jlreq/
[JUSTIFY]
Elika Etemad; Richard Ishida. Approches to Full Justification. URL: http://www.fyrkh.live/International/articles/typography/justification
[TYPOGRAPHY]
Richard Ishida. International text layout and typography index. 20 May 2019. WD. URL: http://www.fyrkh.live/TR/typography/
[XML10]
Tim Bray; et al. Extensible Markup Language (XML) 1.0 (Fifth Edition). 26 November 2008. REC. URL: http://www.fyrkh.live/TR/xml/
[ZHMARK]
標點符號用法 (Punctuation Mark Usage). 1995. 中華人民共和國國家標準

Property Index

Name Value Initial Applies to Inh. %ages Anim-ation type Canonical order Com-puted value
hanging-punctuation none | [ first || [ force-end | allow-end ] || last ] none inline boxes yes n/a discrete per grammar specified keyword(s)
hyphens none | manual | auto manual inline boxes yes n/a discrete n/a specified keyword
letter-spacing normal | <length> normal inline boxes yes n/a by computed value type n/a an absolute length
line-break auto | loose | normal | strict | anywhere auto inline boxes yes n/a discrete n/a specified keyword
overflow-wrap normal | break-word | anywhere normal inline boxes yes n/a discrete n/a specified keyword
tab-size <number> | <length> 8 block containers yes n/a by computed value type n/a the specified number or absolute length
text-align start | end | left | right | center | justify | match-parent | justify-all start block containers yes see individual properties discrete n/a see individual properties
text-align-all start | end | left | right | center | justify | match-parent start block containers yes n/a discrete n/a keyword as specified, except for match-parent which computes as defined above
text-align-last auto | start | end | left | right | center | justify | match-parent auto block containers yes n/a discrete n/a specified keyword
text-indent [ <length-percentage> ] && hanging? && each-line? 0 block containers yes refers to block container’s own inline-axis inner size by computed value type per grammar computed <length-percentage> value, plus any specified keywords
text-justify auto | none | inter-word | inter-character auto inline boxes yes n/a discrete n/a specified keyword
text-transform none | [capitalize | uppercase | lowercase ] || full-width || full-size-kana none inline boxes yes n/a discrete n/a specified keyword
white-space normal | pre | nowrap | pre-wrap | break-spaces | pre-line normal inline boxes yes n/a discrete n/a specified keyword
word-break normal | keep-all | break-all | break-word normal inline boxes yes n/a discrete n/a specified keyword
word-spacing normal | <length> normal inline boxes yes N/A by computed value type n/a an absolute length
word-wrap normal | break-word | anywhere normal inline boxes yes n/a discrete n/a specified keyword

Issues Index

Should block and cluster scripts be merged? They have different tolerances for space-justification vs inter-character justification, but both admit both. ?
THIS CHANGES LIST IS WAY INCOMPLETE PLEASE SEE Disposition of Comments. ?
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