CSS Text Module Level 3

Editor’s Draft,

More details about this document
This version:
https://drafts.csswg.org/css-text-3/
Latest published version:
https://www.w3.org/TR/css-text-3/
Previous Versions:
Implementation Report:
https://test.csswg.org/harness/results/css-text-3_dev/grouped/
Test Suites:
http://test.csswg.org/suites/css3-text/nightly-unstable/
https://wpt.fyi/results/css/css-text/
Feedback:
CSSWG Issues Repository
Editors:
Elika J. Etemad / fantasai (Apple)
(Invited Expert)
Florian Rivoal (Invited Expert)
Suggest an Edit for this Spec:
GitHub Editor
Test Coverage Analysis:
https://drafts.csswg.org/css-text-3/test-coverage

Copyright © 2024 World Wide Web Consortium. W3C® liability, trademark and permissive document license rules apply.

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

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

This document is governed by the 03 November 2023 W3C Process Document.

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

Tests

The test coverage information in this specification covers wpt/css/css-text/ and subdirectories, as well as those tests in wpt/css/CSS2/ and subdirectories that relate to this specification.

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 the CSS Fonts Module. [CSS-FONTS-3]

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

Bidirectional and vertical text are addressed in the CSS Writing Modes Module. [CSS-WRITING-MODES-4].

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

Tests

The following tests are crash tests that relate to general usage of the features described in this specification but are not tied to any particular normative statement.

1.1. Module Interactions

Tests

Tests not needed for this section.

This module, together with the CSS Text Decoration Module, replaces and extends the text-level features defined in Cascading Style Sheets Level 2 chapter 16. [CSS-TEXT-DECOR-3] [CSS2]

In addition to the terms defined below, other terminology and concepts used in this specification are defined in Cascading Style Sheets Level 2 and the CSS Writing Modes Module. [CSS2] and [CSS-WRITING-MODES-4].

1.2. Value Definitions

Tests

Tests not really needed for this section; could possibly test that css-wide keywords apply to every property.

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with 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 as their property value. For readability they have not been repeated explicitly.

1.3. Languages and Typesetting

Tests

Tests not needed for this section: these are definitions, they get tested through their application, not by themselves.

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

Many typographic effects vary by linguistic context. 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.

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. 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 declare the content language using the global lang attribute in HTML or the universal xml:lang attribute in XML. See the rules for determining the content language of an HTML element in HTML, and the rules for determining the content language of an XML element in XML 1.0. [HTML] [XML10]

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. Depending on the document language’s facilities for identifying the content language, this information can be explicit or implied. See the normative Appendix F: Identifying the Content Writing System.

Note: Some languages have more than one writing system tradition; in other cases a language can be transliterated into a foreign writing system. Authors should subtag such cases so that the UA can adapt appropriately.

For example, Korean (ko) can be written in Hangul (-Hang), Hanja (-Hani), or a combination (-Kore). Historical documents written solely in Hanja do not use word spaces and are formatted more like modern Chinese than modern Korean. In other words, for typographic purposes ko-Hani behaves more like zh-Hant than ko (ko-Kore).

As another example Japanese (ja) is typically written in a combination (-Japn) of Hiragana (-Hira), Katakana (-Kana), and Kanji (-Hani). However, it can also be “romanized” into Latin (-Latn) for special purposes like language-learning textbooks, in which case it should be formatted more like English than Japanese.

As a third example contemporary Mongolian is written in two scripts: Cyrillic (-Cyrl, officially used in Mongolia) and Mongolian (-Mong, more common in Inner Mongolia, part of China). These have very different formatting requirements, with Cyrillic behaving similar to Latin and Greek, and Mongolian deriving from both Arabic and Chinese writing conventions.

1.4. Characters and Letters

Tests

For the most part, tests not really needed for this section: these are definitions, they get tested through their applications, by themselves. The few testable assertions that are made have coverage.

Possible additions:

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. 한=Han) can be considered a character. However, the square symbol is really composed of multiple letters each representing a phoneme (e.g. ㅎ=h, ㅏ=a, ㄴ=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. 한), to the individual phonemic character (e.g. ㅎ), or to smaller units such as a base letterform (e.g. ㅇ) 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  ำ THAI CHARACTER SARA AM differently; or the behavior 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.).

Tests

Unicode Standard Annex #29: Text Segmentation defines a unit called the grapheme cluster which approximates the typographic character. [UAX29] 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.

Tests

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:

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. See Appendix E: Characters and 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 or ligatures by element boundaries may give inconsistent or undesired results.

1.5. Text Processing

Tests

This section has adequate coverage. Exhaustive coverage unrealistic, since this section is effectively a dependency on all of Unicode. Some tests nonetheless provided for key functionality (such as the effect of certain control characters on Arabic shaping).

CSS is built on Unicode. [UNICODE] UAs that support Unicode must adhere to all normative requirements of the Unicode Core Standard, except where explicitly overridden by CSS. UAs implemented on the basis of a non-Unicode text encoding model are still expected to fulfill the same text handling requirements by assuming an appropriate mapping and analogous behavior.

Tests

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.

Tests

2. Transforming Text

Tests

This section and its subsections have good test coverage overall, and very good i18n coverage in particular.

Missing tests:

Possible additions:

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: text
Inherited: yes
Percentages: n/a
Computed value: specified keyword
Canonical order: n/a
Animation type: discrete
Tests

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.

Tests

Authors must not rely on text-transform for semantic purposes; rather the correct casing and semantics should be encoded in the source document text and markup.

Tests

Values have the following meanings:

none
No effects.
Tests
capitalize
Puts the first typographic letter unit of each word, if lowercase, in titlecase; other characters are unaffected.
Tests
uppercase
Puts all letters in uppercase.
Tests
lowercase
Puts all letters in lowercase.
Tests
full-width
Puts all typographic character units in full-width form. If a character does not have a corresponding full-width form, it is left as is. This value is typically used to typeset Latin letters and digits as if they were ideographic characters.
Tests
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.
Tests
The following example converts the ASCII characters used in abbreviations in Japanese text to their full-width variants so that they lay out and line break like ideographs: 
abbr:lang(ja) { text-transform: full-width; }

Note: The purpose of text-transform is to allow for presentational casing transformations without affecting the semantics of the document. Note in particular that text-transform casing operations are lossy, and can distort the meaning of a text. While accessibility interfaces may wish to convey the apparent casing of the rendered text to the user, the transformed text cannot be relied on to accurately represent the underlying meaning of the document.

In this example, the first line of text is capitalized as a visual effect. 
section > p:first-of-type::first-line {
  text-transform: uppercase;
}

This effect cannot be written into the source document because the position of the line break depends on layout. But also, the capitalization is not reflecting a semantic distinction and is not intended to affect the paragraph’s reading; therefore it belongs in the presentation layer.

In this example, the ruby annotations, which are half the size of the main paragraph text, are transformed to use regular-size kana in place of small kana. 
rt { font-size: 50%; text-transform: full-size-kana; }
:is(h1, h2, h3, h4) rt { text-transform: none; /* unset for large text*/ }

Note that while this makes such letters easier to see at small type sizes, the transformation distorts the text: the reader needs to mentally substitute small kana in the appropriate places—​not unlike reading a Latin inscription where all “U”s look like “V”s.

For example, if text-transform: full-size-kana were applied to the following source, the annotation would read “じゆう” (jiyū), which means “liberty”, instead of “じゅう” (jū), which means “ten”, the correct reading and meaning for the annotated “十”.

<ruby>十<rt>じゅう</ruby>

2.1.1. Mapping Rules

For capitalize, what constitutes a “word“ is UA-dependent; [UAX29] is suggested (but not required) for determining such word boundaries. 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.

Tests

Note: Authors cannot depend on capitalize to follow language-specific titlecasing conventions (such as skipping articles in English).

The UA must use the full case mappings for Unicode characters, including any conditional casing rules, as defined in the Default Case Algorithms 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.

Tests
For example, in Turkish there are two “i”s, one with a dot—​“İ” and “i”—​and one without—​“I” and “ı”. Thus the usual case mappings between “I” and “i” are replaced with a different set of mappings to their respective dotless/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.
Tests

The definition of full-width and half-width forms can be found in Unicode Standard Annex #11: East Asian Width. [UAX11] The mapping to full-width form is defined by taking code points with the <wide> or the <narrow> tag in their Decomposition_Mapping in Unicode Standard Annex #44: Unicode Character Database. [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.

Tests

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

2.1.2. Order of Operations

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

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

Text transformation happens after § 4.1.1 Phase I: Collapsing and Transformation but before § 4.1.2 Phase II: Trimming and Positioning. This means that full-width only transforms spaces (U+0020) to U+3000 IDEOGRAPHIC SPACE within preserved white space.

Tests

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

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

Tests

This section has good overall test coverage, particularly through tests for § 4 White Space Processing & Control Characters and subsections.

Missing tests:

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

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.
Tests
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.
Tests
nowrap
Like normal, this value collapses white space; but like pre, it does not allow wrapping.
Tests
pre-wrap
Like pre, this value preserves white space; but like normal, it allows wrapping.
Tests
break-spaces
The behavior is identical to that of pre-wrap, except that:

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 it preserves segment breaks in the source as forced line breaks.
Tests

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 End-of-line other space separators
normal Collapse Collapse Wrap Remove Hang
pre Preserve Preserve No wrap Preserve No wrap
nowrap Collapse Collapse No wrap Remove Hang
pre-wrap Preserve Preserve Wrap Hang Hang
break-spaces Preserve Preserve Wrap Wrap Wrap
pre-line Preserve Collapse Wrap Remove Hang
Tests

See White Space Processing Rules for details on how white space collapses.

See Line Breaking for details on wrapping behavior.

4. White Space Processing & Control Characters

Tests

This section has reasonably good test coverage.

Missing tests:

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 (which is controlled with the white-space property) interprets white space characters only for rendering: it has no effect on the underlying document data.

Note: Depending on the document language, 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” or “newline sequence”—​or if none are defined, 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.

In the case of HTML, newlines are normalized to line feed characters (U+000A) for representation in the DOM, so when an HTML document is represented as a DOM tree each line feed (U+000A) is treated as a segment break. [HTML] [DOM]

Note: In most common CSS implementations, HTML does not get styled directly. Instead, it is processed into a DOM tree, which is then styled. Unlike HTML, the DOM does not give any particular meaning to carriage returns (U+000D), so they are not treated as segment breaks. If carriage returns (U+000D) are inserted into the DOM by means other than HTML parsing, they then get treated as defined below.

Tests

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 Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification § visuren#anonymous. [CSS2]

Control characters (Unicode category Cc)—​other than tabs (U+0009), line feeds (U+000A), carriage returns (U+000D) and sequences that form a segment break—​must be rendered as a visible glyph which the UA must synthesize 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. [UNICODE]

Tests

Carriage returns (U+000D) are treated identically to spaces (U+0020) in all respects.

Tests

Note: For HTML documents, carriage returns present in the source code are converted to line feeds at the parsing stage (see HTML § 13.2.3.5 Preprocessing the input stream and the definition of normalize newlines in Infra and therefore do no appear as U+000D CARRIAGE RETURN to CSS. [HTML] [INFRA]) However, the character is preserved—​and the above rule observable—​when encoded using an escape sequence (&#x0d;).

4.1. The White Space Processing Rules

Tests

This section has good test coverage, all parts are well exercised. Most tests to be found in subsections.

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.

Tests

Note: The set of characters considered document white space (part of the document content) and those 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 defines a number of additional space separator characters. [UNICODE] In this specification all characters in the Unicode general category Zs except space (U+0020) and no-break space (U+00A0) are collectively referred to as other space separators.

Tests

4.1.1. Phase I: Collapsing and Transformation

Tests

This section has good test coverage, all parts are well exercised.

For each inline (including anonymous inlines; see Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification § visuren#anonymous [CSS2]) within an inline formatting context, white space characters are processed as follows prior to line breaking and bidi reordering, ignoring bidi formatting characters (characters with the Bidi_Control property [UAX9]) as if they were not there:

Tests
Tests
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:

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.

Tests

4.1.2. Phase II: Trimming and Positioning

Tests

This section has good test coverage, all parts are well exercised.

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

  1. A sequence of collapsible spaces at the beginning of a line is removed.
    Tests
  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 starting content edge of the preserved tab’s nearest block container ancestor. The tab size is given by the tab-size property.
    Tests

    Note: See the Unicode rules on how tabulation (U+0009) interacts with bidi. [UAX9]

    Tests
  3. A sequence of collapsible spaces at the end of a line is removed, as well as any trailing U+1680   OGHAM SPACE MARK whose white-space property is normal, nowrap, or pre-line.
    Tests

    Note: Due to Unicode Bidirectional Algorithm rule L1, a sequence of collapsible spaces located at the end of the line prior to bidi reordering will also be at the end of the line after reordering. [UAX9] [CSS-WRITING-MODES-4]

    Tests
  4. If there remains any sequence of white space, other space separators, and/or preserved tabs at the end of a line (after bidi reordering [CSS-WRITING-MODES-4]):
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.

4.1.3. Segment Break Transformation Rules

Tests

This section has reasonable test coverage, though some assertions are only tested indirectly through test for other features that rely on this, rather than by dedicated tests.

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).

Tests

For other values of white-space, segment breaks are collapsible, and are collapsed as follows:

  1. First, any collapsible segment break immediately following another collapsible segment break is removed.
    Tests
  2. Then any remaining segment break is either transformed into a space (U+0020) or removed depending on the context before and after the break. The rules for this operation are UA-defined in this level.
    Tests

    Note: The white space processing rules have already removed any tabs and spaces around the segment break before this context is evaluated.

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
be more easily read and edited
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 and edited 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.

這個段落是那麼長，
在一行寫不行。最好
用三行寫。

這個段落是那麼長，在一行寫不行。最好用三行寫。

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

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

Note: Historically, HTML and CSS have unconditionally converted segment breaks to spaces, which has prevented content authored in languages such as Chinese from being able to break lines within the source. Thus UA heuristics need to be conservative about where they discard segment breaks even as they strive to improve support for such languages.

4.2. Tab Character Size: the tab-size property

Tests

This section has good test coverage.

Missing test:

Name: tab-size
Value: <number [0,∞]> | <length [0,∞]>
Initial: 8
Applies to: text
Inherited: yes
Percentages: n/a
Computed value: the specified number or absolute length
Canonical order: n/a
Animation type: by computed value type
Tests

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

Tests

5. Line Breaking and Word Boundaries

Tests

Tests mostly not needed for this section: these are definitions, they get tested through their application, not by themselves.

Can be a good section to host tests for i18n requirements not covered in detail by the spec.

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.

Tests

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.

Tests

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

Note: Unicode Standard Annex #14: Unicode Line Breaking Algorithm defines a baseline behavior for line breaking for all scripts in Unicode, which is expected to be further tailored. [UAX14] More information on line breaking conventions can be found in Requirements for Japanese Text Layout [JLREQ] and Formatting Rules for Japanese Documents [JIS4051] for Japanese, Requirements for Chinese Text Layout [CLREQ] and General Rules for Punctuation [ZHMARK] for Chinese. See also the Internationalization Working Group’s Language Enablement Index which includes more information on additional languages. [TYPOGRAPHY] Any guidance on additional appropriate references would be much appreciated.

Tests

Generic line-breaking tests not particularly anchored to any specific spec language

5.1. Line Breaking Details

Tests

This section has partial test coverage.

Missing tests:

Untestable(?):

When determining line breaks:

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

Tests

This section has partial test coverage.

Missing tests:

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

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, treating non-letter typographic character units belonging to the NU, AL, AI, or ID Unicode line breaking classes as typographic letter units for this purpose (only). [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.)

Tests
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).

각 줄의 마지막에 한글이 올 때 줄 나눔 기
준을 “글자” 또는 “어절” 단위로 한다.

각 줄의 마지막에 한글이 올 때 줄 나눔
기준을 “글자” 또는 “어절” 단위로 한다.

Ethiopic similarly has two styles of line-breaking, either only breaking at word separators (word-break: normal), or also allowing breaks between letters within a word (word-break: break-all).

ተወልዱ፡ኵሉ፡ሰብእ፡ግዑዛን፡ወዕሩያን፡
በማዕረግ፡ወብሕግ።ቦሙ፡ኅሊና፡ወዐቅል፡
ወይትጌበሩ፡አሐዱ፡ምስለ፡አሀዱ፡
በመንፈሰ፡እኍና።

ተወልዱ፡ኵሉ፡ሰብእ፡ግዑዛን፡ወዕሩያን፡በማ
ዕረግ፡ወብሕግ።ቦሙ፡ኅሊና፡ወዐቅል፡ወይትጌ
በሩ፡አሐዱ፡ምስለ፡አሀዱ፡በመንፈሰ፡እኍና።

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. For Ethiopic, which also exhibits two different behaviors, such breaks within words are not allowed.
Tests
break-all
Breaking is allowed within “words”: specifically, in addition to soft wrap opportunities allowed for normal, any typographic letter units (and 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.
Tests

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

Tests

Note: This option enables the other common behavior for Ethiopic. It is also often used 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.

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.
Tests

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: 
这是一些汉字 and some Latin و کمی خط عربی และตัวอย่างการเขียนภาษาไทย በጽሑፍ፡ማራዘሙን፡አንዳንድ፡

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

word-break: normal 
这·是·一·些·汉·字·and·some·Latin·و·کمی·خط·عربی·และ·ตัวอย่าง·การเขียน·ภาษาไทย·በጽሑፍ፡·ማራዘሙን፡·አንዳንድ፡
word-break: break-all 
这·是·一·些·汉·字·a·n·d·s·o·m·e·L·a·t·i·n·و·ﮐ·ﻤ·ﻰ·ﺧ·ﻁ·ﻋ·ﺮ·ﺑ·ﻰ·แ·ล·ะ·ตั·ว·อ·ย่·า·ง·ก·า·ร·เ·ขี·ย·น·ภ·า·ษ·า·ไ·ท·ย·በ·ጽ·ሑ·ፍ፡·ማ·ራ·ዘ·ሙ·ን፡·አ·ን·ዳ·ን·ድ፡
word-break: keep-all 
这是一些汉字·and·some·Latin·و·کمی·خط·عربی·และ·ตัวอย่าง·การเขียน·ภาษาไทย·በጽሑፍ፡·ማራዘሙን፡·አንዳንድ፡

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).

Tests

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.

Tests

5.3. Line Breaking Strictness: the line-break property

Tests

This section has extensive test coverage for CJK.

Missing tests:

Untestable(?):

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

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 line breaking classes or mandated by the word-break property. [UAX14] The different wrapping opportunities must not be prioritized. Hyphenation is not applied.
Tests

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.

Tests

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, for these three keywords, this specification does require that: