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CSS Writing Modes Level 3 defines CSS features to support for various international writing modes, such as left-to-right (e.g. Latin or Indic), right-to-left (e.g. Hebrew or Arabic), bidirectional (e.g. mixed Latin and Arabic) and vertical (e.g. Asian scripts).
Inherently bottom-to-top scripts are not handled in this version. See [UTN22] for an explanation of relevant issues.
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This document was produced by the CSS Working Group (part of the Style Activity).
This document was produced by a group operating under the 5 February 2004 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.
The following features are at-risk and may be dropped during CR:
use-glyph-orientation
’ of ‘text-orientation
’
ascii-digits
’, ‘digits
’, ‘alpha
’, ‘latin
’, and ‘alphanumeric
’ values of ‘text-combine-horizontal
’.
text-combine-mode
’ property
CSS Writing Modes Level 3 defines CSS features to support for various international writing modes, such as left-to-right (e.g. Latin or Indic), right-to-left (e.g. Hebrew or Arabic), bidirectional (e.g. mixed Latin and Arabic) and vertical (e.g. Asian scripts).
A writing mode in CSS is determined by the
‘writing-mode
’, ‘direction
’, and
‘text-orientation
’ properties. It is
defined primarily in terms of its inline base direction and block flow direction:
The inline base direction is the
primary direction in which content is ordered on a line and defines on
which sides the "start" and "end" of a line are. The ‘direction
’
property specifies the inline base direction of an element and, together
with the ‘unicode-bidi
’ property and the inherent
directionality of any text content, determines the ordering of
inline-level content within a line.
The block flow direction is the
direction in which block-level boxes stack and the direction in which line
boxes stack within a block container. The ‘writing-mode
’
property determines the block flow direction.
A horizontal writing mode is one with horizontal lines of text, i.e. a downward or upward block flow. A vertical writing mode is one with vertical lines of text, i.e. a leftward or rightward block flow.
These terms should not be confused with vertical block flow (which is a downward or upward block flow) and horizontal block flow (which is leftward or rightward block flow). To avoid confusion, CSS specifications avoid this latter set of terms.
Writing systems typically have one or two native writing modes. Some examples are:
The ‘text-orientation
’ component of the writing
mode determines the line
orientation and the typesetting mode, and controls details
of text layout such as the glyph orientation.
See Unicode Technical Note #22 [UTN22] (HTML version) for a more in-depth introduction to writing modes and vertical text.
This module replaces and extends the ‘unicode-bidi
’
and ‘direction
’ features defined in [CSS21] sections 8.6
and 9.10.
This specification follows the CSS property definition conventions from [CSS21]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS modules may expand the definitions of these value types: for example [CSS3COLOR], when combined with this module, expands the definition of the <color> value type as used in this specification.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.
While the characters in most scripts are written from left to right, certain scripts are written from right to left. In some documents, in particular those written with the Arabic or Hebrew script, and in some mixed-language contexts, text in a single (visually displayed) block may appear with mixed directionality. This phenomenon is called bidirectionality, or "bidi" for short.
Bidirectionality
The Unicode standard (Unicode Standard Annex #9)
defines a complex algorithm for determining the proper ordering of
bidirectional text. The algorithm consists of an implicit part based on
character properties, as well as explicit controls for embeddings and
overrides. CSS relies on this algorithm to achieve proper bidirectional
rendering. The ‘direction
’ and ‘unicode-bidi
’
properties allow authors to specify how the elements and attributes of a
document language map to this algorithm.
User agents that support bidirectional text must apply the Unicode bidirectional algorithm to every sequence of inline-level boxes uninterrupted by a forced (bidi class B) paragraph break or block boundary. This sequence forms the paragraph unit in the bidirectional algorithm.
Except when the ‘plaintext
’ value of ‘unicode-bidi
’
is in effect, the paragraph embedding level is set according to the value
of the ‘direction
’ property of the paragraph's
element rather than by the heuristic given in steps P2 and P3 of the
Unicode algorithm. The paragraph's element is usually the containing
block, but in the case of a paragraph contained by bidi isolation it is the isolating inline element instead.
Because the base directionality of a text depends on the structure and
semantics of the document, the ‘direction
’ and ‘unicode-bidi
’
properties should in most cases be used only to map bidi information in
the markup to its corresponding CSS styles. If a document language
provides markup features to control bidi, authors and users should use
those features and not specify CSS rules to override them.
The HTML 4 specification ([HTML401], section 8.2) defines bidirectionality behavior for HTML elements. The HTML 4 specification also contains more information on bidirectionality issues.
Because HTML UAs can turn off CSS styling, we advise HTML
authors to use the HTML ‘dir
’
attribute and <bdo> element to ensure correct bidirectional layout
in the absence of a style sheet.
direction
’ propertyName: | direction |
---|---|
Value: | ltr | rtl |
Initial: | ltr |
Applies to: | all elements |
Inherited: | yes |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
This property specifies the base directionality of text and elements on
a line, and the directionality of embeddings and overrides (see ‘unicode-bidi
’)
for the Unicode bidirectional algorithm. In addition, it affects the
ordering of table
column layout, the direction of horizontal overflow, and
the default alignment of text within a line, and other things that depend
on the base inline base direction.
Values for this property have the following meanings:
The ‘direction
’ property has no effect on bidi
reordering when specified on inline elements whose ‘unicode-bidi
’
property's value is ‘normal
’.
The value of the ‘direction
’ property on the root element is
also propagated to the initial containing block and, together with the
‘writing-mode
’ property, determines the
document's principal writing mode. (See below.)
Note that the ‘direction
’ property of the HTML BODY
element is not propagated to the viewport. That special behavior
only applies to the background and overflow properties.
The ‘direction
’ property, when specified for
table column elements, is not inherited by cells in the column since
columns are not the ancestors of the cells in the document tree. Thus, CSS
cannot easily capture the "dir" attribute inheritance rules described in
[HTML401],
section 11.3.2.1.
unicode-bidi
’ propertyName: | unicode-bidi |
---|---|
Value: | normal | embed | [ isolate || bidi-override ] | plaintext |
Initial: | normal |
Applies to: | all elements, but see prose |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
Values for this property have the following meanings:
direction
’ property. Inside the element,
reordering is done implicitly. This corresponds to adding a LRE (U+202A),
for ‘direction: ltr
’, or RLE (U+202B),
for ‘direction: rtl
’, at the start of
the element and a PDF (U+202C) at the end of the element. This value has no effect on elements that are not
inline.
direction
’
property, and for the purpose of bidi resolution in its containing bidi
paragraph (if any), the element itself is treated as if it were an Object
Replacement Character (U+FFFC). (If the element is broken across multiple
lines, then each box of the element is treated as an Object Replacement
Character.)
direction
’
property; the implicit part of the bidirectional algorithm is ignored.
This corresponds to adding a LRO (U+202D), for ‘direction: ltr
’, or RLO (U+202E), for ‘direction: rtl
’, at the start of the element and a
PDF (U+202C) at the end of the element.
For the purposes of the Unicode bidirectional algorithm, the base
directionality of each bidi paragraph for which the element forms the
containing block is determined not by the element's computed ‘direction
’ as
usual, but by following the heuristic in rules P2 and P3 of the Unicode
bidirectional algorithm. For inline elements, this value behaves as for
‘isolate
’,
except, as with block containers, the base directionality is determined
by following the Unicode heuristic instead of by using the ‘direction
’
value.
The final order of characters within in each bidi paragraph is the same as if the bidi control codes had been added as described above, markup had been stripped, and the resulting character sequence had been passed to an implementation of the Unicode bidirectional algorithm for plain text that produced the same line-breaks as the styled text.
In this process, replaced elements with ‘display:
inline
’ are treated as neutral characters, unless their
‘unicode-bidi
’ property has a value other
than ‘normal
’, in which case they are treated as
strong characters in the ‘direction
’ specified for the element. All
other atomic inline-level boxes are treated as neutral characters always.
If an inline element is broken around a bidi paragraph boundary (e.g. if split by a block or forced paragraph break), then the bidi control codes corresponding to the end of the element are added before the interruption and the codes corresponding to the start of the element are added after it. (In other words, any embedding levels or overrides started by the element are closed at the paragraph break and reopened on the other side of it.)
Because the Unicode algorithm has a limit of 61 levels of
embedding, care should be taken not to use ‘unicode-bidi
’ with a value other
than ‘normal
’ unless appropriate. In particular,
a value of ‘inherit
’ should be
used with extreme caution. However, for elements that are, in general,
intended to be displayed as blocks, a setting of ‘unicode-bidi: isolate
’ is preferred to keep the
element together in case display is changed to inline (see example below).
The following example shows an XML document with bidirectional text. It illustrates an important design principle: document language designers should take bidi into account both in the language proper (elements and attributes) and in any accompanying style sheets. The style sheets should be designed so that bidi rules are separate from other style rules, and such rules should not be overridden by other style sheets so that the document language's bidi behavior is preserved.
In this example, lowercase letters stand for inherently left-to-right characters and uppercase letters represent inherently right-to-left characters. The text stream is shown in logical backing store order.
<HEBREW>
<PAR>HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5</PAR>
<PAR>HEBREW6 <EMPH>HEBREW7</EMPH> HEBREW8</PAR>
</HEBREW>
<ENGLISH>
<PAR>english9 english10 english11 HEBREW12 HEBREW13</PAR>
<PAR>english14 english15 english16</PAR>
<PAR>english17 <HE-QUO>HEBREW18 english19 HEBREW20</HE-QUO></PAR>
</ENGLISH>
Since this is arbitrary XML, the style sheet is responsible for setting the writing direction. This is the style sheet:
/* Rules for bidi */ HEBREW, HE-QUO {direction: rtl; unicode-bidi: embed;} ENGLISH {direction: ltr; unicode-bidi: embed;} /* Rules for presentation */ HEBREW, ENGLISH, PAR {display: block;} EMPH {font-weight: bold;}
The HEBREW element is a block with a right-to-left base direction, the ENGLISH element is a block with a left-to-right base direction. The PARs are blocks that inherit the base direction from their parents. Thus, the first two PARs are read starting at the top right, the final three are read starting at the top left. Please note that HEBREW and ENGLISH are chosen as element names for explicitness only; in general, element names should convey structure without reference to language.
The EMPH element is inline-level, and since its value for ‘unicode-bidi
’ is ‘normal
’ (the initial
value), it has no effect on the ordering of the text. The HE-QUO element,
on the other hand, creates an embedding.
The formatting of this text might look like this if the line length is long:
5WERBEH 4WERBEH english3 2WERBEH 1WERBEH 8WERBEH 7WERBEH 6WERBEH english9 english10 english11 13WERBEH 12WERBEH english14 english15 english16 english17 20WERBEH english19 18WERBEH
Note that the HE-QUO embedding causes HEBREW18 to be to the right of english19.
If lines have to be broken, it might be more like this:
2WERBEH 1WERBEH -EH 4WERBEH english3 5WERB -EH 7WERBEH 6WERBEH 8WERB english9 english10 en- glish11 12WERBEH 13WERBEH english14 english15 english16 english17 18WERBEH 20WERBEH english19
Because HEBREW18 must be read before english19, it is on the line above english19. Just breaking the long line from the earlier formatting would not have worked. Note also that the first syllable from english19 might have fit on the previous line, but hyphenation of left-to-right words in a right-to-left context, and vice versa, is usually suppressed to avoid having to display a hyphen in the middle of a line.
Since bidi reordering can split apart and reorder text that is logically contiguous, bidirectional text can cause an inline boxes to be split and reordered within a line.
Note that in order to be able to flow inline boxes in a uniform direction (either entirely left-to-right or entirely right-to-left), anonymous inline boxes may have to be created.
For each line box, UAs must take the inline boxes generated for each
element and render the margins, borders and padding in visual order (not
logical order). The start-most box on the
first line box in which the element appears has the start edge's margin, border, and padding; and the
end-most box on the last line box in which the element appears has the end edge's margin, border, and padding. For
example, in the ‘horizontal-tb
’ writing mode:
direction
’ property is ‘ltr
’, the left-most
generated box of the first line box in which the element appears has the
left margin, left border and left padding, and the right-most generated
box of the last line box in which the element appears has the right
padding, right border and right margin.
direction
’ property is ‘rtl
’, the right-most
generated box of the first line box in which the element appears has the
right padding, right border and right margin, and the left-most generated
box of the last line box in which the element appears has the left
margin, left border and left padding.
Analogous rules hold for vertical writing modes.
The ‘box-decoration-break
’ property can override
this behavior to draw box decorations on both sides of each box. [CSS3BG]
This subsection is non-normative.
In addition to extensions to CSS2.1’s support for bidirectional text, this module introduces the rules and properties needed to support vertical text layout in CSS.
Unlike languages that use the Latin script which are primarily laid out horizontally, Asian languages such as Chinese and Japanese can be laid out vertically. The Japanese example below shows the same text laid out horizontally and vertically. In the horizontal case, text is read from left to right, top to bottom. For the vertical case, the text is read top to bottom, right to left. Indentation from the left edge in the left-to-right horizontal case translates to indentation from the top edge in the top-to-bottom vertical case.
Comparison of vertical and horizontal Japanese: iBunko application (iOS)
For Chinese and Japanese lines are ordered either right to left or top to bottom, while for Mongolian and Manchu lines are ordered left to right.
The change from horizontal to vertical writing can affect not just the layout, but also the typesetting. For example, the position of a punctuation mark within its spacing box can change from the horizontal to the vertical case, and in some cases alternate glyphs are used.
Vertical text that includes Latin script text or text from other scripts normally displayed horizontally can display that text in a number of ways. For example, Latin words can be rotated sideways, or each letter can be oriented upright:
Examples of Latin in vertical Japanese: Daijirin Viewer 1.4 (iOS)
In some special cases such as two-digit numbers in dates, text is fit compactly into a single vertical character box:
Mac Fan, December 2010, p.49
Layouts often involve a mixture of vertical and horizontal elements:
Mixture of vertical and horizontal elements
Vertical text layouts also need to handle bidirectional text layout; clockwise-rotated Arabic, for example, is laid out bottom-to-top.
writing-mode
’ propertyName: | writing-mode |
---|---|
Value: | horizontal-tb | vertical-rl | vertical-lr |
Initial: | horizontal-tb |
Applies to: | All elements except table row groups, table column groups, table rows, and table columns |
Inherited: | yes |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
This property sets the block flow direction. Possible values:
The ‘writing-mode
’ property determines the
direction of block flow. This determines the progression of block-level
boxes in a block formatting context; the progression of line boxes in a
block container that contains inlines; and the progression of rows in a
table. By virtue of determining the stacking direction of line boxes, the
‘writing-mode
’ property also determines
whether line boxes and thus the writing mode is horizontal or vertical.
When set on the root element, the ‘writing-mode
’ property together with the
‘direction
’ property determines the principal writing mode of the document.
This writing mode is used, for example, to determine the default page
progression direction. (See [CSS3PAGE].) The ‘writing-mode
’
value of the root element is also propagated to the initial containing
block and sets the block flow direction of the initial block formatting
context.
Note that the ‘writing-mode
’ property of the HTML BODY
element is not propagated to the viewport. That special behavior
only applies to the background and overflow properties.
If an element has a different block flow direction than its containing block:
display
’ of ‘inline
’, its ‘display
’ computes to ‘inline-block
’. [CSS21]
display
’ of ‘run-in
’, its ‘display
’ computes to ‘block
’. [CSS21]
If such an element is a block container, then it establishes a new block formatting context.
The content of replaced elements do not rotate due to the writing mode: images, for example, remain upright. However replaced content involving text (such as MathML content or form elements) should match the replaced element's writing mode and line orientation if the UA supports such a vertical writing mode for the replaced content.
In the following example, two block elements (1 and 3) separated by an image (2) are presented in various flow writing modes.
Here is a diagram of horizontal writing mode (writing-mode:
horizontal-tb
):
Here is a diagram for the right-to-left vertical writing mode commonly
used in East Asia (writing-mode: vertical-rl
):
And finally, here is a diagram for the left-to-right vertical writing
mode used for Manchu and Mongolian (writing-mode:
vertical-lr
):
In the following example, some form controls are rendered inside a
block with ‘vertical-rl
’ writing mode. The form controls
are rendered to match the writing mode.
<style> form { writing-mode: vertical-rl; } </style> ... <form> <p><label>姓名 <input value="艾俐俐"></label> <p><label>语文 <select><option>English <option>français <option>فارسی <option>中文 <option>日本语</select></label> </form>
In this example, ‘writing-mode
’ sets the list markers
upright using the ‘::marker
’
pseudo-element. Vertical alignment ensures that longer numbers will still
align with the right of the first line of text. [CSS3LIST]
::marker { writing-mode: horizontal-tb; vertical-align: text-top; color: blue; }
Example of horizontal list markers in a vertical list
writing-mode
’
ValuesSVG1.1 [SVG11]
defines some additional values: ‘lr
’,
‘lr-tb
’, ‘rl
’, ‘rl-tb
’,
‘tb
’, and ‘tb-rl
’.
These values are deprecated in any context except SVG1 documents. Implementations that wish to support these values in the context of CSS must treat them as follows:
SVG1/Obsolete | CSS |
---|---|
‘lr ’
| ‘horizontal-tb ’
|
‘lr-tb ’
| |
‘rl ’
| |
‘tb ’
| ‘vertical-rl ’
|
‘tb-rl ’
|
The SVG1.1 values were also present in an older revision of
the CSS ‘writing-mode
’ specification, which is
obsoleted by this specification. The additional ‘tb-lr
’ value of that revision is replaced by
‘vertical-lr
’.
In SVG1.1, these values set the inline progression direction, in
other words, the direction the current text position advances each time a
glyph is added. This is a geometric process that happens after bidi reordering, and thus has no effect
on the interpretation of the ‘direction
’ property (which is independent
of ‘writing-mode
’). (See Relationship
with bidirectionality. [SVG11])
There are varying interpretations on whether this process causes "writing-mode: rl" to merely shift the text string or reverse the order of all glyphs in the text.
When different kinds of inline-level content are placed together on a
line, the baselines of the content and the settings of the ‘vertical-align
’ property control how they are
aligned in the transverse direction of the line box. This section
discusses what baselines are, how to find them, and how they are used
together with the ‘vertical-align
’
property to determine the alignment of inline-level content.
This section is non-normative.
A baseline is a line along the inline axis of a line box along which individual glyphs of text are aligned. Baselines guide the design of glyphs in a font (for example, the bottom of most alphabetic glyphs typically align with the alphabetic baseline), and they guide the alignment of glyphs from different fonts or font sizes when typesetting.
Different writing systems prefer different baseline tables.
Preferred baselines in various writing systems
A well-constructed font contains a baseline table, which indicates the position of one or more baselines within the font's design coordinate space. (The design coordinate space is scaled with the font size.)
In a well-designed mixed-script font, the glyphs are positioned in the coordinate space to harmonize with one another when typeset together. The baseline table is then constructed to match the shape of the glyphs, each baseline positioned to match the glyphs from its preferred scripts.
The baseline table is a property of the font, and the positions of the various baselines apply to all glyphs in the font.
Different baseline tables can be provided for alignment in horizontal and vertical text. UAs should use the vertical tables in vertical typesetting modes and the horizontal tables otherwise.
In this specification, only the following baselines are considered:
A future CSS module will deal with baselines in more detail and allow the choice of other dominant baselines and alignment options.
If an atomic inline (such as an inline-block, inline-table, or replaced inline element) is not capable of providing its own baseline information, then the UA synthesizes a baseline table thus:
The dominant baseline (which can change based on the writing mode) is used in CSS for alignment in two cases:
vertical-align
’ value of
‘baseline
’,
child is aligned to the parent by matching the parent's dominant baseline
to the same baseline in the child. (E.g. if the parent's dominant
baseline is alphabetic, then the child's alphabetic baseline is matched
to the parent's alphabetic baseline, even if the child's dominant
baseline is something else.) For values of ‘sub
’, ‘super
’,
‘<length>
’, and ‘<percentage>
’, the baselines are aligned as
for ‘baseline
’, but the child is shifted according
to the offset given by its ‘vertical-align
’ value.
Given following sample markup:
<p><span class="outer">Ap <span class="inner">ji</span></span></p>
And the following style rule:
span.inner { font-size: .75em; }
The baseline tables of the parent (.outer
) and the child
(.inner
) will not match up due to the font size
difference. Since the dominant baseline is the alphabetic baseline, the
child box is aligned to its parent by matching up their alphabetic
baselines.
If we assign ‘vertical-align:
super
’ to the .inner
element from the example
above, the same rules are used to align the .inner
child
to its parent; the only difference is in addition to the baseline
alignment, the child is shifted to the superscript position.
span.inner { vertical-align: super; font-size: .75em; }
Each writing system has one or more native orientations. Modern scripts can therefore be classified into three orientational categories:
In modern typographic systems, all glyphs are assigned a horizontal orientation, which is used when laying out text horizontally. To lay out vertical text, the UA needs to transform the text from its horizontal orientation. This transformation is the bi-orientational transform, and there are two types:
Scripts with a native vertical orientation have an intrinsic bi-orientational transform, which orients them correctly in vertical text: CJK (Chinese/Japanese/Korean) characters translate, that is, they are always upright. Other scripts, such as Mongolian, rotate. (See Appendix B for a list of intrinsic bi-orientational transforms.)
Scripts without a native vertical orientation can be either rotated (set
sideways) or translated (set upright): the transform used is a stylistic
preference depending on the text's usage, rather than a matter of
correctness. The ‘text-orientation
’ property's ‘upright-right
’ and
‘upright
’ values
are provided to specify rotation vs. translation of horizontal-only text.
The ‘sideways-left
’, ‘sideways-right
’,
and ‘sideways
’
values of ‘text-orientation
’ are provided for
decorative layout effects and to work around limitations in CSS support
for bottom-to-top scripts.
Ideally, punctuation should be either sideways or upright
depending on whether the primary script is horizontal-only or vertical.
However, this information (which, like the base directionality, is a
property of the content) is not available to us. (UTN 22 used the concept
of a vertical directionality, given via ‘direction
’ or the HTML dir
attribute to handle this issue.) The current spec works around this by
using the East Asian Width property; but this approach only works if
vertical scripts do not share punctuation with horizontal-only scripts.
text-orientation
’ propertyName: | text-orientation |
---|---|
Value: | upright-right | upright | sideways-right | sideways-left | sideways | use-glyph-orientation |
Initial: | upright-right |
Applies to: | all elements except table row groups, rows, column groups, and columns |
Inherited: | yes |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
This property specifies the orientation of characters within a line and sets the orientation of the line. Current values only have an effect in vertical writing modes.
For readability, the term character is used in place of extended grapheme cluster in this section. See Characters and Properties for further details.
Values have the following meanings:
In vertical writing modes, characters from horizontal-only scripts are set sideways, i.e. 90° clockwise from their standard orientation in horizontal text. Characters from vertical scripts are set with their intrinsic orientation.
In vertical writing modes, this value puts the element in a vertical typographic mode and is typical for layout of primarily vertical-script text.
In vertical writing modes, characters from horizonal-only scripts are rendered upright, i.e. in their standard horizontal orientation. Shaping characters from such scripts are shaped in their isolated forms. Characters from vertical scripts are set with their intrinsic orientation and shaped normally. When available, vertical glyph variants and vertical font metrics are used to set the text. The UA must synthesize vertical font metrics for grapheme clusters that do not have any.
For the purposes of bidi reordering, this value causes all characters
to be treated as strong LTR. This value causes the used value of
‘direction
’ to be ‘ltr
’.
In vertical writing modes, this value puts the element in a vertical typographic mode.
In vertical writing modes, this causes text to be set as if in a horizontal layout (using horizontal glyph variants and metrics), but rotated 90° clockwise. This value puts the element in a horizontal typographic mode.
In vertical writing modes, this causes text to be set as if in a horizontal layout (using horizontal glyph variants and metrics), but rotated 90° counter-clockwise. This value puts the element in a horizontal typographic mode.
If set on a non-replaced inline whose parent is not ‘sideways-left
’,
this forces bidi isolation: the ‘normal
’ and ‘embed
’ values of ‘unicode-bidi
’ compute to ‘isolate
’, and ‘bidi-override
’
computes to ‘bidi-override isolate
’.
Layout of text is exactly as for ‘sideways-right
’ except that the text content
and baseline table of each of the element's inline boxes is mirrored
around a vertical axis along the center of its content box. The
positions of text decorations propagated from an ancestor inline
(including the block container's root inline) are not mirrored, but any
text decorations introduced by the element are positioned using the
mirrored baseline table.
Similarly, if an inline child of the element has a ‘text-orientation
’ value other than
‘sideways-left
’, an analogous transformation
(and bidi isolation) is applied.
This value is equivalent to ‘sideways-right
’ in ‘vertical-rl
’
writing mode and equivalent to ‘sideways-left
’ in ‘vertical-lr
’
writing mode. It can be useful when setting horizontal script text
vertically in a primarily horizontal-only document.
[SVG11] defines
‘glyph-orientation-vertical
’ and
‘glyph-orientation-horizontal
’
properties that were intended to control text orientation. These
properties are deprecated and do not apply to non-SVG elements.
If an implementation supports these properties, the ‘use-glyph-orientation
’ value when set on SVG
elements indicates that the SVG ‘glyph-orientation-vertical
’ and ‘glyph-orientation-horizontal
’ behavior
control the layout of text. Such UAs must set ‘text-orientation: glyph-orientation
’ on all SVG
text content elements in their default UA style sheet for SVG.
In all other contexts, and for implementations that do not support the
glyph orientation properties, the ‘use-glyph-orientation
’ behavior is the same
as for ‘upright-right
’.
This value is at-risk and may be dropped during CR.
| | | |
‘upright-right ’
| ‘upright ’
| ‘sideways-left ’
| ‘sideways-right ’
|
‘text-orientation
’ values (‘writing-mode
’
is ‘vertical-rl
’)
The orientation of characters belonging to the Common, Inherited, and Unknown script categories is defined in Appendix C. Feedback is requested on those rules.
Need a clear normative definition of default character orientation for all Unicode characters that does not rely on data supplied by fonts.
In the following example, the root element of a horizontal-only
document is set to use ‘sideways
’. In the rest of the document, the
author can just set ‘writing-mode
’ without worrying about
whether the text is ‘vertical-rl
’ or ‘vertical-lr
’.
:root { text-orientation: sideways; } caption { caption-side: left; writing-mode: vertical-lr; } thead th { writing-mode: vertical-lr; } h1.banner { position: absolute; top: 0; right: 0; writing-mode: vertical-rl; }
[CSS21] defines
the box layout model of CSS in detail. However, it only defines the box
model for the ‘horizontal-tb
’ writing mode. CSS box layout in
writing modes other than ‘horizontal-tb
’ is analogous to the box layout
defined in CSS2.1 if directions and dimensions are abstracted and remapped
appropriately. This section defines abstract directional and dimensional
terms and their mappings in order to define box layout for other writing
modes, and to provide terminology for future specs to define their layout
concepts abstractly.
The terms left, right, top, and bottom are always interpreted physically, i.e. with respect to the page independent of writing mode. Two abstract mappings are possible for these directions: line-relative and flow-relative, which are defined below.
Although they derive from the behavior of text, these directional terms
exist even for boxes that do not contain any line boxes: they are
calculated directly from the values of the ‘writing-mode
’,
‘text-orientation
’, and ‘direction
’
properties.
Although the block flow direction given by ‘writing-mode
’
determines whether the line is oriented horizontally or vertically, it
doesn't say anything about how the contents within the line are arranged.
The line-relative directions are
over, under, line-left, and line-right. The line orientation, which is given by a
combination of ‘text-orientation
’ and ‘writing-mode
’,
determines which side of the line is the "top" and thus which sides are
under (ascender side) and over
(descender side) the line. The line orientation also affects the
interpretation of alignment (‘vertical-align
’) in the transverse dimension
of the line.
In addition to its over and under sides, a line box, even a
vertically-oriented one, also has a "left" and "right" side, which we will
call the line-left and line-right sides of the box (as distinct
from the physical left and physical right sides of the box). The line-left edge of a box is nominally the edge from
which LTR text would start. The line-right edge of a box is nominally the edge from
which RTL text would start. Depending on
the ‘writing-mode
’ and ‘text-orientation
’ properties, the
line-left side of a box could be on the physical left, top, or bottom.
Note also that while the over and
under directions often map to the same
directions as before and after
respectively, this mapping is reversed for some combinations of ‘writing-mode
’
and ‘text-orientation
’.
The flow-relative directions are
before, after, start, and end. In an LTR ‘horizontal-tb
’ writing mode, they correspond to
the top, bottom, left, and right directions, respectively.
The before edge of a box is nominally the edge that
comes earlier in the block progression, as determined by the ‘writing-mode
’
property. Similarly the after edge is the edge that
comes later in the progression.
The start edge of a box is nominally the edge from
which text of its inline base direction will start. For boxes with a used
‘direction
’ value of ‘ltr
’, this means the line-left edge. For boxes with a used
‘direction
’ value of ‘rtl
’, this means the line-right edge. The edge opposite the start
edge is the end edge.
Note that while determining the before and after
edges of a box depends only on the ‘writing-mode
’ property, determining the start and end edges of
a box depends not only on the ‘writing-mode
’ property but also the
‘direction
’ and ‘text-orientation
’ properties.
An English (LTR-TB) block:
<----- width / measure -----> top side/ before side +------------------------------+ A left side/ | ---inline direction ---> | right side/ | start side | | | end side | | | block * horizontal * | height/ | | direction *writing mode* | extent | V | | +------------------------------+ V bottom side/ after side
A vertical Japanese block (TTB-RL):
<----- width / extent ------> top side/ start side +------------------------------+ A left side/ | <---block direction--- | right side/ | after side | | | before side | | * vertical * inline| | height/ | *writing mode* direction| | measure | V | | +------------------------------+ V bottom side/ end side
The following table summarizes the abstract-to-physical mappings:
‘writing-mode ’
| ‘horizontal-tb ’
| ‘vertical-rl ’
| ‘vertical-lr ’
| |||||||
---|---|---|---|---|---|---|---|---|---|---|
‘text-orientation ’
| — | ‘sideways-left ’
| *right | ‘sideways-left ’
| *right | |||||
‘direction ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
|
extent | height | width | ||||||||
measure | width | height | ||||||||
before | top | right | left | |||||||
after | bottom | left | right | |||||||
start | left | right | bottom | top | top | bottom | bottom | top | top | bottom |
end | right | left | top | bottom | bottom | top | top | bottom | bottom | top |
over | top | left | right | left | right | |||||
under | bottom | right | left | right | left | |||||
line-left | left | bottom | top | bottom | top | |||||
line-right | right | top | bottom | top | bottom |
CSS box layout in vertical writing modes is analogous to layout in the horizontal writing modes, following the principles outlined below:
Layout calculation rules (such as those in CSS2.1, Section 10.3) that apply to the horizontal dimension in horizontal writing modes instead apply to the vertical dimension in vertical writing modes. Likewise, layout calculation rules (such as those in CSS2.1, Section 10.6) that apply to the vertical dimension in horizontal writing modes instead apply to the horizontal dimension in vertical writing modes. Thus:
Layout rules that refer to the width use the height instead, and vice versa.
Layout rules that refer to the ‘*-left
’ and ‘*-right
’ box properties (border, margin, padding)
use ‘*-top
’ and ‘*-bottom
’ instead, and vice versa. Which side of
the box the property applies to doesn't change: only which values are
inputs to which layout calculations changes. The ‘margin-left
’ property still affects the
lefthand margin, for example; however in a ‘vertical-rl
’
writing mode it takes part in margin collapsing in place of ‘margin-bottom
’.
Layout rules that depend on the ‘direction
’ property to choose between
left and right (e.g. overflow, overconstraint resolution, the initial
value for ‘text-align
’, table
column ordering) are abstracted to the start and end sides and applied appropriately.
For example, in vertical writing modes, table rows are vertical and
table columns are horizontal. In a ‘vertical-rl
’ ‘upright-right
’
‘rtl
’ table, the
first column would be on the bottom (the start side), and the first row
on the right (the before side). The table's ‘margin-right
’ and ‘margin-left
’ would collapse with margins
before (on the right) and after (on the left) the table, respectively,
and if the table had ‘auto
’ values for
‘margin-top
’ and ‘margin-bottom
’ it would be centered
vertically within its block flow.
Table in ‘vertical-rl
’ RTL writing mode
For features such as text alignment, floating, and list marker positioning, that primarily reference the left or right sides of the line box or its longitudinal parallels and therefore have no top or bottom equivalent, the line left and line right sides are used as the reference for the left and right sides respectively.
Likewise for features such as underlining, overlining, and baseline
alignment (the unfortunately-named ‘vertical-align
’), that primarily reference the
top or bottom sides of the linebox or its transversal parallels and
therefore have no left or right equivalent, the over
and under sides are used as the reference for the top
and bottom sides respectively.
The details of these mappings are provided below.
Certain properties behave logically as follows:
border-spacing
’ property represent spacing
between columns and rows respectively, not necessarily the horizontal and
vertical spacing respectively. [CSS21]
line-height
’ property
always refers to the logical height. [CSS21]
The height properties (‘height
’,
‘min-height
’, and ‘max-height
’) refer to the physical height, and
the width properties (‘width
’,
‘min-width
’, and ‘max-width
’) refer to the physical width.
However, the rules used to calculate box dimensions and positions are
logical.
For example, the calculation rules in CSS2.1 Section 10.3 are used for the inline dimension measurements: they apply to the measure (which could be either the physical width or physical height) and to the the start and end margins, padding, and border. Likewise the calculation rules in CSS2.1 Section 10.6 are used in the block dimension: they apply to the extent and to the before and after margins, padding, and border. [CSS21]
As a corollary, percentages on the margin and padding properties, which are always calculated with respect to the containing block width in CSS2.1, are calculated with respect to the measure of the containing block in CSS3.
When an element has a different ‘writing-mode
’ from its containing block
two cases are possible:
vertical-rl
’ and ‘vertical-lr
’).
horizontal-tb
’ and ‘vertical-rl
’).
To handle the second case, CSS layout calculations are divided into two phases: sizing a box, and positioning the box within its flow. In the sizing phase—calculating the width and height of the box—the dimensions of the box and the containing block are mapped to the measure and extent and calculations performed accordingly using the writing mode of the element. In the positioning phase—calculating the positioning offsets, margins, borders, and padding—the dimensions of the box and its containing block are mapped to the measure and extent and calculations performed according to the writing mode of the containing block.
For example, if a vertical block is placed inside a horizontal block, then when calculating the physical height (which is the measure) of the child block the physical height of the parent block is used to calculate the measure of the child's containing block, even though the physical height is the extent, not the measure, of the parent block.
Since auto margins are resolved consistent with the containing block's writing mode, a box establishing an orthogonal flow, can, once sized, be aligned or centered within its containing block just like other block-level elements by using auto margins.
It is common in CSS for a containing block to have a defined measure,
but not a defined extent. This typically happens in CSS2.1 when a
containing block has an ‘auto
’ height,
for example: its width is given by the calculations in 10.3.3, but
its extent depends on its contents. In such cases the available measure is defined as the measure of
the containing block; but the available
extent, which would otherwise be the extent of the containing block,
is infinite.
Orthogonal flows allow the opposite to happen: for the available extent to be defined, but the available measure to be infinite. In such cases a percentage of the containing block measure cannot be defined, and thus the initial containing block's size is used instead as a fallback measure to calculate such percentages.
If the computed measure of an element establishing an orthogonal flow is
‘auto
’, then the used measure is
calculated as the fit-content
(shrink-to-fit) size using the initial containing block's size as the
available measure.
If the UA supports CSS Multi-column Layout [CSS3COL], then for the case where
the element's extent or available extent is defined but the element's
measure is ‘auto
’:
column-count
’ and
‘column-width
’ are both
‘auto
’, a used ‘column-width
’ is calculated for the element
as the fill-available
measure using the fallback
measure as the available
measure.
The used measure of the resulting multi-column element is then calculated: if the content neither wraps nor paginates within the multi-column element, then the used measure is the max-content measure of the element's contents; else it is calculated from the used column width, column count, and column gap.
The used extent of the element is either the used column extent (if multiple columns were used) or the max-content extent of the content.
This should behave the same as the auto-sizing algorithm defined in the previous section, except overflowing content, instead of continuing off the side of the containing block, is wrapped into columns in the flow direction of the containing block, thus avoiding T-shaped documents.
This section is informative.
With regards to pagination, the rules in CSS2.1 still hold in vertical writing modes and orthogonal flows: page break opportunities do not occur inside line boxes, only between them. UAs that support [CSS3COL] may break in the (potentially zero-width) gap between columns, however.
Note that if content spills outside the pagination stream established by the root element, the UA is not required to print such content. Authors wishing to mix writing modes with long streams of text are thus encouraged to use CSS columns to keep all content flowing in the document's pagination direction.
In other words, if your document would require two scrollbars on the screen it probably won't all print. Fix your layout, e.g. by using columns so that it all scrolls (and therefore paginates) in one direction if you want to make sure it'll all print. T-shaped documents tend not to print well.
Flow-relative directions are calculated with respect to the writing mode
of the containing block of the element and used to abstract
layout rules related to the box properties (margins, borders, padding) and
any properties related to positioning the box within its containing block
(‘float
’, ‘clear
’, ‘top
’, ‘bottom
’, ‘left
’, ‘right
’) For inline-level elements, the
writing mode of the parent element is used instead.
For example, the margin that is dropped when a box's inline dimension is over-constrained is the end margin as determined by the writing mode of the containing block.
The margin collapsing rules apply exactly with the before margin substituted for the top margin and the after margin substituted for the bottom margin. Similarly the before padding and border are substituted for the top padding and border, and the after padding and border substituted for the bottom padding and border. Note this means only before and after margins ever collapse.
Flow-relative directions are calculated with respect to the writing mode of the element and used to abstract layout related to the element's contents:
text-align
’ property aligns to the start edge
of the line box.
text-indent
’ property
indents from the start edge of the line box.
The line-relative directions are
over, under, line-left, and line-right.
In an LTR ‘horizontal-tb
’
writing mode, they correspond to the top, bottom, left, and right
directions, respectively.
The line-right and line-left directions are calculated with respect to
the writing mode of the element and used to interpret the ‘left
’ and ‘right
’ values of the
following properties:
text-align
’ property [CSS21]
The line-right and line-left directions are calculated with respect to
the writing mode of the containing block of the element and used
to interpret the ‘left
’ and ‘right
’ values of the following properties:
The over and under directions are calculated with respect to the writing mode of the element and used to define the interpretation of the "top" (over edge) and "bottom" (under edge) of the line box as follows:
vertical-align
’
property, the "top" of the line box is the over edge; the "bottom" of the
line box is the under edge. Positive length and percentage values shift
the baseline towards the over edge. [CSS21]
text-decoration
’
property, the underline is drawn on the under side of the text; the
overline is drawn on the over side of the text. [CSS21] Note that
the CSS Text Module defines this in more detail and provides additional
controls for controlling the position of underlines and overlines. [CSS3TEXT]
The following values are purely physical in their definitions and do not respond to changes in writing mode:
rect()
’ notation of the
‘clip
’ property [CSS21]
box-shadow
’ and ‘text-shadow
’ properties
caption-side
’ keywordsProperty: | ‘caption-side ’
|
---|---|
New Values: | ‘before ’ |
‘after ’
|
Initial: | before |
Applies to: | same as CSS2.1 |
Inherited: | same as CSS2.1 |
Percentages: | same as CSS2.1 |
Media: | same as CSS2.1 |
Computed value: | specified value |
This module introduces two new values to the ‘caption-side
’ property: ‘before
’ and ‘after
’, which position the
caption before and after the table box, respectively. For tables with
‘horizontal-tb
’ writing mode, they are
equivalent to the existing ‘top
’ and ‘bottom
’ values, respectively. [CSS21]
For implementations that support the ‘top-outside
’ and ‘bottom-outside
’ model, corresponding
‘before-outside
’ and ‘after-outside
’ will be similarly introduced.
Implementations that support the ‘top
’ and ‘bottom
’ values of the ‘caption-side
’ property but do not support side
captions (i.e. ‘left
’ and ‘right
’ captions in horizontal writing modes)
must treat ‘top
’ and
‘bottom
’ as
‘before
’, when
the table is in a vertical writing mode.
For implementations that do support side captions (i.e. the ‘left
’ and ‘right
’ values from the
obsolete CSS 2.0 specification [CSS2]), this module also introduces
the ‘start
’ and
‘end
’ values, which
behave similarly and which position the caption on the start and end sides
of the table box, calculated with respect to the writing mode of the table
element. For such implementations, the ‘top
’ and ‘bottom
’ values must place the caption on the
top and bottom sides of the table box, respectively.
The CSS2.0 side caption model had some problems and will likely have a different definition in CSS3.
In paged media CSS2.1 classifies all pages as either left or right pages. The page progression direction, which determines whether the left or right page in a spread is first in the flow and whether the first page is by default a left or right page, depends on the writing direction as follows:
writing-mode
’
is ‘vertical-rl
’ or if the root element's
‘writing-mode
’ is ‘horizontal-tb
’ and
its ‘direction
’ is ‘rtl
’.
writing-mode
’
is ‘vertical-lr
’ or if the root element's
‘writing-mode
’ is ‘horizontal-tb
’ and
its ‘direction
’ is ‘ltr
’.
(Unless otherwise overridden, the first page of a document begins on the second half of a spread, e.g. on the right page in a left-to-right page progression.)
text-combine-horizontal
’ propertyName: | text-combine-horizontal |
---|---|
Value: | none | all | [ [digits <integer> | ascii-digits <integer> ] || [ alpha <integer> | latin <integer> ] || alphanumeric <integer> ] |
Initial: | none |
Applies to: | non-replaced inline elements |
Inherited: | yes |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
This property allows the combination of multiple characters into the space of a single character. This property only has an effect in vertical writing modes. Values have the following meanings:
text-combine-horizontal:
none
’ on the element and any descendants.
text-combine-horizontal: all
’. For this property,
a horizontal digit is any character
belonging to a Number category (N*) that does not belong to a vertical script.
text-combine-horizontal: all
’.
This definition is would replace ‘digits
’ as a simplification.
text-combine-horizontal: all
’. For this property,
a horizonal letter is any character
belonging to a Letter category (L*) that does not belong to a vertical script.
text-combine-horizontal:
all
’. For this property, a Latin
letter is any character belonging to a Letter category (L*) that
also belongs to the Latin script.
This definition is would replace ‘alpha
’ as a simplification.
text-combine-horizontal: all
’.
All values except ‘all
’
and ‘none
’ are marked at-risk. Which
ones should we take to CR?
When combining text as for ‘text-combine-horizontal: all
’, the glyphs of the
combined text are stacked horizontally (without line breaks,
letter-spacing, etc., but using the specified font settings), similar to
the contents of an inline-box with a horizontal writing mode and a
line-height of 1em. The effective size of the composition is assumed to be
1em square; anything outside the square is not measured for layout
purposes. The UA should center the glyphs horizontally and vertically
within the measured 1em square. The baseline of the resulting composition
chosen such that the square is centered between the text-over and
text-under baselines of its parent inline box prior to any baseline
alignment shift. For text layout purposes, e.g. bidi ordering,
line-breaking, emphasis marks, text-decoration, etc. the resulting
composition is treated as a single glyph representing the Object
Replacement Character U+FFFC.
In some fonts, the ideographic glyphs are given a compressed design such that they are 1em wide but shorter than 1em tall. To accommodate such fonts, the UA may vertically scale the contents of the composition to match the advance height of 水 U+6C34.
Any CSS fullwidth transformations (‘text-transform: full-width
’ [CSS3TEXT] or ‘font-variant-east-asian-width: full-width
’ [CSS3FONT]) are
turned off for combined text of more than one character.
In East Asian documents, the ‘text-combine-horizontal
’ effect is often used
to display Latin-based strings such as components of a date or letters of
an initialism, always in a horizontal writing mode regardless of the
writing mode of the line:
Example of horizontal-in-vertical tate-chu-yoko
The figure is the result of the rules
date { text-combine-horizontal: digits 2; }
and the following markup:
<date>平成20年4月16日に</date>
In Japanese, this effect is known as tate-chu-yoko.
The following example shows that applying ‘text-combine-horizontal: digits 2
’ to an entire
document, rather than to a segment with a known type of numeric content,
can have unintended consequences:
<p>あれは10,000円ですよ!</p>
Example of mis-applied tate-chu-yoko
text-combine-mode
’ propertyName: | text-combine-mode |
---|---|
Value: | auto | compress | [ no-compress || use-glyphs ] |
Initial: | auto |
Applies to: | non-replaced inline elements |
Inherited: | yes |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
This property controls how multiple characters are combined into the
space of a single character when specified to do so via ‘text-combine-horizontal
’. Values have the
following meanings:
Since even fonts that have fractional-width glyphs available do not
have such glyphs for all characters, the UA must ensure the expected
advance width for ‘use-glyphs
’ by
either compressing or padding (equally on both sides) each glyph
individually if it does not match the required advance width. (This step
does not apply if ‘no-compress
’ is
specified.)
use-glyphs
’, however, this indicates
to perform glyph substitution if possible per ‘use-glyphs
’ but not to compress the glyphs if they
do not fit the size requirements. This value may cause the glyphs to
overflow the line significantly.
Major changes include:
unicode-bidi
’
values and defined ‘plaintext
’ to use heuristics for inline
elements as well.
vertical-right
’ value of
‘text-orientation
’ to ‘upright-right
’.
rotate
’ values of
‘text-orientation
’ to ‘sideways
’.
auto
’ value of ‘text-orientation
’ to ‘use-glyph-orientation
’ and marked it at-risk.
vrt2
feature, fix various errors and omissions in synthesis rules, etc.
text-combine
’
property to ‘text-combine-horizontal
’ and added
ability to auto-combine by character classes.
text-combine-mode
’ property to control
scaling method used to compose horizontal-in-vertical text.
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.
Conformance to CSS Writing Modes Level 3 is defined for three conformance classes:
A style sheet is conformant to CSS Writing Modes Level 3 if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.
A renderer is conformant to CSS Writing Modes Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Writing Modes Level 3 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 CSS Writing Modes Level 3 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.
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 component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.
To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.
Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
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.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
The specification will remain Candidate Recommendation for at least six months.
John Daggett, Martin Heijdra, Yasuo Kida, Tatsuo Kobayashi, Toshi Kobayashi, Ken Lunde, Nat McCully, Paul Nelson, Kenzou Onozawa, Michel Suignard, Taro Yamamoto, Steve Zilles
Unicode defines three codepoint-level properties that are referenced in CSS Writing Modes:
In several sections (as noted), the term character is defined as extended grapheme cluster per [UAX29]. It is roughly equivalent to what a language user considers to be a character or a basic unit of the script (which might not be a single Unicode codepoint). The UA may further tailor this definition as allowed by Unicode.
Unicode defines properties for characters, but for ‘text-orientation
’ and Vertical Typesetting Synthesis,
it is necessary to determine the properties of a grapheme cluster. For the
purposes of CSS Writing Modes, the properties of a grapheme cluster are
given by its base character—except in two cases:
This section is normative.
This appendix gives the orientational properties of scripts in Unicode 6.0 [UNICODE]. Any script not listed explicitly is assumed to be horizontal-only. The script classification of Unicode characters is given by [UAX24].
Code | Name | Transform |
---|---|---|
Bopo | Bopomofo | translate |
Egyp | Egyptian Hieroglyphs | translate |
Hira | Hiragana | translate |
Kana | Katakana | translate |
Hani | Han | translate |
Hang | Hangul | translate |
Mong | Mongolian | rotate |
Phag | Phags Pa | rotate |
Yiii | Yi | translate |
Exceptions: For the purposes of this specification, all fullwidth (F) and wide (W) characters are treated as belonging to a vertical script with a translate bi-orientational transform. All halfwidth (H) characters are treated as belonging to a vertical script with a rotate bi-orientational transform. [UAX11] Neutral (N), narrow (Na) and ambiguous (A) Letters (L*) belonging to the Common script are treated as belonging to a horizontal-only script.
Ogham is also a rotating bi-orientational script, but because
it is a bottom-to-top script, for the purposes of this specification it is
treated as left-to-right horizontal. A future version of CSS may define
proper handling of bottom-to-top scripts. Authors can work around this
lack of support with the ‘sideways-left
’ value of ‘text-orientation
’.
This section defines an algorithm for automatic typesetting of vertical text. For readability, the term character is used in place of extended grapheme cluster in this section. See Characters and Properties for further details.
This section needs careful review. Please send feedback and suggestions for improvement, particularly for the U+2016–U+205F range.
When ‘text-orientation
’ is either ‘upright-right
’ or
‘upright
’, the
following settings are recommended:
Thus a THREE-PER-EM SPACE (U+2004) can be expected to provide a 1/3-em advance in the inline dimension, and brackets can be expected to encase their contents.
text-orientation
’ and Appendix B.
When ‘text-orientation
’ is ‘upright-right
’, the
following settings are recommended for characters not otherwise-specified
above:
When ‘text-orientation
’ is ‘upright
’, set all
characters upright (using vertical font settings if available) unless
otherwise specified above.
In OpenType, vertical font settings are provided by the
vhea
, vmtx
, and VORG
tables, as
well as the vert
and vrt2
GSUB features. If any
of these are present, the font is considered to have vertical font
settings available.
It is a Unicode error that ScriptExtensions.txt does not include the Aegean and Common Indic characters listed above as exceptions; they shouldn't need special treatment once this is fixed.
This section is normative.
CSS layout has several different concepts of automatic sizing that are used in various layout calculations. This section defines some more precise terminology to help connect the layout behaviors of this spec to the calculations used in other modules, and some new keywords for the width and height properties to allow authors to assign elements the dimensions resulting from these size calculations.
Properties: | ‘width ’, ‘min-width ’, ‘max-width ’, ‘height ’, ‘min-height ’, ‘max-height ’
|
---|---|
New Values: | ‘min-content ’ | ‘max-content ’ |
‘fill-available ’ | ‘fit-content ’
|
Initial: | as defined in [CSS21] |
Applies to: | as defined in [CSS21] |
Inherited: | as defined in [CSS21] |
Percentages: | as defined in [CSS21] |
Media: | as defined in [CSS21] |
Computed value: | specified value if keyword specified, else as defined in [CSS21] |
There are four types of automatically-determined sizes in CSS (which are represented in the width and height properties by the keywords defined above):
max(min-content, min(max-content, fill-available))
if the
available measure is finite, and as the max-content measure otherwise. The fit-content extent is calculated from the
same expression applied to the block dimension.
For the layout models in CSS2.1, both the min-content extent and max-content extent of non-replaced elements
are defined as the content extent as defined (for horizontal writing
modes) in CSS2.1§10.6.3
and CSS2.1§17.5.3
for elements with ‘height: auto
’.
For replaced elements, the min-content
and max-content sizes are the same and
correspond used size of the replaced element according to the ‘auto
’ width and height calculations.
The min-content and max-content sizes of a multi-column element are undefined per [CSS3COL]. A future specification may define them.
Property: | ‘column-width ’
|
---|---|
New Values: | ‘min-content ’ | ‘max-content ’ |
‘fill-available ’ | ‘fit-content ’
|
Initial: | as defined in [CSS3COL] |
Applies to: | as defined in [CSS3COL] |
Inherited: | as defined in [CSS3COL] |
Percentages: | as defined in [CSS3COL] |
Media: | as defined in [CSS3COL] |
Computed value: | specified value if keyword specified, else as defined in [CSS3COL] |
When used as values for ‘column-width
’, the new keywords specify
the optimal column width:
min-content
’
max-content
’
fill-available
’
fit-content
’
max(min-content, min(max-content, fill-available))
.
The style sheet rules that would achieve the bidi behaviors specified in [HTML401] for the HTML Strict doctype are given below:
/* HTML dir attribute creates an embedding */ *[dir="ltr"] { direction: ltr; unicode-bidi: embed; } *[dir="rtl"] { direction: rtl; unicode-bidi: embed; } /* BDO element creates an override */ bdo[dir="ltr"] { direction: ltr; unicode-bidi: bidi-override; } bdo[dir="rtl"] { direction: rtl; unicode-bidi: bidi-override; } /* HTML4.01:8.2.6 - preserve bidi behavior if 'display' is changed */ html, body, div, address, blockquote, p, ul, ol, li, dl, dt, dd, fieldset, form, h1, h2, h3, h4, h5, h6, { unicode-bidi: isolate; }
Property | Values | Initial | Applies to | Inh. | Percentages | Media |
---|---|---|---|---|---|---|
direction | ltr | rtl | ltr | all elements | yes | N/A | visual |
‘caption-side’ | ‘before’ | ‘after’ | before | same as CSS2.1 | same as CSS2.1 | same as CSS2.1 | same as CSS2.1 |
‘column-width’ | ‘min-content’ | ‘max-content’ | ‘fill-available’ | ‘fit-content’ | as defined in [CSS3COL] | as defined in [CSS3COL] | as defined in [CSS3COL] | as defined in [CSS3COL] | as defined in [CSS3COL] |
‘width’, ‘min-width’, ‘max-width’, ‘height’, ‘min-height’, ‘max-height’ | ‘min-content’ | ‘max-content’ | ‘fill-available’ | ‘fit-content’ | as defined in [CSS21] | as defined in [CSS21] | as defined in [CSS21] | as defined in [CSS21] | as defined in [CSS21] |
text-combine-horizontal | none | all | [ [digits <integer> | ascii-digits <integer> ] || [ alpha <integer> | latin <integer> ] || alphanumeric <integer> ] | none | non-replaced inline elements | yes | N/A | visual |
text-combine-mode | auto | compress | [ no-compress || use-glyphs ] | auto | non-replaced inline elements | yes | N/A | visual |
text-orientation | upright-right | upright | sideways-right | sideways-left | sideways | use-glyph-orientation | upright-right | all elements except table row groups, rows, column groups, and columns | yes | N/A | visual |
unicode-bidi | normal | embed | [ isolate || bidi-override ] | plaintext | normal | all elements, but see prose | no | N/A | visual |
writing-mode | horizontal-tb | vertical-rl | vertical-lr | horizontal-tb | All elements except table row groups, table column groups, table rows, and table columns | yes | N/A | visual |