Abstract
CSS Writing Modes Level 3 defines CSS 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).
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2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in
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The following features are at-risk and may be dropped during CR:
Table of Contents
1. Introduction to Writing Modes
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:
- Latin-based systems are typically written using a left-to-right inline
direction with a downward (top-to-bottom) block flow direction.
- Arabic-based systems are typically written using a right-to-left
inline direction with a downward (top-to-bottom) block flow direction.
- Mongolian-based systems are typically written using a top-to-bottom
inline direction with a rightward (left-to-right) block flow direction.
- Han-based systems are commonly written using a left-to-right inline
direction with a downward (top-to-bottom) block flow direction,
or a top-to-bottom inline direction with a leftward
(right-to-left) block flow direction. Many magazines and newspapers will
mix these two writing modes on the same page.
The ‘text-orientation’ component of the writing
mode determines the line
orientation, 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.
1.1. Module Interactions
This module replaces and extends the ‘unicode-bidi’ and ‘direction’ features
defined in [CSS21]
sections 8.6 and 9.10.
1.2. Values
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.
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.
User agents that support bidirectional text must apply the Unicode
bidirectional algorithm to every sequence of inline-level boxes
uninterrupted by any block boundary or “bidi
type B” forced paragraph break.
This sequence forms the paragraph unit in the bidirectional
algorithm. Additionally, any such sequence forming part or all of the
contents of a bidi-isolated inline
element also forms a bidi paragraph.
Two CSS properties, ‘direction’ and ‘unicode-bidi’,
provide explicit embedding, isolation, and override controls in the CSS
layer. 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 instead and not specify CSS rules to override
them.
In general, the paragraph embedding level is set according to the ‘direction’ property
of the paragraph's containing block rather than by the heuristic given in
steps P2 and P3 of the Unicode algorithm. [UAX9] When the computed ‘unicode-bidi’ of
the paragraph's containing block is ‘plaintext’, however, the Unicode heuristics (rules
P2 and P3) are used instead.
The HTML specifications ([HTML401], section 8.2, and [HTML5], section 10.3.5)
define bidirectionality behavior for HTML elements.
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.
2.1. Specifying Directionality:
the ‘direction’
property
| Name:
| 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 inline base direction or directionality of
any bidi paragraph, embedding, isolate, or override established by the
element. (See ‘unicode-bidi’.) In
addition, it informs the ordering of table column layout, the
direction of horizontal overflow, and
the default alignment of text within a line, and other layout effects that
depend on the element's inline base direction.
Values for this property have the following meanings:
- ltr
- Left-to-right directionality.
- rtl
- Right-to-left directionality.
The ‘direction’ property has no effect on bidi
reordering when specified on inline elements whose ‘unicode-bidi’
property's value is ‘normal’, because the element does not open an
additional level of embedding with respect to the bidirectional algorithm.
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.
2.2. Embeddings and
Overrides: the ‘unicode-bidi’ property
| Name:
| unicode-bidi
|
| Value:
| normal | embed | isolate | bidi-override | isolate-override |
plaintext
|
| Initial:
| normal
|
| Applies to:
| all elements, but see prose
|
| Inherited:
| no
|
| Percentages:
| N/A
|
| Media:
| visual
|
| Computed value:
| specified value
|
Normally (i.e. when ‘unicode-bidi’ is ‘normal’) an inline element is transparent to the
unicode bidi algorithm; content is ordered as if the element's boundaries
were not there. Other values of the ‘unicode-bidi’ property cause inline elements
to create scopes within the algorithm, and to override the intrinsic
directionality of text.
The following informative table summarizes the element-internal and
element-external effects of ‘unicode-bidi’:
Values for this property have the following (normative) meanings:
- normal
- The element does not open an additional level of embedding with
respect to the bidirectional algorithm. For inline elements, implicit
reordering works across element boundaries.
- embed
- If the element is inline, this value creates a directional embedding by opening an
additional level of embedding with respect to the bidirectional
algorithm. The direction of this embedding level is given by the ‘
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.
- isolate
- On an inline element, this bidi-isolates its
contents. This is similar to a directional embedding (and increases the
embedding level accordingly) except that each sequence of inline-level
boxes uninterrupted by any block boundary or forced paragraph break is
treated as an isolated sequence:
- the content within the sequence is ordered as if inside an
independent paragraph with the base directionality specified by the
element's ‘
direction’ property.
- for the purpose of bidi resolution in its containing bidi paragraph,
the sequence is treated as if it were a single Object Replacement
Character (U+FFFC).
In effect, neither is the content inside the element bidi-affected by the
content surrounding the element, nor is the content surrounding the
element bidi-affected by the content or specified directionality of the
element. However, forced paragraph
breaks within the element still create a corresponding break in
the containing paragraph.
In Unicode 7.0 and beyond, this will correspond to adding
an LRI (U+2066), for ‘direction: ltr’, or RLI
(U+2067), for ‘direction: rtl’, at the start
of the element, and a PDI (U+2069) at the end of the element.
This value has no effect on elements that are not inline.
- bidi-override
- This value puts the element's immediate content in a directional override. For an inline, this
means that the element acts like a directional embedding in the
bidirectional algorithm, except that reordering within it is strictly in
sequence according to the ‘
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. If the
element is a block container, the override is applied to an anonymous
inline element that surrounds all of its content.
- isolate-override
- This combines the isolation
behavior of ‘
isolate’
with the override behavior of ‘bidi-override’: to surrounding content, it is
equivalent to ‘isolate’, but within the element content is
ordered as if ‘bidi-override’ were specified.
- plaintext
-
This value behaves as ‘isolate’ except that for the purposes of the
Unicode bidirectional algorithm, the base directionality of each of the
element's bidi paragraphs (if a block
container) or isolated
sequences (if an inline) is determined by following the
heuristic in rules P2 and P3 of the Unicode bidirectional algorithm
(rather than by using the ‘direction’ property of the element).
In Unicode 7.0 and beyond, for inline elements this will
correspond to adding an FSI (U+2068) at the start of the element, and a
PDI (U+2069) at the end of the element.
Because the ‘unicode-bidi’ property does not inherit,
setting ‘bidi-override’ or ‘plaintext’ on a block element will not affect any
descendant blocks. Therefore these values are best used on blocks and
inlines that do not contain any block-level structures.
Note that ‘unicode-bidi’ does not affect the ‘direction’ property
even in the case of ‘plaintext’, and thus does not affect ‘direction’-dependent
layout calculations.
The final order of characters within 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 is either ‘embed’ or ‘bidi-override’, 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 assigned to the end of the element are added before
the interruption and the codes assigned 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.)
For example, where <BR/> is a forced paragraph break the bidi
ordering is identical between
<para>...<i1><i2>...<BR/>...</i2><i1>...</para>
and
<para>...<i1><i2>...</i2><i1><BR/><i1><i2>...</i2><i1>...</para>
for all values of ‘unicode-bidi’ on inline elements <i1>
and <i2>.
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 the
‘display’ is changed to ‘inline’ (see example below).
2.3. Example of Bidirectional
Text
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.
2.4. Box model for inline
elements in bidirectional context
Since bidi reordering can split apart and reorder text that is logically
contiguous, bidirectional text can cause an inline box 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:
- When the parent's ‘
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.
- When the parent's ‘
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]
3. Introduction to Vertical
Text
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.
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:
In some special cases such as two-digit numbers in dates, text is fit
compactly into a single vertical character box:
Layouts often involve a 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.
3.1. Block Flow Direction:
the ‘writing-mode’ property
| Name:
| 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 specifies whether lines of text are laid out horizontally
or vertically and the direction in which blocks progress. Possible values:
- horizontal-tb
- Top-to-bottom block flow
direction. The writing mode
is horizontal.
- vertical-rl
- Right-to-left block flow
direction. The writing mode
is vertical.
- vertical-lr
- Left-to-right block flow
direction. The writing mode
is vertical.
The ‘writing-mode’ property specifies the block flow direction, which
determines the progression of block-level boxes in a block formatting
context; the progression of line boxes in a block container that contains
inlines; the progression of rows in a table; etc. By virtue of determining
the stacking direction of line boxes, the ‘writing-mode’
property also determines whether the line boxes' orientation (and thus the
writing mode) is horizontal or
vertical. The ‘text-orientation’ property then determines how
text is laid out within the line box.
The principal writing mode of the
document is determined by the ‘writing-mode’ and ‘direction’ values
specified on the root element. This writing mode is used, for example, to
determine the default page progression direction. (See [CSS3PAGE].) Like ‘direction’, 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:
- If the element has a specified ‘
display’ of ‘inline’,
its ‘display’ computes to ‘inline-block’. [CSS21]
- If the element has a specified ‘
display’ of ‘run-in’,
its ‘display’ computes to ‘block’. [CSS21]
- If the 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>日本語</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; }
SVG1.1 [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:
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.
4. Inline-level Alignment
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.
4.1. Introduction to
Baselines
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.
[Picture of alphabetic text in two font sizes with the
baseline and emboxes indicated.]
Different writing systems prefer different baseline tables.
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.)
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 writing
modes and the horizontal tables otherwise.
4.2. Text Baselines
In this specification, only the following baselines are considered:
- alphabetic
- The alphabetic baseline, which
typically aligns with the bottom of uppercase Latin glyphs.
- central
- The central baseline, which typically
crosses the center of the em box. If the font is missing this baseline,
it is assumed to be halfway between the ascender (over) and descender (under) edges of the em box.
In vertical writing mode, the central
baseline is used as the dominant baseline when ‘text-orientation’ is ‘mixed’ or ‘upright’. Otherwise the alphabetic baseline is used.
A future CSS module will deal with baselines in more detail
and allow the choice of other dominant baselines and alignment options.
4.3. Atomic Inline
Baselines
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:
- alphabetic
- The alphabetic baseline is assumed to be at the under margin edge.
- central
- The central baseline is assumed to be halfway between the under and over margin
edges of the box.
4.4. Baseline Alignment
The dominant baseline (which can change based on the writing mode) is used
in CSS for alignment in two cases:
- Aligning glyphs from different fonts within the same inline
box. The glyphs are aligned by matching up the positions of the
dominant baseline in their corresponding fonts.
- Aligning a child inline-level box within its parent.
For the ‘
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; }
5. Introduction to
Vertical Text Layout
Each writing system has one or more native orientations. Modern scripts
can therefore be classified into three orientational categories:
- horizontal-only
- Scripts that have horizontal, but not vertical, native orientation.
Includes: Latin, Arabic, Hebrew, Devanagari
- vertical-only
- Scripts that have vertical, but not horizontal, native orientation.
Includes: Mongolian, Phags Pa
- bi-orientational
- Scripts that have both vertical and horizontal native orientation.
Includes: Han, Hangul, Japanese Kana
A vertical script is one that has a native
vertical orientation: i.e. one that is either vertical-only or that is
bi-orientational. A horizontal script is
one that has a native horizontal orientation: i.e. one that is either
horizontal-only or that is bi-orientational. (See Appendix B for a categorization of scripts
by native orientation.)
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:
- rotate
- Rotate the glyph from horizontal to vertical
- translate
- Translate the glyph from horizontal to vertical
Scripts with a native vertical orientation have an intrinsic
bi-orientational transform, which orients them correctly in vertical text:
most CJK (Chinese/Japanese/Korean) characters translate, that is, they are
always upright. Characters from other scripts, such as Mongolian, rotate.
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 ‘mixed’ 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.
5.1. Orienting Text: the
‘text-orientation’ property
| Name:
| text-orientation
|
| Value:
| mixed | upright | sideways-right | sideways-left | sideways |
use-glyph-orientation
|
| Initial:
| mixed
|
| 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 text within a line. Current
values only have an effect in vertical writing modes; the property has no
effect on elements in horizontal 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:
- mixed
-
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. See Vertical
Orientations for further details.
This value is typical for layout of primarily vertical-script text.
- upright
-
In vertical writing modes, characters from horizontal-only scripts are
rendered upright, i.e. in their standard horizontal orientation.
Characters from vertical scripts are set with their intrinsic
orientation and shaped normally. See Vertical Orientations for further
details.
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’.
- sideways-right
-
In vertical writing modes, this causes text to be set as if in a
horizontal layout, but rotated 90° clockwise.
- sideways-left
-
In vertical writing modes, this causes text to be set as if in a
horizontal layout, but rotated 90° counter-clockwise.
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 ‘isolate-override’.
Layout of text is exactly as for ‘sideways-right’ except that the baseline table of
each of the element's inline boxes is mirrored around a vertical axis
along the center of its content box and text layout is rotated 180° to
match. 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.
- sideways
-
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.
- use-glyph-orientation
-
[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: use-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 ‘mixed’.
This value is at-risk and may be dropped during CR.
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; }
Changing the value of this property may affect inline-level
alignment. Refer to Text Baselines for more
details.
5.1.1. Mixed
Vertical Orientations
[UTR50] defines a
property for the default character orientation of mixed-orientation
vertical text. When ‘text-orientation’ is ‘mixed’, the UA must render a character upright if its orientation property
is U, T, or Tu; or typeset it sideways (90° clockwise from
horizontal) if its orientation property is R. For
Tr characters, which are
intended to be either transformed or rotated sideways, the UA may assume
that appropriate glyphs for upright typesetting are given in the font and
render them upright; alternately it may check for such glyphs first, and
fall back to typesetting them sideways if the appropriate glyphs are
missing.
In some systems (e.g. when using OpenType fonts), to correctly orient a
character belonging to the Mongolian or
Phags-pa script upright, the UA must actually typeset it sideways.
This is because the "upright" orientation in the Unicode code
charts (which assume vertical typesetting) and the "upright" orientation
of the glyphs of these scripts in most OpenType fonts (which assume
horizontal typesetting) don't match.
As of the time of publication, Unicode has not yet
officially updated its orientation data to account for the many errata
that were accepted in June 2012. Therefore, the official UTR50 data
(version 5) should not be used. (Instead, see this
draft of version 6.) When Unicode publishes a proper update, this spec
will update its references accordingly.
5.1.2. Vertical
Typesetting and Font Features
When typesetting text in ‘mixed’ and ‘upright’ orientations:
- upright characters
- Are typeset upright with vertical font metrics. The UA must
synthesize vertical font metrics for fonts that lack them. (This
specification does not define heuristics for synthesizing such metrics.)
Additionally, font features (such as alternate glyphs and other
transformation) intended for use in vertical typesetting must be used.
(E.g. the OpenType ‘
vert’ feature must be
enabled.) Furthermore, characters from horizontal cursive scripts (such
as Arabic) are shaped in their isolated forms when typeset upright.
Note that even when typeset "upright", some glyphs should
appear rotated. For example, dashes and enclosing punctuation (such as
〈 LEFT ANGLE BRACKET U+3008) should be oriented relative to the inline
axis. In OpenType, this is typically handled by glyph substitution,
although not all fonts have alternate glyphs for all relevant
codepoints. (East Asian fonts usually provide alternates for East Asian
codepoints, but Western fonts typically lack any vertical typesetting
features.) Unicode published draft data on which characters should appear sideways as the
SVO property in this
data file; however, this property has been abandoned for future
revisions of [UTR50].
- sideways characters
- Are typeset rotated 90° sideways with horizontal metrics, and
vertical typesetting features are not used. However, if the font has
features meant to be enabled for sideways text that is typeset in
vertical lines (e.g. to adjust brush stroke angles or alignment), those
features are used.
All text in ‘sideways’, ‘sideways-right’, and ‘sideways-left’
orientations is typeset using horizontal font metrics and the normal set
of features used for horizontal text runs. Vertical metrics, vertical
glyph variations, and any other features meant for text typeset in
vertical lines are not used.
The OpenType ‘vrt2’ feature,
which is intended for mixed-orientation typesetting, is not used by CSS.
It delegates the responsibility for orienting glyphs to the font designer.
CSS instead dictates the orientation through [UTR50] and orients glyphs by
typesetting them sideways or upright as appropriate.
This section needs additional work. Suggestions are welcome.
6. Abstract Box Terminology
CSS2.1 [CSS21]
defines the box layout model of CSS in detail, but only for the ‘horizontal-tb’ writing
mode. Layout is analogous in writing modes other than ‘horizontal-tb’; however
directional and dimensional terms in CSS2.1 must be 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 next section explains how to apply them to CSS2.1 layout
calculations and how to handle orthogonal
flows.) Although they derive from the behavior of text, these abstract
mappings 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.
There are three sets of directional terms in CSS:
- physical
- Interpreted relative to the page, independent of writing mode. The
physical directions are left, right, top, and
bottom.
- flow-relative
- Interpreted relative to the flow of content. The flow-relative
directions are start, end, before, and after.
- line-relative
- Interpreted relative to the orientation of the line box. The
line-relative directions are line-left,
line-right, over, and under.
The physical dimensions are
width and height, which correspond to measurements along the
x-axis (vertical dimension) and y-axis (horizontal
dimension), respectively. Abstract
dimensions are identical in both flow-relative and line-relative
terms, so there is only one set of these terms.
6.1. Abstract
Dimensions
The abstract dimensions are defined
below:
- block dimension
- The dimension perpendicular to the flow of text with in a line, the
vertical dimension in horizontal writing modes, and the
horizontal dimension in vertical writing modes.
- inline dimension
- The dimension parallel to the flow of text within a line, i.e. the
horizontal dimension in horizontal writing modes, and the
vertical dimension in vertical writing modes.
- block-axis
- The axis in the block dimension, i.e. the vertical axis in
horizontal writing modes and the horizontal axis in vertical
writing modes.
- inline-axis
- The axis in the inline dimension, i.e. the horizontal axis in
horizontal writing modes and the vertical axis in vertical writing
modes.
- extent or logical
height
-
- A measurement in the block dimension: refers to the physical height
(vertical dimension) in horizontal writing modes, and to the physical
width (horizontal dimension) in vertical writing modes.
- measure or logical
width
-
- A measurement in the inline dimension: refers to the physical width
(horizontal dimension) in horizontal writing modes, and to the physical
height (vertical dimension) in vertical writing modes. (The term measure derives from its use in
typography.)
6.2. Flow-relative
Directions
The flow-relative directions, before, after, start, and end, are
defined relative to the flow of content on the page. In an LTR ‘horizontal-tb’ writing mode, they correspond to the
top, bottom, left, and right directions, respectively. They are defined as
follows:
- before or block-before
- Nominally the side that comes earlier in the block progression, as
determined by the ‘
writing-mode’ property: the physical top in
‘horizontal-tb’
mode, the right in ‘vertical-rl’, and the left in ‘vertical-lr’.
- after or block-after
- The side opposite before.
- start
- Nominally the side from which text of its inline base direction will
start. For boxes with a used ‘
direction’ value of ‘ltr’, this means the line-left side. For boxes with a used ‘direction’ value of
‘rtl’, this means the line-right side.
- end
- The side opposite start.
The naming of the block-axis logical directions is under
discussion. The before/after set of names is used in XSL-FO, but is
unfortunately somewhat confusing with DOM-relative directions (e.g. the
::before/::after pseudo-elements), and is not sufficiently distinct from
start/end, which is used for the inline axis. Feedback is welcome, see Koji's
summary email.
Note that while determining the before and after
sides of a box depends only on the ‘writing-mode’ property, determining the start and end sides 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
6.3. Line-relative
Directions
The line orientation determines which
side of a line box is the logical “top” (ascender side). It is given
by a combination of ‘text-orientation’ and ‘writing-mode’.
Usually the line-relative “top” corresponds to the block-before side, but this is not always
the case: in Mongolian typesetting (and thus by default in ‘vertical-lr’ writing
modes), the line-relative “top” corresponds to the block-after side. Hence the need for
distinct terminology.
In addition to a line-relative “top” and “bottom” to map things
like ‘vertical-align: top’, CSS also needs to
refer to a line-relative “left” and “right” in order to map things
like ‘text-align: left’. Thus there are four
line-relative directions, which are
defined relative to the line
orientation as follows:
- over
- Nominally the side that corresponds to the ascender side or “top”
side of a line box. (The side overlines are typically drawn on.)
- under
- Opposite of over: the line-relative
“bottom” or descender side. (The side underlines are typically drawn
on.)
- line-left
- Nominally the side from which LTR
text would start.
- line-right
- Nominally the side from which RTL
text would start. (Opposite of line-left.)
See the table below for the exact
mappings between physical and line-relative directions.
6.4.
Abstract-to-Physical Mappings
The following table summarizes the abstract-to-physical mappings:
7. Abstract Box Layout
7.1. Principles of Layout
in Vertical Writing Modes
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’ ‘mixed’ ‘rtl’ table, the first column would be on the
bottom (the start side), and the first row on the right (the block-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.
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.
7.2. Dimensional Mapping
Certain properties behave logically as follows:
- The first and second values of the ‘
border-spacing’ property represent spacing
between columns and rows respectively, not necessarily the horizontal and
vertical spacing respectively. [CSS21]
- The ‘
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 block-before and block-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.
7.3. Orthogonal Flows
When an element has a different ‘writing-mode’ from its containing block two
cases are possible:
When an element has a writing mode that is perpendicular to its
containing block it is said to be in, or establish, an orthogonal flow.
To handle this 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 are 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
are performed according to the writing mode of the containing
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.
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 as the
child's containing block measure, even though the physical height is the
extent, not the measure, of the parent block.
On the other hand, because the containing block is in a horizontal
writing mode, the vertical margins on the child participate in
margin-collapsing, even though they are in the inline-axis of the child,
and horizontal auto margins will expand to fill the containing block,
even though they are in the block-axis of the child.
Add a picture.
It is common in CSS for a containing block to have a definite measure,
but not a definite 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
indefinite. In such cases a percentage of the containing block measure
cannot be defined, and inline-axis computations cannot be resolved. In
these cases, the initial containing block's size is used as a
fallback variable in place of the available measure for
calculations that require a definite available measure.
7.3.1. Auto-sizing in
Orthogonal Flows
This section needs careful review for whether it is a)
correct and b) sensible.
If the UA does not support CSS Multi-column Layout [CSS3COL] and the element is a
block container, when the computed measure of the element establishing an
orthogonal flow is ‘auto’, then the used inner
measure is calculated as:
min(max-content, max(min-content,
min(fill-available, fill-fallback))), where:
- min-content
- the min-content
measure of the element
- max-content
- the max-content
measure of the element
- fill-available
- the fill-available fit into the element's containing block's size in
the element's inline axis
- fill-fallback
- the fill-available fit into the initial containing block's size in the
element's inline axis
See [CSS3-SIZING] for further
details.
7.3.2. Multi-column
Layout in Orthogonal Flows
If the UA supports CSS Multi-column Layout [CSS3COL] and the element is a
block container or multi-column element, for the case where the element's
extent or available extent is definite but the element's measure is
‘auto’:
- If ‘
column-count’ and ‘column-width’ are both ‘auto’, calculate the used ‘column-width’ as the inner measure for ‘auto’-sized elements, as defined above.
- If the columns' extent is indefinite, the fill-available
extent of the element is used.
- The used ‘
column-count’ then follows
from filling the resulting columns with the element's content.
The used measure of the resulting multi-column element is then
calculated: if the content neither line-wraps nor fragments 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 fragmentation, the rules in CSS2.1 still hold in
vertical writing modes and orthogonal flows: 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.
7.4.
Flow-Relative Mappings
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 block-before margin substituted for
the top margin and the block-after
margin substituted for the bottom margin. Similarly the block-before padding and border are
substituted for the top padding and border, and the block-after padding and border substituted
for the bottom padding and border. Note this means only block-before and block-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:
- The initial value of the ‘
text-align’
property aligns to the start edge of the line box.
- The ‘
text-indent’ property indents
from the start edge of the line box.
- For tables, the ordering of columns begins on the start side of the
table, and the ordering of rows begins on the block-before side of the table.
7.5. Line-Relative Mappings
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:
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:
- For the ‘
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]
- For the ‘
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]
7.6. Purely Physical
Mappings
The following values are purely physical in their definitions and do not
respond to changes in writing mode:
- the ‘
rect()’ notation of the ‘clip’ property [CSS21]
- the background properties [CSS21] [CSS3BG]
- the border-image properties [CSS3BG]
- the offsets of the ‘
box-shadow’ and
‘text-shadow’ properties
7.7. Table Caption Mappings:
the ‘caption-side’ keywords
| Property:
| ‘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.
8. Page Flow: the page
progression direction
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:
(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.)
9. Glyph Composition
9.1.
Horizontal-in-Vertical Composition: the ‘text-combine-horizontal’ property
| Name:
| text-combine-horizontal
|
| Value:
| none | all
|
| 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:
- none
- No special processing.
- all
- In vertical writing mode, attempt to display the text contents of the
element horizontally within the vertical line box. If the contents are
wider than 1em, the UA must attempt to fit the contents within 1em, but
may use any method to do so. The resulting composition is treated as a
single glyph for the purposes of layout and decoration. If the content
contains any element boundaries this is treated as ‘
text-combine-horizontal: none’ on the element and any
descendants.
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:
The figure is the result of the rules
.tcy { text-combine-horizontal: all; }
and the following markup:
<p>平成<span class="tcy">20</span>年
<span class="tcy">4</span>月<span class="tcy">16</span>日に</p>
In Japanese, this effect is known as tate-chu-yoko.
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 bidi reordering, the
composition is treated the same as a character with ‘text-orientation: upright’. For line breaking, the
composition is treated as a regular inline with its actual contents. For
other text layout purposes, e.g. emphasis marks, text-decoration, spacing,
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.
Changes
Major changes include:
- Replaced ‘
unicode-bidi’ value of ‘isolate bidi-override’ with single keyword ‘isolate-override’.
- Clarified that bidi isolated content does not affect plaintext
heuristics of the containing paragraph.
- Lots of other clarifications to bidi.
- Renamed ‘
mixed-right’ value of ‘text-orientation’ to ‘mixed’.
- Updated references to UTR50 for orientation of characters in Unicode
and improved text defining vertical typesetting rules.
- Fixed errors and clarified auto-sizing rules for orthogonal flows.
- Replaced Appendix D with references to the new [CSS3-SIZING] module.
- Removed ‘
text-combine-mode’ property.
- Removed all ‘
text-combine-horizontal’ values except
‘none’ and ‘all’.
- Defined effect of ‘
text-combine-horizontal’ on line-breaking.
Major changes include:
- Hooked up vertical typesetting details to UTR50.
- Removed concept of "typographic modes".
- Altered orthogonal sizing to take into
account the fill-available size; now the minimum of the fill-available
and ICB size is used to resolve ‘
auto’
sizes.
- Renamed ‘
digits’ to ‘numeric’ and ‘ascii-digits’
to ‘digits’ for ‘text-combine-horizontal’.
- Defined interaction of ‘
text-combine-horizontal’ and ‘text-transform’.
10.1. Document Conventions
Conformance requirements are expressed with a combination of descriptive
assertions and RFC 2119 terminology. The key words “MUST”, “MUST
NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the
normative parts of this document are to be interpreted as described in RFC
2119. However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with
class="example", like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from
the normative text with class="note", like this:
Note, this is an informative note.
Conformance to CSS Writing Modes Level 3 is defined for three
conformance classes:
- style
sheet
- A CSS
style sheet.
- renderer
- A UA
that interprets the semantics of a style sheet and renders documents that
use them.
- authoring tool
- A UA
that writes a style sheet.
A style sheet is conformant to 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.
10.3. Partial Implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must treat as
invalid (and ignore as
appropriate) any at-rules, properties, property values, keywords, and
other syntactic constructs for which they have no usable level of support.
In particular, user agents must not selectively ignore
unsupported 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.
10.4. Experimental
Implementations
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.
10.5. Non-Experimental
Implementations
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.
10.6. CR Exit Criteria
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:
- independent
- each implementation must be developed by a different party and cannot
share, reuse, or derive from code used by another qualifying
implementation. Sections of code that have no bearing on the
implementation of this specification are exempt from this requirement.
- interoperable
- passing the respective test case(s) in the official CSS test suite,
or, if the implementation is not a Web browser, an equivalent test. Every
relevant test in the test suite should have an equivalent test created if
such a user agent (UA) is to be used to claim interoperability. In
addition if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those equivalent
tests in the same way for the purpose of interoperability. The equivalent
tests must be made publicly available for the purposes of peer review.
- implementation
- a user agent which:
- implements the specification.
- is available to the general public. The implementation may be a
shipping product or other publicly available version (i.e., beta
version, preview release, or “nightly build”). Non-shipping product
releases must have implemented the feature(s) for a period of at least
one month in order to demonstrate stability.
- is not experimental (i.e., a version specifically designed to pass
the test suite and is not intended for normal usage going forward).
The specification will remain Candidate Recommendation for at least six
months.
Acknowledgements
John Daggett, Martin Heijdra, Laurentiu Iancu, Yasuo Kida, Tatsuo
Kobayashi, Toshi Kobayashi, Ken Lunde, Nat McCully, Eric Muller, Paul
Nelson, Kenzou Onozawa, Dwayne Robinson, Michel Suignard, Taro Yamamoto,
Steve Zilles
Appendix A. Characters and
Properties
Unicode defines three codepoint-level properties that are referenced in
CSS Writing Modes:
- East Asian
width
- Defined in [UAX11] and given as the
East_Asian_Width property in the Unicode Character Database [UAX44].
- General
Category
- Defined in [UAX44] and given as the
General_Category property in the Unicode Character Database [UAX44].
- Script
property
- Defined in [UAX24] and given as the Script
property in the Unicode Character Database [UAX44]. (UAs should include any
ScriptExtensions.txt assignments in this mapping.)
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 appropriate to match typographic convention. For
example, when typesetting ‘upright’, Tibetan tsek and shad marks are kept with
the preceding letters, rather than treated as an independent cluster.
Unicode defines properties for characters, but for ‘text-orientation’, 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:
- Grapheme clusters formed with an Enclosing Mark (Me) of the Common
script are considered to be Other Symbols (So) in the Common script. They
are assumed to have the same Unicode properties as the Replacement
Character U+FFFD.
- Grapheme clusters formed with a Space Separator (Zs) as the base are
considered to be Modifier Symbols (Sk). They are assumed to have the same
East Asian Width property as the base, but take their other properties
from the first combining character in the sequence.
Appendix B: Bidi Rules for HTML 4
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; }
Appendix C: Vertical Scripts in
Unicode
This section is informative.
This appendix lists the vertical and bi-orientational scripts in Unicode
6.0 [UNICODE]
and their transformation from horizontal to vertical orientation. Any
script not listed explicitly is assumed to be horizontal-only. The
script classification of Unicode characters is given by [UAX24].
Vertical Scripts in Unicode
| Code
| Name
| Transform (Clockwise)
| Vertical Intrinsic Direction
|
| Bopo
| Bopomofo
| 0°
| ttb
|
| Egyp
| Egyptian Hieroglyphs
| 0°
| ttb
|
| Hira
| Hiragana
| 0°
| ttb
|
| Kana
| Katakana
| 0°
| ttb
|
| Hani
| Han
| 0°
| ttb
|
| Hang
| Hangul
| 0°
| ttb
|
| Merc
| Meroitic Cursive
| 0°
| ttb
|
| Mero
| Meroitic Hieroglyphs
| 0°
| ttb
|
| Mong
| Mongolian
| 90°
| ttb
|
| Ogam
| Ogham
| -90°
| btt
|
| Orkh
| Old Turkic
| -90°
| ttb
|
| Phag
| Phags Pa
| 90°
| ttb
|
| Yiii
| Yi
| 0°
| ttb
|
Exceptions: For the purposes of this specification, all
fullwidth (F) and wide (W) characters are treated as belonging to a
vertical script, and halfwidth characters (H) are treated as belonging ot
a horizontal script. [UAX11]
CSS3 Writing Modes cannot correctly handle either Ogham or
Old Turkic. It is recommended that ‘text-orientation:
sideways-left’ be used to typeset these scripts. A future version
of CSS may define automatic handling of these scripts.
Note that for vertical-only characters (such as Mongolian and
Phags Pa letters), the glyphs in the Unicode code charts are shown in
their vertical orientation. In horizontal text, they are typeset in a 90°
counter-clockwise rotation from this orientation.
References
Normative references
-
- [CSS21]
- Bert Bos; et al. Cascading Style
Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June
2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607
- [CSS3-SIZING]
- Tab Atkins Jr.; Elika J. Etemad. CSS
Intrinsic & Extrinsic Sizing Module Level 3. 27 September
2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-sizing-20120927/
- [CSS3BG]
- Bert Bos; Elika J. Etemad; Brad Kemper. CSS
Backgrounds and Borders Module Level 3. 24 July 2012. W3C
Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-background-20120724/
- [CSS3COL]
- Håkon Wium Lie. CSS
Multi-column Layout Module. 12 April 2011. W3C Candidate
Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2011/CR-css3-multicol-20110412
- [RFC2119]
- S. Bradner. Key
words for use in RFCs to Indicate Requirement Levels. Internet
RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
- [SVG11]
- Erik Dahlström; et al. Scalable
Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011.
W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-SVG11-20110816/
- [UAX11]
- Asmus Freytag. East Asian
Width. 17 January 2012. Unicode Standard Annex #11. URL: http://www.unicode.org/reports/tr11/
- [UAX24]
- Mark Davis; Ken Whistler. Unicode Script
Property. 13 January 2012. Unicode Standard Annex #24. URL: http://www.unicode.org/reports/tr24/
- [UAX29]
- Mark Davis. Unicode Text
Segmentation. 24 January 2012. Unicode Standard Annex #29.
URL: http://www.unicode.org/reports/tr29/
- [UAX44]
- Mark Davis; Ken Whistler. Unicode Character
Database. 23 January 2012. Unicode Standard Annex #44. URL: http://www.unicode.org/reports/tr44/
- [UAX9]
- Mark Davis. Unicode Bidirectional
Algorithm. 16 January 2012. Unicode Standard Annex #9. URL: http://www.unicode.org/reports/tr9/
- [UNICODE]
- The Unicode Consortium. The
Unicode Standard. 2003. Defined by: The Unicode Standard,
Version 4.0 (Boston, MA, Addison-Wesley, ISBN 0-321-18578-1), as updated
from time to time by the publication of new versions URL: http://www.unicode.org/standard/versions/enumeratedversions.html
- [UTR50]
- Eric Muller. Unicode
Properties for Vertical Text Layout. 17 May 2012. Proposed
Draft Unicode Technical Report #50. URL: http://www.unicode.org/reports/tr50/tr50-5.html
Other references
-
- [CSS2]
- Ian Jacobs; et al. Cascading Style
Sheets, level 2 (CSS2) Specification. 11 April 2008. W3C
Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS2-20080411
- [CSS3COLOR]
- Tantek Çelik; Chris Lilley; L. David Baron. CSS Color
Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607
- [CSS3LIST]
- Tab Atkins Jr. CSS Lists
and Counters Module Level 3. 24 May 2011. W3C Working Draft.
(Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-lists-20110524
- [CSS3PAGE]
- Håkon Wium Lie; Melinda Grant. CSS3 Module:
Paged Media. 10 October 2006. W3C Working Draft. (Work in
progress.) URL: http://www.w3.org/TR/2006/WD-css3-page-20061010
- [CSS3TEXT]
- Elika J. Etemad; Koji Ishii. CSS Text
Module Level 3. 13 November 2012. W3C Working Draft. (Work in
progress.) URL: http://www.w3.org/TR/2012/WD-css3-text-20121113/
- [HTML401]
- Dave Raggett; Arnaud Le Hors; Ian Jacobs. HTML 4.01
Specification. 24 December 1999. W3C Recommendation. URL: http://www.w3.org/TR/1999/REC-html401-19991224
- [HTML5]
- Ian Hickson. HTML5.
25 May 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-html5-20110525/
- [UTN22]
- Elika J. Etemad. Robust
Vertical Text Layout. 25 April 2005. Unicode Technical Note
#22. URL: http://unicode.org/notes/tn22/
Property Index
| Property
| Values
| Initial
| Applies to
| Inh.
| Percentages
| Media
|
| direction
| ltr | rtl
| ltr
| all elements
| yes
| N/A
| visual
|
| text-combine-horizontal
| none | all
| none
| non-replaced inline elements
| yes
| N/A
| visual
|
| text-orientation
| mixed | upright | sideways-right | sideways-left | sideways |
use-glyph-orientation
| mixed
| all elements except table row groups, rows, column groups, and
columns
| yes
| N/A
| visual
|
| unicode-bidi
| normal | embed | isolate | bidi-override | isolate-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
|
