Copyright © 2010 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
This module specifies the text layout model in CSS and the properties that control it. It covers bidirectional and vertical text.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-writing-modes” in the subject, preferably like this: “[css3-writing-modes] …summary of comment…”
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.
This is the first public working draft of the CSS Writing Modes module as a separate specification. The functionality was split off from the CSS Text module of 14 May 2003.
writing-mode
’
property
text-combine
’ property
This section is non-normative.
This module defines support for various international writing directions, such as left-to-right (e.g., Latin scripts), right-to-left (e.g., Hebrew or Arabic), bidirectional (e.g., mixing Latin with 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.
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 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, while for Mongolian and Manchu 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:
Example of horizontal-in-vertical two-digit numbers
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.
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, the CSS specifications avoid this latter set of terms.
Writing systems typically have one or two native writing modes. Some examples are:
Some additional characteristics, the line orientation and glyph
orientation, are primarily used to handle scripts placed in a non-native
writing mode. The line orientation
determines which side of the line is the "top" and thus which sides are
under or over the line. This
determines the interpretation of alignment in the transverse dimension of
the line. It also determines the default glyph
orientation for scripts in a non-native orientation. These
characteristics are controlled by the ‘text-orientation
’ property.
In the following example, two blocks 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 Uighur and Mongolian (writing-mode: lr
):
See Unicode Technical Note #22 [UTN22] (HTML version) for a more in-depth introduction to writing modes and vertical text.
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. 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 boxes uninterrupted by
a forced (bidi
class B) line break or block boundary. This sequence forms the
"paragraph" unit in the bidirectional algorithm. The paragraph embedding
level is set according to the value of the ‘direction
’
property of the containing block rather than by the heuristic given in
steps P2 and P3 of the Unicode algorithm.
Because the base directionality of a text depends on the structure and semantics of the document, these 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 reordering in
inline-level 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 ] || [ plaintext | bidi-override ] ] |
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.
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 "paragraph" for which the element is the
containing block element is determined not by the element's computed
‘direction
’ as usual, but by following
rules P1, P2, and P3 of the Unicode bidirectional algorithm. However, if
no direction-determining character is found in step P2, then the value
of the ‘direction
’ property is used instead.
Note this value has no effect on inline elements.
The final order of characters in each block-level element 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,
non-textual entities such as images 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.
Please 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), more inline boxes (including anonymous inline boxes) may have to be created, and some inline boxes may have to be split up and reordered before flowing.
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.
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).
When the element'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 element'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.
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:
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 them
in the context of CSS must treat these values as follows:
SVG1 | CSS |
---|---|
lr | horizontal-tb |
lr-tb | |
rl | |
tb | vertical-rl |
tb-rl |
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.
See this demo to
check out your implementation's interpretation! (Note that most SVG
implementations don't support the ‘direction
’ property, and thus your results
may be skewed on that account. Examine the red line of text: if the
numbers are not in reverse order, your implementation doesn't support
"direction: rtl".)
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. As a result it also determines which side of a box is its before edge (i.e. the edge that comes earlier in the
progression) and which side is its after edge (i.e.
the edge that comes later in the progression).
The ‘writing-mode
’ property, by virtue of
determining the stacking direction of line boxes, determines whether line
boxes and thus the writing mode of text are horizontal or vertical.
However, it does not determine the orientation of the line boxes' contents
or the start and end sides of the line. See Line orientation.
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 ‘direction
’ 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.
CSS box layout in vertical writing modes is analogous to layout in the horizontal writing modes, following the principles outlined below. See Abstract Box layout for a more complete discussion.
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 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. See Line-Relative Directions for details.
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. (See Line-Relative Directions.)
Every line box has an orientation. Whether its orientation is horizontal
or vertical is determined by the ‘writing-mode
’ property. Given that, which
side is its "top", or over edge, is
determined by the ‘text-orientation
’ property on its
containing block, and is independent of the block flow direction.
In addition to its over (ascender side) and under (descender side) edges, 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 to distinguish from the physical left and physical right sides of the box.
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.
These directional mappings exist even for boxes that do not contain any
line boxes: they are calculated by the values of the ‘writing-mode
’,
‘text-orientation
’, and ‘direction
’
properties only.
Note that determining the start
and end edges of a box depends not only on its
‘writing-mode
’ and ‘direction
’
properties, but also its ‘text-orientation
’ property.
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
’.
text-orientation
’ propertyName: | text-orientation |
---|---|
Value: | vertical-right | upright | rotate-right | rotate-left | rotate-normal | auto |
Initial: | vertical-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 in a non-native
‘writing-mode
’ and sets the orientation of
the line. Current values only have an effect in vertical writing modes.
Values have the following meanings:
In vertical writing modes, grapheme clusters from scripts that do not have an intrinsic vertical orientation are rotated 90° clockwise from their standard orientation in horizontal text. When available, vertical glyph variants and vertical font metrics are used to set all punctuation and characters from any script that is not rotated. This value is typical for layout of primarily vertical-script text.
In vertical writing modes, grapheme clusters that do not have an intrinsic vertical orientation are rendered upright, i.e. in their standard horizontal orientation. Shaping characters are shaped in their isolated forms. When available, vertical glyph variants and vertical font metrics are used to set the text. The UA should 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 causes text to be set as if in a
horizontal layout (using horizontal glyph variants and metrics), but
rotated 90° clockwise. This value is typical for ‘vertical-rl
’ text in a primarily
horizontal-script document.
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 is typical for
‘vertical-lr
’ text in a primarily
horizontal-script document.
If set on a non-replaced inline whose parent is not ‘rotate-left
’, this
forces ‘isolate
’ to be added to the
computed value of ‘unicode-bidi
’. Layout of text is exactly
as for ‘rotate-right
’ except that the entire text
content and baseline table of the element is mirrored: each box of the
inline is mirrored around a vertical axis such that its content box does
not move. (However the contents of atomic inlines are not mirrored; only
their alignment is changed.) Similarly, if a child of the element has a
‘text-orientation
’ value other than
‘rotate-left
’, an analogous transformation is
applied.
This value is equivalent to ‘rotate-right
’ in ‘vertical-rl
’ writing mode and equivalent to
‘rotate-left
’ in ‘vertical-lr
’ writing mode.
[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 ‘auto
’ 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: auto
’ 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 ‘auto
’ behavior is the same as for ‘vertical-right
’.
Baseline alignment is not yet defined.
Add section explaining native script orientations. Note that all wide characters are treated the same as ideographic. Link to definition of grapheme clusters in [UAX29].
Add appendix that describes interaction with OpenType features and font layout?
[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 boxes and dimensions are abstracted and remapped
appropriately. This module defines the following abstract directional and
dimensional terms and their mappings in order to define box layout for
other writing modes:
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: the logical width (which could be either the physical width or physical height) and the start and end margins, padding, and border. Likewise the calculation rules in CSS2.1 Section 10.6 are used for measurements in the block dimension. [CSS21]
As a corollary, percentages on the margin and padding properties, which are calculated with respect to the containing block width regardless of their dimension, are calculated with respect to the logical width of the containing block.
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, for the purposes of calculating the layout of the box, the dimensions corresponding to the logical height and logical width of the containing block are determined using the writing mode of the box under consideration, not the writing mode of the element associated with the containing block.
What's a reasonable way of calculating ‘auto
’ logical widths? The
fallout of the above statements is not reasonable. Three good options are:
use the max-content size; use 100vh margin-box; use the same logical width
that would be calculated if the block flows were parallel.
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: logical and line-relative. Which one is chosen depends on whether the usage is primarily with respect to the block, or primarily with respect to the line box.
The logical 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.
An English (LTR-TB) block:
<---- width / logical width ---> top side/ before side +------------------------------+ A left side/ | ---inline direction ---> | right side/ | start side | | | end side | | | block * horizontal * | height/ | | direction *writing mode* | logical height | V | | +------------------------------+ V bottom side/ after side
A vertical Japanese block (TTB-RL):
<--- width / logical height ---> top side/ start side +------------------------------+ A left side/ | <---block direction--- | right side/ | after side | | | before side | | * vertical * inline| | height/ | *writing mode* direction| | logical width | V | | +------------------------------+ V bottom side/ end side
Logical directions are calculated with respect to the writing mode of
the element and used to abstract layout related to padding and border
properties. For example, if an element had computed values of ‘direction: ltr; writing-mode: vertical-lr; text-orientation:
vertical-right
’, ‘padding-top
’ would give its start padding, and
‘padding-left
’ would give its
before padding.
Logical directions are calculated with respect to the writing mode of
the parent of the element and used to abstract layout related to
the margin properties and the ‘top
’, ‘bottom
’, ‘left
’, and ‘right
’ properties. (For the root element,
which has no parent, the values of the writing mode of the element is used
instead.)
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 padding and border on the same side as the before margin is substituted for the top padding and border, and the padding and border on the same side as the after margin for the bottom padding and border. Note this means only before and after margins ever collapse.
The parent element is used instead of the containing block, because the benefit of using the containing block is very rare, but the cost to implement it is rather high for implementations that do logical-physical mapping at cascade time.
The start and end directions are also used for inline layout as follows:
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 table summarizes the abstract-to-physical mappings:
‘writing-mode ’
| ‘horizontal-tb ’
| ‘vertical-rl ’
| ‘vertical-lr ’
| |||||||
---|---|---|---|---|---|---|---|---|---|---|
‘text-orientation ’
| — | ‘rotate-left ’
| *right | ‘rotate-left ’
| *right | |||||
‘direction ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
| ‘ltr ’
| ‘rtl ’
|
length | 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 |
The following values are purely physical in their definitions and do not respond to changes in writing mode:
background-repeat
’ other than ‘repeat-y
’ and ‘repeat-x
’ (described above)
rect()
’ notation of the
‘clip
’ property [CSS21]
box-shadow
’ and ‘text-shadow
’ properties
caption-side
’ propertyThis 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]
Implementations that only support the ‘top
’ and ‘bottom
’ values of the ‘caption-side
’ property must treat them as
‘before
’, when
the table is in a vertical writing mode.
For implementations that support side captions (i.e. the ‘left
’ and ‘right
’ values), 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.
text-combine
’ propertyName: | text-combine |
---|---|
Value: | none | [ horizontal <number>? ] |
Initial: | none |
Applies to: | non-replaced inline elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
This property allows the combination of multiple characters into the space of a single character. 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. Values have the following meanings:
In vertical writing mode, attempt to display the text contents of the element horizontally within the vertical line box, ideally within the space of one ideographic character (1em square), as follows:
If the UA has compressed glyphs available for the contents of the element, then it should use those glyphs to attempt sizing the contents to 1em square. For example, a two digit number should use halfwidth or proportional glyphs, a three-digit number use 1/3-em glyphs (if available, else halfwidth glyphs), etc. The glyphs are stacked horizontally (similar to the contents of an inline-box with a horizontal writing mode and a line-height of 1em) and the baseline of the resulting composition chosen such that it is centered between the content edges of its parent inline box.
If the size of the composed glyph exceeds the element's used
line-height, then { the UA may scale the
contents to fit (else overflow the line) | the contents are
instead rendered as if ‘text-combine
’ were ‘none
’ }
In horizontal mode, or if the number of grapheme clusters in the
element exceeds the number specified (if any), this value is equivalent
to ‘none
’.
In East Asian documents, the ‘text-combine:
upright
’ 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
In Japanese, this effect is known as tate-chu-yoko.
Some people have requested a way to have numbers automatically text-combine'd. Maybe a text-auto-combine property? Note that whether a number should be tate-chu-yoko'd is often context-sensitive: this would give very weird results when applied to an arbitrary paragraph.
John Daggett, Martin Heijdra, Paul Nelson, Michel Suignard, Steve Zilles
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; }