CSS Writing Modes Module Level 3

Abstract

This module specifies the text layout model in CSS and the properties that control it. It covers bidirectional and vertical text.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.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.

Table of Contents

• 1. Introduction
  • 1.1. Introduction to Vertical Text
• 2. Writing Modes and Terminology
• 3. Inline Direction and Bidirectionality
  • 3.1. Specifying Directionality: the ‘direction’ property
  • 3.2. Embeddings and Overrides: the ‘unicode-bidi’ property
  • 3.3. Example of Bidirectional Text
  • 3.4. Box model for inline elements in bidirectional context
• 4. Block Flow Direction: the ‘writing-mode’ property
  • 4.1. Box Layout in Vertical Writing Modes
• 5. Line Orientation
  • 5.1. Orienting Text: the ‘text-orientation’ property
• 6. Abstract Box Layout
  • 6.1. Logical vs Physical Dimensions
    • 6.1.1. Orthogonal Flows
  • 6.2. Logical vs Physical Directions
    • 6.2.1. Logical Directions
    • 6.2.2. Line-Relative Directions
    • 6.2.3. Abstract-to-Physical Mappings
    • 6.2.4. Purely Physical Properties
    • 6.2.5. The ‘caption-side’ property
• 7. Glyph Composition: the ‘text-combine’ property
• Acknowledgements
• Appendix B: Bidi Rules for HTML
• References
  • Normative references
  • Other references

1. Introduction

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.

1.1. Introduction to Vertical Text

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:

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.

2. Writing Modes and Terminology

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:

• 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.

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.

See Unicode Technical Note #22 [UTN22] (HTML version) for a more in-depth introduction to writing modes and vertical text.

3. Inline Direction and Bidirectionality

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.

3.1. Specifying Directionality: the ‘direction’ property

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:

ltr

Left-to-right directionality.

rtl

Right-to-left directionality.

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.

3.2. Embeddings and Overrides: the ‘unicode-bidi’ property

Values for this property have the following meanings:

normal

The element does not open an additional level of embedding with respect to the bidirectional algorithm. For inline-level elements, implicit reordering works across element boundaries.

embed

If the element is inline-level, this value opens 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.

bidi-override

For inline-level elements this creates an override. For block-level, table-cell, table-caption, or inline-block elements this creates an override for inline-level descendants not within another block-level, table-cell, table-caption, or inline-block element. This means that inside the element, reordering 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.

isolate

For the purposes of the Unicode bidirectional algorithm, the contents of the element are considered to be inside a separate, independent paragraph, and for the purpose of bidi resolution in its containing bidi paragraph (if any), the element itself is treated as if it were an Object Replacement Character (U+FFFC). (If the element is broken across multiple lines, then each box of the element is treated as an Object Replacement Character.)

plaintext

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.

3.3. Example of Bidirectional Text

<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>
    

/* 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;}
    

               5WERBEH 4WERBEH english3 2WERBEH 1WERBEH

                                8WERBEH 7WERBEH 6WERBEH

english9 english10 english11 13WERBEH 12WERBEH

english14 english15 english16

english17 20WERBEH english19 18WERBEH
    

       2WERBEH 1WERBEH
  -EH 4WERBEH english3
                 5WERB

   -EH 7WERBEH 6WERBEH
                 8WERB

english9 english10 en-
glish11 12WERBEH
13WERBEH

english14 english15
english16

english17 18WERBEH
20WERBEH english19
    

3.4. Box model for inline elements in bidirectional context

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.

4. Block Flow Direction: the ‘writing-mode’ property

This property sets the block flow direction. Possible values:

horizontal-tb

Top-to-bottom block flow. The writing mode is horizontal.

vertical-rl

Right-to-left block flow. The writing mode is vertical.

vertical-lr

Left-to-right block flow. The writing mode is vertical.

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:

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:

• 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 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.

4.1. Box 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. 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.)

5. Line Orientation

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’.

5.1. Orienting Text: the ‘text-orientation’ property

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:

vertical-right

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.

upright

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’.

rotate-right

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.

rotate-left

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.

rotate-normal

This value is equivalent to ‘rotate-right’ in ‘vertical-rl’ writing mode and equivalent to ‘rotate-left’ in ‘vertical-lr’ writing mode.

auto

[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?

6. Abstract Box 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:

6.1. Logical vs Physical Dimensions

block flow 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.

inline-axis

The axis in the block flow dimension, i.e. the vertical axis in horizontal writing modes and the horizontal axis in vertical writing modes.

block-axis

The axis in the inline dimension, i.e. the horizontal axis in horizontal writing modes and the vertical axis in vertical writing modes.

length or logical height

A measurement in the block flow 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.)

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: 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.

6.1.1. Orthogonal Flows

When an element has a different ‘writing-mode’ from its containing block two cases are possible:

• The two writing modes are parallel to each other. (For example, ‘vertical-rl’ and ‘vertical-lr’).
• The two writing modes are perpendicular to each other. (For example, ‘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.

6.2. Logical vs Physical Directions

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.

6.2.1. Logical Directions

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.

                  <---- 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
    

                  <--- 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:

• 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.

6.2.2. Line-Relative Directions

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 ‘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 ‘float’ property [CSS21]
• the ‘clear’ property [CSS21]

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]

6.2.3. Abstract-to-Physical Mappings

The following table summarizes the abstract-to-physical mappings:

6.2.4. Purely Physical Properties

The following values are purely physical in their definitions and do not respond to changes in writing mode:

• all values of ‘background-repeat’ other than ‘repeat-y’ and ‘repeat-x’ (described above)
• the ‘rect()’ notation of the ‘clip’ property [CSS21]
• the offsets of the ‘box-shadow’ and ‘text-shadow’ properties

6.2.5. The ‘caption-side’ property

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]

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.

7. Glyph Composition: the ‘text-combine’ property

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:

none

No special processing.

horizontal

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’.

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.

Acknowledgements

John Daggett, Martin Heijdra, Paul Nelson, Michel Suignard, Steve Zilles

Appendix B: Bidi Rules for HTML

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; }
  

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 8 September 2009. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2009/CR-CSS2-20090908

[SVG11]

Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 22 June 2010. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2010/WD-SVG11-20100622

[UAX29]

Mark Davis. Text Boundaries. 25 March 2005. Unicode Standard Annex #29. URL: http://www.unicode.org/unicode/reports/tr29/tr29-9.html

Other references

[CSS3PAGE]

Melinda Grant; Håkon Wium Lie. 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]

Paul Nelson; Elika J. Etemad. CSS Text Level 3. 6 March 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-text-20070306

[HTML401]

David Raggett; Ian Jacobs; Arnaud Le Hors. HTML 4.01 Specification. 24 December 1999. W3C Recommendation. URL: http://www.w3.org/TR/1999/REC-html401-19991224

[UTN22]

Elika J. Etemad. Robust Vertical Text Layout. 25 April 2005. Unicode Technical Note #22. URL: http://unicode.org/notes/tn22/