Title: CSS Ruby Annotation Layout Module Level 1
Shortname: css-ruby
Level: 1
Status: ED
Work Status: Revising
Group: csswg
ED: https://drafts.csswg.org/css-ruby-1/
TR: https://www.w3.org/TR/css-ruby-1/
Test Suite: https://w3c.github.io/i18n-tests/results/css-ruby
Previous Version: https://www.w3.org/TR/2021/WD-css-ruby-1-20211202/
Previous Version: https://www.w3.org/TR/2021/WD-css-ruby-1-20210310/
Previous Version: https://www.w3.org/TR/2020/WD-css-ruby-1-20200429/
Previous Version: https://www.w3.org/TR/2014/WD-css-ruby-1-20140805/
Previous Version: https://www.w3.org/TR/2013/WD-css3-ruby-20130919/
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Koji Ishii, Google, kojiishi@gmail.com, w3cid 45369
Editor: Xidorn Quan, Mozilla https://www.mozilla.org/, https://www.upsuper.org/, w3cid 73936
Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net/, w3cid 43241
Abstract: “Ruby”, a form of interlinear annotation, are short runs of text alongside the base text.
Abstract: They are typically used in East Asian documents to indicate pronunciation or to provide a short annotation.
Abstract: This module describes the rendering model and formatting controls related to displaying ruby annotations in CSS.
spec:css-text-3; type:dfn; text:character
spec:css-box-4; type:dfn; text:content area
Introduction
What is ruby?
This subsection is not normative.
Ruby is the commonly-used name for a run of text
that appears alongside another run of text (referred to as the “base”)
and serves as an annotation or a pronunciation guide associated with that run of text.
A run of Japanese text with phonetic ruby annotations indicating the pronunciation of each ideograph.
The following figures show two examples of Ruby,
a simple case and one with more complicated structure.
In this example, a single annotation is used to annotate a base text
consisting of multiple characters.
Example of ruby used in Japanese (simple case)
In Japanese typography, this case is sometimes called
taigo ruby (Japanese: 対語ルビ, i.e. per-word ruby) or group-ruby,
because the annotation as a whole is associated
with multi-character word (as a whole).
In this second example,
two levels of annotations are attached to the base text:
the hiragana characters on top refer to the pronunciation of each of the base kanji characters,
while the words “Keio” and “University” on the bottom are give the English translation.
Complex ruby with annotation text over and under the base characters
Notice that to reflect the correct association
of hiragana characters and their corresponding Kanji base characters,
the spacing within the base-level text is adjusted.
(This happens around the fourth Kanji character in the figure above,
which has a three-character phonetic gloss.)
To avoid variable spacing of the base text in the example above,
the hiragana annotations can be styled as a [=merged annotation=],
which will look more like the group-ruby example earlier.
However because the base-annotation [=pairings=] are recorded in the ruby structure,
if the text breaks across lines, the annotation characters will stay
correctly paired with their respective base characters.
In HTML, ruby structure and markup to represent it is described
in the Ruby Markup Extension specification.
This module describes the CSS rendering model
and formatting controls relevant to ruby layout of such markup.
A more in-depth introduction to [=Ruby=] and its formatting
can be found in the “What is Ruby“ article [[QA-RUBY]].
Extensive information about the main ways [=ruby=] has traditionally been formatted in Japanese
can be found in JIS X-4051 [[JIS4051]] (in Japanese)
and in “Ruby and Emphasis Dots” in Requirements for Japanese Text Layout [[JLREQ]] (in English and Japanese);
Rules for Simple Placement of Japanese Ruby [[SIMPLE-RUBY]]
also describes (in English) one possible approach for Japanese ruby formatting.
“Interlinear annotations” in Requirements for Chinese Text Layout [[CLREQ]]
describes the related practices for Chinese typography (in Chinese and English).
Module interactions
This module extends the inline box model of CSS Level 2 [[!CSS2]]
to support ruby.
None of the properties in this module apply to the ''::first-line'' or
''::first-letter'' pseudo-elements;
however 'ruby-position' can inherit through ''::first-line'' to affect
ruby annotations on the first line.
Value Definitions
This specification follows the CSS property definition conventions from [[!CSS2]]
using the value definition syntax from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the [=CSS-wide keywords=] keywords as their property value.
For readability they have not been repeated explicitly.
Diagram conventions
This subsection is not normative.
Many typographical conventions in East Asian typography depend
on whether the character rendered is wide (CJK) or narrow (non-CJK).
There are a number of illustrations in this document
for which the following legend is used:
Wide-cell glyph (e.g. Han) that is the nth character in the text sequence.
They are typically sized to 50% when used as annotations.
Narrow-cell glyph (e.g. Roman) which is the nth glyph in the text sequence.
The orientation which the above symbols assume in the diagrams
corresponds to the orientation that the glyphs they represent
are intended to assume when rendered by the user agent.
Spacing between these characters in the diagrams is incidental,
unless intentionally changed to make a point.
Ruby Box Model
The CSS ruby model is based on
the W3C HTML5 Ruby Markup model
and the XHTML Ruby Annotation Recommendation [[RUBY]].
In this model, a ruby structure consists of
one or more [=ruby base=] elements representing the base (annotated) text,
associated with one or more levels of [=ruby annotation=] elements representing the annotations.
The structure of ruby is similar to that of a table:
there are “rows” (the [=base text level=], each [=annotation level=])
and “[=columns=]” (each [=ruby base=] and its corresponding [=ruby annotations=]).
The Japanese compound word “上手” annotated with its pronunciation “じょうず”. Each syllable is associated individually with its base character.
Sets of consecutive bases and their consecutive annotations are grouped together into [=ruby segments=].
Within a [=ruby segment=], a [=ruby annotation=] may span multiple [=ruby bases=].
The Chinese city name “旧金山” annotated with its pronunciation in Pinyin “jiùjīnshān” and its English name “San Francisco”. Each Pinyin syllable is associated individually with its base character, however the English name is associated with the name as a whole.
Note: In HTML, a single <ruby> element may contain multiple [=ruby segments=].
(In the XHTML Ruby model, a single <ruby> element can only contain one [=ruby segment=].)
Ruby-specific 'display' Values
For document languages (such as XML applications) that do not have pre-defined ruby elements,
authors must map document language elements to ruby elements;
this is done with the 'display' property.
The following new 'display' values assign ruby layout roles to an arbitrary element:
ruby
Specifies that an element generates a ruby container box.
(Corresponds to HTML/XHTML <ruby> elements.)
ruby-base
Specifies that an element generates a ruby base box.
(Corresponds to HTML/XHTML <rb> elements.)
ruby-text
Specifies that an element generates a ruby annotation box.
(Corresponds to HTML/XHTML <rt> elements.)
ruby-base-container
Specifies that an element generates a ruby base container box.
(Corresponds to XHTML <rbc> elements; generated as an anonymous box in HTML.)
ruby-text-container
Specifies that an element generates a ruby annotation container box.
(Corresponds to HTML/XHTML <rtc> elements.)
Authors using a language (such as HTML)
that supports dedicated ruby markup
should use that markup rather than
styling arbitrary elements (like <span>)
with ruby 'display' values.
Using the correct markup ensures that screen readers
and non-CSS renderers can interpret the ruby structures.
Note: [=Replaced elements=] with a 'display' value of
''ruby-base'',
''ruby-text'',
''ruby-base-container'',
and ''ruby-text-container''
are treated as [=inline-level boxes=],
as per [[css-display-3#layout-specific-display]];
[=replaced elements=] with a 'display' value of ''ruby'' or ''inline ruby''
behave according to their [=outer display type=],
as per [[css-display-3#outer-role]].
(See also [[#block-ruby]].)
The Ruby Formatting Context
[=Ruby containers=] are non-atomic [=inline-level=] boxes.
Like regular [=inline boxes=] (see [[css-inline-3#model]]),
they break across lines,
and their [=containing block=] is the nearest [=block container=] ancestor.
And just as the contents of an [=inline box=]
participate in the same [=inline formatting context=] that contains the [=inline box=] itself,
a [=ruby container=] and its base-level contents
participate in the same [=inline formatting context=] that contains the [=ruby container=] itself.
However [=ruby containers=] also establish a ruby formatting context
that builds further structure around their segment of the [=inline formatting context=]
in order to host their annotations.
Note: this [=formatting context=] is not an [=independent formatting context=].
[=Ruby bases=], [=ruby annotations=], [=ruby base containers=], and [=ruby annotation containers=]
are internal ruby boxes:
like [=internal table elements=],
they have specific roles in ruby layout,
and participate in their [=ruby container=]’s [=ruby formatting context=].
In addition to their role in the [=ruby formatting context=],
[=ruby bases=] simultaneously participate
in the same base-level [=inline formatting context=] as the [=ruby container=],
while [=ruby annotations=] participate
in separate annotation-level [=inline formatting contexts=]
established by the [=ruby container=].
As with the contents of [=inline boxes=],
the [=containing block=] for the contents of a [=ruby container=] (and all its [=internal ruby boxes=])
is the [=containing block=] of the [=ruby container=].
So floats, for example, are trapped by the [=ruby container=]’s [=containing block=],
not any of the ruby box types.
Non-Inline Ruby
If an element has an [=inner display type=] of ''ruby''
and an [=outer display type=] other than ''display/inline'',
then it generates two boxes:
a principal box of the required [=outer display type=] type,
and an inline-level [=ruby container=].
All properties specified on the element apply to the principal box
(and if inheritable, inherit to the [=ruby container box=]).
This allows styling the element as a block,
while correctly maintaining the internal ruby structure.
Note: Absolute positioning or floating an element causes its 'display' value
to compute to a block-level equivalent. (See [[!CSS-DISPLAY-3]] or [[!CSS2]] section 9.7.)
For the [=internal ruby display types=],
this causes their 'display' value to compute to ''display/block''.
Anonymous Ruby Box Generation
The CSS model does not require that the document language
include elements that correspond to each of these components.
Missing parts of the structure are implied through the anonymous box generation rules
similar to those used to normalize tables. [[!CSS2]]
[=Inlinify=] block-level boxes:
Any in-flow boxes directly contained by a
[=ruby container=],
[=ruby base container=],
[=ruby annotation container=],
[=ruby base box=],
or [=ruby annotation box=]
are “[=inlinified=]” per [[!CSS-DISPLAY-3]]),
and their 'display' value computed accordingly,
so that they contain only inline-level content.
For example,
the 'display' property of an in-flow element with ''display: block''
parented by an element with ''display: ruby-text''
computes to ''inline-block''.
Generate anonymous ruby containers:
Any consecutive sequence of
improperly-contained
[=ruby base containers=],
[=ruby annotation containers=],
[=ruby bases=],
and/or
[=ruby annotations=]
(and any intervening [=white space=])
is wrapped in an anonymous [=ruby container=].
For the purpose of this step:
an improperly-contained [=ruby base=] is one not parented by a [=ruby base container=] or [=ruby container=]
an improperly-contained [=ruby annotation=] is one not parented by a [=ruby annotation container=] or [=ruby container=]
an improperly-contained [=ruby base container=] or [=ruby annotation container=]
is one not parented by a [=ruby container=]
Wrap misparented inline-level content:
Any consecutive sequence of text and inline-level boxes
directly parented by a [=ruby container=] or [=ruby base container=]
is wrapped in an anonymous [=ruby base=].
Similarly, any consecutive sequence of text and inline-level boxes
directly parented by a [=ruby annotation container=]
is wrapped in an anonymous [=ruby annotation=].
(For this purpose, misparented [=internal table elements=]
are treated as [=inline-level content=]
since, being parented by ruby boxes,
they will be ultimately wrapped by an [=inline-level=] [=table wrapper box=].)
However, if an anonymous box so constructed contains only [=white space=],
it is considered intra-ruby white space
and is either discarded
or preserved
as described below.
Trim leading/trailing white space:
Any [=intra-ruby white space=]
that is not the sole [=in-flow=] child of its parent
and is the first or last [=in-flow=] child of
a [=ruby container=], [=ruby annotation container=], or [=ruby base container=]
is removed, as if it had ''display: none''
Remove inter-level white space:
Any [=intra-ruby white space=]
whose immediately adjacent [=in-flow=] siblings match one of the patterns below
is inter-level white space
and is removed, as if it had ''display: none''.
Previous box
Next box
any
[=ruby annotation container=]
not [=ruby annotation=]
[=ruby annotation=]
Interpret intra-level white space:
Any [=intra-ruby white space=] box
whose immediately adjacent [=in-flow=] siblings match one of the patterns below
is assigned the box type and subtype defined in the table below:
Previous box
Next box
Box type
Subtype
[=ruby base=]
[=ruby base=]
[=ruby base=]
inter-base white space
[=ruby annotation=]
[=ruby annotation=]
[=ruby annotation=]
inter-annotation white space
[=ruby annotation=] or [=ruby annotation container=]
[=ruby base=] or [=ruby base container=]
[=ruby base=]
inter-segment white space
[=ruby base=] or [=ruby base container=]
[=ruby base container=]
[=ruby base container=]
[=ruby base=] or [=ruby base container=]
The intra-level white space boxes defined above
are treated specially for [=pairing=] and layout.
See below.
Suppress line breaks:
Convert all forced line breaks inside [=ruby annotations=] (regardless of 'white-space' value)
as defined for collapsible segment breaks in CSS Text Level 3 § 4.1.2.
Issue: The goal of this is to simplify the layout model by suppressing any line breaks within ruby annotations.
Alternatively we could try to define some kind of acceptable behavior for them.
Generate anonymous level containers:
Any consecutive sequence of [=ruby bases=] and [=inter-base white space=]
(and not [=inter-segment white space=])
not parented by a [=ruby base container=]
is wrapped in an anonymous [=ruby base container=].
Similarly, any consecutive sequence of [=ruby annotations=] and [=inter-annotation white space=]
not parented by a [=ruby annotation container=]
is wrapped in an anonymous [=ruby annotation container=].
Once all ruby layout structures are properly parented,
the UA can start to associate bases with their annotations.
Note: The UA is not required to create any of these anonymous boxes
(or the anonymous empty [=intra-level white space=] boxes in [[#ruby-pairing]])
in its internal structures,
as long as [=pairing=] and layout behaves as if they existed.
The following markup diagram representing the various boxes
shows where [=intra-ruby white space=] is preserved or discarded:
<ruby>×<rbc>×<rb></rb>⬕<rb></rb>×</rbc>☒<rtc>×<rt></rt>⬔<rt></rt>×</rtc>◼<rbc>×<rb></rb>…</rtc>×</ruby>
where
* × represents discarded leading/trailing white space
* ☒ represents discarded inter-level white space
* ⬕ represents [=inter-base white space=]
* ⬔ represents [=inter-annotation white space=]
* ◼ represents [=inter-segment white space=]
Annotation Pairing
Annotation pairing is the process of associating
[=ruby annotations=] with [=ruby bases=].
Each [=ruby annotation=] is associated with one or more [=ruby bases=],
and is said to span those bases.
(A [=ruby annotation=] that [=spans=] multiple bases is called
a spanning annotation.)
A [=ruby base=] can be associated with
only one [=ruby annotation=] per [=annotation level=].
However, if there are multiple [=annotation levels=],
it can be associated with multiple [=ruby annotations=].
Once [=pairing=] is complete, ruby columns are defined,
each represented by a single [=ruby base=]
and one [=ruby annotation=] (possibly an empty, anonymous one)
from each [=interlinear=] [=annotation level=] in its [=ruby segment=].
Segment Pairing and Annotation Levels
A ruby structure is divided into ruby segments,
each consisting of a single [=ruby base container=]
followed by one or more [=ruby annotation containers=].
Each [=ruby annotation container=] in a [=ruby segment=]
represents one level of annotation for the base text:
the first one represents the first level of annotation,
the second one represents the second level of annotation,
and so on.
The [=ruby base container=] represents the base level.
The [=ruby base container=] in each segment is thus paired
with each of the [=ruby annotation containers=] in that segment.
In order to handle degenerate cases, some empty anonymous containers are assumed:
If the first child of a [=ruby container=] is a [=ruby annotation container=],
an anonymous, empty [=ruby base container=] is assumed to exist before it.
Similarly, if the [=ruby container=] contains consecutive [=ruby base containers=],
anonymous, empty [=ruby annotation containers=] are assumed to exist between them.
[=Inter-segment white space=] is effectively a [=ruby segment=] of its own.
Unit Pairing and Spanning Annotations
Within a [=ruby segment=],
each [=ruby base=] in the [=ruby base container=]
is paired with one [=ruby annotation=]
from each [=ruby annotation container=] in its [=ruby segment=].
If a [=ruby annotation container=] contains only
a single, anonymous [=ruby annotation=],
then that [=ruby annotation=] is paired with (i.e. [=spans=] across)
all of the [=ruby bases=] in its [=ruby segment=].
Otherwise, each [=ruby annotation=] is paired,
in document order, with the corresponding [=ruby base=] in that segment.
If there are not enough [=ruby annotations=] in a [=ruby annotation container=],
the remaining [=ruby bases=] are paired with anonymous empty annotations
inserted at the end of the [=ruby annotation container=].
If there are not enough [=ruby bases=],
any remaining [=ruby annotations=] pair with empty, anonymous bases
inserted at the end of the [=ruby base container=].
If an implementation supports ruby markup with explicit spanning
(e.g. XHTML Complex Ruby Annotations),
it must adjust the [=pairing=] rules to pair [=spanning annotations=]
to their bases appropriately.
[=Intra-level white space=] does not participate in standard annotation [=pairing=].
However, if the immediately-adjacent [=ruby bases=] or [=ruby annotations=]
are paired
with two [=ruby bases=] or [=ruby annotations|annotations=]
that surround corresponding [=intra-level white space=] in another level,
then the so-corresponding [=intra-level white space=] boxes are also paired.
with a single spanning [=ruby annotation=],
then the [=intra-level white space=] is also paired to that [=ruby annotation=]
with two [=ruby bases=] or [=ruby annotations|annotations=]
with no intervening [=intra-level white space=],
then the [=intra-level white space=] box pairs with
an anonymous empty [=intra-level white space=] box assumed to exist between them.
The following diagram shows the pairing of
regular [=base=] and [=annotation=] boxes
as well as the pairing of [=intra-ruby white space=] (represented as ws):
|[ s p a n n i n g a n n o t a t i o n ]||[ a1 ]|[ws]|[ a2 ]|[]|[ a3 ]|[ws]|[ a4 ]||[ b1 ]|[ws]|[ b2 ]|[ws]|[ b3 ]|[]|[ b4 ]|
Blue brackets ([ ]) represent [=base boxes=],
red brackets ([ ]) represent [=annotation boxes=],
gray bars (|) represent the limits of [=ruby columns=],
[ws] represents [=intra-ruby white space=],
and [] represents an empty anonymous base or annotation
automatically generated to pair with [=intra-ruby white space=] in another level.
[=Ruby containers=], [=base containers=], and [=annotation containers=] are omitted.
Hiding Annotations: ''visibility: collapse'' and auto-hidden ruby
A [=ruby annotation=] whose 'visibility' is ''visibility/collapse''
is a hidden annotation.
Additionally,
if a [=ruby annotation=] has the exact same text content as its base,
it is automatically [=hidden=]
(auto-hidden)
by the UA.
[=Hiding=] a [=ruby annotation=] does not affect annotation [=pairing=].
However the [=hidden annotation=] is not visible,
and it has no impact on layout
other than to separate adjacent sequences of [=ruby annotation boxes=] within its level,
as if they belonged to separate segments
and the [=hidden annotation=]’s base were not a [=ruby base=] but an intervening inline.
Auto-hiding allows correct inlined display of annotations
for Japanese words that are a mix of kanji and hiragana.
For example, the word 振り仮名 should be inlined as
However, when displayed as ruby, the “り” should be hidden
Hiragana ruby for 振り仮名. Notice there is no hiragana annotation above り, since it is already in hiragana.
The Japanese word in this example is composed of three kanji characters,
one of which is taught in first grade,
while the other two are more advanced.
(Characters colored to show base-pair correspondences.)
<ruby><rb>昆<rb>虫<rb>記<rt>こん<rt class=easy>ちゅう<rt>き</ruby>
Fully annotated three-character word
Although some readers might need pronunciation guidance
on all three characters,
for other audiences it is more appropriate
to hide the annotation on the easier character.
Applying ''visibility: collapse'' enables this hiding:
Middle annotation as ''visibility: collapse''
The [=hiding=] behavior of ''visibility: collapse''
differs from ''visibility: hidden''--
which makes the annotation invisible,
but does not remove its impact on layout:
Middle annotation as ''visibility: hidden''
It also differs from ''display: none''
because ''visibility: collapse'' preserves pairing relationships,
whereas ''display: none'' removes the box from the tree entirely,
disturbing the pairing of any annotations after it:
Middle annotation as ''display: none''
When the [=computed value=] of 'ruby-merge' on the [=annotation container=]
is ''merge'',
[=hiding=] is disabled.
When that value is ''ruby-merge/auto'',
the user agent may decide whether to disable [=hiding=] of its annotations,
but it is recommended to enable [=hiding=]
if the user agent’s layout algorithm
produces the results similar to ''separate''.
The content comparison for [=auto-hiding=]
takes place prior to white space collapsing ('white-space') and text transformation ('text-transform')
and ignores elements (considers only the textContent of the boxes).
Note: Future levels of CSS Ruby may add controls for [=auto-hiding=],
but in this level it is always forced.
White Space Collapsing
As discussed in [[#box-fixup]],
white space within a ruby structure is discarded:
at the beginning and end of a [=ruby container=], [=annotation container=], or [=base container=],
between a [=base container=] and its following [=annotation container=],
between [=annotation containers=].
For example, the following markup will display without any spaces:
Between [=ruby segments=], between [=ruby bases=], and between [=ruby annotations=], however,
white space is not discarded,
and is maintained for rendering
as [=inter-base white space|inter-base=],
[=inter-annotation white space|inter-annotation=],
or [=inter-segment white space=].
(See Interpret intra-level white space, above.)
The rules preserving white space allow ruby to be used with space-separated scripts such as Latin.
For example,
Where undiscarded white space is [=collapsible white space|collapsible=],
it will collapse across adjacent boxes in each line box
following the standard white space processing rules [[!CSS-TEXT-3]].
Annotations in separate [=ruby segments=]
or separated by [=hidden annotations=]
are not considered adjacent;
however all base-level content
(including [=inter-character annotations=],
which are treated as [=atomic inlines=])
is.
For [=collapsible white space=] between [=ruby segments=] ([=inter-segment white space=]),
the contextual text for determining [[css-text-3#line-break-transform|segment break transformations]]
is thus given by the [=ruby bases=] on either side,
not necessarily the text on either side of the white space in source document order
(which could include [=interlinear annotations=]).
Note: The white space processing rules
cause a white space sequence containing a [=segment break=] (such as a line feed)
to collapse to nothing between Han and Kana characters.
This means that Chinese and Japanese ruby can safely use white space for indentation of ruby markup.
For example, the following markup will display without any spaces:
However, white space that does not contain a [=segment break=] does not collapse completely away,
so this markup will display with a space between the first and second ruby pairs:
When a ruby structure is laid out,
its [=base level=] is initially laid out on the line
exactly as if its [=ruby bases=] were a regular sequence of [=inline boxes=]
and the [=ruby container=] a regular [=inline box=] wrapped around it.
If a [=ruby container=] has any [=inter-character annotations=],
they are laid out into the [=base level=] as detailed in [[#inter-character-layout]].
Subsequently,
the [=base container=] is sized and
[=interlinear annotations=] are laid out
as detailed in [[#interlinear-layout]].
As in other CSS layout models,
relative positioning, transforms, and other graphical effects
apply after this layout of the boxes.
Interlinear Ruby Layout
[=Interlinear=] [=ruby annotations=] within a level
are initially laid out as if they were [=inline boxes=]
participating in the same [=inline formatting context=],
effectively establishing a [=line box=]
for that [=level=] of annotation in the [=ruby container=].
[=Annotations=] and [=bases=] are aligned to each other
by adjusting their spacing as described below.
Inline-axis Interlinear Layout
In the inline axis, [=interlinear=] [=ruby annotations=] are aligned
with respect to their [=ruby base boxes=]
in accordance with their [=annotation container=]’s 'ruby-merge' value.
When 'ruby-merge' is ''ruby-merge/separate'',
each [=ruby column=] is sized
to the widest content ([=ruby base=] or [=ruby annotation=]) in that [=column=].
In the case of [=spanning annotations=]
(whether actually spanning or pretending to span per ''ruby-merge: merge''),
if the content of a [=spanning annotation=] would be wider
than the [=columns=] that it spans,
then the difference is distributed equally among the spanned [=columns=].
If a [=ruby segment=] contains multiple [=spanning annotations=],
this distribution of additional space
is performed starting with the [=spanning annotations=]
that span the least number of bases,
and then in increasing number of bases spanned.
Each [=ruby base=] and [=ruby annotation=]
is then sized to exactly span its column(s).
Note: If there are multiple annotations in different levels spanning the same number of bases
that overlap but do not coincide,
the distribution of space is undefined.
Note this is not possible with HTML markup,
but can happen in markup languages with explicit spanning such as in [[RUBY]].
If any [=ruby annotation=] in an [=annotation container=] with ''ruby-merge: auto''
is wider than its [=ruby base|base=],
then [=ruby annotations=] in that [=annotation container=]
may extend outside of their [=column=](s).
When they do, their influence on
the measure of the [=columns=] they intersect
is up to the user agent,
provided that the [=ruby segment=] is made wide enough to fit all its content.
[=Inter-character=] annotations are interleaved between columns:
they factor into measurement of annotations that span both adjacent columns,
but are not included in either column
and are never affected by the sizing or positioning
of [=interlinear annotations=].
Within each [=base=] and [=annotation=] box,
how the extra space is distributed
when its content is narrower than the measure of the box
is specified by its 'ruby-align' property.
The following diagrams illustrate these rules,
covering typical situations
as well as a few more complex ones.
In each case,
[=base boxes=] and [=annotation boxes=] are assumed to have ''ruby-align: center'',
while [=annotation containers=] are assumed to have ''ruby-align: space-between''.
Blue brackets ([ ]) represent [=base boxes=],
red brackets ([ ]) represent [=annotation boxes=],
and gray bars (|) represent the limits of [=ruby columns=].
[=Ruby containers=], [=base containers=], and [=annotation containers=] are omitted.
Separate / non-spanning:
|[ a1 ]|[ a2 ]|[annotation-3]||[base 1]|[base 2]|[ base 3 ]|
Boxes within each [=column=] are sized to match the widest box of that [=column=].
The value of 'ruby-align' on each [=base box=] and [=annotation box=] is used
to distribute the extra space in each box.
Spanning (short):
|[ a1 ]|[ short span ]||[base 1]|[base 2]|[base 3]|
When a [=spanning annotation=] is shorter than the spanned [=bases=],
there is no extra space to distribute to these [=bases=].
Extra space within the [=spanning annotation=]
is distributed according to 'ruby-align' on that [=annotation box=],
as for non spanning ones.
Spanning (long):
|[ a1 ]|[spanning annotation]||[base 1]|[ base 2 ]|[ base 3 ]|
When the [=spanning annotation=] is longer than the spanned [=base boxes=],
the extra space is distributed equally between them.
Merged (short):
|[merged annotation]||[base 1]|[base 2]|[base 3]|
A [=merged annotation=] behaves similarly to a [=spanning=] one,
except that distribution of any extra space in it
is determined by the value of 'ruby-align' on the [=annotation container=],
not on any of its [=annotation boxes=].
Several levels, with spanning annotation:
|[ a1 ]|[ annotation-2 ]|[ a3 ]||[long annotation spanning all content]||[ base 1 ]|[ base 2 ]|[ base 3 ]|
|[ xx ]|[annotation spanning bases]||[ a1 ]|[ annotation-2 ]|[ a3 ]||[base 1]|[ base 2 ]|[ base 3 ]|
In these two examples with multiple [=levels=],
each [=column=] is sized to its longest content,
and [=spanning annotations=] that are still longer
than the sum of the [=columns=] that they span
grow them further,
adding to each equally.
Several levels, with multiple spanning annotations:
|[ xx ]|[annotation spanning bases]||[ a1 ]|[annotation-2]|[ a3 ]||[lengthy annotation spanning base content]||[base 1]|[ base 2 ]|[base 3]|
When there are multiple [=spanning annotations=],
those that span fewest [=bases=] are processed first.
In this case, the green one,
which spans two bases,
is processed before the orange one,
which spans three.
Changing this order would change the distribution of space.
To help identify which [=spanning annotation=] is responsible
for which extra spacing,
in this diagram
the color of the text in each [=spanning annotation=] is matched
with the background color of spacing it adds to other boxes.
Block-axis Interlinear Layout
ISSUE: Define the extent to which 'vertical-align' affects these ruby boxes.
See Issue 4987.
Each [=base container=] is then sized and positioned
to contain exactly all of its [=ruby bases=]’ margin boxes--
and all their associated [=inter-character=] [=ruby annotation=] margin boxes, if any--
as well as the [=base containers|base=] and [=annotation containers=]
of any descendant [=ruby containers=]
also participating in this [=inline formatting context=].
(If a [=base container=] has no [=in-flow=] content,
it is sized and positioned
as if it contained a single empty [=ruby base=].)
Each [=interlinear=] [=annotation container=] is sized and positioned
to contain exactly all of its [=ruby annotations=]’ margin boxes
as well as the [=base containers|base=] and [=annotation containers=]
of any descendant [=ruby containers=]
also participating in this [=annotation level=]’s [=inline formatting context=].
(If an [=annotation container=] has no [=in-flow=] content,
it is sized and positioned
as if it contained a single empty [=ruby annotation=].)
These [=annotation containers=] are then stacked outward
over or under (depending on 'ruby-position') their corresponding [=base container=],
without any intervening space,
thus determining the block-axis positioning of the [=ruby annotations=]
with respect to their [=ruby bases=].
Issue: Should block-axis margins collapse?
This makes layout more robust,
but is inconsistent with how inlines behave along the inline-axis.
Inter-character Ruby Layout
[=Inter-character annotations=] have special layout.
[=Inter-character=] [=ruby annotation boxes=]
are spliced into and measured as part of the layout of the [=base level=].
Each [=ruby annotation=] is inserted to the right of the [=ruby base=] it is paired with;
a [=spanning=] [=inter-character annotation=] is placed after
the rightmost of all the bases that it spans.
[=Inter-character=] [=ruby annotations=] are laid out
exactly like [=inline blocks=],
except as described below.
Note: As per the definition of ''inter-character'',
[=inter-character annotations=] always have a ''vertical-rl'' 'writing-mode',
and only exist in [=horizontal writing mode|horizontal=] [=ruby containers=].
Each [=ruby annotation=] is placed immediately to the right of the relevant [=ruby base=],
before any [=inter-base white space=] (or [=inter-segment white space=]).
If multiple [=inter-character=] [=ruby annotation boxes=] are placed against the same [=ruby base=],
their [=margin boxes=] are stacked rightwards without intervening space,
in increasing [=level=] order.
If the [=automatic size|automatic height=] of the [=ruby annotation box=]’s [=content area=]
would be shorter than the height of the [=content box=]
of the [=ruby base=] after which it is placed,
its [=content box=] height is increased to match that height exactly.
The alignment of the [=ruby annotation box=]
depends on the 'ruby-align' property on the [=ruby annotation=]:
If 'ruby-align' is ''ruby-align/start'',
the top edge of the [=ruby annotation=]'s [=content box=] is aligned
with the top edge of the [=ruby base=]'s [=content box=].
Otherwise,
the center of the [=ruby annotation=]'s [=content box=] is vertically aligned
with the center of the [=ruby base=]'s [=content box=].
The [=ruby annotation container box=] of an [=inter-character=] annotation
is sized and placed so that its content box coincides exactly with that of the [=ruby base container box=].
Note: The size and position of [=inter-character=] [=ruby annotation container boxes=]
has no effect on layout.
The above definition is merely so that programmatically inquiring about them
produces deterministic results.
For the purpose of [[#ruby-align-property|alignment]]
and column sizing (as discussed in [[#ruby-layout]]),
[=inter-character=] [=ruby annotations=] are not considered
to be part of the same [=column=] as the base to with they are attached;
rather, they effectively form a [=column=] of their own.
Styling Ruby Boxes
[=Ruby bases=] and [=ruby containers=] are treated as [=inline boxes=],
and all properties that apply to [=inline boxes=],
unless otherwise specified,
also apply to them in the same way.
However, the line-relative shift values of 'vertical-align'
(''vertical-align/top'', ''vertical-align/bottom'')
must be ignored (treated as zero).
All properties that apply to [=inline boxes=],
unless otherwise specified,
also apply to [=ruby annotations=] in the same way,
except 'line-height', which does not apply.
Also, the line-relative shift values of 'vertical-align'
(''vertical-align/top'', ''vertical-align/bottom'')
must be ignored (treated as zero).
Neither the [=margin=], [=padding=], [=border=] properties
nor the any properties that do not apply to [=inline boxes=]
apply to [=base containers=] or [=annotation containers=].
Additionally, 'line-height' does not apply to [=annotation containers=].
The UA is not required to support
any of the background properties or outline properties,
or any other property that illustrates the bounds of the box
on [=ruby base container boxes=] or [=ruby annotation container boxes=].
The UA may implement these boxes simply as abstractions for inheritance
and control over the layout of their contents.
Breaking Across Lines
When there is not enough space for an entire [=ruby container=] to fit on the line,
the ruby may be broken wherever all levels simultaneously allow a break.
(See [[CSS-TEXT-3#line-breaking]] for details on line breaking.)
Ruby most often breaks between base-annotation sets,
but if the line-breaking rules allow it, can also break within a [=ruby base=]
(and, in parallel, its associated [=ruby annotation boxes=]).
Whenever ruby breaks across lines, [=ruby annotations=] must stay
with their respective [=ruby bases=].
The line must not break between a [=ruby base=] and its [=annotations=],
even in the case of [=inter-character annotations=].
''inter-character'' ruby line breaking opportunity
After line-breaking,
each fragment is [[#ruby-layout|laid out]] independently,
and [[#ruby-align-property|ruby alignment]] takes place within each fragment.
Breaking Between Bases
In typical cases, [=ruby base boxes=] and [=ruby annotation boxes=]
are styled to forbid internal line wrapping and do not contain forced breaks.
(See Appendix A.)
In such cases the [=ruby container=] can only break between adjacent [=ruby bases=],
and only if no [=ruby annotations=] span those [=ruby bases=].
Ruby line breaking opportunity
Whether ruby can break between two adjacent [=ruby bases=]
is controlled by normal line-breaking rules for the base text,
exactly as if the [=ruby bases=] were adjacent inline boxes.
(The annotations are ignored when determining soft wrap opportunities for the [=base level=].)
For example, if two adjacent ruby bases are “蝴” and “蝶”,
the line may break between them,
because lines are normally allowed to break between two Han characters.
However, if 'word-break' is ''keep-all'', that line break is forbidden.
<ruby>蝴<rt>hú</rt>蝶<rt>dié</rt>
Inter-base white space is significant for evaluating line break opportunities between [=ruby bases=].
As with white space between inlines,
it collapses when the line breaks there,
following the rules detailed in [[CSS-TEXT-3#white-space-rules]].
Similarly, annotation white space is also trimmed at a line break.
Due to the space, the line may break between “one” and “two“.
If the line breaks there, that space—and the space between “1” and “2”—disappears,
in accordance with standard CSS white space processing rules.
Breaking Within Bases
For longer base texts, it is sometimes appropriate to allow breaking within a base-annotation pair.
For example, if an English sentence is annotated with its Japanese translation,
allowing the text to wrap allows for reasonable line breaking behavior in the paragraph.
Issue: Insert scanned example so people don't think this is just the ramblings of an insane spec-writer.
Line-breaking within a [=ruby base=] is only allowed if the 'white-space' property
of the [=ruby base=] and all its parallel [=annotations=] allow it,
and there exists a [=soft wrap opportunity=] within (i.e. not at the start or end)
the content of each base/annotation box.
Since there is no structural correspondence between fragments of content
within [=ruby bases=] and [=annotations=],
the UA may break at any set of opportunities;
but it is recommended that the UA attempt to proportionally balance
the amount of content inside each fragment.
There are no line breaking opportunities within [=inter-character annotations=].
Bidi Reordering
The Unicode bidirectional algorithm reorders logically-stored text for visual presentation
when characters from scripts of opposing directionalities are mixed
within a single paragraph.
To preserve the correspondence of [=ruby annotations=]
to their respective [=ruby bases=],
a few restrictions must be imposed:
The contents of a [=ruby base=] or [=ruby annotation=] must remain contiguous.
[=Ruby annotations=] must be reordered together with their [=ruby bases=].
All [=ruby bases=] spanned by a single [=ruby annotation=] must remain contiguous.
To this end,
Bidi isolation is forced on all [=internal ruby boxes=] and the [=ruby container=]:
the ''unicode-bidi/normal'' and ''embed'' values of 'unicode-bidi' compute to ''isolate'',
and ''bidi-override'' computes to ''isolate-override''.
Note: This means that implicit bidi reordering does not work across ruby bases,
so authors will need to ensure that the [=ruby container=]’s declared directionality
does indeed match its contents.
During layout, [=ruby segments=] are ordered within the [=ruby container=]
by the 'direction' property of their [=ruby container=].
Within a segment, [=ruby bases=] and [=ruby annotations=]
that are not [=merged=]
are ordered within their respective containers
by the 'direction' property of the segment’s [=ruby base container=].
[=Merged annotations=] are ordered
by the 'direction' property of their [=annotation container=],
exactly as if they were a sequence of inline boxes within their [=annotation container=].
Note: This means the 'direction' property on [=ruby annotation containers=]
is ignored for the purpose of layout of non-[=merged=] annotations.
However, it can still inherit into the container's children
and thereby affect the [=inline base direction=]
of any [=ruby annotations=] it contains.
As with other inline-level content,
the bidi reordering of [=internal ruby boxes=] happens after line-breaking
so that content is divided across lines according to its logical order.
Note: It could be useful to adjust these rules slightly
so that when 'ruby-merge' is ''merge'' on a particular [=annotation container=],
bidi isolation is not forced onto the individual annotations,
enabling them to be processed together.
However, this would add complexity to implementations,
which does not seem justified in the absence of demand to handle this situation.
Anyone with a need for this to be handled
is encouraged to contact the CSS Working Group.
See [[CSS3-WRITING-MODES]] for a more in-depth discussion of bidirectional text in CSS.
Line Spacing
The 'line-height' property controls spacing between lines in CSS.
When inline content on line is shorter than the 'line-height',
half-leading is added on either side of the content,
as specified in [[CSS-INLINE-3#line-height]].
In order to ensure consistent spacing of lines,
documents with ruby typically ensure that the 'line-height' is large enough
to accommodate ruby between lines of text.
Therefore, ordinarily, [=ruby annotation containers=] and [=ruby annotation boxes=]
do not contribute to the measured height of a line's inline contents;
any alignment (see 'vertical-align') and line-height calculations
are performed using only the [=ruby base container=],
exactly as if it were a normal inline.
However, if the 'line-height' specified on the [=ruby container=]
is less than the distance between
the top of the top [=ruby annotation container=]
and the bottom of the bottom [=ruby annotation container=],
then additional leading is added
on the appropriate side(s) of the [=ruby base container=]
such that if a block consisted of three lines
each containing ruby identical to this,
none of the [=ruby containers=] would overlap.
Note: This does not ensure that the [=ruby annotations=] remain within the line box.
It merely ensures that if all lines had equal spacing
and equivalent amounts and positioning of [=ruby annotations=],
there would be enough room to avoid overlap.
Authors should ensure appropriate 'line-height' and 'padding' to accommodate ruby,
and be particularly careful at the beginning or end of a block
and when a line contains inline-level content
(such as images, inline blocks, or elements shifted with 'vertical-align')
taller than the paragraph's default font size.
Ruby annotations will often overflow the line;
authors should ensure content over/under a ruby-annotated line
is adequately spaced to leave room for the ruby.
Note: More control over how ruby affects alignment and line layout
will be part of the CSS Line Layout Module Level 3.
Note, it is currently in the exploratory stages of development;
descriptions of new features should not yet be relied upon.
Name: ruby-position
Value: [ alternate || [ over | under ] ] | inter-character
Initial: alternate
Applies to: ruby annotation containers
Inherited: yes
Computed value: specified keyword
Animation type: discrete
This property controls position of the [=ruby annotation=] with respect to its base.
Values have the following meanings:
alternate
Different [=levels=] of annotations alternate between ''over'' and ''under''.
If the [=annotation container=] is the first [=level=] of annotation in its [=ruby segment=],
or if all prior [=levels=] are [=inter-character=],
then ''alternate'', either on its own or in combination with ''over'',
behaves the same as ''over'',
while ''alternate'' in combination with ''under''
behaves the same as ''under''.
Otherwise,
if the previous level of [=interlinear=] annotation is ''over'',
''alternate'' behaves like ''under'',
and vice versa.
(In this case, whether ''alternate'' is specified alone
or in combination with ''over'' or ''under''
makes no difference.)
over
The ruby annotation appears [=line-over=] the base.
Ruby over Japanese base text in horizontal layout
Ruby to the right of Japanese base text in vertical layout
under
The ruby annotation appears [=line-under=] the base.
This is a relatively rare setting used in ideographic East Asian writing systems,
most easily found in educational text.
Ruby under Japanese base text in horizontal layout
Ruby to the left of Japanese base text in vertical layout
inter-character
If the [=writing mode=] of the enclosing [=ruby container=] is [=vertical writing mode|vertical=],
this value has the same effect as ''over''.
Otherwise, the ruby annotation becomes an inter-character annotation.
The annotation appears on the right of the base in horizontal text.
This forces the computed value of 'writing-mode' of the [=ruby annotation=] children of this [=ruby annotation container=] to be ''vertical-rl''.
Note: The computed value of 'writing-mode' on the [=ruby annotation container=] itself is not affected.
This is to avoid circular dependencies between computed values
on the 'writing-mode', 'display', and 'ruby-position' properties on the same element.
This value is provided for the special case of traditional Chinese
as used especially in Taiwan:
ruby (made of bopomofo glyphs) in that context
appears vertically along the right side of the base glyph,
even when the layout of the base characters is horizontal:
“Bopomofo” ruby in traditional Chinese
(ruby annotation shown in blue for clarity) in horizontal layout
Note: As inheritance works on the element tree without accounting for anonymous boxes created for ruby layout,
when using [=inter-character annotations=]
authors need to be careful to avoid markup patterns involving
an element-based [=ruby annotation container=],
an anonymous [=ruby annotation=],
and further descendant elements,
as those descendants would inherit their writing mode from the [=ruby annotation container=],
and not from the [=ruby annotation=] whose 'writing-mode' has been changed to ''vertical-rl''.
ruby { ruby-position: inter-character; }
<ruby>base<rtc><em>problematic</em> annotation</ruby>
In the above markup, an anonymous [=ruby annotation=] box is created as a child of the <rtc> element
to wrap the whole “problematic annotation”.
Since it is a child of an [=annotation container=] whose 'ruby-position' is ''inter-character'',
its 'writing-mode' will be ''vertical-rl'', which is expected.
However, the <em> element inherits its 'writing-mode' directly from the <rtc> element,
which has not been forced to ''vertical-rl''.
In this example, the explicit [=ruby annotation container=] element was not necessary,
so using a [=ruby annotation=] element instead would avoid the problem:
<ruby>base<rt><em>problematic</em> annotation</ruby>
If an explicit [=ruby annotation container=] element is needed,
then using a [=ruby annotation=] element as well would also address the problem:
<ruby>base<rtc><rt><em>problematic</em> annotation</ruby>
[=Annotation containers=] that are not [=inter-character annotations=]
are called interlinear annotations.
If multiple [=ruby annotation containers=] have the same 'ruby-position',
they stack outwards from the base text.
Sharing Annotation Space: the 'ruby-merge' property
Name: ruby-merge
Value: separate | merge | auto
Initial: separate
Applies to: [=interlinear=] ruby annotation containers
Inherited: yes
Computed value: specified keyword
Animation type: by computed value type
This property controls how ruby annotation boxes should be rendered
when there are more than one in a ruby container box:
whether each pair should be kept separate,
the annotations should be merged and rendered as a group,
or the separation should be determined based on the space available.
Note: [=Inter-character annotations=] are always separate, and this property does not apply.
Possible values:
separate
Each ruby annotation box is rendered within
the same [=column=](s) as its corresponding base box(es),
i.e. without overlapping adjacent bases on either side.
This style is called “mono ruby” in [[JLREQ]].
''ruby-merge: separate'' with center alignment
For example, the following two lines render the same:
All [=ruby annotation boxes=] within the same [=ruby segment=] on the same line are concatenated
as [=inline boxes=] within their [=annotation container=],
and laid out in a single anonymous [=ruby annotation box=]
spanning all their associated [=ruby base boxes=].
When laid out on a single line,
this style renders similar to “group ruby” in [[JLREQ]].
However, when it breaks across lines,
[=ruby annotations=] are kept together with their respective [=ruby bases=].
''ruby-merge: merge'' with center alignment
The following two lines render the same
if both characters fit on one line:
However, the second one renders the same as ''ruby-position: separate''
when the two bases are split across lines.
auto
The user agent may use any algorithm to determine how each ruby annotation box
is rendered to its corresponding base box,
with the intention that if all annotations fit over their respective bases,
the result is identical to ''separate'',
but if some annotations are wider than their bases
the space is shared in some way
to avoid imposing space between bases.
''ruby-merge: auto'' with center alignment
Note: This behavior is intended for compound words,
see “Jukugo Ruby” in “What is ruby?”. [[QA-RUBY]]
There are various conventions for rendering this type of ruby,
see Placement of Jukugo-ruby in [[SIMPLE-RUBY]],
Positioning of Jukugo-ruby in [[JLREQ]],
and 4.12.3(c) 熟語ルビの処理 in [[JISX4051]]
for examples of varying complexity.
The simplest of these is
to render as ''separate'' if all ruby annotation boxes fit
within the advances of their corresponding base boxes,
and render as ''merge'' otherwise.
Note: Text does not shape or form ligatures
across [=ruby annotations=] or [=ruby base|bases=],
even merged ones,
due to [=bidi isolation=].
See [[#bidi]] and [[css-text-3#boundary-shaping]].
Ruby Text Distribution: the 'ruby-align' property
Name: ruby-align
Value: start | center | space-between | space-around
Initial: space-around
Applies to: ruby bases, ruby annotations, ruby base containers, ruby annotation containers
Inherited: yes
Computed value: specified keyword
Animation type: by computed value type
This property specifies how text is distributed within the various ruby boxes
when their contents do not exactly fill their respective boxes.
Note that space distributed by 'ruby-align' is unrelated to, and independent of,
any space distributed due to justification.
For [=inter-character=] annotations,
this property can also affect the alignment of the box itself
(see [[#inter-character-layout]]).
It otherwise only affects the alignment of content within the box,
not the size or position of the box itself.
Values have the following meanings:
start
The ruby content is aligned with the start edge of its box.
''ruby-align/start'' ruby distribution
ISSUE: "Katatsuki ruby" (肩付きルビ) is close to,
but not quite the same as,
this ''ruby-align/start'' value.
In particular,
its behavior when overhanging its base
can differ from start alignment
depending on surrounding context,
see JLREQ.
Also, it's only ever used in vertical writing,
and the JLTF considers it not particularly important,
so it may not be worth the effort to make this value smart enough to deal with katatsuki ruby.
If ''ruby-align/start'' is needed for some other purpose,
we should keep it.
Otherwise, maybe just drop it?
center
The ruby content is centered within its box.
''ruby-align/center'' ruby distribution
space-between
The ruby content expands as defined for normal text justification
(as defined by 'text-justify'),
except that if there are no [=justification opportunities=]
the content is centered.
''ruby-align/space-between'' ruby distribution
space-around
As for ''ruby-align/space-between''
except that there exists an extra [=justification opportunities=]
whose space is distributed half before and half after the ruby content.
''ruby-align/space-around'' ruby distribution
The [[#default-ua-ruby|default UA style sheet]] applies
''text-justify: ruby'' to [=ruby annotations=],
defining [=justification opportunities=]
between every adjacent pair of CJK [=characters=]
but not between adjacent pairs of Latin or Bopomofo [=characters=].
Thus, the ''space-around'' and ''space-between'' values
will nevertheless result in Latin or Bopomofo characters centered:
Non-CJK ruby text in ''ruby-align/space-around'' and ''ruby-align/space-between'' ruby distribution are centered
The distribution of space can be controlled via 'text-justify'. [[CSS-TEXT-3]]
Content in [=ruby bases=] and [=ruby annotations=],
except those that span due to ''ruby-merge: merge'',
is aligned within its box based on the value of 'ruby-align' on that box.
Content in [=merged annotations=]
is aligned within the [=ruby annotation container=]
based on the value of 'ruby-align' on the [=annotations container=],
ignoring the value of 'ruby-align' on the individual [=ruby annotations=].
Issue: What if a [=merged annotation=] causes the [=ruby segment=] to be wider?
Does it cause each base to grow as if it were spanning them, as currently specified?
This does not allow e.g. centering the text in a multi-base base container
when its merged annotation is longer.
Instead, maybe we should allow ruby-merge to apply to base containers as well,
but this would require us either to allow a single base to span multiple annotations
(if the base is merged but some annotation levels are not),
or to require that if the base is merged, then all annotation levels must be merged as well.
The way content is aligned when 'ruby-merge' is ''ruby-merge/auto'' is up to the user agent,
but must be identical to that of ''ruby-merge: separate''
when all annotations fit over their respective bases.
Ruby Text Decoration
Text decoration does not propagate from the base text to the annotations.
When text decoration is specified on an ancestor of the ruby,
it is drawn across the entire content area of the ruby base container,
including any extra space added on either side of the ruby base contents to accommodate long annotations.
When text decoration is specified on a ruby base itself,
this extra space is not decorated,
similar to how a box's own padding is not decorated when text decoration is specified directly on that box. [[!CSS3-TEXT-DECOR]]
Text decoration may be specified directly on ruby base containers
and ruby annotation containers:
in such cases it is propagated to all of the container's bases or annotations (respectively),
and is also drawn between them for continuity.
The positions of ruby annotations may be adjusted
to avoid potential conflicts
with overline and underline decorations applied to the base text.
In the basic case of consistent font size and baseline alignment,
an underline or overline is positioned
between the [=base level=] and any annotations on that side.
Issue: This section needs some clarification about
drawing decorations between the content of adjacent bases/annotations.
Depends on if those boxes are as wide as their [=column=] or not.
Edge Effects
Overhanging Ruby: the 'ruby-overhang' property
Name: ruby-overhang
Value: auto | none
Initial: auto
Applies to: ruby annotation containers
Inherited: yes
Computed value: specified keyword
Animation type: by computed value type
The 'ruby-overhang' property controls whether [=ruby annotations=]
may overlap adjacent text outside the [=ruby container=].
Values have the following meanings:
auto
When a [=ruby annotation container=] is longer than
its corresponding [=ruby base container=],
the [=ruby annotation container=] may
partially overlap adjacent boxes.
Whether, how much, and under which conditions to overhang
are determined by the UA.
none
A [=ruby annotation container=] is never allowed
to extend past the [=ruby annotation container=].
When [=ruby annotations=] are not allowed to overhang,
the [=ruby container=] behaves like a traditional inline box,
i.e. only its own contents are rendered within its boundaries
and adjacent elements do not cross the box boundary:
Simple ruby whose text is not allowed to overhang adjacent text
However, if a [=ruby annotation container=] is allowed to overhang,
neighbouring content may overlap the [=ruby container box=],
allowing its [=ruby annotation=](s) to partially render over/under
surrounding inline-level content.
Overhang is only allowed to the extent
that it does not cause collisions between
the neighboring content
and the ruby container’s [=annotation boxes=]
or its overlapped [=base=]’s contents.
Simple ruby whose text is allowed to overhang adjacent text
Note: Whether [=ruby annotations=] related to a [=ruby base=]
can overhang another [=ruby base=] is controlled by 'ruby-merge'.
Typically,
the alignment of the contents of the base or the annotation
is not affected by overhanging behavior:
alignment and space distribution (see 'ruby-align')
is achieved the same way regardless of the overhang allowance,
and is computed before the space available for overlap is determined.
However, UAs may consider allowed overhang
when determining space distribution and/or alignment of annotations and bases.
Issue: I suspect overhanging interacts with alignment in some cases;
might need to look into this later.
The user agent may use [[JIS4051]] recommendation of
using ''0.5ic'' (half of a full-width character) in the annotation’s font
as the maximum overhang length.
Detailed rules for how ruby text can overhang adjacent characters
for Japanese are described by [[JLREQ]].
Line-edge Alignment
When a [=ruby annotation box=] that is longer than its [=ruby base=]
is at the start or end edge of a line,
the user agent may force the side of the [=ruby annotation=] that touches the edge of the line
to align to the corresponding edge of the base.
This type of alignment is described by [[JLREQ]].
This level of the specification does not provide a mechanism to control this behavior.
Line-edge alignment
Appendix A: Sample Style Sheets
This section is informative.
A.1 Default UA Style Sheet
The following represents a default UA style sheet
for rendering HTML and XHTML ruby markup as ruby layout:
Note: Authors should not use the above rules:
a UA that supports ruby layout should provide these by default.
UAs implementing a user-controlled “minimum font size” feature
should consider scaling that minimum down for ruby annotations.
A.2 Inlining Ruby Annotations
The following represents a sample style sheet
for rendering HTML and XHTML ruby markup as inline annotations:
Unfortunately, because Selectors cannot match against text nodes,
it's not possible with CSS to express rules that will automatically and correctly
add parentheses to unparenthesized ruby annotations
for all possible ruby markup patterns in HTML.
(This is because HTML ruby allows implying the [=ruby base=] from raw text
without a corresponding element.)
However, these rules will add parentheses around each annotation sequence
in cases where either <rb>
or <rtc> is used rigorously:
/* Parens around <rtc> */
rtc::before { content: "（"; }
rtc::after { content: "）"; }
/* Parens before first <rt> not inside <rtc> */
rb + rt::before,
rtc + rt::before { content: "（"; }
/* Parens after <rt> not inside <rtc> */
rb ~ rt:last-child::after,
rt + rb::before { content: "）"; }
rt + rtc::before { content: "）（"; }
Alternatively, if it is known that a purely alternating style of markup is used
(<ruby>A<rt>a</rt>B<rt>b</rt>C<rt>c</rt><ruby>)
where there are no adjacent ruby annotations,
the following rules will add parentheses around each annotation:
/* Parens around each <rt> */
rt::before { content: "（"; }
rt::after { content: "）"; }
Glossary
Bopomofo or Zhuyin Fuhao (Chinese: ㄅㄆㄇㄈ, 注音符號, or 注音符号)
Characters and tone markings developed for use as phonetics in Chinese,
especially standard Mandarin.
These are often, but not exclusively, used for ruby annotations.
Example of Bopomofo used as phonetic inter-character annotations in Chinese
Bopomofo tone marks are spacing characters that occur
(in memory)
at the end of each bopomofo syllable.
They are typically displayed in a separate track
to the right of or above the other bopomofo characters,
and the position of the tone mark depends
on the number of characters in the syllable.
The neutral tone mark, however,
is placed before (and in line with) the bopomofo, not alongside it.
Note: The user agent and font subsystem are responsible for
ensuring the correct relative alignment and positioning of glyphs,
including [=bopomofo tone marks=], when displaying text--
whether the text occurs in ruby annotations or as normal inline content.
Such glyph placement is not a function of CSS ruby layout.
Bopomofo [=letters=] belong to the Bopomofo [=Unicode script=]
(and are currently mapped in the U+3100–312F and U+31A0–31BF blocks);
the [=Bopomofo tone marks=] are
U+02C9 (ˉ), U+02CA (ˊ), U+02C7 (ˇ), U+02CB (ˋ), U+02EA (˪), U+02EB (˫), U+02D9 (˙).
Collectively these are all considered Bopomofo characters
for the purpose of CSS.
Hanja (Korean: 漢字)
Subset of the Korean writing system that utilizes ideographic
characters borrowed or adapted from the Chinese writing system.
Also see Kanji.
Hiragana (Japanese: 平仮名)
Japanese syllabic script, or character of that script.
Rounded and cursive in appearance.
Subset of the Japanese writing system,
used together with kanji and katakana.
In recent times,
mostly used to write Japanese words
when kanji are not available or appropriate,
and word endings and particles.
Also see Katakana.
Ideograph
A character that is used to represent an idea, word, or word component,
in contrast to a character from an alphabetic or syllabic script.
The most well-known ideographic script is used (with some variation)
in East Asia (China, Japan, Korea,...).
Kana (Japanese: 仮名)
Collective term for hiragana and katakana.
Kanji (Japanese: 漢字)
Japanese term for ideographs; ideographs used in Japanese.
Subset of the Japanese writing system,
used together with hiragana and katakana.
Also see Hanja.
Katakana (Japanese: 片仮名)
Japanese syllabic script, or character of that script.
Angular in appearance.
Subset of the Japanese writing system,
used together with kanji and hiragana.
In recent times, mainly used to write foreign words.
Also see Hiragana.
Acknowledgments
This specification would not have been possible without the help from:
David Baron,
Robin Berjon,
Susanna Bowen,
Stephen Deach,
Martin Dürst,
Hideki Hiura (樋浦 秀樹),
Masayasu Ishikawa (石川雅康),
Taichi Kawabata,
Chris Pratley,
Xidorn Quan,
Takao Suzuki (鈴木 孝雄),
Frank Yung-Fong Tang,
Chris Thrasher,
Masafumi Yabe (家辺勝文),
Boris Zbarsky,
Steve Zilles.
Special thanks goes to the previous editors:
Michel Suignard and Marcin Sawicki of Microsoft,
and Richard Ishida of W3C.
Changes
This section documents the changes since previous publications.
Changes since the 2 December 2021 WD
Made the computed-value adjustment of 'writing-mode' on [=inter-character annotations=]
apply to the [=ruby annotation box=] rather than the [=ruby annotation container box=].
Added zh-Hant to rule applying ''font-size: 30%'' to [=ruby annotations=] in HTML.
Added text-justify: ruby to the UA default style sheet.
(Issue 771Issue 779)
Changes since the 29 April 2020 WD
Added ''ruby-position/alternate'' keyword to 'ruby-position'
and made it the [=initial value=].
Renamed collapse value of 'ruby-merge' to ''ruby-merge/merge''.
(Issue 5004)
Defined ''visibility: collapse'' to explicitly [=hide=] an annotation
the same way [=auto-hidden=] annotations are hidden.
(Issue 5927)
Specified more precisely which properties apply to the various ruby display types
([[#box-style]])
and defined ruby, ruby base, and ruby annotation boxes
to behave the same as regular [=inline boxes=] except as otherwise specified.
(Issue 4935,
Issue 4936,
Issue 4976,
Issue 4979)
Substantially revamped [[#ruby-layout]] to more clearly define
[=interlinear=] and [=inter-character=] ruby layout
and their interaction with 'ruby-align'.
[=Inter-character=] ruby is now based on [=inline block=] layout,
and its interaction with [=interlinear=] ruby
is also now well-defined.
Inline-axis space distribution for [=interlinear=] ruby
is now more precisely defined for spanning annotations
(modeled on ''grid-template-columns/max-content'' grid tracks).
Defined handling of nested ruby
by accounting for their base and annotation containers
in the block-axis sizing of [=interlinear=] ruby.
(Issue 4986,
Issue 4980)
Specified more precisely how 'ruby-align' works.
Loosened requirement that ruby overhang not affect the alignment of ruby contents,
added requirement that it not cause ruby content to collide with adjacent content.
Clarified interaction of bidi reordering and [=merged annotations=].
([[#bidi]])
Tightened up normative prose throughout.
Improved [[#intro|introduction]], examples, and other informative text.
Apply 'ruby-position' to ''::first-line''.
(Issue 2998)
Clarify containing block of ruby boxes and contents is, as in the case of other inline boxes, the nearest block container.
Clarify handling of misparented internal table elements.
Trim leading/trailing white space in ruby containers, base base containers, and annotation containers to prevent it from interfering with pairing.
Define layout effect of empty base and annotation containers.
Disable margins, padding, and borders on ruby base containers and ruby annotation containers (but not on ruby bases and ruby annotations).
Changes since the 19 September 2013 WD
Rewrite anonymous box generation rules and white space handling rules,
defined specialized [=pairing=] of anonymous white space boxes.
Take nested ruby handling out of [=pairing=].
(Will be handling it via sizing/layout.)
Define bidi layout of ruby structures.
Changes since the 30 June 2011 WD
Remove ruby-span and mentions of rbspan.
Explicit spanning is not used in HTML ruby in favor of implicit spanning.
This can't handle some pathological double-sided spanning cases,
but there seems to be no requirement for these at the moment.
(For implementations that support full complex XHTML Ruby,
they can imply spanning from the markup the same magic way
that we handle cell spanning from tables.
It doesn't seem necessary to include controls this in Level 1.)
Defer ruby-overhang and ruby-align: line-end to Level 2.
It's somewhat complicated, advanced feature.
Proposal is to make this behavior UA-defined and provide some examples of acceptable options.
Close issue requesting display: rp: use display: none.
The Internationalization WG added an issue
requesting a display value for <{rp}> elements.
They're supposed to be hidden when <{ruby}> is displayed as ruby.
But this is easily accomplished already with display: none.
Change 'ruby-position' values to match 'text-emphasis-position'.
Other than ''inter-character'', which we need to keep,
it makes more sense to align ruby positions with 'text-emphasis-position',
which can correctly handle various combinations of horizontal/vertical preferences.
Remove unused values of 'ruby-align'.
left, right, and end are not needed.
Replace auto, distribute-letter, and distribute-space
from 'ruby-align' with ''space-between'' and ''space-around''.
The auto value relied on inspecting content to determine behavior;
this can be avoided by just using ''space-around'' with standard justification rules
(which allow spacing between CJK but not between Latin).
Replaced distribute-letter and distribute-space
with ''space-between'' and ''space-around''
for consistency with distribution keywords in [[CSS-FLEXBOX-1]] and [[CSS-ALIGN-3]]
and to avoid any links to the definition of text-justify: distribute.
Added 'ruby-merge' property to control jukugo rendering.
This is a stylistic effect, not a structural one;
the previous model assumed that it was structural
and suggested handling it by changing markup. :(
Remove inline from 'ruby-position'.
This is do-able via display: inline
on all the ruby-related elements,
see Appendix A.
