spec:css-writing-modes-3; type:dfn; text:start

spec:css-writing-modes-3; type:dfn; text:end

@media (min-width: 40em) {

#mapping-diagram figure {

width: 49%;

float: left;

}

#mapping-diagram figure:first-of-type {

margin-right: 2%;

}

}

Note: See [[!css-writing-modes-4]] for a proper introduction to writing modes;

this module assumes familiarity with its terminology.

Because different writing systems are written in different directions,

a variety of <a>writing modes</a> exist:

left to right, top to bottom;

right to left, top to bottom;

bottom to top, right to left;

etc.

logical concepts like the “start” of a page or line

map differently to physical concepts like the “top” of a line or “left edge” of a paragraph.

Some aspects of a layout are actually relative to the writing directions,

and thus will vary when the page is translated to a different system;

others are inherently relative to the page's physical orientation.

<div class="example">

For example,

lists, headings, and paragraphs are typically left-aligned in English;

but actually they are start-aligned, because in Arabic the same constructs are right-aligned,

and a multilingual document will need to accommodate both writing systems accordingly.

The following code exemplifies how using logical syntax can help you write code

that works across different writing systems:

<figure>

<img src="./images/example01.png" width="300" height="109" />

<figcaption>Rendering of the below code in a compatible browser</figcaption>

</figure>

<xmp highlight="html">

<blockquote dir="auto">Quotation in English</blockquote>

<blockquote dir="auto">اقتباس في العربية</blockquote>

</xmp>

<xmp highlight="css">

blockquote {

text-align: start; /* left in latin, right in arabic */

margin-inline-start: 0px; /* margin-left in latin, margin-right in arabic */

border-inline-start: 5px solid gray; /* border-left in latin, border-right in arabic */

padding-inline-start: 5px; /* padding-left in latin, padding-right in arabic */

}

</xmp>

Documents might need both logical and physical properties. For instance

the drop shadows on buttons on a page must remain consistent throughout,

so their offset will be chosen based on visual considerations and physical directions,

and not vary by writing system.

</div>

Since CSS was originally designed with only physical coordinates in its controls,

this module introduces text-flow&ndash;relative equivalents

so that declarations in a CSS style sheet can be expressed

in <a>flow-relative</a> terms.

It defines the mapping and cascading of equivalent properties,

some new properties and values equivalent to those in CSS2.1,

and the principles used to derive their syntaxes.

Future CSS specifications are expected to incorporate both sets of coordinates

in their property and value definitions,

so this module will not track the introduction of <a>flow-relative</a> variants

of newer CSS features.

<a href="http://www.w3.org/TR/css-writing-modes/">CSS Writing Modes</a>’ <a href="https://www.w3.org/TR/css-writing-modes-3/#abstract-box">Abstract Box Terminology</a> section

defines how to map between flow-relative and physical terms.

This mapping,

which depends on the <a>used values</a> of

'writing-mode', 'direction', and 'text-orientation',

controls the interpretation of [=flow-relative=] keywords and properties.

<div id="mapping-diagram">

<figure>

<img src="diagrams/sizing-ltr-tb.svg">

<figcaption>

Correspondence of physical and flow-relative terms in typical English text layout

</figcaption>

</figure>

<figure>

<img src="diagrams/sizing-ttb-rl.svg">

<figcaption>

Correspondence of physical and flow-relative terms in vertical Chinese text layout

</figcaption>

</figure>

</div>

Note: Due to its interaction with ''text-orientation: upright'',

the <a lt="used value">used</a> 'direction'

depends on the <a>computed values</a> of 'writing-mode' and 'text-orientation'.

<strong>Things That Are Unstable</strong>

Since implementation of parts of this module is effectively required

for shipping an implementation of <a href="https://www.w3.org/TR/css-writing-modes-3/">CSS Writing Modes</a> on the Web

(in order to correctly implement the default HTML styles),

the CSSWG resolved that

the requisite features in

<a href="#directional-keywords"></a> and <a href="#box"></a>

are approved for shipping.

(See <a href="https://lists.w3.org/Archives/Public/www-style/2017Dec/0043.html">FPWD announcement</a> for additional background.)

However, there are a few significant open issues:

<ul>

<li>The ''border/logical'' keyword on shorthands,

because the name of the keyword may change or it may be replaced by some other syntactic marker.

(This feature will be deferred from this level for further development

if there is no clearly satisfactory mechanism proposed,

see <a href="https://github.com/w3c/csswg-drafts/issues/1282">Issue 1282</a>.)

<li>Whether flow-relative longhands inherit from their namesake on the parent,

or are mapped to a physical property and inherit from that property.

(See <a href="https://github.com/w3c/csswg-drafts/issues/3029">Issue 3029</a>.)

<li>Whether shorthands like 'margin' expand to both sets of longhands,

or only the ones that were set.

(See <a href="https://github.com/w3c/csswg-drafts/issues/3030">Issue 3030</a>.)

</ul>

Comments, suggestions, and use cases are welcome on these issues.

Please file them in GitHub, tweet them to @csswg, or send them to [email protected].

This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]

using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].

Value types not defined in this specification are defined in CSS Values &amp; 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 <a>CSS-wide keywords</a> as their property value.

For readability they have not been repeated explicitly.

Many CSS properties have historically accepted <dfn>directional keyword</dfn> values

that are [=physical=]

(<css>top</css>, <css>bottom</css>, <css>left</css>, <css>right</css>).

This specification introduces [=directional keyword=] values that are [=flow-relative=]:

<css>block-start</css>, <css>block-end</css>, <css>inline-start</css>, <css>inline-end</css>.

A property's effect can be either 1-dimensional or 2-dimensional.

When contextually constrained to one dimension,

the flow-relative keywords are abbreviated

(to <css>start</css> and <css>end</css>).

CSS Level 2 properties are here redefined to also accept

<a>flow-relative</a> directional keywords.

Such values can be used in place of the corresponding physical values.

For properties that take multiple keywords,

combinations of flow-relative and physical values are not allowed

(unless otherwise specified in a future specification).

Note: Newer CSS specifications are expected in most cases to define

[=flow-relative=] or [=line-relative=] values

instead of or in addition to any [=physical=] ones.

In general, the mapping of such relative values are expected to use

the [=writing mode=] of the [=containing block=] when affecting the box itself,

and that of the box itself when affecting its contents.

Regardless, which [=writing modes=] is used for the mapping

needs to be explicitly defined.

<pre class="propdef partial">

These two values are added only for implementations that support

''caption-side/left'' and ''caption-side/right'' values for 'caption-side'.

The ''caption-side/left'' and ''caption-side/right'' values themselves

are defined to be <a>line-relative</a>.

The existing ''caption/top'' and ''caption/bottom'' values are idiosyncratically redefined

as assigning to the <a>block-start</a> and <a>block-end</a> sides of the table, respectively.

The mapping on this property uses the <a>writing mode</a> of the caption’s <a>containing block</a>

(that is, the <a>table wrapper box</a>).

<pre class="propdef partial">

The mapping on these properties uses the <a>writing mode</a> of the element’s <a>containing block</a>.

Note: These properties are 1-dimensional in CSS2,

but are planned to be expanded to two dimensions,

and therefore are given unabbreviated <a>flow-relative</a> keywords.

<pre class="propdef partial">

These values are normatively defined in [[!css-text-3]].

In CSS, all pages are classified by user agents as either left pages or right pages. [[!CSS2]]

Which page is first in a spread, however,

depends on whether the page progression is left-to-right or right-to-left.

To allow control of page breaking to the page

that is on the earlier or later side of a spread,

rather than to the left or right side of a spread,

this module introduces the following additional keywords

for the 'page-break-after' and 'page-break-before' properties [[!CSS2]]:

<dl dfn-type="value" dfn-for="logical-page">

<dt><dfn>recto</dfn>

<dd>

Equivalent to 'right' in left-to-right page progressions

and 'left' in right-to-left page progressions.

<dt><dfn>verso</dfn>

<dd>

Equivalent to 'left' in left-to-right page progressions

and 'right' in right-to-left page progressions.

</dl>

These values are computed as specified

and are further defined in [[!css-break-3]].

Although authors typically place page numbers using physical placements,

the contents of headers often follows conventions depending

on which page in the spread is earlier.

Therefore the following flow-relative <a href="https://www.w3.org/TR/CSS2/page.html#page-selectors">page selectors</a>

are also added to support flow-relative page selection:

<dl dfn-type="value" dfn-for="logical-page-selector">

<dt><dfn>:recto</dfn>

<dd>

Equivalent to ':right' in left-to-right page progressions

and ':left' in right-to-left page progressions.

<dt><dfn>:verso</dfn>

<dd>

Equivalent to ':left' in left-to-right page progressions

and ':right' in right-to-left page progressions.

</dl>

The flow-relative page selectors have specificity equal to

the ':left' and ':right' page selectors.

For many formatting effects,

the axis or direction affected is encoded in the property name

rather than in its value.

The type of directional or axis mapping

(<a>flow-relative</a> or <a>physical</a>)

of each such property

is called its <dfn export>mapping logic</dfn>.

Historically, all properties have been encoded in [=physical=] terms;

this specification introduces new CSS properties

that are <a>flow-relative</a> equivalents

of CSS2’s <a>physical</a> box model properties.

Note: Newer CSS specifications are expected in most cases to define

[=flow-relative=] or [=line-relative=] properties

instead of or in addition to any [=physical=] ones.

Each set of parallel

[=flow-relative=] properties and [=physical=] properties

(ignoring [=shorthand=] properties)

related by setting equivalent styles on the various sides or dimensions of a box,

forms a <dfn export>logical property group</dfn>.

For example, the 'padding-*' properties

form a single <a>logical property group</a>,

the 'margin-*' properties

form a separate <a>logical property group</a>,

the 'border-*-style' properties

form another <a>logical property group</a>,

etc.

(Each [=longhand property=] can belong to at most one <a>logical property group</a>.)

Within each [=logical property group=],

corresponding [=flow-relative=] and [=physical=] properties

are paired using the element’s own

[=computed value|computed=] [=writing mode=].

Although the [=specified value=] of each property remains distinct,

paired properties share a [=computed value=].

This shared value is determined by [=cascading=]

the declarations of both properties together as one;

in other words, the [=computed value=] of both properties in the pair

is derived from the [=specified value=] of the property declared with

higher priority in the CSS [=cascade=]. [[!CSS-CASCADE-3]]

<p class="note">

Note that this requires implementations to maintain

relative order of declarations within a CSS declaration block,

which was not previously required for CSS cascading.

It also requires that 'writing-mode', 'direction', and 'text-orientation'

be computed as a prerequisite for cascading together

the [=flow-relative=] and [=physical=] declarations

of a [=logical property group=]

to find their [=computed values=].

</p>

<div class="example">

For example, given the following rule:

<pre class="lang-css">

p {

margin-inline-start: 1px;

margin-left: 2px;

margin-inline-end: 3px;

}

</pre>

In a paragraph with computed 'writing-mode' being ''horizontal-tb''

and computed 'direction' being ''ltr'',

the computed value of 'margin-left' is ''2px'',

since for that 'writing-mode' and 'direction',

'margin-inline-start' and 'margin-left' share a computed value,

and the declaration of 'margin-left'

is after the declaration of 'margin-inline-start'.

However, if the computed 'direction' were instead ''rtl'',

the computed value of 'margin-left' is ''3px'',

since 'margin-inline-end' and 'margin-left' share a computed value,

and the declaration of 'margin-inline-end'

is after the declaration of 'margin-left'.

</div>

[[!CSSOM]] APIs that return computed values

(such as <code>getComputedStyle()</code>)

must return the same value for each individual property

in such a pair.

Note: Depending on the element’s own <a>writing mode</a>

for mapping every <a>flow-relative</a> property

to its <a>physical</a> equivalent

simplifies the cascading calculations

and gives a straightforward model for authors to reason about.

However, it is problematic in many cases,

see for example <a href="https://www.w3.org/mid/[email protected]">this discussion</a>.

Authors may need to use nested elements

to get the correct mapping behavior

when changing an element’s <a>writing mode</a> from its parent.

Inheritance of each property is from its corresponding property on the parent.

For example, although the [=inline-start=] margin of an''direction/rtl'' box is its right margin,

'margin-inline-start' on this box will inherit

the 'margin-inline-start' of an ''direction/ltr'' parent

even though that happens to be the parent’s <em>left</em> margin.

Unless otherwise specified,

[=shorthand properties=] that encompass both logical and physical longhands

(such as the 'all' shorthand)

set their physical longhands last.

For example, ''all: inherit'' will set all of the 'margin' properties to ''inherit'',

but since the physical longhands are set last,

the child's margins will inherit from their physical counterparts in the parent.

<pre class="propdef">

These properties correspond to the

'height' and 'width' properties.

The mapping depends on the element's 'writing-mode'.

<pre class="propdef">

These properties correspond to the

'min-height' and 'min-width' properties.

The mapping depends on the element's 'writing-mode'.

<pre class="propdef">

These properties correspond to the

'max-height' and 'max-width' properties.

The mapping depends on the element's 'writing-mode'.

<pre class="propdef">

These properties correspond to the 'margin-top', 'margin-bottom', 'margin-left', and 'margin-right' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation'.

<pre class="propdef shorthand">

These two <a>shorthand properties</a> set the

'margin-block-start' &amp; 'margin-block-end'

and

'margin-inline-start' &amp; 'margin-inline-end',

respectively.

The first value represents the <a>start</a> edge style,

and the second value represents the <a>end</a> edge style.

If only one value is given, it applies to both the <a>start</a> and <a>end</a> edges.

The 'top', 'left', 'bottom', 'right' physical properties,

their 'inset-block-start', 'inset-block-end', 'inset-inline-start', 'inset-inline-end' flow-relative correspondents,

and the 'inset-block', 'inset-inline', and 'inset' shorthands,

are collectively known as the <dfn export>inset properties</dfn>.

<pre class="propdef">

These properties correspond to the

'top', 'bottom', 'left', and 'right' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation'.

<pre class="propdef shorthand">

These two <a>shorthand properties</a> set the

'inset-block-start' &amp; 'inset-block-end'

and

'inset-inline-start' &amp; 'inset-inline-end',

respectively.

The first value represents the <a>start</a> edge style,

and the second value represents the <a>end</a> edge style.

If only one value is given, it applies to both the <a>start</a> and <a>end</a> edges.

<pre class="propdef shorthand">

This <a>shorthand property</a> sets the

'top', 'right', 'bottom', and 'left' properties.

Values are assigned to its <a>sub-properties</a> as for 'margin'.

<pre class="propdef">

These properties correspond to the

'padding-top', 'padding-bottom', 'padding-left', and 'padding-right' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation'.

<pre class="propdef shorthand">

These two <a>shorthand properties</a> set the

'padding-block-start' &amp; 'padding-block-end'

and

'padding-inline-start' &amp; 'padding-inline-end',

respectively.

The first value represents the <a>start</a> edge style,

and the second value represents the <a>end</a> edge style.

If only one value is given, it applies to both the <a>start</a> and <a>end</a> edges.

<pre class="propdef">

These properties correspond to the

'border-top-width', 'border-bottom-width', 'border-left-width', and 'border-right-width' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation'.

<pre class="propdef shorthand">

These two <a>shorthand properties</a> set the

'border-block-start-width' &amp; 'border-block-end-width'

and

'border-inline-start-width' &amp; 'border-inline-end-width',

respectively.

The first value represents the <a>start</a> edge width,

and the second value represents the <a>end</a> edge width.

If only one value is given, it applies to both the <a>start</a> and <a>end</a> edges.

<pre class="propdef">

These properties correspond to the

'border-top-style', 'border-bottom-style', 'border-left-style', and 'border-right-style' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation'.

<pre class="propdef shorthand">

These two <a>shorthand properties</a> set the

'border-block-start-style' &amp; 'border-block-end-style'

and

'border-inline-start-style' &amp; 'border-inline-end-style',

respectively.

The first value represents the <a>start</a> edge style,

and the second value represents the <a>end</a> edge style.

If only one value is given, it applies to both the <a>start</a> and <a>end</a> edges.

<pre class="propdef">

These properties correspond to the

'border-top-color', 'border-bottom-color', 'border-left-color', and 'border-right-color' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation'.

<pre class="propdef shorthand">

These two <a>shorthand properties</a> set the

'border-block-start-color' &amp; 'border-block-end-color'

and

'border-inline-start-color' &amp; 'border-inline-end-color',

respectively.

The first value represents the <a>start</a> edge color,

and the second value represents the <a>end</a> edge color.

If only one value is given, it applies to both the <a>start</a> and <a>end</a> edges.

<pre class="propdef shorthand">

These properties correspond to the

'border-top', 'border-bottom', 'border-left', and 'border-right' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation'.

<pre class="propdef shorthand">

These two <a>shorthand properties</a> set the

'border-block-start' &amp; 'border-block-end'

or

'border-inline-start' &amp; 'border-inline-end',

respectively,

both to the same style.

<!-- intentionally cannot set two sides independently;

see discussion in https://github.com/w3c/csswg-drafts/issues/3519 -->

<pre class="propdef">

These properties correspond to the

'border-top-left-radius', 'border-bottom-left-radius', 'border-top-right-radius', and 'border-bottom-right-radius' properties.

The mapping depends on the element's 'writing-mode', 'direction', and 'text-orientation',

with the first start/end giving the block axis side,

and the second the inline-axis side

(i.e. patterned as 'border-<var>block</var>-<var>inline</var>-radius').

The shorthand properties for margin, padding, and border

set values for physical properties by default.

But authors can specify

the <dfn value for="margin, padding, border-color, border-style, border-width">logical</dfn> keyword

at the beginning of the property value

to indicate that the values map to the flow-relative properties instead of the physical ones.

<p class="issue">

The proposed syntax for this feature is <a href="https://github.com/w3c/csswg-drafts/issues/1282">under discussion</a>

and is almost guaranteed to change from what is described here.

This section remains in the draft to promote discussion of alternatives,

to document the affected properties,

and to specify the expected impact on the interpretation

of whatever syntactic switch is ultimately chosen.

The following [[!CSS2]] shorthand properties

accept the ''margin/logical'' keyword:

<ul>

<li>'inset'

<li>'margin'

<li>'padding'

<li>'border-width'

<li>'border-style'

<li>'border-color'

<li>'scroll-padding'

<li>'scroll-margin'

</ul>

The syntax for these properties is effectively changed by replacing

<pre class="prod">&lt;<var>value-type</var>&gt;{1,4}</pre>

with

<pre class="prod">logical? &lt;<var>value-type</var>&gt;{1,4}</pre>

When the ''margin/logical'' keyword is present in the value,

the values that follow are assigned to its flow-relative longhands

as follows:

<ul>

<li>If only one value is set, the value applies to all four flow-relative <a>longhands</a>.

<li>If two values are set, the first is for block-start and block-end,

the second is for inline-start and inline-end.

<li>If three values are set, the first is for block-start,

the second is for inline-start and inline-end,

and the third is for block-end.

<li>If four values are set, they apply to

the block-start, inline-start, block-end, and inline-end sides

in that order.

</ul>

<div class="example">

In the following example, the two rules are equivalent:

<pre class="lang-css">

blockquote {

margin: logical 1em 2em 3em 4em;

}

blockquote {

margin-block-start: 1em;

margin-inline-start: 2em;

margin-block-end: 3em;

margin-inline-end: 4em;

}

</pre>

</div>

Cameron McCormack,

David Baron,

Oriol Brufau,

Shinyu Murakami,

Tab Atkins

Changes since the <a href="https://www.w3.org/TR/2018/WD-css-logical-1-20180827/">previous Working Draft</a> include:

<ul>

<li>Miscellaneous minor clarifications

</ul>

Changes between the earlier Editors Drafts and the <a href="https://www.w3.org/TR/2017/WD-css-logical-1-20170518/">18 May 2017 First Public Working Draft</a> include:

<ul>

<li>Making all properties cascade using the <a>writing mode</a> specified on the element, not on its parent.

<li>Making the ordering of longhands within 'margin'-like shorthands put inline-start before inline-end.

<li>Adding the '*-inline' and '*-block' shorthand forms for margins/borders/padding.

<li>Renaming the <css>offset-*</css> properties to 'inset-*' and marking an issue for discussion.

<li>Adding an Introduction section.

<li>Updating to current terminology of CSS Writing Modes.

<li>Miscellaneous prose cleanup.

</ul>

<h2 class=no-num id=privacy>Privacy Considerations</h2>

No new privacy considerations have been reported on this specification.

<h2 class=no-num id=security>Security Considerations</h2>

No new security considerations have been reported on this specification.