CSS Logical Properties and Values Level 1

Introduction

Note: See [CSS3-WRITING-MODES] 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 writing modes 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.

Since CSS was originally designed with only physical coordinates in its controls, this module introduces text-flow–relative equivalents so that declarations in a CSS style sheet can be expressed in flow-relative 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 flow-relative variants of newer CSS features.

CSS Writing Modes’ Abstract Box Terminology section defines how to map between flow-relative and physical terms. This mapping controls the interpretation of flow-relative keywords and properties.

1. Flow-Relative Values: block-start, block-end, inline-start, inline-end

Properties that accept physical directional keyword values (top, bottom, left, or right) are redefined to also accept the appropriate flow-relative directional keywords. In such cases, the flow-relative 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).

Properties can be either 1-dimensional or 2-dimensional. When contextually constrained to one dimension, the flow-relative keywords are abbreviated.

1.1. Logical Values for the caption-side Property

These two values are added only for implementations that support left and right values for caption-side. The existing top and bottom values are idiosyncratically redefined as assigning to the block-start and block-end sides of the table, respectively. The mapping on this property uses the writing mode of the element’s containing block (that is, the table wrapper box).

1.2. Flow-Relative Values for the float and clear Properties

The mapping on this property uses the writing mode of the element’s containing block.

Note: These properties are 1-dimensional in CSS2, but are planned to be expanded to two dimensions, and therefore are given unabbreviated flow-relative keywords.

1.3. Flow-Relative Values for the text-align Property

These values are normatively defined in [CSS3TEXT].

1.4. Flow-Relative Values for the resize Property

2. Flow-Relative Page Classifications

In CSS, all pages are classified by user agents as either left pages or right pages. [CSS21] 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 [CSS21]:

recto

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

verso

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

These values are further defined in [CSS3-BREAK].

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 page selectors are also added to support flow-relative page selection:

:recto

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

:verso

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

The flow-relative page selectors have specificity equal to the ':left' and ':right' page selectors.

3. Flow-Relative Box Model Properties

This specification introduces new CSS properties that are flow-relative equivalents of physical box model properties.

The specified values of these properties are separate from the specified values of the parallel physical properties, but the flow-relative and physical properties share computed values. Which pairs of properties share computed values depends on the element’s computed values of writing-mode, direction, and text-orientation.

Depending on the element’s own writing mode simplifies the cascading calculations and gives a straightforward model for authors to reason about. However, it is problematic in many cases, see for example this discussiona.

A computed value that has flow-relative and physical properties as input is determined by applying the CSS cascade to declarations of both. Overriding is not determined by whether a declaration is flow-relative or physical, but only by the rules of the CSS cascade [CSS3-CASCADE].

Note that this requires implementations to maintain relative order of declarations within a CSS declaration block, which was not previously required for CSS cascading.

p {
  margin-inline-start: 1px;
  margin-left: 2px;
  margin-inline-end: 3px;
}

[[CSSOM!]] APIs that return computed values (such as getComputedStyle()) must return the same value for equivalent pairs of such properties.

3.1. Logical Height and Logical Width: the block-size and inline-size properties

These properties correspond to the width and height properties. The mapping depends on the element’s writing-mode.

These properties correspond to the min-width and min-height properties. The mapping depends on the element’s writing-mode.

These properties correspond to the max-width and max-height properties. The mapping depends on the element’s writing-mode.

3.2. Flow-relative Margins: the margin-block-start, margin-block-end, margin-inline-start, margin-inline-end properties and margin-block and margin-inline shorthands

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.

These two shorthand properties set the margin-block-start & margin-block-end and margin-inline-start & margin-inline-end, respectively. The first value represents the start edge style, and the second value represents the end edge style. If only one value is given, it applies to both the start and end edges.

3.3. Flow-relative Offsets: the inset-block-start, inset-block-end, inset-inline-start, inset-inline-end properties and inset-block, inset-inline, and inset shorthands

These properties correspond to the top, bottom, left, and right properties. The mapping depends on the element’s writing-mode, direction, and text-orientation.

These two shorthand properties set the inset-block-start & inset-block-end and inset-inline-start & inset-inline-end, respectively. The first value represents the start edge style, and the second value represents the end edge style. If only one value is given, it applies to both the start and end edges.

This shorthand property sets the top, right, bottom, and left properties. Values are assigned to its sub-properties as for margin.

3.4. Flow-relative Padding: the padding-block-start, padding-block-end, padding-inline-start, padding-inline-end properties and padding-block and padding-inline shorthands

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.

These two shorthand properties set the padding-block-start & padding-block-end and padding-inline-start & padding-inline-end, respectively. The first value represents the start edge style, and the second value represents the end edge style. If only one value is given, it applies to both the start and end edges.

3.5. Flow-relative Borders

3.5.1. Flow-relative Border Widths: the border-block-start-width, border-block-end-width, border-inline-start-width, border-inline-end-width properties and border-block-width and border-inline-width shorthands

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.

These two shorthand properties set the border-block-start-width & border-block-end-width and border-inline-start-width & border-inline-end-width, respectively. The first value represents the start edge width, and the second value represents the end edge width. If only one value is given, it applies to both the start and end edges.

3.5.2. Flow-relative Border Styles: the border-block-start-style, border-block-end-style, border-inline-start-style, border-inline-end-style properties and border-block-style and border-inline-style shorthands

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.

These two shorthand properties set the border-block-start-style & border-block-end-style and border-inline-start-style & border-inline-end-style, respectively. The first value represents the start edge style, and the second value represents the end edge style. If only one value is given, it applies to both the start and end edges.

3.5.3. Flow-relative Border Colors: the border-block-start-color, border-block-end-color, border-inline-start-color, border-inline-end-color properties and border-block-color and border-inline-color shorthands

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.

These two shorthand properties set the border-block-start-color & border-block-end-color and border-inline-start-color & border-inline-end-color, respectively. The first value represents the start edge style, and the second value represents the end edge style. If only one value is given, it applies to both the start and end edges.

3.5.4. Flow-relative Border Shorthands: the border-block-start, border-block-end, border-inline-start, border-inline-end properties and border-block and border-inline shorthands

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.

These two shorthand properties set the border-block-start & border-block-end and border-inline-start & border-inline-end, respectively.

3.6. Four-Directional Shorthand Properties: the margin, padding, border-width, border-style, and border-color shorthands

The shorthand properties for margin, padding, and border set values for physical properties by default. But authors can specify the logical keyword at the beginning of the property value to indicate that the values map to the flow-relative properties instead of the physical ones.

other candidates of the keyword are: relative, script, writing-mode, beas, or the value itself (e.g., vertical-lr-ltr)

The following [CSS21] shorthand properties (and additionally the new inset shorthand defined above) accept the logical keyword:

• margin
• padding
• border-width
• border-style
• border-color

The syntax for these properties is effectively changed by replacing

<value-type>{1,4}

with

logical? <value-type>{1,4}

When the logical keyword is present in the value, the values that follow are assigned to the flow-relative properties as follows:

• If only one value is set, the value applies to all four flow-relative longhands.
• If two values are set, the first is for block-start and block-end, the second is for inline-start and inline-end.
• 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.
• If four values are set, they apply to the block-start, inline-start, block-end, and inline-end sides in that order.

blockquote {
  margin: logical 1em 2em 3em 4em;
}
blockquote {
  margin-block-start:  1em;
  margin-inline-start: 2em;
  margin-block-end:    3em;
  margin-inline-end:   4em;
}

4. Logical Background and Border Images

[CSS3BG] is handled separately because it can fill an area with specified images, and the area to fill can be rotated or flipped depending on the text flow.

4.1. Background Image Transform: The background-image-transform property

is this the right default? we need to investigate which is more common

This property defines whether background images are transformed to match to the value of writing-mode property, and whether background-size widths and heights are flow-relative or physical. Values have the following meanings:

logical

The values for the background-size property are flow-relative. The background images are transformed to match to the flow-relative axis.

physical

The values for the background-size property are physical. The background images remain unchanged.

rotate

Similar to logical, except that the inline base direction is ignored. The result is affected only by the block flow direction.

4.2. The background-repeat property

The repeat-x and repeat-y values are physical, but in CSS3 this property can also accept double values to specify horizontal and vertical behaviors separately. The double values are considered flow-relative if the logical keyword is specified, otherwise physical.

should also add repeat-horizontal and repeat-vertical for the physical value?

4.3. Border Image Transform: The border-image-transform property

is this the right initial default?

This property defines whether border images are transformed to match to the value of writing-mode property, with the reference writing mode being writing-mode: horizontal-tb; direction: ltr. Values have the following meanings:

logical

The values for the border-image-* properties are flow-relative. The border images are transformed to match to the flow-relative axis.

physical

The values for the border-image-* properties are physical. The border images remain unchanged.

rotate

Similar to logical, except that the inline base direction is ignored. The result is affected only by the block flow direction.

The following properties use the value of this property to determine how directional mappings are done:

• border-image-slice
• border-image-width
• border-image-outset
• border-image-repeat
• border-image

5. Acknowledgements

Cameron McCormack, David Baron, Shinyu Murakami, Tab Atkins

6. Changes

Changes since the earler Editors Drafts include:

• Making all properties cascade using the writing mode specified on the element, not on its parent.
• Making the ordering of longhands within margin-like shorthands put inline-start before inline-end.
• Adding the *-inline and *-block shorthand forms for margins/borders/padding.
• Renaming the outset-* properties to inset-* and marking an issue for discussion.
• Adding an Introduction section.
• Updating to current terminology of CSS Writing Modes.
• Miscellaneous prose cleanup.