CSS Logical Properties and Values Level 1 (original) (raw)

1. Introduction

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 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.

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:

Rendering of the below code in a compatible browser

Quotation in English

اقتباس في العربية

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 */ }

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.

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, which depends on the used values of writing-mode, direction, and text-orientation, controls the interpretation of flow-relative keywords and properties.

Note: Due to its interaction with text-orientation: upright, the used direction depends on the computed values of writing-mode and text-orientation.

Things That Are Unstable Since implementation of parts of this module is effectively required for shipping an implementation of CSS Writing Modes on the Web (in order to correctly implement the default HTML styles), the CSSWG resolved that the requisite features in § 2 Flow-Relative Values: block-start, block-end, inline-start, inline-end and § 4 Flow-Relative Box Model Properties are approved for shipping. (See FPWD announcement for additional background.)

However, there are a few significant open issues:

• The 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 Issue 1282.)
• Whether flow-relative longhands inherit from their namesake on the parent, or are mapped to a physical property and inherit from that property. (See Issue 3029.)
• Whether shorthands like margin expand to both sets of longhands, or only the ones that were set. (See Issue 3030.)

Comments, suggestions, and use cases are welcome on these issues. Please file them in GitHub, tweet them to @csswg, or send them to www-style@w3.org.

1.1. 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 as their property value. For readability they have not been repeated explicitly.

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

Many CSS properties have historically accepted directional keyword values that are physical (top, bottom, left, right). This specification introduces directional keyword values that are flow-relative: block-start, block-end, inline-start, inline-end.

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 start and end).

CSS Level 2 properties are here redefined to also accept flow-relative 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.

2.1. 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 left and right values themselves are defined to be line-relative.

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 caption’s containing block (that is, the table wrapper box).

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

The mapping on these properties 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.

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

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

3. Flow-Relative Page Classifications

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]:

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 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 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.

4. Flow-Relative Box Model Properties

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 (flow-relative or physical) of each such property is called its mapping logic. Historically, all properties have been encoded in physical terms; this specification introduces new CSS properties that are flow-relative equivalents of CSS2’s physical 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 logical property group. For example, the padding-* properties form a single logical property group, the margin-* properties form a separate logical property group, the border-*-style properties form another logical property group, etc. (Each longhand property can belong to at most one logical property group.)

Within each logical property group, corresponding flow-relative and physical properties are paired using the element’s own 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]

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.

For example, given the following rule:

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

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.

[CSSOM] APIs that return computed values (such as getComputedStyle()) must return the same value for each individual property in such a pair.

Note: Depending on the element’s own writing mode for mapping every flow-relative property to its physical 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 this discussion. Authors may need to use nested elements to get the correct mapping behavior when changing an element’s writing mode from its parent.

Inheritance of each property is from its corresponding property on the parent. For example, although the inline-start margin of anrtl box is its right margin, margin-inline-start on this box will inherit the margin-inline-start of an ltr parent even though that happens to be the parent’s left 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.

4.1. Logical Height and Logical Width: the block-size/inline-size, min-block-size/min-inline-size, and max-block-size/max-inline-size properties

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

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

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

4.2. 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.

4.3. 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

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 inset properties.

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.

4.4. 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.

4.5. Flow-Relative Borders

4.5.1. 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.

4.5.2. 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.

4.5.3. 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 color, and the second value represents the end edge color. If only one value is given, it applies to both the start and end edges.

4.5.4. 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 or border-inline-start & border-inline-end, respectively, both to the same style.

4.6. Flow-Relative Corner Rounding: the border-start-start-radius, border-start-end-radius, border-end-start-radius, border-end-end-radius properties

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-block-inline-radius').

4.7. 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.

The proposed syntax for this feature is under discussion 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 logical keyword:

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

The syntax for these properties is effectively changed by replacing

{1,4}

with

logical? {1,4}

When the logical keyword is present in the value, the values that follow are assigned to its flow-relative longhands 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.

In the following example, the two rules are equivalent:

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

5. Acknowledgements

Cameron McCormack, David Baron, Oriol Brufau, Shinyu Murakami, Tab Atkins

6. Changes

Changes since the previous Working Draft include:

• Miscellaneous minor clarifications

Changes between the earlier Editors Drafts and the 18 May 2017 First Public Working Draft 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 offset-* properties to inset-* and marking an issue for discussion.
• Adding an Introduction section.
• Updating to current terminology of CSS Writing Modes.
• Miscellaneous prose cleanup.

Privacy Considerations

No new privacy considerations have been reported on this specification.

Security Considerations

No new security considerations have been reported on this specification.

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.