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Showing content from https://www.w3.org/TR/2017/WD-css-logical-1-20170518/ below:

CSS Logical Properties and Values Level 1

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.

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. However 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 controls the interpretation of flow-relative keywords and properties.

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 although the draft is not very stable overall, the requisite features in

and

are approved for shipping. However, there are a number of unstable features in this draft (which are not required for implementing the HTML default style sheet, incidentally), and these are:

• The float and clear keywords, because it is not yet clear what the upcoming 2-dimensional syntax of float will be. (This has been resolved as of April 2017.)
• The inset-* properties, because the name of the prefix is undecided. (It used to be offset-*, but ended up conflicting with another CSS feature.)
• The logical keyword on shorthands, because the name of the keyword may change or it may be replaced by some other syntactic marker.
• The background-image-transform and border-image-transform properties, because they have, as far as the editor is aware, received no review from anyone and may or may not be well-designed.

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

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.

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).

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.

These values are normatively defined in [CSS3TEXT].

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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;
}

[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.

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.

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?

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

Cameron McCormack, David Baron, Shinyu Murakami, Tab Atkins

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.

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.

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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.

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