CSS Line Grid Module

Abstract

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc. This module contains the features of CSS level 3 relating to DESCRIBE HERE. It includes and extends the functionality of CSS level 2 [CSS21], which builds on CSS level 1 [CSS1]. The main extensions compared to level 2 are SUMMARIZE HERE.

Status of this document

This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don't cite this document other than as work in progress.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “unknown-shortname” in the subject, preferably like this: “[unknown-shortname] …summary of comment…”

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

The following features are at risk: …

Table of contents

• 1. Introduction
  • 1.1. Background
  • 1.2. Module Interactions
  • 1.3. Values
• 2. Defining a Line Grid: the ‘line-grid’ property
• 3. Snapping to a Grid
  • 3.1. Snapping Line Boxes: the ‘line-snap’ property
  • 3.2. Line Snap Allowance: the ‘line-slack’ property
  • 3.3. Snapping Block Boxes: the ‘box-snap’ property
• 4. Layout in Grid Units
  • 4.1. Grid Length Units: the ‘gd’ unit
  • 4.2. Rounding Lengths to Multiple of Units: ‘rounddown()’ and ‘roundup()’
• 5. Conformance
  • 5.1. Document Conventions
  • 5.2. Conformance Classes
  • 5.3. Partial Implementations
  • 5.4. Experimental Implementations
  • 5.5. CR Exit Criteria
• Acknowledgments
• References
  • Normative references
  • Other references
• Index
• Property index

1. Introduction

1.1. Background

This section is not normative.

This specification provides features to align lines and blocks to invisible grids in the document.

Aligning lines and blocks to grids provides the following benefits:

• Vertical rhythm is kept for better readability.
• Lines are aligned between columns in multi-column documents.
• The top and the bottom margins of pictures are made equal, while keeping the vertical rhythm of text before and after the pictures.
• Layout lines are at the same position on every page in paged media. Keeping the position of the bottom line of a page has benefits for design and readability. This also improves the readability of duplex printing, two pages spreads, and displaying on slow display devices like e-ink.
• East Asian layouts require vertical rhythm more often than other scripts do, even in single column, non-paged media documents, as defined in [JLREQ].

There are several types of objects in a document that can break the vertical rhythm. Examples include lines with different sizes of text, pictures, and tables.

Vertical rhythm kept through pictures and different size of text in a multi-column document.

Large text wraps within line grids.

When a different size of text, such as a headings, wraps, it is usually aligned to grids as a block and the lines within the block do not align.

East Asian layouts may require width be a multiple of em without fractions.

East Asian layouts may require grid-like features in inline progression direction as well.

It is often desirable in East Asian layouts to make the line width a multiple of em without fractions. Because most East Asian characters have 1em advance and most East Asian documents are justified, this minimizes cases where justification needs to expand character spacing.

This module provides the following capabilities:

• Defining grids in the line progression direction.
• Controling how lines and blocks align to the grids.
• Rounding lengths to multiples of units.

It is important to control these capabilities independently, so that, for example, aligning to grids can be turned off for tables, but can then be turned back on for aligning the following text to the grids.

1.2. Module Interactions

This module extends the line box model defined in [CSS21] sections 9.4.2 and 10.8.

1.3. Values

This specification follows the CSS property definition conventions from [CSS21]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS modules may expand the definitions of these value types: for example [CSS3COLOR], when combined with this module, expands the definition of the <color> value type as used in this specification.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.

2. Defining a Line Grid: the ‘line-grid’ property

Specifies which line grid the line boxes of this element (if any) will align.

If there does not already exist a line grid with the specified name, then this property creates a line grid using the font and line layout settings of this element. The line grid consists of a series of horizontal lines corresponding to all the baselines (alphabetic, text-top, text-bottom, mathematic, central, hanging, etc.) and to the line-over and line-under edges, positioned where they would fall if the contents of this element consisted entirely of line boxes filled with text (no sub-elements) using the first available font. If the element is paginated, the line grid is restarted on each page; since line boxes cannot be paginated, no page begins with the bottom part of a line's grid.

As a side effect of the initial value and this definition, the root element always sets up a line grid, so that ‘line-snap’ can be used even if the author has not explicitly set up a line grid.

3. Snapping to a Grid

3.1. Snapping Line Boxes: the ‘line-snap’ property

This property applies to all the line boxes directly contained by the element, and, when not ‘none’, causes each line box to shift downward (possibly by zero) until it snaps to the line grid specified by ‘line-grid’. (The unshifted position is the position that would be determined by normal line stacking rules, with consideration of any new controls defined by other modules such as [CSS3LINE].) Values have the following meanings:

‘none’

Line boxes do not snap to the grid; they stack normally.

‘baseline’

The dominant baseline snaps with the matching baseline on the line grid applying to the element.

‘contain’

Two baselines are used to align the line box: the line box is snapped so that its ‘central’ baseline (halfway between the ‘text-over’ and ‘text-under’ baselines) is centered between one of the line grid's ‘text-over’ baselines and a subsequent (but not necessarily consecutive) ‘text-under’ baseline.

In some cases lines of equal line height will not align perfectly to a baseline grid: this happens, for example, when fonts (of the same size) with different baseline tables are mixed on a line. For this reason, if shifting the line by the largest difference between the smallest ascent and largest ascent of a single size used on the line would result in a smaller shift, then the contents of the line box are shifted up within the line box so as to allow the line to snap without jumping downward to the next grid line.

3.2. Line Snap Allowance: the ‘line-slack’ property

This property specifies the slack in snapping lines to a grid.

‘none’

No slack: each line box is snapped to the grid.

<integer>

Nominally, each nth line box is snapped to the grid or each nth grid line has a line box snapped to it, whichever results in smaller shifts, for the specified integer as n. A value of ‘1’ allows, for example, lines of half the grid height to fit twice as densely, while a value of ‘2’ also allows lines 1.5 times the grid height to match the grid 2:3. A value less than ‘1’ is invalid.

When ‘line-snap’ is given as a <baseline>, the ‘line-slack’ property interpolates a grid against the main line grid and snaps the element's line boxes against this grid. The interpolated grid is drawn by creating a new line grid using the baseline table and ‘line-height’ of the current element and increasing the inter-line spacing of this new grid to snap according to the element's ‘line-snap’ and ‘line-slack’ values. The first snapped line is chosen to minimize the shift of the first line box of the element. The interpolated grid is only used for this block's line boxes; descendant boxes interpolate their own grid against the main grid.)

Define something sane for ‘line-snap: contain’.

User Agents are not required to support values greater than 4; unsupported used values higher than 4 must be clamped to 4.

Can it be extended to handle vertical justification?

3.3. Snapping Block Boxes: the ‘box-snap’ property

Specifies which edges, if any, are snapped to the line box grid. If four values are specified, they apply to the before, end, after, and start edges of the box, respectively. If the start edge is omitted, it is the same as the end. If the after edge is omitted, it is the same as the before. If the end edge is omitted, it is assigned ‘none’. Values have the following meanings:

none

The edge is not snapped to any grid.

margin-box

The margin edge is snapped to the nearest grid line.

border-box

The border edge is snapped to the nearest grid line.

half-border

The edge halfway between the border and padding edges is snapped to the nearest grid line.

<identifier>

Names the grid to snap to. Defaults to the effective ‘line-grid’. If the named grid does not exist (or has no lines in the appropriate dimension), then the affected edge is not snapped.

This allows the author to specify a grid that is not a line grid.

When snapping to baselines on a line grid, either the text-over or text-under baseline is chosen: whichever one is on the matching side of the central baseline. For example, when snapping the before edge in horizontal writing mode, the over edge is chosen. In some cases the under edge might be used instead for the before edge: for example, when the writing mode of the line grid doesn't match that of the affected element, or when due to the ‘text-orientation’ settings the under side corresponds to the after edge.

To snap a before, start, or end edge of a block-level element or its after margin-box edge to a grid line, the effective margin is increased at that edge. To snap an after border-box or half-border edge to a grid line, the effective content-box height is increased. If, however, the box is an empty block that could be collapsed through, then this property has no effect. [CSS21]

When applied to table row group and table row boxes, ‘box-snap’ only affects the before and after edges, and only if those edges are not at the beginning or end of the table, respectively. To snap a before edge on a table row or row group, the preceding row's height is increased. To snap an after edge on a table row or row group, the affected row's height is increased.

When applied to table column group and table column boxes, ‘box-snap’ only affects the start and end edges, and only if those edges are not at the start or end of the table, respectively. How the space is redistributed among columns to satisfy snapping constraints is not defined, however:

• In an auto-sized table no column may be smaller than its minimum content width.
• The resulting table must not exceed its original measure if it had a non-‘auto’ measure.
• The adjusted widths must not cause the table to overflow its containing block any more than it would with ‘box-snap: none’.

To satisfy these constraints, some column edges may remain unsnapped.

For internal table elements in separated borders tables, ‘margin-box’ represents the edge at the midpoint of the border spacing. For collapsed tables, ‘margin-box’ and ‘border-box’ are treated as ‘half-border’. When two coinciding edges have conflicting snap values, the later value wins.

4. Layout in Grid Units

This section is mostly unrevised brainstorming.

4.1. Grid Length Units: the ‘gd’ unit

The existence of a grid in an element makes it possible and very useful to express various measurements in that element in terms of grid units. Grid units are used very frequently in East Asian typography, especially for the left, right, top and bottom element margins.

Therefore, a new length unit is necessary, gd, to enable the author to specify the various measurements in terms of the grid.

For example, consider the following style:

P { layout-grid: strict both 20pt 15pt; margin: 1gd 3gd 1gd 2gd }

This way, all P elements would effectively acquire a 15pt top margin, a 60pt right margin, a 15pt bottom margin and a 40pt left margin.

If no grid is specified, the gd unit should be treated the same as the em unit.

Just copied from old WD, need to refresh

p {
  layout-grid-line: 1.5;
  layout-grid-mode: baseline;
  margin-top: 1gd;
  margin-bottom: 1gd;
}

h1 {
  line-grid-mode: block;
  min-height: 3gd; /* 3 Gyou-dori */
  margin-top: 1gd;
}

‘gem’ length unit is also requested

4.2. Rounding Lengths to Multiple of Units: ‘rounddown()’ and ‘roundup()’

The ‘width’ and ‘height’ properties accept ‘rounddown(length)’ and ‘roundup(length)’ values.

The ‘rounddown(length)’ computes to ‘auto’, and then the result will be rounded down to the largest multiple of the specified unit that is equal to or less than the original computed value.

Similarly, the ‘roundup(length)’ computes to ‘auto’, and then the result will be rounded up to the smallest multiple of the specified unit that is equal to or greater than the original computed value.

The fraction is distributed evenly to the computed values of margins on each side. If it is used for ‘height’, half the fraction is added to the top and the bottom margins. If for ‘width’, to the left and the right margins.

When the element is a float, the fraction is added to the opposite side of the float direction instead. For example, if the ‘float’ property is ‘left’, the fraction is added to the right margins.

Is this design appropriate to solve text with floats case?

Are there requirements for table cell width?

Should have an option to left/right align instead of center? Can author align running head and page numbers with body with this design?

Use cases for non-East Asian layouts?

body {
  margin: 3cm 3.5cm 3cm 3cm;
  text-align: justify;
  width: rounddown(1em);
}
.sidefigure {
  float: right;
  width: roundup(1em);
}

5. Conformance

5.1. Document Conventions

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.

5.2. Conformance Classes

Conformance to CSS TEMPLATE Module is defined for three conformance classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

A style sheet is conformant to CSS TEMPLATE Module if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.

A renderer is conformant to CSS TEMPLATE Module if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the properties defined by CSS TEMPLATE Module 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 CSS TEMPLATE Module if it writes syntactically correct style sheets, according to the generic CSS grammar and the individual grammars of each property in this module.

5.3. Partial Implementations

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.

5.4. Experimental Implementations

To avoid clashes with future CSS features, the CSS specifications reserve a prefixed syntax for proprietary property and value extensions to CSS. The CSS Working Group recommends that experimental implementations of features in CSS Working Drafts also use vendor-prefixed property or value names. This avoids any incompatibilities with future changes in the draft. Once a specification reaches the Candidate Recommendation stage, implementors should implement the non-prefixed syntax for any feature they consider to be correctly implemented according to spec.

5.5. CR Exit Criteria

[Change or remove the following CR exit criteria if the spec is not a module, but, e.g., a Note or a profile. This text was decided on 2008-06-04.]

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

interoperable

passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

implementation

a user agent which:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

Acknowledgments

[acknowledgments]

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

Other references

[CSS1]

Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS1-20080411

[CSS3COLOR]

Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607

[CSS3LINE]

Michel Suignard; Eric A. Meyer. CSS3 module: line. 15 May 2002. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2002/WD-css3-linebox-20020515

[JLREQ]

Yasuhiro Anan; et al. Requirements for Japanese Text Layout. 29 November 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-jlreq-20111129/

Index

• authoring tool, 5.2.
• ‘baseline’, 3.1.
• border-box, 3.3.
• box-snap, 3.3.
• ‘contain’, 3.1.
• half-border, 3.3.
• <identifier>, 3.3.
• <integer>, 3.2.
• length unit
  • rounddown(), 4.2., 4.2.
• line-grid, 2.
• line-slack, 3.2.
• line-snap, 3.1.
• margin-box, 3.3.
• none, 3.3.
• ‘none’, 3.1., 3.2.
• renderer, 5.2.
• style sheet
  • as conformance class, 5.2.

Property index