CSS Line Grid Module Level 1

Abstract

Status of this document

Table of Contents

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.
• Controlling how lines and blocks align to the grids.

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 whether this box creates a new baseline grid for its descendants or uses the same baseline grid as its parent. (Each box always has an associated line grid. However, whether a box or its contents snap to a line grid is determined by line-snap and box-snap.)

The values of this property have the following meanings:

match-parent

Box assumes the line grid of its parent.

create

Box creates a new line grid using its own font and line layout settings. 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 box is paginated, the line grid is restarted on each page; since line boxes cannot be fragmented, no page begins with the bottom part of a line’s grid.

The names of these values is currently up for debate. Current suggestions for match-parent include match-parent and normal; those for create include create and new.

The original proposal for line grids allowed an element to create a named grid. This property could still be extended to do this in the future.

Håkon points out that there might be a need to have line grids aligned to the page box rather than the page content box. The current proposal has no way to switch between the two.

There might need to be an offset for more complicated designs. How to set this offset is problematic: usually it’s not a fixed length, but the distance to clear some header content. This could be added to a later level of 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 (usually 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].) Shifting line boxes in this way affects layout – it is not merely a display translation. If a line box is shifted downward, then subsequent line boxes will be laid out using the new shifted position as input to their line stacking rules.

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 is centered between one of the line grid’s text-over-edge baselines and a subsequent (but not necessarily consecutive) text-under-edge 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.

Line boxes almost always shift downward (towards the block-end direction) when snapping to a line grid. Here there are three lines with 20px line-height and line-snap:baseline that should snap to a 30px line grid. Each line box shifts down so that the baselines align with the grid lines.

In the figures below, there are two additional lines from h3 elements with line-snap:none. These lines do not shift to align to the grid, but their positions can change based on the shifting of lines around them. In this example, lines 1 2 and 3 shift down to snap their baselines to the grid lines, and line B has normal line box placement just below the line above.

The block containing all of these lines might not be top-aligned within its container. In the figures below, the block containing the elements is centered. In a centered situation, you have to align baselines while maintaining centering. This can be done in two shift-and-center steps.

First, shift the snapping lines as if the block was top-aligned (as in figure 9 above), then remove the shift for the very first snapping line. After removing the first shift, try centering the block. This is almost certain to throw the baseline alignment off. You can see one such result in the partial shifting figure below.

Second, measure the distance from the first snapped line’s baseline to the grid lines above and below, looking for the closest grid line to that baseline.

If the closest grid line is in the block-start direction, then add space below the last line in the block equal to twice that distance. Then the block is centered again, which will align all of the snapped lines to the grid.

If the closest grid line is in the block-end direction, then the first snapped line is shifted downward by twice that distance. Then the block is centered again, which will again align all of the snapped lines to the grid.

An end-aligned block also uses two steps, but is simpler than the centered case.

First, shift the snapping lines as if there were no end-alignment (as in figure 9 above), then end-align the block.

Second, shift the entire block contents upwards until the last snapped line aligns to a grid line. In this example, the shift is very minor.

3.2 Snapping Block Boxes: the box-snap property

This is a rough draft of trying to solve the box-snapping problem.

Some optional box values (margin-box, border-box) could be added to the before and after values to allow snapping various box model edges to the line grid.

An auto value could be useful - one that defaults to center, but snaps to before if it’s the first block in a fragment container, and snaps to after if it’s the last block in a fragment container.

Specifies how the block is snapped to the baseline grid.

Values have the following meanings:

none

The block is not snapped to any grid.

block-start

The block-start edge is snapped to the nearest grid line.

block-end

The block-end edge is snapped to the nearest grid line.

center

The block is centered centered between one of the baseline grid’s text-over baselines and a subsequent (but not necessarily consecutive) text-under baseline.

first-baseline

The first line box’s dominant baseline is snapped to the nearest grid line.

last-baseline

The last line box’s dominant baseline is snapped to the nearest grid line.

When snapping to baselines on a line grid, either the text-over-edge or text-under-edge baseline is chosen: whichever one is on the matching side of the central baseline. For example, when snapping the block-start edge in horizontal writing mode, the text-over-edge is chosen. In some cases the text-under-edge might be used instead for the block-start 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 text-under-edge corresponds to the block-start edge.

To snap a block-level element to a grid line, the effective margin is increased at that edge. 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.

Acknowledgments

This module was made possible by the advice and contributions of Dave Cramer, Dave Hyatt, Bem Jones-Bey, and the CSS Working Group members.

Change Log

Since April 3rd 2014

• Added line-snapping examples
• Modified box-snap values
• Removed line-slack
• Removed grid offset
• Removed grid units
• Removed 2d box snapping
• Removed rounding functions

Conformance

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.

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.

Conformance classes

Conformance to this specification 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 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.

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.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

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.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

References

Normative References

Informative References

Index

Property index

Issues Index