CSS Rhythmic Sizing

W3C First Public Working Draft,

This version:
https://www.w3.org/TR/2017/WD-css-rhythm-1-20170326/
Editor's Draft:
https://drafts.csswg.org/css-rhythm/
Test Suite:
http://test.csswg.org/suites/css-rhythm-1_dev/nightly-unstable/
Issue Tracking:
Inline In Spec
GitHub Issues
Editors:
(Google)
Elika J. Etemad / fantasai (Invited Expert)

Abstract

This module contains CSS features for aligning content size to multiple of unit size.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document is a First Public Working Draft.

Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-rhythm” in the title, preferably like this: “[css-rhythm] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group.

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.

This document is governed by the 1 March 2017 W3C Process Document.

1. Introduction

This specification provides features to control sizes of CSS objects according to the rules desired by use cases.

Controlling sizes of CSS objects to be multiple of a unit is desired in many cases. This level of the specification focuses on following cases.

By controlling heights of line boxes, lines of text in different fonts can create consistent visuals to help readability.

Also by stacking such line boxes, authors can align lines across columns, pages, scroll-snapped blocks, or multiple blocks placed absolutely, to produce vertical rhythm.

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

2. Adjusting Line Box Heights: the line-height-step property

Name: line-height-step
Value: none | <length>
Initial: none
Applies to: block containers
Inherited: yes
Percentages: N/A
Media: visual
Computed value: the absolute length, 0 for none
Canonical order: per grammar
Animatable: no

This property defines the step unit for line box heights. When the step unit is set to a positive <length>, the line box heights are rounded up to the closest multiple of the unit. Negative <length> values are invalid.

[CSS21] §10.8 Line height calculations defines how to compute the height of a line box from its inline-level content. The rounding is applied to the resulting height of the line box, and the additional space is distributed to over-side and under-side of the line box equally, so that the original line box appears at the center of the multiple of step unit. This adjustment is done by assuming that there is an inline-level box that has adjusted A' and D' in the line box. This inline-level box does not affect alignment points of the vertical-align property, except values that align relative to the line box.

Rounding up the computed line box height.

none and 0 are equivalent. Is this ok?
Should this be animatable? There doesn’t seem to be use cases but needed for consistency?

In the following example, the height of line box in each paragraph is rounded up to the step unit.

:root {
  font-size: 12pt;
  --my-grid: 18pt;
  line-height-step: var(--my-grid);
}
h1 {
  font-size: 20pt;
  margin-top: calc(2 * var(--my-grid));
}
p {
  margin: 0;
}

The line box in <h1> does not fit into one step unit and thus occupies two, but it is still centered within the two step unit.

Authors can keep margins or other properties to be multiple of step unit using var() and calc() as in the example above.

If author prefers, tools like Sass can make such declarations shorter.

$gu: 18px;

@function gu($n) {
  @return $n * $gu;
}

h1 {
  font-size: 20pt;
  margin: gu(1.2) auto gu(1.8);
}
It is usually recommended to set the line-height lower than the step unit. The used line height can increase due to several factors such as the use of vertical-align or font fallback.

3. Adjusting Block-level Box Heights

This proposal can be simplified down to just the block-step-size property, represented solely through its shortened form as block-step. This level will likely at most contain block-step-size and block-step-insert, leaving block-step-align and block-step-round to be added if the future demands. The full design is described herein for current discussion and future reference.

This proposal is currently defined to apply only to block-level boxes. This limitation is solely to simplify the first iteration; it should eventually be extended to all layout modes that honor specified heights.

3.1. Specifying the Step Size: the block-step-size property

Name: block-step-size
Value: none | <length>
Initial: none
Applies to: block-level boxes
Inherited: no
Percentages: N/A
Media: visual
Computed value: keyword or absolute length
Canonical order: per grammar
Animatable: ???

This property defines the step unit for a block-level box’s block size. When the step unit is set to a positive <length>, the box’s outer height is rounded (see block-step-round) to the closest multiple of the unit. Negative <length> values are invalid.

Values other than none cause the box to establish a new formatting context.

In situations where margins collapse, only the box’s own margin is considered in calculating its outer size.

3.2. Specifying the Spacing Type: the block-step-insert property

Name: block-step-insert
Value: margin | padding
Initial: margin
Applies to: block-level boxes
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Canonical order: per grammar
Animatable: no

This property specifies whether extra spacing derived from applying block-step-size is inserted inside (like padding) or outside (like margin) the box’s border.

Values have the following meanings:

margin
Any extra space resulting from a block-step-size-induced adjustment is inserted outside the box’s border, as extra margin.
padding
Any extra space resulting from a block-step-size-induced adjustment is inserted inside the box’s border, as extra padding.

3.3. Specifying Alignment: the block-step-align property

Name: block-step-align
Value: auto | center | start | end
Initial: auto
Applies to: block-level boxes
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Canonical order: per grammar
Animatable: no

This property specifies whether extra spacing derived from applying block-step-size is inserted before, inserted after, or split between both sides of the box.

Values have the following meanings:

auto
If block-step-insert is margin: if align-self is start, end, or center, treat as that value, otherwise treat as center.
center
Any extra space resulting from a block-step-size-induced adjustment is split, and applied half on either side of the box.
start
Any extra space resulting from a block-step-size-induced adjustment is inserted on the start side of the box.
end
Any extra space resulting from a block-step-size-induced adjustment is inserted on the end side of the box.

3.4. Rounding Method: the block-step-round property

Name: block-step-round
Value: up | down | nearest
Initial: up
Applies to: block-level boxes
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Canonical order: per grammar
Animatable: no

This property specifies whether adjustments due to block-step-size insert positive or negative space.

Values have the following meanings:

up
The outer size of the box is increased (positive space is inserted) to fulfill the block-step-size constraint.
down
The outer size of the box is decreased (negative space is inserted) to fulfill the block-step-size constraint.
nearest
The outer size of the box is either increased (as for up) or decreased (as for downwhichever results in the smallest absolute change—to fulfill the block-step-size constraint. If both options would result in the same amount of change, the size is increased.

3.5. Block Step Adjustment Shorthand: the block-step shorthand

Name: block-step
Value: <block-step-size> || <block-step-insert> || <block-step-align> || <block-step-round>
Initial: See individual properties
Applies to: block-level boxes
Inherited: no
Percentages: N/A
Media: visual
Computed value: See individual properties
Canonical order: per grammar
Animatable: See individual properties

This shorthand property allows for setting block-step-size, block-step-insert, block-step-align, and block-step-round in one declaration. Omitted values are set to the property’s initial value.

Authors are advised to use this shorthand rather than the longhands unless there is a specific need for its individual longhands to cascade independently.

4. Privacy and Security Considerations

This specification introduces no new privacy leaks, or security considerations beyond "implement it correctly".

5. Acknowledgments

This specification would not have been possible without the help from: Takao Baba, Chris Eppstein, Shinyu Murakami, Tsutomu Nanjo, Charlie Neely, Florian Rivoal, Hiroshi Sakakibara, Alan Stearns, and the CSS Working Group members.

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:

This is an example of an informative example.

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.

Requirements for Responsible Implementation of CSS

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

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 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.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

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.

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

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ALIGN-3]
Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. URL: https://www.w3.org/TR/css-align-3/
[CSS-CASCADE-4]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. URL: https://www.w3.org/TR/css-cascade-4/
[CSS-INLINE-3]
Dave Cramer; Elika Etemad; Steve Zilles. CSS Inline Layout Module Level 3. URL: https://www.w3.org/TR/css-inline-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. URL: https://www.w3.org/TR/css-values-3/
[CSS-WRITING-MODES-4]
CSS Writing Modes Module Level 4 URL: https://www.w3.org/TR/css-writing-modes-4/
[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS2
[CSS22]
Bert Bos. Cascading Style Sheets Level 2 Revision 2 (CSS 2.2) Specification. URL: https://www.w3.org/TR/CSS22/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Informative References

[CSS-VARIABLES-1]
Tab Atkins Jr.. CSS Custom Properties for Cascading Variables Module Level 1. URL: https://www.w3.org/TR/css-variables-1/

Property Index

Name Value Initial Applies to Inh. %ages Media Ani­mat­able Canonical order Com­puted value
line-height-step none | <length> none block containers yes N/A visual no per grammar the absolute length, 0 for none
block-step-size none | <length> none block-level boxes no N/A visual ??? per grammar keyword or absolute length
block-step-insert margin | padding margin block-level boxes no N/A visual no per grammar as specified
block-step-align auto | center | start | end auto block-level boxes no N/A visual no per grammar as specified
block-step-round up | down | nearest up block-level boxes no N/A visual no per grammar as specified
block-step <block-step-size> || <block-step-insert> || <block-step-align> || <block-step-round> See individual properties block-level boxes no N/A visual See individual properties per grammar See individual properties

Issues Index

none and 0 are equivalent. Is this ok?
Should this be animatable? There doesn’t seem to be use cases but needed for consistency?
This proposal can be simplified down to just the block-step-size property, represented solely through its shortened form as block-step. This level will likely at most contain block-step-size and block-step-insert, leaving block-step-align and block-step-round to be added if the future demands. The full design is described herein for current discussion and future reference.
This proposal is currently defined to apply only to block-level boxes. This limitation is solely to simplify the first iteration; it should eventually be extended to all layout modes that honor specified heights.