Overview.bs

<style>
.example .figure img {
  background-color: white;
  padding: 20px;
  margin: 20px;
}
</style>
<pre class=metadata>
Title: CSS Rhythmic Sizing
Shortname: css-rhythm
Level: 1
Group: CSSWG
Status: ED
Work Status: exploring
Editor: Koji Ishii, Google, kojiishi@gmail.com, w3cid 45369
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
ED: https://drafts.csswg.org/css-rhythm/
TR: https://www.w3.org/TR/css-rhythm-1/
Abstract: This module contains CSS features for aligning content size
  to multiple of unit size.
</pre>
<pre class=link-defaults>
spec:css2; type:property; text:max-height
spec:css2; type:property; text:max-width
spec:css2; type:property; text:min-width
spec:css-writing-modes-4; type:dfn; text:end
spec:css-writing-modes-4; type:dfn; text:start
spec:css-break-3; type:dfn; text:fragment
</pre>

Introduction {#intro}
=====================

	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.

	* Adjust heights of block-level boxes to multiples of the specified length.
	* Adjust heights of line boxes to multiples of the specified length.

	By stacking blocks at multiples,
	authors can align content across columns, pages, scroll-snapped bocks
	or multiple blocks placed absolutely
	to maintain vertical rhythm.

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

	<figure>
		<a href="examples/snap-height.html">
			<img src="images/snap-height-sample.png"></a>
		<figcaption>Vertical rhythm kept through pictures and different size of text in a multi-column document.</figcaption>
	</figure>

East Asian Casual Vertical Rhythms {#eastasia}
----------------------------------------------

In East Asia, a casual variant of vertical rhythm is widely used.

Vertical rhythm is typically used in professional typography.
While it improves readability,
its spacing constraints require careful and well-thought design of spaces.

The East Asian casual variant was
originally a product of technical constraints of traditional word processors in '80s.
But when the technical constraints were lifted in more modern technologies,
with the help of square-like visual of ideographic characters,
East Asian authors preferred to keep parts of the characteristics.

In this variant of vertical rhythm,
the requirement is loosened for the ease of use for non-professional authors.
Text is still on the rhythm,
so that the majority of ideographic characters are mostly on grid,
but when author specifies borders, margins, or some other objects that may break the rhythm,
the rhythm is shifted rather than forced.
The strict vertical rhythm often surprises non-professional authors by forced jumps in such cases,
while this variant combines rhythm on text and the ease of use for non-professional authors.

This variant was very widely accepted in East Asia since the middle of '90s,
such that most major word processors used in East Asia provided similar features
by default.

In East Asian publishing typography,
the vertical rhythm is one of important properties,
but its priority compared to other properties
varies by types of documents.
In single column documents,
the priority is weaker than that of multi-column documents.
Text should be on the vertical rhythm,
but it is often preferred for borders, margins,
or other properties to win over the rhythm.
In such cases, the rhythm is shifted, similar to the casual variant.

In this specification,
when the 'line-height-step' property is used without combination of the 'block-step' property
or the 'line-grid' property,
it produces the similar effect as the East Asian casual vertical rhythm.

It may also serve good for East Asian publishing typography,
depends on the desired strength of the vertical rhythm.

For other cases of vertical rhythm,
it is expected that
the 'block-step' property or the 'line-grid' property are used,
or that the 'line-height-step' property is used together with them.

Value Definitions {#values}
---------------------------

	This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
	using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
	Value types not defined in this specification are defined in CSS Values &amp; 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 <a>CSS-wide keywords</a> as their property value.
	For readability they have not been repeated explicitly.

Adjusting Block-level Box Heights {#block-height}
=================================

	The most common typographic rhythm problems are block-level intrusions:
	when continuous paragraph-level text is interrupted
	by differently-sized content such as illustrations and headings,
	then the line-to-line rhythm can be thrown off.
	The block step properties allow such elements
	to be fitted to the rhythmic interval
	by constraining their height to a multiple of a specified step size.
	This allows content before and after the interruption
	to maintain a continuous rhythm.

		<figure>
			<img src="images/block-step-size-before.png"
			width="500">
			<figcaption>Heads and blockquotes have varying font sizes and line heights,
			resulting in uneven text across columns.</figcaption>
		</figure>



		<figure>
			<img src="images/block-step-size-after.png"
			width="500">
			<figcaption>Space inserted before and after blocks (shown with colored borders)
			restores vertical rhythm.</figcaption>
		</figure>

	While consistent use of the block step properties
	can produce the strictly gridded layout needed
	for parallel content flows,
	per-element specification of the step size
	also allows some interruptions to take their natural size in the flow,
	restarting the vertical rhythm afterwards.
	This can be useful in single-column layouts
	where the vertical rhythm is important to maintain visual continuity,
	but there is nothing alongside to align to.
	In these cases,
	large interruptions which visually disconnect the flow before and after
	can prioritize optimal spacing around the item
	over strict adherence to a continuous grid
	(by specifying ''block-step: none'', the initial value).

	ISSUE: 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.

	ISSUE: 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.

Specifying the Step Size: the 'block-step-size' property {#block-step-size}
--------------------------------------------------------

	<pre class="propdef">
	Name: block-step-size
	Value: none | <<length [0,∞]>>
	Initial: none
	Applies to: block-level boxes
	Inherited: no
	Percentages: N/A
	Computed Value: specified keyword or absolute length
	Animation type: by computed value type
	</pre>

	This property defines the <a>step unit</a> for a block-level box’s <a>block size</a>.
	When the <a>step unit</a> 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 ''block-step-size/none''
	cause the box to [=establish an independent formatting context=].

	In situations where margins <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a>,
	only the box’s own margin is considered
	in calculating its outer size.

	When a box <a>fragments</a>,
	step sizing is applied per fragment.

Specifying the Spacing Type: the 'block-step-insert' property {#block-step-insert}
--------------------------------------------------------

	<pre class="propdef">
	Name: block-step-insert
	Value: margin | padding
	Initial: margin
	Applies to: block-level boxes
	Inherited: no
	Percentages: N/A
	Computed Value: specified keyword
	Animation type: discrete
	</pre>

	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:

	<dl dfn-for="block-step-insert" dfn-type="value">
		<dt><dfn>margin</dfn>
		<dd>
			Any extra space resulting from a 'block-step-size'-induced adjustment
			is inserted outside the box’s border, as extra margin.

		<dt><dfn>padding</dfn>
		<dd>
			Any extra space resulting from a 'block-step-size'-induced adjustment
			is inserted inside the box’s border, as extra padding.
	</dl>

Specifying Alignment: the 'block-step-align' property {#block-step-align}
-----------------------------------------------------

	<pre class="propdef">
	Name: block-step-align
	Value: auto | center | start | end
	Initial: auto
	Applies to: block-level boxes
	Inherited: no
	Percentages: N/A
	Computed Value: specified keyword
	Animation type: discrete
	</pre>

	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:

	<dl dfn-for="block-step-align" dfn-type="value">
		<dt><dfn>auto</dfn>
		<dd>
			If 'block-step-insert' is ''margin'':
			if 'align-self' is ''start'', ''end'', or ''center'', treat as that value,
			otherwise treat as ''center''.

		<dt><dfn>center</dfn>
		<dd>
			Any extra space resulting from a 'block-step-size'-induced adjustment
			is split, and applied half on either side of the box.
		<dt><dfn>start</dfn>
		<dd>
			Any extra space resulting from a 'block-step-size'-induced adjustment
			is inserted on the <a>end</a> side of the box.
		<dt><dfn>end</dfn>
		<dd>
			Any extra space resulting from a 'block-step-size'-induced adjustment
			is inserted on the <a>start</a> side of the box.
	</dl>

	In all cases,
	additional spacing cannot be added to margins
	that are truncated or omitted due to unforced fragmentation breaks
	(see [[CSS3-PAGE]] and [[CSS-BREAK-3]]);
	therefore if 'block-step-insert' is ''margin'',
	all extra space derived from applying 'block-step-size'
	must be inserted on the opposite side of the fragment
	(regardless of 'block-step-align').

Rounding Method: the 'block-step-round' property {#block-step-round}
------------------------------------------------

	<pre class="propdef">
	Name: block-step-round
	Value: up | down | nearest
	Initial: up
	Applies to: block-level boxes
	Inherited: no
	Percentages: N/A
	Computed Value: specified keyword
	Animation type: discrete
	</pre>

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

	Values have the following meanings:

	<dl dfn-for="block-step-align" dfn-type="value">
		<dt><dfn>up</dfn>
		<dd>
			The outer size of the box is <em>increased</em>
			(positive space is inserted)
			to fulfill the 'block-step-size' constraint.

		<dt><dfn>down</dfn>
		<dd>
			The outer size of the box is <em>decreased</em>
			(negative space is inserted)
			to fulfill the 'block-step-size' constraint.

		<dt><dfn>nearest</dfn>
		<dd>
			The outer size of the box is either
			<em>increased</em> (as for ''up'') or <em>decreased</em> (as for ''down''--
			whichever 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.
	</dl>

Block Step Adjustment Shorthand: the 'block-step' shorthand {#block-step}
-----------------------------------------------------------

	<pre class="propdef shorthand">
	Name: block-step
	Value: <<'block-step-size'>> || <<'block-step-insert'>> || <<'block-step-align'>> || <<'block-step-round'>>
	Applies to: block-level boxes
	Inherited: no
	Percentages: N/A
	</pre>

	This <a>shorthand property</a> 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.

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

Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
===============================================================================

	<pre class='propdef'>
	Name: line-height-step
	Value: <<length [0,∞]>>
	Initial: 0px
	Applies to: block containers
	Inherited: yes
	Percentages: N/A
	Computed Value: absolute length
	Animation type: by computed value type
	</pre>

This property defines the <dfn>step unit</dfn> for line box heights.
A <a>step unit</a> is the length of the vertical rhythm,
usually the distance from one baseline to the next baseline of the body text.
Body text should fit into one <a>step unit</a>,
and taller lines such as headings
should have heights of two or more <a>step unit</a>s.
Vertical rhythm is created
by making heights of all lines an integer multiple of the <a>step unit</a>.

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

	[[!CSS21]] <a href="https://drafts.csswg.org/css2/visudet.html#line-height">&#xA7;10.8 Line height calculations</a>
	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
	<a>over</a>-side and <a>under</a>-side of the line box equally,
	so that the original line box appears at the center of the
	multiple of <a>step unit</a>.
	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.

	<figure>
	<img src="images/adjust-line-height.svg">
	<figcaption>Rounding up the computed line box height.</figcaption>
	</figure>

	<div class="issue">Should this be animatable?
	There doesn't seem to be use cases but needed for consistency?</div>

	<div class="example">
		<figure style="float:right">
			<img src="images/line-grid-center.svg" height="300">
		</figure>

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

		<pre class="lang-css">
			: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;
			}
		</pre>

		The line box in <code>&lt;h1&gt;</code> does not fit into one <a>step unit</a>
		and thus occupies two,
		but it is still centered within the two <a>step unit</a>.
	</div>

	<div class="example">
	Authors can keep margins or other properties to be multiple of <a>step unit</a>
	using ''var()'' and ''calc()'' as in the example above.

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

		<pre class="lang-css">
		  $gu: 18px;

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

		  h1 {
		    font-size: 20pt;
		    margin: gu(1.2) auto gu(1.8);
		  }
		</pre>
	</div>

	<div class="note">
	It is usually recommended to set the 'line-height' lower than
	the <a>step unit</a>.
	The used line height can increase due to several factors such as
	the use of 'vertical-align' or font fallback.
	</div>

<!--
Notes on Block-level Boxes {#inline-block}
------------------------------------------

	<i>This section is not normative.</i>

	This level of the specification does not provide features
	to adjust heights of block-level boxes.

	<div class="example">
	The following CSS turns <code>&lt;h2&gt;</code> to inline-blocks,
	so that the 'line-height-step' property can control its height.

	<pre class="lang-css">
	:root {
	  line-height-step: 18pt;
	}
	h2 {
	  display: inline-block;
	  width: 100%;
	  line-height-step: 0;
	  line-height: 1.2;
	}
	</pre>

	When an <code>&lt;h2&gt;</code> is long enough to wrap,
	text inside the <code>&lt;h2&gt;</code> uses ''line-height: 1.2'', while
	the height of the <code>&lt;h2&gt;</code> block is rounded up
	to the multiple of ''18pt''.
	See <a href="examples/snap-height.html">a sample in action</a>.
	</div>
-->

Privacy and Security Considerations {#priv-sec}
===============================================

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

Acknowledgments {#acks}
=======================

	This specification would not have been possible without the help from:
	Masaharu Akutsu,
	Yoko Aoki,
	Takao Baba,
	Chris Eppstein,
	Ichiro Inaba,
	Jxck,
	Noriko Kase,
	Motoya Kinoshita,
	Shinyu Murakami,
	Tsutomu Nanjo,
	Kiyoshi Narishima,
	Charlie Neely,
	Takuya Nishimura,
	Katsuhiro Osumi,
	Florian Rivoal,
	Hiroshi Sakakibara,
	Alan Stearns,
	Masataka Yakura,
	KADOKAWA Corporation,
	PixelGrid Inc.,
	and the CSS Working Group members.