Overview.src.html

<h1>CSS Line Grid Module Level 1</h1>

<pre class='metadata'>
Status: ED
Shortname: css-line-grid
Level: 1
Group: csswg
TR: http://www.w3.org/TR/css-line-grid-1/
ED: http://dev.w3.org/csswg/css-line-grid/
Editor: Elika Etemad, Invited Expert, http://fantasai.inkedblade.net/contact
Editor: Koji Ishii, Invited Expert, kojiishi@gluesoft.co.jp
Editor: Alan Stearns, Adobe Systems&#44; Inc., stearns@adobe.com
Abstract: This module contains CSS features for aligning content to a baseline grid.
</pre>

<style type="text/css">
	.sidenote-block {
		border: 1px solid gray;
		margin: auto;
		text-align: left;
		padding: 4pt;
		position: relative;
		max-width: 400pt;
	}
	.sidenote-body {
		margin-left: 120pt;
		font-size: 12pt;
		line-height: 18pt;
	}
	.sidenote-note {
		font-size: 10pt;
		line-height: 18pt;
		position: absolute; left: 4pt; top: 4pt;
		width: 110pt;
	}
</style>

<h2 id='intro'>
Introduction</h2>

<h3 id='context'>
Background</h3>

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

	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:

	<ul>
		<li>Vertical rhythm is kept for better readability.</li>
		<li>Lines are aligned between columns in multi-column documents.</li>
		<li>The top and the bottom margins of pictures are made equal,
			while keeping the vertical rhythm of text before and after the pictures.</li>
		<!--
		<li>Aligning to grids can sometimes be turned off for objects like tables,
			but then turned back on for the following text
			to the same grids as the one for the text before the objects.</li>
		-->
		<li>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.</li>
		<li>East Asian layouts require vertical rhythm
			more often than other scripts do,
			even in single column, non-paged media documents,
			as defined in [[JLREQ]].</li>
	</ul>

	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.

	<div class="figure">
		<img src="line-grid-multicol.png"
			width="480" height="246"
			alt="Vertical rhythm kept through pictures and different size of text in a multi-column document"
			/>
		<p class="caption">Vertical rhythm kept through pictures and different size of text in a multi-column document.</p>
	</div>

	<div class="example">
		<div class="sidefigure">
			<img src="line-grid-wrap.png"
				width="276" height="244"
				alt="Large text wraps within line grids"
				/>
			<p class="caption">Large text wraps within line grids.</p>
		</div>

		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.

		<br style="clear:both;"/>
	</div>

	<div id="ex-sidenote" class="figure">
		<div class="sidenote-block">
			<div class="sidenote-body">

				Sidenotes (and footnotes for that matter) are often set
				at a smaller size than the basic text.
				This smaller text should still line up with the basic text.
				Authors can achieve this effect
				by calculating appropriate font-size, line-height,
				and margins*.
			</div>
			<div class="sidenote-note">

				Only if author controls everything.
				It can easily be broken by user stylesheet, for instance.
			</div>
		</div>
		<p class="caption">Sidenotes are set at a smaller size, but still line up with the basic text.</p>
	</div>

	<div id="ex-width" class="sidefigure">
		<img src="width-multiple-of-em.png"
			width="180" height="142"
			alt="East Asian layouts may require width be a multiple of em without fractions"
			/>
		<p class="caption">East Asian layouts may require width
			be a multiple of <em>em</em> without fractions.</p>
	</div>

	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>em</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:

	<ul>
		<li>Defining grids in the line progression direction.</li>
		<li>Controlling how lines and blocks align to the grids.</li>
	</ul>

	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.


<h3 id='placement'>
Module Interactions</h3>

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

<h3 id='values'>
Values</h3>

	This specification follows the
	<a href="http://www.w3.org/TR/CSS21/about.html#property-defs">
		CSS property definition conventions
	</a> 
	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 &lt;color&gt; 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
	<a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
	keyword as their property value. 
	For readability it has not been repeated explicitly.
	
<h2 id='line-grid'>
Defining a Line Grid: the 'line-grid' property</h2>

	<pre class='propdef'>
		Name: line-grid
		Value: match-parent | create
		Initial: match-parent
		Applies to: block containers
		Inherited: no
		Animatable: no
		Percentages: N/A
		Media: visual
		Computed Value: as specified
	</pre>

	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:

	<dl dfn-type="value" dfn-for="line-grid">
		<dt><dfn>match-parent</dfn>
		<dd>Box assumes the line grid of its parent.

		<dt><dfn>create</dfn>
		<dd>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.
	</dl>

	Issue: 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''.

	<p class="note">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.

	Issue: <a href="http://www.w3.org/mid/21262.26596.32230.32937@gargle.gargle.HOWL">Håkon points out</a> 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.
	
	<p class="note">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.

<h2 id='grid-snapping'>
Snapping to a Grid</h2>

<h3 id='line-snap'>
Snapping Line Boxes: the 'line-snap' property</h3>

	<pre class='propdef'>
		Name: line-snap
		Value: none | baseline | contain
		Initial: none
		Applies to: all elements
		Inherited: yes
		Animatable: no
		Percentages: N/A
		Media: visual
		Computed Value: as specified
	</pre>

	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:

	<dl dfn-type="value" dfn-for="line-snap">
	<dt><dfn>none</dfn>
		<dd>Line boxes do not snap to the grid; they stack normally.
	<dt><dfn>baseline</dfn>
		<dd>The dominant baseline snaps with the matching baseline on the
		line grid applying to the element.
	<dt><dfn>contain</dfn>
		<dd>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.
	</dl>

	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.

	<div class="example">

	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.

		<figure style="float:left;">
			<img src="images/simple-no-snap.png" 
				alt="line positions before snapping"/>
			<figcaption>
				Before line snapping
			</figcaption>
		</figure>
		<figure style="float:left;">
			<img src="images/simple-snapped.png" 
				alt="line positions after snapping"/>
			<figcaption>
				After line snapping
			</figcaption>
		</figure>
		<p style="clear:left;">
			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.
		</p>
		<figure style="float:left;">
			<img src="images/top-no-snap.png" 
				alt="line positions before snapping"/>
			<figcaption>
				Before line snapping
			</figcaption>
		</figure>
		<figure style="float:left;">
			<img src="images/top-snapped.png" 
				alt="line positions after snapping"/>
			<figcaption>
				After line snapping
			</figcaption>
		</figure>
	</div>

	<div class="example">

		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 there were no centering 
		(as in figure 8 above), 
		then remove the shift 
		for the very first snapping line 
		and center the block. 
		You can see the result 
		in the partial shifting figure below.

		In the second step, 
		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.

		<figure style="float:left;">
			<img src="images/first-center-part-snapped.png" 
				alt="line positions at step 1"/>
			<figcaption>
				Partial shifting
			</figcaption>
		</figure>
		<figure style="float:left;">
			<img src="images/first-center-snapped.png" 
				alt="line positions after full snapping"/>
			<figcaption>
				Full line snapping
			</figcaption>
		</figure>

		<p style="clear:left;">
			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.
		</p>
		<figure style="float:left;">
			<img src="images/second-center-part-snapped.png" 
				alt="line positions at step 1"/>
			<figcaption>
				Partial shifting
			</figcaption>
		</figure>
		<figure style="float:left;">
			<img src="images/second-center-snapped.png" 
				alt="line positions after full snapping"/>
			<figcaption>
				Full line snapping
			</figcaption>
		</figure>
	</div>

	<div class="example">

		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 centering 
		(as in figure 8 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.

		<figure style="float:left;">
			<img src="images/bottom-no-snap.png" 
				alt="line positions before snapping"/>
			<figcaption>
				Before snapping
			</figcaption>
		</figure>
		<figure style="float:left;">
			<img src="images/bottom-snapped.png" 
				alt="line positions after snapping"/>
			<figcaption>
				After line snapping
			</figcaption>
		</figure>
	</div>

<h3 id='box-snap'>
Snapping Block Boxes: the 'box-snap' property</h3>

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

	Issue: 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.

	Issue: 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.

	<pre class='propdef'>
		Name: box-snap
		Value: none | before | after | center | first-baseline | last-baseline 
		Initial: none
		Applies to: block-level boxes and internal table elements except table cells
		Inherited: yes
		Animatable: no
		Percentages: N/A
		Media: visual
		Computed Value: as specified
	</pre>

	Specifies how the block is snapped to the baseline grid.
	
	Values have the following meanings:

	<dl dfn-type="value" dfn-for="box-snap">
	<dt><dfn>none</dfn>
		<dd>The block is not snapped to any grid.</dd>
	<dt><dfn>before</dfn>
		<dd>The before edge is snapped to the nearest grid line.
	<dt><dfn>after</dfn>
		<dd>The after edge is snapped to the nearest grid line.
	<dt><dfn>center</dfn>
		<dd>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.
	<dt><dfn>first-baseline</dfn>
		<dd>The first line box's dominant baseline is snapped to the nearest grid line.
	<dt><dfn>last-baseline</dfn>
		<dd>The last line box's dominant baseline is snapped to the nearest grid line.
	</dl>

	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 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 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 
	<a href="http://www.w3.org/TR/CSS21/box.html">collapsed through</a>, 
	then this property has no effect. [[!CSS21]]
  <!--To snap an after border-box or half-border edge to a grid
	line, the effective content-box height is increased.--> 

	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:
	<ul>
		<li>In an auto-sized table no column may be smaller than its minimum content width.
		<li>The resulting table must not exceed its original measure if it had a non-auto measure.
		<li>The adjusted widths must not cause the table to overflow its containing block any more than it would with ''box-snap: none''.
	</ul>
	
	To satisfy these constraints, some column edges may remain unsnapped.

<!-- 
	Line snapping for columnsis processed from start to end: column widths
	are increased or decreased in order to snap the column edges.
	By preference, the widths are increased.
	However if the table has an explicit size or is auto-sized and the resulting
	table overflows its containing block, then columns whose widths were
	increased must, one by one, switch their snap to decreasing the column
	width, until the table reaches its specified size or 
-->
<!--
	<p>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.
-->


<h2 class=no-num id='acknowledgments'>Acknowledgments</h2>

	This module was made possible by the advice and contributions of
	&hellip;