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<!doctype html>
<html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<title>CSS Line Grid Module Level 1</title>
<link href="../default.css" rel="stylesheet" type="text/css">
<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
<style>
body {
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
background-attachment: fixed;
color: black;
font-family: sans-serif;
margin: 0 auto;
max-width: 50em;
padding: 2em 1em 2em 70px;
}
:link { color: #00C; background: transparent }
:visited { color: #609; background: transparent }
a[href]:active { color: #C00; background: transparent }
a[href]:hover { background: #ffa }
a[href] img { border-style: none }
h1, h2, h3, h4, h5, h6 { text-align: left }
h1, h2, h3 { color: #005A9C; }
h1 { font: 170% sans-serif }
h2 { font: 140% sans-serif }
h3 { font: 120% sans-serif }
h4 { font: bold 100% sans-serif }
h5 { font: italic 100% sans-serif }
h6 { font: small-caps 100% sans-serif }
.hide { display: none }
div.head { margin-bottom: 1em }
div.head h1 { margin-top: 2em; clear: both }
div.head table { margin-left: 2em; margin-top: 2em }
p.copyright { font-size: small }
p.copyright small { font-size: small }
pre { margin-left: 2em }
dt { font-weight: bold }
ul.toc, ol.toc {
list-style: none;
}
</style>
<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>
</head>
<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="http://www.w3.org/">
<img alt="W3C" height="48" src="https://www.w3.org/Icons/w3c_home" width="72">
</a>
</p>
<h1 class="p-name no-ref" id="title">CSS Line Grid Module Level 1</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft,
<time class="dt-updated" datetime="2015-01-14">14 January 2015</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:</dt>
<dd><a class="u-url" href="http://dev.w3.org/csswg/css-line-grid/">http://dev.w3.org/csswg/css-line-grid/</a></dd>
<dt>Latest version:</dt>
<dd><a href="http://www.w3.org/TR/css-line-grid-1/">http://www.w3.org/TR/css-line-grid-1/</a></dd>
<dt>Previous Versions:</dt>
<dd><a href="http://www.w3.org/TR/2014/WD-css-line-grid-1-20140916/" rel="previous">http://www.w3.org/TR/2014/WD-css-line-grid-1-20140916/</a></dd>
<dt>Feedback:</dt>
<dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss-line-grid%5D%20feedback">www-style@w3.org</a> with subject line “<kbd>[css-line-grid] <var>… message topic …</var></kbd>” (<a href="http://lists.w3.org/Archives/Public/www-style/" rel="discussion">archives</a>)</span></dd>
<dt>Issue Tracking:</dt>
<dd><a href="#issues-index">Inline In Spec</a></dd>
<dt class="editor">Editors:</dt>
<dd class="editor">
<div class="p-author h-card vcard"><a class="p-name fn u-url url" href="http://fantasai.inkedblade.net/contact">Elika Etemad</a> (<span class="p-org org">Invited Expert</span>)</div>
</dd>
<dd class="editor">
<div class="p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:kojiishi@gluesoft.co.jp">Koji Ishii</a> (<span class="p-org org">Invited Expert</span>)</div>
</dd>
<dd class="editor">
<div class="p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:stearns@adobe.com">Alan Stearns</a> (<span class="p-org org">Adobe Systems, Inc.</span>)</div>
</dd>
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2014 <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="http://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>), All Rights Reserved. W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.
</p>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<div class="p-summary" data-fill-with="abstract">
<p>This module contains CSS features for aligning content to a baseline grid.</p>
<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.</div>
<h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
<div data-fill-with="status">
<p>
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.
</p>
<p>
The (<a href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public mailing list
<a href="mailto:www-style@w3.org?Subject=%5Bcss-line-grid%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>
(see <a href="http://www.w3.org/Mail/Request">instr
72A1
uctions</a>)
is preferred for discussion of this specification.
When sending e-mail,
please put the text “css-line-grid” in the subject,
preferably like this:
“[css-line-grid] <em>…summary of comment…</em>”
</p>
<p>
This document was produced by the <a href="http://www.w3.org/Style/CSS/members">CSS Working Group</a>
(part of the <a href="http://www.w3.org/Style/">Style Activity</a>).
</p>
<p>
This document was produced by a group operating under
the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent Policy</a>.
W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/32061/status" rel="disclosure">public list of any patent disclosures</a>
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 <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a>
must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent Policy</a>.
</p>
<p>
This document is governed by the <a href="http://www.w3.org/2014/Process-20140801/">1 August 2014 W3C Process Document</a>.
</p></div>
<div data-fill-with="at-risk"></div>
<h2 class="no-num no-toc no-ref heading settled" id="contents"><span class="content">Table of Contents</span></h2>
<div data-fill-with="table-of-contents" role="navigation">
<ul class="toc" role="directory">
<li><a href="#intro"><span class="secno">1</span> <span class="content">
Introduction</span></a>
<ul class="toc">
<li><a href="#context"><span class="secno">1.1</span> <span class="content">
Background</span></a></li>
<li><a href="#placement"><span class="secno">1.2</span> <span class="content">
Module Interactions</span></a></li>
<li><a href="#values"><span class="secno">1.3</span> <span class="content">
Values</span></a></li>
</ul>
</li>
<li><a href="#line-grid-property"><span class="secno">2</span> <span class="content">
Defining a <span data-dfn-type="dfn" data-noexport="">Line Grid</span>: the <span class="property" data-link-type="propdesc" title="line-grid">line-grid</span> property</span></a></li>
<li><a href="#grid-snapping"><span class="secno">3</span> <span class="content">
Snapping to a Grid</span></a>
<ul class="toc">
<li><a href="#line-snap"><span class="secno">3.1</span> <span class="content">
Snapping Line Boxes: the <span class="property" data-link-type="propdesc" title="line-snap">line-snap</span> property</span></a></li>
<li><a href="#box-snap"><span class="secno">3.2</span> <span class="content">
Snapping Block Boxes: the <span class="property" data-link-type="propdesc" title="box-snap">box-snap</span> property</span></a></li>
</ul>
</li>
<li><a href="#snap-details"><span class="secno">4</span> <span class="content">
Snapping Details</span></a>
<ul class="toc">
<li><a href="#alignment-interactions"><span class="secno">4.1</span> <span class="content">
Interacting with other alignments</span></a></li>
<li><a href="#leading-adjustments"><span class="secno">4.2</span> <span class="content">
Leading adjustments</span></a></li>
</ul>
</li>
<li><a href="#acknowledgments"><span class="secno"></span> <span class="content">Acknowledgments</span></a></li>
<li><a href="#change-log"><span class="secno"></span> <span class="content">
Change Log</span></a>
<ul class="toc">
<li><a href="#20140916"><span class="secno"></span> <span class="content">
Since <span>September 16th 2014</span></span></a></li>
<li><a href="#20140403"><span class="secno"></span> <span class="content">
Since <span>April 3rd 2014</span></span></a></li>
</ul>
</li>
<li><a href="#conformance"><span class="secno"></span> <span class="content">
Conformance</span></a>
<ul class="toc">
<li><a href="#conventions"><span class="secno"></span> <span class="content">
Document conventions</span></a></li>
<li><a href="#conformance-classes"><span class="secno"></span> <span class="content">
Conformance classes</span></a></li>
<li><a href="#partial"><span class="secno"></span> <span class="content">
Partial implementations</span></a></li>
<li><a href="#experimental"><span class="secno"></span> <span class="content">
Experimental implementations</span></a></li>
<li><a href="#testing"><span class="secno"></span> <span class="content">
Non-experimental implementations</span></a></li>
</ul>
</li>
<li><a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ul class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a></li>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a></li>
</ul>
</li>
<li><a href="#index"><span class="secno"></span> <span class="content">Index</span></a></li&
8096
gt;
<li><a href="#property-index"><span class="secno"></span> <span class="content">Property Index</span></a></li>
<li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a></li>
</ul></div>
<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">
Introduction</span><a class="self-link" href="#intro"></a></h2>
<h3 class="heading settled" data-level="1.1" id="context"><span class="secno">1.1. </span><span class="content">
Background</span><a class="self-link" href="#context"></a></h3>
<p><em>This section is not normative.</em></p>
<p>This specification provides features
to align lines and blocks
to invisible grids in the document.</p>
<p>Aligning lines and blocks to grids
provides the following benefits:</p>
<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>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 <a data-biblio-type="informative" data-link-type="biblio" href="#biblio-jlreq" title="JLREQ">[JLREQ]</a>.</li>
</ul>
<p>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.</p>
<div class="figure">
<img alt="Vertical rhythm kept through pictures and different size of text in a multi-column document" src="images/line-grid-multicol.png" width="480">
<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 alt="Large text wraps within line grids" src="images/line-grid-wrap.png" width="276">
<p class="caption">Large text wraps within line grids.</p>
</div>
<p>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.</p>
<p><br style="clear:both;"></p>
</div>
<div class="figure" id="ex-sidenote">
<div class="sidenote-block">
<div class="sidenote-body">
<p>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 try to achieve this effect
by calculating appropriate font-size, line-height,
and margins*, but lack the proper tools to get the baselines to align.</p>
</div>
<div class="sidenote-note">
<p>Even if the author controls all this,
the baselines won’t align. And careful
calculations can be thrown off by user stylesheets.</p>
</div>
</div>
<p class="caption">Sidenotes are set at a smaller size, and baselines don’t align.</p>
</div>
<figure>
<img alt="sidenote rendering with aligned baselines" src="images/sidenote-aligned.png">
<figcaption>
Sidenote with baselines aligned to the body text.
</figcaption>
</figure>
<div class="sidefigure" id="ex-width">
<img alt="East Asian layouts may require width be a multiple of em without fractions" height="142" src="width-multiple-of-em.png" width="180">
<p class="caption">East Asian layouts may require width
be a multiple of <em>em</em> without fractions.</p>
</div>
<p>East Asian layouts may require grid-like features
in inline progression direction as well.</p>
<p>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.</p>
<p>This module provides the following capabilities:</p>
<ul>
<li>Defining grids in the line progression direction.</li>
<li>Controlling how lines and blocks align to the grids.</li>
</ul>
<p>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.</p>
<h3 class="heading settled" data-level="1.2" id="placement"><span class="secno">1.2. </span><span class="content">
Module Interactions</span><a class="self-link" href="#placement"></a></h3>
<p>This module extends the line box model
defined in <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a> sections 9.4.2 and 10.8.</p>
<h3 class="heading settled" data-level="1.3" id="values"><span class="secno">1.3. </span><span class="content">
Values</span><a class="self-link" href="#values"></a></h3>
<p>This specification follows the
<a href="http://www.w3.org/TR/CSS21/about.html#property-defs">
CSS property definition conventions
</a>
from <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a>.
Value types not defined in this specification
are defined in CSS Level 2 Revision 1 <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a>.
Other CSS modules may expand
the definitions of these value types:
for example <a data-biblio-type="informative" data-link-type="biblio" href="#biblio-css3color" title="CSS3COLOR">[CSS3COLOR]</a>,
when combined with this module,
expands the definition
of the <color> value type
as used in this specification.</p>
<p>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.</p>
<h2 class="heading settled" data-level="2" id="line-grid-property"><span class="secno">2. </span><span class="content">
Defining a <dfn data-dfn-type="dfn" data-noexport="" id="line-grid">Line Grid<a class="self-link" href="#line-grid"></a></dfn>: the <a class="property" data-link-type="propdesc" href="#propdef-line-grid">line-grid</a> property</span><a class="self-link" href="#line-grid-property"></a></h2>
<table class="definition propdef">
<tbody>
<tr>
<th>Name:</th>
<td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-line-grid">line-grid<a class="self-link" href="#propdef-line-grid"></a></dfn></td>
</tr>
<tr>
<th>Value:</th>
<td class="prod">match-parent <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> create</td>
</tr>
<tr>
<th>Initial:</th>
<td>match-parent</td>
</tr>
<tr>
<th>Applies to:</th>
<td>block, flex and grid containers</td>
</tr>
<tr>
<th>Inherited:</th>
<td>no</td>
</tr>
<tr>
<th>Percentages:</th>
<td>N/A</td>
</tr>
<tr>
<th>Media:</th>
<td>visual</td>
</tr>
<tr>
<th>Computed value:</th>
<td>as specified</td>
</tr>
<tr>
<th>Animatable:</th>
<td>no</td>
</tr>
</tbody></table>
<p>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 <a data-link-type="dfn" href="#line-grid">line grid</a>.
However, whether a box or its contents snap
to a <a data-link-type="dfn" href="#line-grid">line grid</a> is determined
by <a class="property" data-link-type="propdesc" href="#propdef-line-snap">line-snap</a> and <a class="property" data-link-type="propdesc" href="#propdef-box-snap">box-snap</a>.)</p>
<p>The values of this property have the following meanings:</p>
<dl data-dfn-for="line-grid" data-dfn-type="value">
<dt><dfn class="css" data-dfn-for="line-grid" data-dfn-type="value" data-export="" id="valdef-line-grid-match-parent">match-parent<a class="self-link" href="#valdef-line-grid-match-parent"></a></dfn>
</dt>
<dd>Box assumes the <a data-link-type="dfn" href="#line-grid">line grid</a> of its parent.
</dd>
<dt><dfn class="css" data-dfn-for="line-grid" data-dfn-type="value" data-export="" id="valdef-line-grid-create">create<a class="self-link" href="#valdef-line-grid-create"></a></dfn>
</dt>
<dd>Box creates a new <a data-link-type="dfn" href="#line-grid">line grid</a>
using its own font and line layout settings.
The <a data-link-type="dfn" href="#line-grid">line grid</a> 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 <a data-link-type="dfn" href="#line-grid">line grid</a> is restarted on each page;
since line boxes cannot be fragmented,
no page begins with the bottom part of a line’s grid.
</dd>
</dl>
<p>The initial containing block establishes a <a data-link-type="dfn" href="#line-grid">line grid</a>
using the font and line layout settings of the root element.
If the root element (which has no parent)
has a value of <a class="css" data-link-type="maybe" href="#valdef-line-grid-match-parent">match-parent</a>,
then it adopts the <a data-link-type="dfn" href="#line-grid">line grid</a> of the initial containing block.</p>
<p>In paged media,
if the page context applying to a particular page
specifies <a class="css" data-link-type="propdesc" href="#propdef-line-grid" title="line-grid">line-grid: create</a>,
then a <a data-link-type="dfn" href="#line-grid">line grid</a> for that page is established
using the page context’s font and line layout settings
applied to the page box (including its margins),
and for layout within that page the initial containing block
is assumed to have adopted this <a data-link-type="dfn" href="#line-grid">line grid</a>.</p>
<div class="example">
Since the initial value of <a class="property" data-link-type="propdesc" href="#propdef-line-grid">line-grid</a> is <a class="css" data-link-type="maybe" href="#valdef-line-grid-match-parent">match-parent</a>,
by default a <a data-link-type="dfn" href="#line-grid">line grid</a> is automatically created
using the root element’s font and line height settings
to fill the initial containing block / page area.
When all pages of a document are the same size,
this will result in a consistent <a data-link-type="dfn" href="#line-grid">line grid</a> that exactly aligns
to the top of each page area.
<p>It is sometimes appropriate
for the <a data-link-type="dfn" href="#line-grid">line grid</a> to begin with the content,
after the root element’s margins/padding/border
and/or other header content.
To do this, the root element (or some other element) can be told to establish the <a data-link-type="dfn" href="#line-grid">line grid</a>.</p>
<pre>main { line-grid: create; }
</pre>
<p>If this <a data-link-type="dfn" href="#line-grid">line grid</a> is then paginated, the <a data-link-type="dfn" href="#line-grid">line grid</a> will start at the top of each page area after the first.</p>
<p>In other cases, it’s desirable for
all pages of a document share a common <a data-link-type="dfn" href="#line-grid">line grid</a>,
even if page margins, border and/or padding change.
The following rule will instead fill the page box area
<em>including</em> the page margin/padding/borders
with the <a data-link-type="dfn" href="#line-grid">line grid</a>:</p>
<pre>@page { line-grid: create; }
</pre>
<p>As this <a data-link-type="dfn" href="#line-grid">line grid</a> is paginated, it will start at the top of each page box after the first;
however it might not align to the top of the page area
(where the content would normally begin).</p>
</div>
<p class="issue" id="issue-68d02fb3"><a class="self-link" href="#issue-68d02fb3"></a>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>
<p class="note" role="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.
</p>
<p class="note" role="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 css-line-grid.
</p>
<h2 class="heading settled" data-level="3" id="grid-snapping"><span class="secno">3. </span><span class="content">
Snapping to a Grid</span><a class="self-link" href="#grid-snapping"></a></h2>
<p>The <a class="property" data-link-type="propdesc" href="#propdef-line-snap">line-snap</a> and <a class="property" data-link-type="propdesc" href="#propdef-box-snap">box-snap</a> properties
cause line boxes and block boxes to shift position
in order to align to the <a data-link-type="dfn" href="#line-grid">line grid</a>.
Shifting boxes in this way affects layout –
it is not merely a display translation.
If a line box is shifted,
then subsequent line boxes will be laid out
using the new shifted position as input
to their line stacking rules.
If a block box is shifted,
then subsequent boxes will be laid out
using the new shifted position as input.</p>
<h3 class="heading settled" data-level="3.1" id="line-snap"><span class="secno">3.1. </span><span class="content">
Snapping Line Boxes: the <a class="property" data-link-type="propdesc" href="#propdef-line-snap">line-snap</a> property</span><a class="self-link" href="#line-snap"></a></h3>
<table class="definition propdef">
<tbody>
<tr>
<th>Name:</th>
<td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-line-snap">line-snap<a class="self-link" href="#propdef-line-snap"></a></dfn></td>
</tr>
<tr>
<th>Value:</th>
<td class="prod">none <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> baseline <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> contain</td>
</tr>
<tr>
<th>Initial:</th>
<td>none</td>
</tr>
<tr>
<th>Applies to:</th>
<td>all elements</td>
</tr>
<tr>
<th>Inherited:</th>
<td>yes</td>
</tr>
<tr>
<th>Percentages:</th>
<td>N/A</td>
</tr>
<tr>
<th>Media:</th>
<td>visual</td>
</tr>
<tr>
<th>Computed value:</th>
<td>as specified</td>
</tr>
<tr>
<th>Animatable:</th>
<td>no</td>
</tr>
</tbody></table>
<p>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 <a data-link-type="dfn" href="#line-grid">line grid</a>
specified by <a class="property" data-link-type="propdesc" href="#propdef-line-grid">line-grid</a>.
(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 <a data-biblio-type="informative" data-link-type="biblio" href="#biblio-css3line" title="CSS3LINE">[CSS3LINE]</a>.)</p>
<p>Values have the following meanings:</p>
<dl data-dfn-for="line-snap" data-dfn-type="value">
<dt><dfn class="css" data-dfn-for="line-snap" data-dfn-type="value" data-export="" id="valdef-line-snap-none">none<a class="self-link" href="#valdef-line-snap-none"></a></dfn>
</dt>
<dd>Line boxes do not snap to the grid; they stack normally.
</dd>
<dt><dfn class="css" data-dfn-for="line-snap" data-dfn-type="value" data-export="" id="valdef-line-snap-baseline">baseline<a class="self-link" href="#valdef-line-snap-baseline"></a></dfn>
</dt>
<dd>The dominant baseline snaps with the matching baseline on the
<a data-link-type="dfn" href="#line-grid">line grid</a> applying to the element.
</dd>
<dt><dfn class="css" data-dfn-for="line-snap" data-dfn-type="value" data-export="" id="valdef-line-snap-contain">contain<a class="self-link" href="#valdef-line-snap-contain"></a></dfn>
</dt>
<dd>
Two baselines are used to align the line box:
the line box is snapped
so that its <a href="http://dev.w3.org/csswg/css-inline/#central">central</a> baseline
is centered between one
of the line grid’s <a href="http://dev.w3.org/csswg/css-inline/#text-over-edge">text-over-edge</a> baselines
and a subsequent (but not necessarily consecutive)
<a href="http://dev.w3.org/csswg/css-inline/#text-under-edge">text-under-edge</a> baseline.
</dd>
</dl>
<div class="example">
<p>Line boxes almost always shift downward
(towards the block-end direction)
when snapping to a <a data-link-type="dfn" href="#line-grid">line grid</a>.
Here there are three lines
with 20px line-height
and line-snap:baseline
that should snap to a 30px <a data-link-type="dfn" href="#line-grid">line grid</a>.
Each line box shifts down
so that the baselines align
with the grid lines.</p>
<figure style="float:left;">
<img alt="line positions before snapping" src="images/simple-no-snap.png">
<figcaption>
Before line snapping
</figcaption>
</figure>
<figure style="float:left;">
<img alt="line positions after snapping" src="images/simple-snapped.png">
<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 alt="line positions before snapping" src="images/top-no-snap.png">
<figcaption>
Before line snapping
</figcaption>
</figure>
<figure style="float:left;">
<img alt="line positions after snapping" src="images/top-snapped.png">
<figcaption>
After line snapping
</figcaption>
</figure>
</div>
<h3 class="heading settled" data-level="3.2" id="box-snap"><span class="secno">3.2. </span><span class="content">
Snapping Block Boxes: the <a class="property" data-link-type="propdesc" href="#propdef-box-snap">box-snap</a> property</span><a class="self-link" href="#box-snap"></a></h3>
<p class="issue" id="issue-dcdebabc"><a class="self-link" href="#issue-dcdebabc"></a>This is a rough draft of trying to solve
the box-snapping problem.</p>
<p class="issue" id="issue-1bcf4d99"><a class="self-link" href="#issue-1bcf4d99"></a>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 <a data-link-type="dfn" href="#line-grid">line grid</a>.</p>
<p class="issue" id="issue-13c3b0a7"><a class="self-link" href="#issue-13c3b0a7"></a>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.</p>
<table class="definition propdef">
<tbody>
<tr>
<th>Name:</th>
<td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-box-snap">box-snap<a class="self-link" href="#propdef-box-snap"></a></dfn></td>
</tr>
<tr>
<th>Value:</th>
<td class="prod">none <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> block-start <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> block-end <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> center <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> baseline <a data-link-for="" data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> last-baseline</td>
</tr>
<tr>
<th>Initial:</th>
<td>none</td>
</tr>
<tr>
<th>Applies to:</th>
<td>block-level boxes and internal table elements except table cells</td>
</tr>
<tr>
<th>Inherited:</th>
<td>yes</td>
</tr>
<tr>
<th>Percentages:</th>
<td>N/A</td>
</tr>
<tr>
<th>Media:</th>
<td>visual</td>
</tr>
<tr>
<th>Computed value:</th>
<td>as specified</td>
</tr>
<tr>
<th>Animatable:</th>
<td>no</td>
</tr>
</tbody></table>
<p>Specifies how the block is snapped to the baseline grid.</p>
<p>Values have the following meanings:</p>
<dl data-dfn-for="box-snap" data-dfn-type="value">
<dt><dfn class="css" data-dfn-for="box-snap" data-dfn-type="value" data-export="" id="valdef-box-snap-none">none<a class="self-link" href="#valdef-box-snap-none"></a></dfn>
</dt>
<dd>The block is not snapped to any grid.</dd>
<dt><dfn class="css" data-dfn-for="box-snap" data-dfn-type="value" data-export="" id="valdef-box-snap-block-start">block-start<a class="self-link" href="#valdef-box-snap-block-start"></a></dfn>
</dt>
<dd>The block-start edge is snapped to the nearest grid line.
</dd>
<dt><dfn class="css" data-dfn-for="box-snap" data-dfn-type="value" data-export="" id="valdef-box-snap-block-end">block-end<a class="self-link" href="#valdef-box-snap-block-end"></a></dfn>
</dt>
<dd>The block-end edge is snapped to the nearest grid line.
</dd>
<dt><dfn class="css" data-dfn-for="box-snap" data-dfn-type="value" data-export="" id="valdef-box-snap-center">center<a class="self-link" href="#valdef-box-snap-center"></a></dfn>
</dt>
<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.
</dd>
<dt><dfn class="css" data-dfn-for="box-snap" data-dfn-type="value" data-export="" id="valdef-box-snap-baseline">baseline<a class="self-link" href="#valdef-box-snap-baseline"></a></dfn>
</dt>
<dd>The first line box’s dominant baseline is snapped to the nearest grid line.
</dd>
<dt><dfn class="css" data-dfn-for="box-snap" data-dfn-type="value" data-export="" id="valdef-box-snap-last-baseline">last-baseline<a class="self-link" href="#valdef-box-snap-last-baseline"></a></dfn>
</dt>
<dd>The last line box’s dominant baseline is snapped to the nearest grid line.
</dd>
</dl>
<p>Snapping block boxes always uses the <a data-link-type="dfn" href="#line-grid">line grid</a> of the box parent
(a block’s own <a data-link-type="dfn" href="#line-grid">line grid</a> has no effect on box snapping).
If 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. <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a></p>
<p>To snap a block-level element to a grid line,
take the unsnapped result of layout
(including margin collapsing, clearance, etc.)
as the block’s initial position,
then shift the block downward
(towards the block-end direction)
until it snaps to the <a data-link-type="dfn" href="#line-grid">line grid</a>.</p>
<p>For the <a class="css" data-link-type="maybe" href="#valdef-box-snap-block-start">block-start</a> and <a class="css" data-link-type="maybe" href="#valdef-box-snap-block-end">block-end</a> values,
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 <a data-link-type="dfn" href="#line-grid">line grid</a>
doesn’t match that of the affected element,
or when due to the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-writing-modes-3/#propdef-text-orientation">text-orientation</a> settings
the text-under-edge corresponds to the block-start edge.</p>
<p>When applied to table row group and table row boxes,
<a class="property" data-link-type="propdesc" href="#propdef-box-snap">box-snap</a> 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.</p>
<p>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:</p>
<ul>
<li>In an auto-sized table no column may be smaller than its minimum content width.
</li>
<li>The resulting table must not exceed its original measure if it had a non-auto measure.
</li>
<li>The adjusted widths must not cause the table to overflow its containing block any more than it would with <a class="css" data-link-type="propdesc" href="#propdef-box-snap" title="box-snap">box-snap: none</a>.
</li>
</ul>