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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang=en>
<head profile="http://www.w3.org/2006/03/hcard"><meta
content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Line Grid Module</title>
<link href="../default.css" rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<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>
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS Line Grid Module</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 19 January
2012</h2>
<dl>
<dt>This version:
<dd><a href="http://www.w3.org/TR/2012/ED-unknown-shortname-20120119/">
http://www.w3.org/TR/2012/ED-unknown-shortname-20120119</a> <!--
<dt>Latest version:
<dd><a href="http://www.w3.org/TR/css-line-grid">
http://www.w3.org/TR/css-line-grid</a>
<dt>Previous version:
<dd><a href="http://www.w3.org/PreviousVersionURI">
http://www.w3.org/PreviousVersionURI</a>
-->
<dt>Editors:
<dd class=vcard><span class=fn>Koji Ishii</span>, <span class=org>Invited
Expert</span>, <span class=email><a
href="mailto:kojiishi@gluesoft.co.jp">kojiishi@gluesoft.co.jp</a></span>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2012 <a
href="http://www.w3.org/"><acronym
title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym
title="Massachusetts Institute of Technology">MIT</acronym></a>, <a
href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>,
<a href="http://www.keio.ac.jp/">Keio</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>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>CSS is a language for describing the rendering of structured documents
(such as HTML and XML) on screen, on paper, in speech, etc. This module
contains the features of CSS level 3 relating to <var
class=issue>DESCRIBE HERE</var>. It includes and extends the functionality
of CSS level 2 <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>, which builds on CSS
level 1 <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>.
The main extensions compared to level 2 are <var
class=issue>SUMMARIZE HERE</var>.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-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>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bunknown-shortname%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“unknown-shortname” in the subject, preferably like this:
“[<!---->unknown-shortname<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/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="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<p>The following features are at risk: …
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
<ul class=toc>
<li><a href="#context"><span class=secno>1.1. </span>Background</a>
<li><a href="#placement"><span class=secno>1.2. </span>Module
Interactions</a>
<li><a href="#values"><span class=secno>1.3. </span>Values</a>
</ul>
<li><a href="#line-grid"><span class=secno>2. </span> Defining a Line
Grid: the ‘<code class=property>line-grid</code>’
property</a>
<li><a href="#snapping-to-a-grid"><span class=secno>3. </span>Snapping to
a Grid</a>
<ul class=toc>
<li><a href="#line-snap"><span class=secno>3.1. </span> Snapping Line
Boxes: the ‘<code class=property>line-snap</code>’
property</a>
<li><a href="#line-slack"><span class=secno>3.2. </span> Line Snap
Allowance: the ‘<code class=property>line-slack</code>’
property</a>
<li><a href="#box-snap"><span class=secno>3.3. </span> Snapping Block
Boxes: the ‘<code class=property>box-snap</code>’
property</a>
</ul>
<li><a href="#layout-in-grid-units"><span class=secno>4. </span>Layout in
Grid Units</a>
<ul class=toc>
<li><a href="#grid-length-unit"><span class=secno>4.1. </span> Grid
Length Units: the ‘<code class=css>gd</code>’ unit</a>
<li><a href="#round-length"><span class=secno>4.2. </span>Rounding
Lengths to Multiple of Units: ‘<code
class=css>rounddown()</code>’ and ‘<code
class=css>roundup()</code>’ </a>
</ul>
<li><a href="#conformance"><span class=secno>5. </span> Conformance</a>
<ul class=toc>
<li><a href="#conventions"><span class=secno>5.1. </span>Document
Conventions</a>
<li><a href="#conformance-classes"><span class=secno>5.2. </span>
Conformance Classes</a>
<li><a href="#partial"><span class=secno>5.3. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>5.4. </span> Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>5.5. </span> CR Exit
Criteria</a>
</ul>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<ul class=toc>
<li class=no-num><a href="#normative-references">Normative
references</a>
<li class=no-num><a href="#other-references">Other references</a>
</ul>
<li class=no-num><a href="#index">Index</a>
<li class=no-num><a href="#property-index">Property index</a>
</ul>
<!--end-toc-->
<h2 id=introduction><span class=secno>1. </span>Introduction</h2>
<h3 id=context><span class=secno>1.1. </span>Background</h3>
<p><em>This section is not normative.</em>
<p>This specification provides features to align lines and blocks to
invisible grids in the document.
<p>Aligning lines and blocks to grids provides the following benefits:
<ul>
<li>Vertical rhythm is kept for better readability.
<li>Lines are aligned between columns in multi-column documents.
<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>East Asian layouts require vertical rhythm more often than other
scripts do, even in single column, non-paged media documents, as defined
in <a href="#JLREQ" rel=biblioentry>[JLREQ]<!--{{JLREQ}}--></a>.
</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.
<div class=figure> <img
alt="Vertical rhythm kept through pictures and different size of text in a multi-column document"
height=246 src=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"
height=244 src=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>
<br style="clear:both;">
</div>
<div class=figure id=ex-sidenote>
<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 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>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>This module provides the following capabilities:
<ul>
<li>Defining grids in the line progression direction.
<li>Controling how lines and blocks align to the grids.
<li>Rounding lengths to multiples of units.
</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.
<h3 id=placement><span class=secno>1.2. </span>Module Interactions</h3>
<p>This module extends the line box model defined in <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections 9.4.2 and 10.8.
<h3 id=values><span class=secno>1.3. </span>Values</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 href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 <a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
modules may expand the definitions of these value types: for example <a
href="#CSS3COLOR" rel=biblioentry>[CSS3COLOR]<!--{{CSS3COLOR}}--></a>,
when combined with this module, expands the definition of the
<color> value type as used in this specification.
<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.
<h2 id=line-grid><span class=secno>2. </span> Defining a Line Grid: the
‘<a href="#line-grid0"><code
class=property>line-grid</code></a>’ property</h2>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=line-grid0>line-grid</dfn>
<tr>
<th><a href="#values">Value</a>:
<td><identifier>
<tr>
<th>Initial:
<td>root-grid
<tr>
<th>Applies to:
<td>block containers
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Animatable:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
<tr>
<th>Canonical order:
<td><abbr title="follows order of property value definition">per
grammar<abbr> </abbr></abbr>
</table>
<p>Specifies which line grid the line boxes of this element (if any) will
align.
<p>If there does not already exist a line grid with the specified name,
then this property creates a line grid using the font and line layout
settings of this element. 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 element is paginated,
the line grid is restarted on each page; since line boxes cannot be
paginated, no page begins with the bottom part of a line's grid.
<p class=note>As a side effect of the initial value and this definition,
the root element always sets up a line grid, so that ‘<a
href="#line-snap0"><code class=property>line-snap</code></a>’ can be
used even if the author has not explicitly set up a line grid.
<h2 id=snapping-to-a-grid><span class=secno>3. </span>Snapping to a Grid</h2>
<h3 id=line-snap><span class=secno>3.1. </span> Snapping Line Boxes: the
‘<a href="#line-snap0"><code
class=property>line-snap</code></a>’ property</h3>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=line-snap0>line-snap</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>none | baseline | contain
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>all elements
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Animatable:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
<tr>
<th>Canonical order:
<td><abbr title="follows order of property value definition">per
grammar<abbr> </abbr></abbr>
</table>
<p>This property applies to all the line boxes directly contained by the
element, and, when not ‘<a href="#none1"><code
class=css>none</code></a>’, causes each line box to shift downward
(possibly by zero) until it snaps to the line grid specified by ‘<a
href="#line-grid0"><code class=property>line-grid</code></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 href="#CSS3LINE"
rel=biblioentry>[CSS3LINE]<!--{{CSS3LINE}}--></a>.) Values have the
following meanings:
<dl>
<dt><dfn id=none>‘<code class=css>none</code>’</dfn>
<dd>Line boxes do not snap to the grid; they stack normally.
<dt><dfn id=baseline>‘<code class=css>baseline</code>’</dfn>
<dd>The dominant baseline snaps with the matching baseline on the line
grid applying to the element.
<dt><dfn id=contain>‘<code class=css>contain</code>’</dfn>
<dd>Two baselines are used to align the line box: the line box is snapped
so that its ‘<code class=css>central</code>’ baseline
(halfway between the ‘<code class=css>text-over</code>’ and
‘<code class=css>text-under</code>’ baselines) is centered
between one of the line grid's ‘<code
class=css>text-over</code>’ baselines and a subsequent (but not
necessarily consecutive) ‘<code class=css>text-under</code>’
baseline.
</dl>
<p>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.
<h3 id=line-slack><span class=secno>3.2. </span> Line Snap Allowance: the
‘<a href="#line-slack0"><code
class=property>line-slack</code></a>’ property</h3>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=line-slack0>line-slack</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>none | <integer>
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>all elements
<tr>
<
D5F4
;th>Inherited:
<td>yes
<tr>
<th>Animatable:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
<tr>
<th>Canonical order:
<td><abbr title="follows order of property value definition">per
grammar<abbr> </abbr></abbr>
</table>
<p>This property specifies the slack in snapping lines to a grid.
<dl>
<dt><dfn id=none0>‘<code class=css>none</code>’</dfn>
<dd>No slack: each line box is snapped to the grid.
<dt><dfn id=ltintegergt><integer></dfn>
<dd>Nominally, each <var>n</var>th line box is snapped to the grid
<em>or</em> each <var>n</var>th grid line has a line box snapped to it,
whichever results in smaller shifts, for the specified integer as
<var>n</var>. A value of ‘<code class=css>1</code>’ allows,
for example, lines of half the grid height to fit twice as densely, while
a value of ‘<code class=css>2</code>’ also allows lines 1.5
times the grid height to match the grid 2:3. A value less than
‘<code class=css>1</code>’ is invalid.
</dl>
<p>When ‘<a href="#line-snap0"><code
class=property>line-snap</code></a>’ is given as a <baseline>,
the ‘<a href="#line-slack0"><code
class=property>line-slack</code></a>’ property interpolates a grid
against the main line grid and snaps the element's line boxes against this
grid. The interpolated grid is drawn by creating a new line grid using the
baseline table and ‘<code class=property>line-height</code>’
of the current element and increasing the inter-line spacing of this new
grid to snap according to the element's ‘<a href="#line-snap0"><code
class=property>line-snap</code></a>’ and ‘<a
href="#line-slack0"><code class=property>line-slack</code></a>’
values. The first snapped line is chosen to minimize the shift of the
first line box of the element. The interpolated grid is only used for this
block's line boxes; descendant boxes interpolate their own grid against
the main grid.)
<p class=issue>Define something sane for ‘<code class=css>line-snap:
contain</code>’. <!--
<p>When 'line-snap' is ''contain'', then 'line-slack' not ''none''
is handled by line-snapping using the ''central'' baseline any time
the interpolated grid is narrower than the main grid. It has no effect
when the interpolated grid is wider than the main grid.
<p>When 'line-snap' is ''contain'', the 'line-slack' property creates
an interpolated grid as above, but instead of snapping according to
a fixed ratio, it increases the spacing of the interpolated grid so
that its first ''text-over'' baseline aligns to one of the main grid's
''text-over'' baselines and its last ''text-under'' baseline aligns
to one of the main grid's ''text-under'' baseline. In this case, the
integer represents the maximum number of line boxes in the block
before slack is ignored. The slack is
-->
<p>User Agents are not required to support values greater than 4;
unsupported used values higher than 4 must be clamped to 4.
<p class=issue> Can it be extended to handle vertical justification?
<h3 id=box-snap><span class=secno>3.3. </span> Snapping Block Boxes: the
‘<a href="#box-snap0"><code
class=property>box-snap</code></a>’ property</h3>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=box-snap0>box-snap</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>[ none | margin-box | border-box | half-border ]? ]{1,4} [ /
<identifier>{1,4} ]
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>block-level boxes and internal table elements except table cells
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Animatable:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
<tr>
<th>Canonical order:
<td><abbr title="follows order of property value definition">per
grammar<abbr> </abbr></abbr>
</table>
<p>Specifies which edges, if any, are snapped to the line box grid. If four
values are specified, they apply to the before, end, after, and start
edges of the box, respectively. If the start edge is omitted, it is the
same as the end. If the after edge is omitted, it is the same as the
before. If the end edge is omitted, it is assigned ‘<a
href="#none1"><code class=css>none</code></a>’. Values have the
following meanings:
<dl>
<dt><dfn id=none1>none</dfn>
<dd>The edge is not snapped to any grid.
<dt><dfn id=margin-box>margin-box</dfn>
<dd>The margin edge is snapped to the nearest grid line.
<dt><dfn id=border-box>border-box</dfn>
<dd>The border edge is snapped to the nearest grid line.
<dt><dfn id=half-border>half-border</dfn>
<dd>The edge halfway between the border and padding edges is snapped to
the nearest grid line.
<dt><dfn id=ltidentifiergt><identifier></dfn>
<dd>Names the grid to snap to. Defaults to the effective ‘<a
href="#line-grid0"><code class=property>line-grid</code></a>’. If
the named grid does not exist (or has no lines in the appropriate
dimension), then the affected edge is not snapped.
<p class=note>This allows the author to specify a grid that is not a line
grid.
</dl>
<p>When snapping to baselines on a line grid, either the <i>text-over</i>
or <i>text-under</i> 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 <i>over</i> edge is chosen. In some cases
the <i>under</i> 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 ‘<code
class=property>text-orientation</code>’ settings the <i>under</i>
side corresponds to the <i>after</i> edge.
<p>To snap a before, start, or end edge of a block-level element or its
after margin-box edge to a grid line, the effective margin is increased at
that edge. To snap an after border-box or half-border edge to a grid line,
the effective content-box height is increased. 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. <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
<p>When applied to table row group and table row boxes, ‘<a
href="#box-snap0"><code class=property>box-snap</code></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 increa
35CC
sed. To snap
an after edge on a table row or row group, the affected row's height is
increased.
<p>When applied to table column group and table column boxes, ‘<a
href="#box-snap0"><code class=property>box-snap</code></a>’ 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-‘<code class=css>auto</code>’ measure.
<li>The adjusted widths must not cause the table to overflow its
containing block any more than it would with ‘<code
class=css>box-snap: none</code>’.
</ul>
<p>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, ‘<a
href="#margin-box"><code class=css>margin-box</code></a>’ represents
the edge at the midpoint of the border spacing. For collapsed tables,
‘<a href="#margin-box"><code class=css>margin-box</code></a>’
and ‘<a href="#border-box"><code
class=css>border-box</code></a>’ are treated as ‘<a
href="#half-border"><code class=css>half-border</code></a>’. When
two coinciding edges have conflicting snap values, the later value wins.
<h2 id=layout-in-grid-units><span class=secno>4. </span>Layout in Grid
Units</h2>
<p class=issue>This section is mostly unrevised brainstorming.
<h3 id=grid-length-unit><span class=secno>4.1. </span> Grid Length Units:
the ‘<code class=css>gd</code>’ unit</h3>
<p>The existence of a grid in an element makes it possible and very useful
to express various measurements in that element in terms of grid units.
Grid units are used very frequently in East Asian typography, especially
for the left, right, top and bottom element margins.
<p>Therefore, a new length unit is necessary, gd, to enable the author to
specify the various measurements in terms of the grid.
<p>For example, consider the following style:
<pre><code
class=css>P { layout-grid: strict both 20pt 15pt; margin: 1gd 3gd 1gd 2gd }</code></pre>
<p>This way, all P elements would effectively acquire a 15pt top margin, a
60pt right margin, a 15pt bottom margin and a 40pt left margin.
<p>If no grid is specified, the gd unit should be treated the same as the
em unit.
<p class=issue>Just copied from old WD, need to refresh
<div class=example id=vert-rhythm-p-margin>
<p>The most common addition of vertical space is that inserted between
paragraphs. If the rhythm of the page is to be maintained, the spacing of
paragraphs should be related to the basic leading. This is achieved by
setting top- and bottom-margins equal to ‘<code
class=css>1gd</code>’.
<pre><code class=css>
<!-- -->p {
<!-- --> layout-grid-line: 1.5;
<!-- --> layout-grid-mode: baseline;
<!-- --> margin-top: 1gd;
<!-- --> margin-bottom: 1gd;
<!-- -->}
<!-- --></code></pre>
</div>
<div class=example id=ex-gyou-dori>
<p>East Asian layout often requires heights specified by the count of the
line grids. In Japanese, this method is called Gyou-dori. Refer to <a
href="#JLREQ" rel=biblioentry>[JLREQ]<!--{{JLREQ}}--></a> for more
details about Gyou-dori.</p>
<pre><code class=css>
<!-- -->h1 {
<!-- --> line-grid-mode: block;
<!-- --> min-height: 3gd; /* 3 Gyou-dori */
<!-- --> margin-top: 1gd;
<!-- -->}
<!-- --></code></pre>
</div>
<p class=issue>‘<code class=css>gem</code>’ length unit is also
requested
<h3 id=round-length><span class=secno>4.2. </span>Rounding Lengths to
Multiple of Units: ‘<code class=css><span class=index id=rounddown
title="length unit::rounddown()">rounddown()</span></code>’ and
‘<code class=css><span class=index id=roundup
title="length unit::rounddown()">roundup()</span></code>’</h3>
<p>The ‘<code class=property>width</code>’ and ‘<code
class=property>height</code>’ properties accept ‘<code
class=css>rounddown(<em>length</em>)</code>’ and ‘<code
class=css>roundup(<em>length</em>)</code>’ values.
<p>The ‘<code class=css>rounddown(<em>length</em>)</code>’
computes to ‘<code class=css>auto</code>’, and then the result
will be rounded down to the largest multiple of the specified unit that is
equal to or less than the original computed value.
<p>Similarly, the ‘<code
class=css>roundup(<em>length</em>)</code>’ computes to ‘<code
class=css>auto</code>’, and then the result will be rounded up to
the smallest multiple of the specified unit that is equal to or greater
than the original computed value.
<p>The fraction is distributed evenly to the computed values of margins on
each side. If it is used for ‘<code
class=property>height</code>’, half the fraction is added to the top
and the bottom margins. If for ‘<code
class=property>width</code>’, to the left and the right margins.
<p>When the element is a float, the fraction is added to the opposite side
of the float direction instead. For example, if the ‘<code
class=property>float</code>’ property is ‘<code
class=css>left</code>’, the fraction is added to the right margins.
<p class=issue>Is this design appropriate to solve text with floats case?
<p class=issue>Are there requirements for table cell width?
<p class=issue>Should have an option to left/right align instead of center?
Can author align running head and page numbers with body with this design?
<p class=issue>Use cases for non-East Asian layouts?
<div class=example>
<p>It is often desirable for East Asian layouts to make the line width to
be 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 is needed to expand lines.</p>
<pre><code class=css>
<!-- -->body {
<!-- --> margin: 3cm 3.5cm 3cm 3cm;
<!-- --> text-align: justify;
<!-- --> width: rounddown(1em);
<!-- -->}
<!-- -->.sidefigure {
<!-- --> float: right;
<!-- --> width: roundup(1em);
<!-- -->}
<!-- --></code></pre>
</div>
<h2 id=conformance><span class=secno>5. </span> Conformance</h2>
<h3 id=conventions><span class=secno>5.1. </span>Document Conventions</h3>
<p>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.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a
href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>
<p>Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with
<code>class="example"</code>, like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word “Note” and are set apart from
the normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.
<h3 id=conformance-classes><span class=secno>5.2. </span> Conformance
Classes</h3>
<p>Conformance to <var class=issue>CSS TEMPLATE Module</var> is defined for
three conformance classes:
<dl>
<dt><dfn id=style-sheet title="style sheet!!as conformance class">style
sheet</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
style sheet</a>.
<dt><dfn id=renderer>renderer</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
that interprets the semantics of a style sheet and renders documents that
use them.
<dt><dfn id=authoring-tool>authoring tool</dfn>
<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
that writes a style sheet.
</dl>
<p>A style sheet is conformant to <var class=issue>CSS TEMPLATE
Module</var> if all of its declarations that use properties defined in
this module have values that are valid according to the generic CSS
grammar and the individual grammars of each property as given in this
module.
<p>A renderer is conformant to <var class=issue>CSS TEMPLATE Module</var>
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the properties defined by <var
class=issue>CSS TEMPLATE Module</var> 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.)
<p>An authoring tool is conformant to <var class=issue>CSS TEMPLATE
Module</var> if it writes syntactically correct style sheets, according to
the generic CSS grammar and the individual grammars of each property in
this module.
<h3 id=partial><span class=secno>5.3. </span> Partial Implementations</h3>
<p>So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers <strong>must</strong> treat as