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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 1 June 2011</h2>
<dl>
<dt>This version:
<dd><a href="http://www.w3.org/TR/2011/ED-unknown-shortname-20110601/">
http://www.w3.org/TR/2011/ED-unknown-shortname-20110601</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> &copy; 2011 <a
href="http://www.w3.org/"><acronym title="World Wide Web
Consortium">W3C</acronym></a><sup>&reg;</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&nbsp;3 relating to <var
class=issue>DESCRIBE HERE</var>. It includes and extends the functionality
of CSS level&nbsp;2 <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>, which builds on CSS
level&nbsp;1 <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>.
The main extensions compared to level&nbsp;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
&#8220;unknown-shortname&#8221; in the subject, preferably like this:
&#8220;[<!---->unknown-shortname<!---->] <em>&hellip;summary of
comment&hellip;</em>&#8221;
<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: &hellip;
<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="#conventions"><span class=secno>1.3. </span>Document
Conventions</a>
<li><a href="#values"><span class=secno>1.4. </span>Values</a>
</ul>
<li><a href="#defining-grids"><span class=secno>2. </span>Defining
Grids</a>
<ul class=toc>
<li><a href="#layout-grid-line-prop"><span class=secno>2.1.
</span>Define Line Grids: the &lsquo;<code
class=property>layout-grid-line</code>&rsquo; property</a>
</ul>
<li><a href="#using-grids"><span class=secno>3. </span>Using Grids</a>
<ul class=toc>
<li><a href="#layout-grid-mode-prop"><span class=secno>3.1. </span>Grid
Mode: the &lsquo;<code class=property>layout-grid-mode</code>&rsquo;
property</a>
<li><a href="#grid-shorthand-the-layout-grid-property"><span
class=secno>3.2. </span>Grid Shorthand: the &lsquo;<code
class=property>layout-grid</code>&rsquo; property</a>
<li><a href="#grid-length-unit"><span class=secno>3.3. </span>The
&lsquo;<code class=css>gd</code>&rsquo; length unit</a>
<li><a href="#forcing-line-width-to-multiple-of-em"><span
class=secno>3.4. </span>Forcing Line Width to Multiple of
<em>em</em></a>
<ul class=toc>
<li><a href="#character-grid-the-layout-grid-char-prop"><span
class=secno>3.4.1. </span>Character Grid: the &lsquo;<code
class=property>layout-grid-char</code>&rsquo; property</a>
<li><a href="#round-length"><span class=secno>3.4.2. </span>Rounding
Lengths to Multiple of Units: &lsquo;<code
class=css>rounddown()</code>&rsquo; and &lsquo;<code
class=css>roundup()</code>&rsquo; </a>
<li><a href="#rounding-lengths-to-multiple-of-units-th"><span
class=secno>3.4.3. </span>Rounding Lengths to Multiple of Units: the
&lsquo;<code class=property>layout-grid-measure</code>&rsquo;
property</a>
</ul>
</ul>
<li><a href="#conformance"><span class=secno>4. </span> Conformance</a>
<ul class=toc>
<li><a href="#conformance-classes"><span class=secno>4.1. </span>
Conformance Classes</a>
<li><a href="#partial"><span class=secno>4.2. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>4.3. </span> Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>4.4. </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=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 class=issue>Explain, normatively, how this module affects the definition
of CSS.
<p>This module replaces and extends the <var class=issue>SUMMARIZE
HERE</var> features defined in <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections <var class=issue>W.X
and Y.Z</var>.
<h3 id=conventions><span class=secno>1.3. </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=values><span class=secno>1.4. </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
&lt;color&gt; 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=defining-grids><span class=secno>2. </span>Defining Grids</h2>
<h3 id=layout-grid-line-prop><span class=secno>2.1. </span>Define Line
Grids: the &lsquo;<a href="#layout-grid-line"><code
class=property>layout-grid-line</code></a>&rsquo; property</h3>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=layout-grid-line>layout-grid-line</dfn>
<tr>
<th>Value:
<td>none | auto | &lt;length&gt; | &lt;percentage&gt;
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>block containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed&nbsp;value:
<td>specified value
</table>
<p>This property defines a line grid for the element and its descendent
elements. Possible values are:
<p class=issue>An implementer said the grid's scope should be limited to
the same BFC, as referring property values and position of parent elements
could be difficult to implement if the element is in different BFC than
the parent. This can mean a float cannot share the grids with its parent,
which is not ideal. How can we solve this?
<dl>
<dt><dfn id=none title="layout-grid-line::none">&lsquo;<code
class=css>none</code>&rsquo;</dfn>
<dd>No line grid is specified.
<dt><dfn id=auto title="layout-grid-line::auto">&lsquo;<code
class=css>auto</code>&rsquo;</dfn>
<dd>The grid line size is determined by the computed value of the
&lsquo;<code class=property>line-height</code>&rsquo; property.
<dt><dfn id=ltlengthgt
title="layout-grid-line::length">&lt;length&gt;</dfn>
<dd>Size of the line grid's unit space (a.k.a. "line pitch").
<dt><dfn id=ltpercentgt
title="layout-grid-line::percent">&lt;percent&gt;</dfn>
<dd>Size of the line grid's unit space relative to the logical height of
the parent element. If the parent has no explicit height, it is treated
as &lsquo;<code class=css>auto</code>&rsquo;. <span class=issue>This was
defined in <a
href="http://www.w3.org/TR/2001/WD-css3-text-20010517/#layout-grid-line-prop">CSS3
module: text WD 20010517</a>, but not sure how much useful this is</span>
</dl>
<p>The defined grid uses the position of the element as the origin of the
grid. It uses the top of the element as the origin of the grid for
horiozntal text flow. Similarly, it uses the right of the element as the
origin of the grid for &lsquo;<code
class=property>writing-mode</code>&rsquo; is &lsquo;<code
class=css>vertical-rl</code>&rsquo;, and the left of the element for
&lsquo;<code class=css>vertical-lr</code>&rsquo;.
<p class=issue>Define behavior for paged media/column break
<p class=issue>TBD: I might need to copy more text from <a
href="http://www.w3.org/TR/2001/WD-css3-text-20010517/#layout-grid-line-prop">CSS3
module: text WD 20010517</a>.
<h2 id=using-grids><span class=secno>3. </span>Using Grids</h2>
<h3 id=layout-grid-mode-prop><span class=secno>3.1. </span>Grid Mode: the
&lsquo;<a href="#layout-grid-mode"><code
class=property>layout-grid-mode</code></a>&rsquo; property</h3>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=layout-grid-mode>layout-grid-mode</dfn>
<tr>
<th>Value:
<td>none | baseline | block | line
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>block containers
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed&nbsp;value:
<td>specified value
</table>
<p>This property controls how lines and blocks are aligned to grids. Values
have the following meanings:
<dl>
<dt><dfn id=none0 title="layout-grid-mode::none">&lsquo;<code
class=css>none</code>&rsquo;</dfn>
<dd>Do not align any objects to the grids.
<dt><dfn id=baseline title="layout-grid-mode::baseline">&lsquo;<code
class=css>baseline</code>&rsquo;</dfn>
<dd>Align baselines to the line grids. <!--
The line in this definition is a line box
with before margin included if it is the first line of a block,
and with after margin if last.
-->
The height is the height of the line box. Increasing heights to fit to
grids is done by increasing the computed height of the line box. <span
class=issue>How to make this value work well with images, headings, etc.
needs further thoughts.</span>
<dt><dfn id=block title="layout-grid-mode::block">&lsquo;<code
class=css>block</code>&rsquo;</dfn>
<dd>Align blocks to the line grids. <!--
The height of the block in this definition includes margins.
-->
The height includes paddings and borders, but not margins. Increasing
heights to fit to grids is done by adding to computed margins.
<dt><dfn id=line title="layout-grid-mode::line">&lsquo;<code
class=css>line</code>&rsquo;</dfn>
<dd>Align line boxes to the line grids. <!--
The line in this definition is a line box
with before margin included if it is the first line of a block,
and with after margin if last.
-->
The height is the height of the line box. Increasing heights to fit to
grids is done by increasing the computed height of the line box.
</dl>
<p class=issue>Handling margins and borders needs review
<p>When this property is set to anything other than &lsquo;<code
class=css>none</code>&rsquo;, the computed height of the object is
increased to the smallest multiple of the grid height that is equal to or
smaller than the original computed height.
<p>When the value of this property is &lsquo;<code
class=css>baseline</code>&rsquo;, the line box is then moved so that its
baseline is aligned to the next closest grid.
<p>When the value of this property is either &lsquo;<code
class=css>block</code>&rsquo; or &lsquo;<code
class=css>line</code>&rsquo;, the object is vertically centered within the
grid row. If the computed height is larger than the grid space, then the
object is centered within the smallest number of grid rows necessary for
the object to fit in. The starting alignment point is moved accordingly.
This is illustrated below, where <em>a</em> represents the numerical
&lsquo;<a href="#layout-grid-line"><code
class=property>layout-grid-line</code></a>&rsquo; value, and <em>b</em>
and <em>c</em> are half the difference between the computed line-heights
and either the layout-grid-line value or twice the value respectively.
<div class=figure> <img alt="Layout of content within a line grid"
src=lgl.gif>
<p class=caption>Layout of content within a line grid, where <em>a</em>
represents the layout-grid-line value, and the alignment baseline is at
the bottom (after-edge).</p>
</div>
<p>As for regular height computation, the ruby box is treated specifically.
In a similar fashion to the line-height determination process, only the
ruby base element is considered for centering purposes, however the above
and/or the after space should be sufficient to fit the ruby text above
and/or after the base text within the grid-line space. If this is not the
case, the grid-line size needs to be increased in multiple grid rows until
the whole ruby box fits. <span class=issue>Is just ignoring ruby text
enough? like rt { layout-grid-mode: ignore; } or nothing needed to be
defined if line-stacking-ruby: auto does the work?</span>
<p>Note that in order for this property to have an effect, line grids must
be defined by the &lsquo;<a href="#layout-grid-line"><code
class=property>layout-grid-line</code></a>&rsquo; property.
<p class=issue>This design does not allow authors to specify the exact top
position of the first line. It is probably ok for web/epub where viewport
is not fixed and therefore the UA has to absorb fractions using margins,
but print folks may want more precise controls.
<p class=issue>Need to think more about use cases in tables. Turning off
line grids for table cells is one easy way and probably is good enough
when all cells are single line, but there probably are more use cases.
<p class=issue>Need to think more about use cases in forms.
<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>
<p>This effect can be achieved by the following code:</p>
<pre><code class=css>
<!-- -->body { line-grid-mode: line; }
<!-- -->h1 { line-grid-mode: block; }
<!-- --></code></pre>
<br style="clear:both;">
</div>
<h3 id=grid-shorthand-the-layout-grid-property><span class=secno>3.2.
</span>Grid Shorthand: the &lsquo;<code
class=property>layout-grid</code>&rsquo; property</h3>
<p class=issue>TBD
<h3 id=grid-length-unit><span class=secno>3.3. </span>The &lsquo;<code
class=css><span class=index id=gd title="length
unit::gd">gd</span></code>&rsquo; length 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 &lsquo;<code
class=css>1gd</code>&rsquo;.
<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>&lsquo;<code class=css>gem</code>&rsquo; length unit is also
requested
<h3 id=forcing-line-width-to-multiple-of-em><span class=secno>3.4.
</span>Forcing Line Width to Multiple of <em>em</em></h3>
<p class=issue>There are 3 ideas for this, and all ideas are listed here
for disucssions.
<h4 id=character-grid-the-layout-grid-char-prop><span class=secno>3.4.1.
</span>Character Grid: the &lsquo;<a href="#layout-grid-char"><code
class=property>layout-grid-char</code></a>&rsquo; property</h4>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=layout-grid-char>layout-grid-char</dfn>
<tr>
<th>Value:
<td>none | auto | &lt;length&gt;
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>block containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed&nbsp;value:
<td>specified value
</table>
<p>This property defines a character grid for the element. Possible values
are:
<p>When a character grid is defined, start and end edges of a line box snap
to the nearest grid in the direction to shorten the line box.
<div class=figure> <object data=line-length.svg height=158
type="image/svg+xml" width=340> </object>
<p class=caption>The gray part of a line box is rounded down to snap to
the grid. The second line box has a float on the left (green), and
therefore its start edge snaps to the grid as well.</p>
</div>
<p class=issue>Should we add &lsquo;<code class=css>edges</code>&rsquo; to
the &lsquo;<code class=property>line-grid-mode</code>&rsquo; property?
<h4 id=round-length><span class=secno>3.4.2. </span>Rounding Lengths to
Multiple of Units: &lsquo;<code class=css><span class=index id=rounddown
title="length unit::rounddown()">rounddown()</span></code>&rsquo; and
&lsquo;<code class=css><span class=index id=roundup title="length
unit::rounddown()">roundup()</span></code>&rsquo;</h4>
<p>The &lsquo;<code class=property>width</code>&rsquo; and &lsquo;<code
class=property>height</code>&rsquo; properties accept &lsquo;<code
class=css>rounddown(<em>length</em>)</code>&rsquo; and &lsquo;<code
class=css>roundup(<em>length</em>)</code>&rsquo; values.
<p>The &lsquo;<code class=css>rounddown(<em>length</em>)</code>&rsquo;
computes to &lsquo;<code class=css>auto</code>&rsquo;, 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 &lsquo;<code
class=css>roundup(<em>length</em>)</code>&rsquo; computes to &lsquo;<code
class=css>auto</code>&rsquo;, 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 &lsquo;<code
class=property>height</code>&rsquo;, half the fraction is added to the top
and the bottom margins. If for &lsquo;<code
class=property>width</code>&rsquo;, 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 &lsquo;<code
class=property>float</code>&rsquo; property is &lsquo;<code
class=css>left</code>&rsquo;, 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>
<h4 id=rounding-lengths-to-multiple-of-units-th><span class=secno>3.4.3.
</span>Rounding Lengths to Multiple of Units: the &lsquo;<a
href="#layout-grid-measure"><code
class=property>layout-grid-measure</code></a>&rsquo; property</h4>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=layout-grid-measure>layout-grid-measure</dfn>
<tr>
<th>Value:
<td>none | auto | &lt;length&gt; || up | down
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>block containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed&nbsp;value:
<td>specified value
</table>
<h2 id=conformance><span class=secno>4. </span> Conformance</h2>
<h3 id=conformance-classes><span class=secno>4.1. </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>4.2. </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
invalid (and <a
href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore as
appropriate</a>) any at-rules, properties, property values, keywords, and
other syntactic constructs for which they have no usable level of support.
In particular, user agents <strong>must not</strong> selectively ignore
unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid (as
unsupported values must be), CSS requires that the entire declaration be
ignored.
<h3 id=experimental><span class=secno>4.3. </span> Experimental
Implementations</h3>
<p>To avoid clashes with future CSS features, the CSS specifications
reserve a <a
href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
syntax</a> for proprietary property and value extensions to CSS. The CSS
Working Group recommends that experimental implementations of features in
CSS Working Drafts also use vendor-prefixed property or value names. This
avoids any incompatibilities with future changes in the draft. Once a
specification reaches the Candidate Recommendation stage, implementors
should implement the non-prefixed syntax for any feature they consider to
be correctly implemented according to spec.
<h3 id=cr-exit-criteria><span class=secno>4.4. </span> CR Exit Criteria</h3>
<p class=issue>[Change or remove the following CR exit criteria if the spec
is not a module, but, e.g., a Note or a profile. This text was <a
href="/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]
<p id=cr-exit-criteria> For this specification to be advanced to Proposed
Recommendation, there must be at least two independent, interoperable
implementations of each feature. Each feature may be implemented by a
different set of products, there is no requirement that all features be
implemented by a single product. For the purposes of this criterion, we
define the following terms:
<dl>
<dt>independent
<dd>each implementation must be developed by a different party and cannot
share, reuse, or derive from code used by another qualifying
implementation. Sections of code that have no bearing on the
implementation of this specification are exempt from this requirement.
<dt>interoperable
<dd>passing the respective test case(s) in the official CSS test suite,
or, if the implementation is not a Web browser, an equivalent test. Every
relevant test in the test suite should have an equivalent test created if
such a user agent (UA) is to be used to claim interoperability. In
addition if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those equivalent
tests in the same way for the purpose of interoperability. The equivalent
tests must be made publicly available for the purposes of peer review.
<dt>implementation
<dd>a user agent which:
<ol class=inline>
<li>implements the specification.
<li>is available to the general public. The implementation may be a
shipping product or other publicly available version (i.e., beta
version, preview release, or “nightly build”). Non-shipping product
releases must have implemented the feature(s) for a period of at least
one month in order to demonstrate stability.
<li>is not experimental (i.e., a version specifically designed to pass
the test suite and is not intended for normal usage going forward).
</ol>
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