csswg-drafts/css-line-grid/Overview.src.html at 85ce3799483cb495ab784bf87b86f79a921d435b · w3c/csswg-drafts · GitHub

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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 http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Line Grid Module</title>
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link rel="stylesheet" type="text/css"
        href="http://www.w3.org/StyleSheets/TR/W3C-ED.css">
  <style type="text/css">
  </style>
</head>

<div class="head">
<!--logo-->

<h1>CSS Line Grid Module</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
  <dt>This version:
    <dd><a href="[VERSION]">
    http://www.w3.org/TR/[YEAR]/[STATUS]-[SHORTNAME]-[CDATE]</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>

<!--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 [[CSS21]],
  which builds on CSS level&nbsp;1 [[CSS1]].
  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>

<!--status-->

<p>The following features are at risk: &hellip;

<h2 class="no-num no-toc" id="contents">Table of contents</h2>

<!--toc-->

<h2>Introduction</h2>

<h3 id="context">Background</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>Alinging lines and blocks to grids gives the following benefits:</p>

  <ul>
    <li>Keep vertical rhythm for better readability.</li>
    <li>Align lines between columns in multi-column documents.</li>
    <li>Make the top and the bottom margins of pictures equal,
      while keeping the vertical rhythm of text before and after the pictures.</li>
    <li>Layout lines at the same position in every page in paged media.
      Keeping the position of the bottom line of a page has benefits for
      design and readability.
      This al
E880
so improves the readability on duplex printing,
      two pages spreads,
      and when displaying on slow display devices like e-ink.</li>
    <li>East Asian layouts require vertical rhythm
      more often than other scripts does,
      even in single column, non-paged media documents,
      as defined in [[JLREQ]].</li>
    <li>It is often desirable in East Asian layouts
      to make the line width to be
      multiple of <em>em</em> without fractions.
      Since most East Asian characters have 1em advance,
      and since most East Asian documents are justified,
      this minimizes the case where justification needs to expand lines.</li>
  </ul>

  <p>There are several types of objects in a document
    that can break the vertical rhythm.
    Examples include lines in different size of text, pictures, and tables.</p>

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

  <p>This module defines following capabilities:</p>
  <ul>
    <li>Defines grids in the line progression direction.</li>
    <li>Controls how lines and blocks align to the grids.</li>
    <li>Rounds lengths down to mulitple of units.</li>
  </ul>

  <p>It is important to control these capabilities independently,
    so that aligning to grids can be turned off for tables for example,
    but can then be turned back on for the following text to the grids.
  </p>


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

  <p class="issue">Explain, normatively, how this module affects the definition of CSS.</p>

  <p>This module replaces and extends the <var class="issue">SUMMARIZE HERE</var>
  features defined in [[!CSS21]] sections <var class="issue">W.X and Y.Z</var>.

<h3 id="conventions">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. [[!RFC2119]]</p>

  <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.</p>

<h3 id="values">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 [[!CSS21]]. Value types not defined in
  this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
  Other CSS modules may expand the definitions of these value types: for
  example [[CSS3COLOR]], when combined with this module, expands the
  definition of the &lt;color&gt; value type as used in this specification.</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.

<h2>Defining Grids</h2>

<h3 id="layout-grid-line-prop">Define Line Grids: the 'layout-grid-line' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>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 class="issue">TBD:
    I expect this to be very similar to the one in
    <a href="http://www.w3.org/TR/2001/WD-css3-text-20010517/#layout-grid-line-prop">CSS3 module: text WD 20010517</a>.</p>
  <p class="issue">Grid origin should be defined by the element that has this property,
    similar to ''display: static''.
    In that case, should this property inherit, or not inherit the property but inherit grid?</p>


<h2>Using Grids</h2>

<h3 id="layout-grid-mode-prop">Grid Mode: the 'layout-grid-mode' property</h3>

  <table class=propdef>
    <tr>
      <th>Name:
      <td><dfn>layout-grid-mode</dfn>
    <tr>
      <th>Value:
      <td>none | 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:</p>

  <dl>
    <dt><dfn title="layout-grid-mode::none">''none''</dfn></dt>
      <dd>Do not align any objects to the grids.</dd>
    <dt><dfn title="layout-grid-mode::block">''block''</dfn></dt>
      <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.
      </dd>
    <dt><dfn title="layout-grid-mode::line">''line''</dfn></dt>
      <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.
      </dd>
  </dl>
  <p class="issue">Handling margins and borders needs review</p>

  <p>When this property is set to anything other than ''none'',
    the object to align 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 'layout-grid-line' value,
    <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.:</p>

  <div class="figure">
    <img src="lgl.gif"
      />
    <p class="caption">Layout of contents within 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 especially.
    In a similar fashion to the line-height determination process,
    only the ruby base element is considered for centering purpose,
    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 of 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>

  <p>Note that in order for this property to have an effect,
    line grids must be defined by the 'layout-grid-line' property.</p>

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

    <p>When a different size of text like headings wraps,
      it is usually aligned to grids as a block and
      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>Grid Shorthand: the 'layout-grid' property</h3>

  <p class="issue">TBD</p>

<h3 id="grid-length-unit">The ''<span class="index" title="length unit::gd">gd</span>'' 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>

  <p>Therefore a new length unit is necessary:
    gd to enable the author to specify the various measurements in terms of the grid.</p>

  <p>For example, consider the following style:</p>
  <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>

  <p>If no grid is specified, the gd unit should be treated the same as the em unit.</p>

  <p class="issue">Just copied from old WD, need to refresh</p>
  <p class="issue">Use cases for non-East Asian layouts?</p>

  <div id="gyou-dori" class="example">
    <p>East Asian layout often requires heights specified by multiple of the line grids.
      In Japanese, this method is called Gyou-dori.
      Refer to [[JLREQ]] for more details of Gyou-dori.</p>
    <pre><code class="css">
<!-- -->h1 {
<!-- -->  line-grid-mode: block;
<!-- -->  min-height: 3gd;
<!-- -->}
<!-- --></code></pre>
  </div>

<h3 id="round-length">Rounding Down Lengths to Multiple of Units:
  ''<span class="index" title="length unit::*gd">*gd</span>''
  and ''<span class="index" title="length unit::*em">*em</span>''</h3>

  <p>The 'width' and 'height' properties
    accept ''*gd'' and ''*em'' values.
    These values compute to ''auto'',
    and then the result will be round down to
    the largest multiple of the specified unit
    that is equal to or less than the original computed value.</p>

  <p>The fraction is distributed evenly to the computed values of margins of each side.
    If it is used for 'height',
    the half the fraction is added to the top and the bottom margins.
    If for 'width', to the left and the right margins.</p>

  <p class="issue">Should this be a property instead?</p>
  <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>
  <p class="issue">Use cases for non-East Asian layouts?</p>

  <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.
      Since most East Asian characters have 1em advance,
      and since most East Asian documents are justified,
      this minimizes the case where justification needs to expand lines.</p>
    <pre><code class="css">
<!-- -->body {
<!-- -->  margin: 3cm 3.5cm 3cm 3cm;
<!-- -->  text-align: justify;
<!-- -->  width: *em;
<!-- -->}
<!-- --></code></pre>
  </div>

<h2 id="conformance">
Conformance</h2>

<h3 id="conformance-classes">
Conformance Classes</h3>

  <p>Conformance to <var class="issue">CSS TEMPLATE Module</var>
  is defined for three conformance classes:
  <dl>
    <dt><dfn 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>renderer</dfn></dt>
      <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></dt>
      <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">
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.</p>

<h3 id="experimental">
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.</p>

<h3 id="cr-exit-criteria">
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>

  <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>
  </dl>

  <p>The specification will remain Candidate Recommendation for at least
  six months.

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

  <p><var class="issue">[acknowledgments]</var></p>

<h2 class=no-num id=references>References</h2>


<h3 class="no-num" id="normative-references">Normative references</h3>
<!--normative-->

<h3 class="no-num" id="other-references">Other references</h3>
<!--informative-->

<h2 class="no-num" id="index">Index</h2>
<!--index-->


<h2 class="no-num" id="property-index">Property index</h2>
<!-- properties -->
</body>
</html>
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sgml-minimize-attributes:t
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End:
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