csswg-drafts/css-template/Overview.src.html at master · smfr/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">

<html lang=en>
<head
profile="http://microformats.org/profile/hcard http://microformats.org/wiki/rel-license">
<meta content="text/html; charset=utf-8" http-equiv=content-type>
<title>CSS Template Layout Module</title>
<link href="../default.css" rel=stylesheet type="text/css">

<style type="text/css">
  /* Just in case an incorrect max-width occurs in default.css */
  body {max-width: none !important}

  /* The TR postprocessor already inserted quotes: */
  .css::before, .property::before, .descriptor::before {content: none}
  .css::after, .property::after, .descriptor::after {content: none}

  .prod { font-family: inherit; font-size: inherit }
  pre.prod { white-space: pre-wrap; margin: 1em 0 1em 2em }
  code { font-size: inherit; }

  .example .visible {border: thin dotted silver}
</style>

<link rel=stylesheet href="https://www.w3.org/StyleSheets/TR/W3C-MO.css">

<body class=h-entry>
<div class=head><!--logo-->
<h1 class=p-name>CSS Template Layout Module</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE: 3 August 2004]</h2>

<dl>
  <dt>This version:

  <dd><a class=u-url href="[VERSION]">[VERSION]</a>

  <dt>Latest version:

  <dd><a href="[LATEST]">https://www.w3.org/TR/css-template-3/</a>

  <dt>Previous version:

  <dd><a href="https://www.w3.org/TR/2015/NOTE-css-template-3-20150326/"
  >https://www.w3.org/TR/2015/NOTE-css-template-3-20150326/</a>

  <dt>Feedback:

  <dd><a
  href="mailto:www-style@w3.org?subject=%5Bcss3-layout%5D%20feedback"
  >www-style@w3.org</a> with subject line &ldquo;[css-template-3]
  <var>&hellip; message topic &hellip;</var>&rdquo; (<a
  rel="discussion"
  href="http://lists.w3.org/Archives/Public/www-style/">archives</a>)

  <dt>Editors:

  <dd class="p-author h-card vcard"><span class="p-name fn">Bert
  Bos</span> (<span class="p-org org h-org">W3C</span>) <a
  class="u-email email" href="mailto:bert@w3.org"
  rel=author>bert@w3.org</a>

  <dd class="p-author h-card vcard"><span class="p-name fn">César
  Acebal</span> (<span class="p-org org h-org">University of
  Oviedo</span>)

  <dt>Editors' draft:

  <dd><a href="https://drafts.csswg.org/css-template/"
  >https://drafts.csswg.org/css-template/</a>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>

<!--=================================================================-->

<h2 class="no-num no-toc" id=abstract>Abstract</h2>

<div class=sidefigure>
<p><img alt="Image: four elements move to four slots in a template"
longdesc=diagram.desc src=diagram.png>

<p class=caption>Above: a typical Web page. Below: the underlying grid
template.
</div>

<p>CSS is a simple, declarative language for creating style sheets that
specify the rendering of HTML and other structured documents. This
Note contains experimental ideas for <em>layout templates</em> and alternative
layout models in CSS.

<p>At the moment, it defines a <em>typographic grid</em> for
CSS. It has features to set up a grid-based template, to style the
<em>slots</em> of the template and to flow content into them.

<p>A grid template can be seen as a cross between table layout
(align blocks of text automatically in two-dimensions) and absolute
positioning (position elements in arbitrary order), but it also has
unique
features of its own.

<p>Grid templates can also be associated with pages in paged media
(via '@page'), to create <em>page templates.</em>

<p>Slots can form <em>chains,</em> to create non-rectangular
regions. A chain of slots is like a series of small pages: content is
broken at possible page breaks and distributed over the slots.

<p><em>Region-based styling</em> allows content to be styled not only
based on its position in the document tree, but also based on the slot
it finds itself in. This is achieved by allowing slot names to appear
in selectors.

<!--=================================================================-->

<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--status-->

<p>This draft combines ideas from earlier drafts on Template Layout,
Grid Positioning [[CSS3GRID]] and Grid Layout [[CSS3-GRID-LAYOUT]].

<p>Issues on this draft are mentioned in the text itself and/or in the
on-line Tracker under <a
href="http://www.w3.org/Style/CSS/Tracker/products/15" >“Details on
Product CSS3 Template Layout.”</a>

<!-- Decided at Sep 2014 ftf: -->
<p><em>This document currently serves as a repository for ideas around
using templates for laying out elements, regions or pages; and for
various alternative layout models. It is expected to be updated, but
it is not expected to become a W3C Recommendation in this
form. Instead, ideas may migrate to other modules of CSS.</em>

<!--=================================================================-->

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

<!--=================================================================-->

<h2>Introduction</h2>

<p><em>(This section is not normative.)</em>

<h3>Summary and use cases</h3>

<div class=sidefigure>
<p><img alt="Image: four elements move to four slots in a template"
longdesc=diagram.desc src=diagram.png>

<p class=caption>Above: a typical Web page. Below: the underlying grid
template. It has four slots, a, b, c and d and the grid template is
defined by 'grid: "a a a a a a a" "b c c c c d d"'.
</div>

<p class=mtb>The styling of a Web page, a form or a graphical user
interface can roughly be divided into two parts: (1) defining the overall
“grid” of the page or window and (2) specifying the fonts, indents,
colors, etc., of the text and other objects. The two are not completely
separate, of course, because indenting or coloring a text influences
the perceived grid as well. Nevertheless, when one separates the parts
of a style that should change when the window gets bigger from the
parts that stay the same, one often finds that the grid changes
(different number of columns and rows, different order for the blocks
of content, less or more open space between those blocks, etc.),
while fonts, colors, indents, numbering styles, and many other things
don't have to change, until the size of the window becomes extreme.

<p>The properties in this specification associate a layout grid with
an element (or with a page, see below). Rather than rendering all
descendants of an element in a
single flow, they give an element multiple flows, called
<span>slots,</span> placed on an invisible grid by means of a
<span>grid template.</span> The descendants are given a 'flow'
property to designate the slot into which they flow.

<p>Because layouts on the Web have to adapt to different window and
paper sizes, the rows and columns of the grid can be made fixed or
flexible in size.

<p class=note>In some typographical traditions, the slots are called
“<span class=index title="field (of a typographical
grid)">fields</span>” or “<span class=index title="spatial
zone">spatial zones.</span>” Traditionally, also, all columns have the
same width, called the “<span class=index title="module (of a
typographical grid)">module</span>” or “<span class=index title="unit
(of a typographical grid)">unit</span>.” The horizontal lines of a
grid that define the rows are sometimes called “<span class=index
title="flow line">flow lines.</span>” Some traditions make the space
between pieces of content an explicit part of the grid, some consider
the space part of the content: Both styles are possible in CSS, the
latter by using the 'margin' property, the former by using the '.'
symbol in a grid template.

<p>The typical use cases for these properties include:

<ul>
<li><p>Complex Web pages, with multiple navigation bars in fixed
positions, areas for advertisements, etc.

<li><p>Complex forms, where the alignment of labels and form fields may
be easier with the properties of this module than with the properties
for tables, floats and margins.

<li><p>GUIs, where buttons, toolbars, labels, icons, etc., are aligned in
complex ways and have to stay aligned (and not wrap, for example) when
the window is resized.

<li><p>Paged displays (e.g., printed media) where each page
is divided into fixed areas for different kinds of content.

<li><p>Complex pages with multiple regions, where the style
of the text does not only depend on the type of element it comes from,
but also on the region it is displayed in (<em>region-based
styling</em>).

<li>Paged displays with complex running headers and footers, e.g.,
running headers that are in between parts of the page content, rather
than at the edge of the page.
</ul>

<p>Once a grid template has been defined, it can also be used to
position elements without adding them to a flow. Setting 'display:
grid' on a grid element makes all children into separate flows and
automatically distributes them over the grid. This is explained in
<cite>CSS Grid Layout</cite> [[!CSS3-GRID-LAYOUT]].

<p>Template-based positioning makes many traditional uses of
absolute positioning unnecessary. Like absolute positioning, grid
templates are especially useful for aligning elements that don't have
simple relationships in the source (parent-child, ancestor-descendant,
immediate sibling). But in contrast to absolute positioning, the
elements are not positioned with the help of horizontal and vertical
coordinates, but by flowing them into a template that is very much
like a table: The size and alignment of elements are governed
implicitly by the rows and columns of the template, and can
automatically adapt to the overall size of the element and to the
content in each row and column.

<div class=example>
<p>In this example, the four children of an element are assigned to
four slots (called a, b, c and d) in a 2×2 template. (All mark-up
examples in this specification are HTML fragments, unless otherwise
stated.)

<div class=sidefigure>
<p><img alt="Image: sample rendering" longdesc=aapje.desc
src=aapje.png>

<p class=caption>Each element occupies one slot. In this template, all
slots have the same size.
</div>

<pre>
&lt;style type="text/css"&gt;
  dl { grid: "a b"
             "c d" }
  #sym1 { flow: a }
  #lab1 { flow: b }
  #sym2 { flow: c }
  #lab2 { flow: d }
&lt;/style&gt;
&lt;dl&gt;
  &lt;dt id=sym1&gt;A
  &lt;dd id=lab1&gt;A is een aapje
  &lt;dt id=sym2&gt;B
  &lt;dd id=lab2&gt;B is de bakker
&lt;/dl&gt;
</pre>

<p><a href="ex-aapje.html">Try it out!</a> (Uses a JavaScript
emulation in browsers that do not implement grid templates natively.)
</div>

<div class=example>
<p>Templates can also help with device-independence. This example uses
Media Queries [[MEDIAQ]] to change the overall layout of a page from
3-column layout for a wide screen to a 1-column layout for a narrow
screen. It assumes the page has been marked-up with logical sections
with IDs.

<pre>
@media all
{
  body { grid: "a a a"
               "b c d" }
  #head { flow: a }
  #nav { flow: b }
  #adv { flow: c }
  #body { flow: d }
}
@media all and (max-width: 500px)
{
  body { grid: "a"
               "b"
               "c" }
  #head { flow: a }
  #nav { flow: b }
  #adv { display: none }
  #body { flow: c }
}
</pre>
</div>

<div class=example>
<p>The slots in a grid do not overlap, unless with negative margins.
Here is how
the <a
href="http://www.csszengarden.com/?cssfile=/026/026.css&amp;page=0" >
“zunflower” design</a> of the CSS Zen Garden could be done:

<pre>
#container {grid: "A B C"}
#container::slot(A) {box-shadow: 0.5em 0.5em 0.5em; z-index: 1}
#container::slot(B) {box-shadow: 0.5em 0.5em 0.5em; margin-left: -2em}
#intro {flow: A}
#supportingText {flow: B}
#linkList {flow: C}
</pre>
</div>

<div class=example>
<p>Template-based positioning borrows some concepts from table layout,
in particular the idea of aligning elements in rows and columns, so
that they constrain one another's size. But there are also
differences. This example shows some of them. Assume this document
fragment:

<pre>
&lt;div class=group>
 &lt;div>aa aa aa aa aa aa&lt;/div>
 &lt;div>bbb&lt;/div>
 &lt;div class=menu>ccccc&lt;/div>
&lt;/div>
</pre>

<p>We can lay it out as three columns, as the following illustrations
show. The style sheet would contain the following.

<pre>
.group {display: table}
.group > div {display: table-cell}
</pre>

<div class=figure>
<p><img alt="[Three unequal cells]" longdesc=table1.desc
src=table1.png>

<p class=caption>Example of rendering with a table.
</div>

<p>We can also use a template, in which case the style sheet would
contain this:

<pre>
.group {grid: "a b c"}
.group > div {flow: a}
.group > div + div {flow: b}
.group > div + div + div {flow: c}
</pre>

<p>By default, the table is as wide as needed to fit its contents. To
make sure it is as wide as its containing block, we need to add

<pre>.group {display: table; <i>width: 100%</i>}</pre>

<p>That is not needed for the grid template, but, on the other hand,
if we want the template to fit its contents, we would need to say so:

<pre>.group {grid: "a b c"; <i>width: fit-content</i>}</pre>

<p>(See [[!CSS3BOX]] for the definition of the 'width' property.) The
columns of the template are by default all the same size. The columns
of the table satisfy certain constraints, but their sizes may
differ. We can make them all the same by adding a rule (see
[[!CSS3TBL]]):

<pre>.group {display: table; width: 100%; <i>table-layout: fixed</i>}</pre>

<div class=figure>
<p><img alt="[Three equal cells]" longdesc=table2.desc src=table2.png>

<p class=caption>Example of rendering with equal columns.
</div>

<p>In both styles, we can set a column to a certain size:

<pre>div:nth-child(3) {<i>width: 3em</i>}</pre>

<p>resp.,

<pre>.group {grid: <i>* * 3em</i>  "a b c"}</pre>

<div class=figure>
<p><img alt="[Two equal cells, third is 3em wide]"
longdesc=table3.desc src=table3.png>

<p class=caption>Example of rendering with a fixed third column and
the other two columns of equal width.
</div>

<p>If there is an unknown number of columns (children of the div.group
element), the style sheet for the table model will automatically take
them into account. The style sheet for the template model, however,
creates a template of exactly three columns and can't handle tables
with an unknown number of columns. The extra elements will be added
into the default slot (in this case the ''a'' slot).

<p>(However, setting 'display' to 'grid' on the grid element creates a
different kind of grid, that <em>does</em> grow, see <a
href="#absolute">“Comparison with 'display: grid' and 'display:
inline-grid'”</a> below.)

<p>In both models, elements can have borders, but only in the table
model can borders be collapsed, which makes setting borders easier in
the table model:

<pre>
.group {display: table; <i>border-collapse: collapse</i>}
.group > div {<i>border: solid</i>}
</pre>

<p>resp.,

<pre>
.group > div {<i>border: solid; border-left: none</i>}
.group > div:first-child {<i>border-left: solid</i>}
</pre>

<p>In the template model, the order of the elements is explicit, and
thus it is possible to reverse the order of the columns:

<pre>
.group > div {flow: <i>c</i>}
.group > div + div {flow: b}
.group > div + div + div {flow: <i>a</i>}
</pre>

<div class=figure>
<p><img alt="[Different contents for the cells]" longdesc=table4.desc
src=table4.png>

<p class=caption>Example of rendering with the contents of the three
columns reversed: the third element is shown in the first slot and the
first element in the third slot.
</div>

<p>In the table model, the order of the rows and columns is given by
the document source and thus cannot be changed.
</div>

<div class=example>
<p>[Editorial note: Remove this example if the 'content' property will
<em>not</em> be allowed on elements, as it is currently expected to
be.]

<p>This example shows a way to move notes to the end of a section.
“Notes” in this example refers to elements in HTML with a class of
“note”. A fragment of HTML such as

<pre>
&lt;div class=section>
  &lt;p>The balubious coster of the fifth secter&lt;span
    class=note> The sixth secter coster is a difter
    manon.&lt;/span> of The Rollow Carpug mentizes a costernica.
  &lt;p>&hellip;
&lt;/div>
</pre>

<p>with this style sheet

<pre>
div.section {
    grid: "*"
          "F"}
.note {
    flow: F;
    content: counter(note) ".\A0" contents;
    counter-increment: note;
    font-size: smaller}
.note::before {
    content: counter(note);
    vertical-align: super;
    font-size: larger}
</pre>

<p>results in a rendering similar to this:

<div class=figure>
<p><img alt="Same text, with the SPAN replaced by “(1)” and its
content moved to the end." longdesc=footnotes.desc src=footnotes.png>

<p class=caption>Rendering of a text with footnotes.
</div>

<p>The “Generated content for paged media” module [[CSS3GCPM]] is
expected to define specific features for footnotes that are both
easier and more powerful than this example.
</div>

<div class=example>
<p>This example shows the use of <span class=index title=chain>chained
regions:</span> text from region 1
continues in region 2, 3 and 4. And it shows how to use
pseudo-elements to style text in a particular region: text in region 1
is bigger than in other regions.

<div class=figure>
<p><img alt="Image of a complex, 3-column layout"
longdesc=regions-intro-rendering.desc src=regions-intro-rendering.png>

<p class=caption>Example rendering
</div>

<p>We assume a document fragment similar to this:

<pre>
&lt;div id=article>
  &lt;h1>Introduction&lt;/h1>
  &lt;p>&lt;img src="sunset" alt="">
  &lt;p>This is an example…

  &lt;h1>More Details&lt;/h1>
  &lt;p>This illustrates…
  &lt;p>Then, the example…
  &lt;p>Finally, this…
&lt;/div>
</pre>

<p>The style sheet makes the DIV into a grid element with five
regions, called A, *, b, c and d. The regions are grouped into two
chains: region A on its own, and the chain consisting of *, b, c and
d.

<pre>
#article {
  <span class=index>grid</span>: "A A c"
        "A A c"
        "* * c"
        "a b c";
  <span class=index>chains</span>: * a b c }
</pre>

<p>The '*' is a special name. It indicates the <em>default slot,</em>
the slot where content goes if it doesn't have an explicit 'flow'
property. We could have used a normal letter and added an explicit
'flow' property to the children of the DIV, but by using * we don't
c as needed). The IMG element goes into region A. We assume for
simplicity that there is only one IMG element:

<pre>#article img { flow: A }</pre>

<p>The text in region * is bolder, larger and a different color than
in the other regions. Also, an H1 that falls into this region is
rendered differently from other H1s:

<pre>
<span class=index>@region</span> ::slot(*)
{
  * {
    font-weight: bold;
    color: #0C3D5F;
    font-size: larger }
  h1 {
    color: crimson;
    display: run-in }
}
</pre>

<p>(For brevity, the example doesn't show the style rules to set the
color and background, to make the text justified, add the left border
to the second H1, etc.)

<p>As the image shows, the text of the last paragraph isn't complete
and presumably continues somewhere else, but that part is outside the
scope of this example. In paged media, we could attach the grid
template to a page instead of an element and let the text continue on
the next page; we could also make slot c into a multi-column element
so that the text forms additional columns to the right of current
image (see [[CSS3GCPM]] for some ideas), or there could be a scrollbar
on slot c.
</div>

<div class=example>
<p>This example shows three ways to align the contents of a
fixed-height paragraph to the bottom of the box:

<dl>
<dt>Method 1 <dd>
<pre>P {height: 20em; grid: "." "*" fit-content}</pre>
<dt>Method 2 <dd>
<pre>P {height: 20em; grid: "*"}
P::slot(*) {vertical-align: bottom}</pre>
<dt>Method 3 <dd>
[Editorial note: Remove method 3 if <span
class=index>vertical-align</span> will <em>not</em> be generalized to
apply to all blocks.]
<pre>P {height: 20em; vertical-align: bottom}</pre>
</dl>

<p>However, the styles differ in case the content is taller than 20 em
and overflows: the first style has a baseline-aligned content which
will thus overflow <em>below</em> the element's box; the second
example has bottom-aligned content, which will thus overflow
<em>above</em> the element's box.
</div>

<div class=example>
<p>This example (based on a challenge by Andrew Fedoniouk) shows how a
grid template can be used to display six items in two rows of
three. Assume the following document:
<pre>
&lt;p&gt;
  &lt;span&gt;...&lt;/span&gt;
  &lt;span&gt;...&lt;/span&gt;
  &lt;span&gt;...&lt;/span&gt;
  &lt;span&gt;...&lt;/span&gt;
  &lt;span&gt;...&lt;/span&gt;
  &lt;span&gt;...&lt;/span&gt;
&lt;/p&gt;
</pre>
<p>The simplest way to break this into two lines (or more) is to add a
hard line break after the third SPAN: ''span:nth-child(3)::after
{content: "\A"; white-space: pre}''. That works fine, unless the SPANs
are inline blocks (''span {display: inline-block}''). Another way is
to float all SPANs and add a 'clear' property on the fourth: ''span
{float: left} span:nth-child(4) {clear: left}''. But if the SPANs have
different heights or baselines, their vertical alignment will be
different than if they were on the same line.

<p>With a grid template, we can say that the P has not one, but two
flows, a and b, the second one positioned below the first; and then we
assign the fourth and subsequent SPANs to
the “b” flow. (The others automatically go into the default, first
flow.)
<pre>
p {grid: "a" "b"}
span:nth-child(n+4) {flow: b}
</pre>

<p>This almost works. There is just a subtle issue with the spaces
between the SPANs. We moved the last three SPANs to flow b, but we
have no way to move the spaces between them. They remain in the
default (first) slot. (They collapse, as there is nothing left between
them, so we don't actually see them anymore.) To get spaces between
the SPANs in slot b, we need to generate new spaces:

<pre>span:nth-child(n+4)::after {content: " "}</pre>

<p class=issue>This space handling is not very intuitive. One might
expect there to be a way to say “select everything from this element
onward” or maybe a pseudo-element selector for the text between
elements. Or a new property to specify different ways to handle the
anonymous text inside a grid element. For the moment, there is no such
thing. Suggestions are welcome.

<p>If, on the other hand, we <em>don't</em> want spaces between the
SPANs, then the question becomes how to get rid of the spaces between
the SPANs in the first row. There are several ways of doing that. One
way is to explicitly flow the three first SPANs to slot “a”:

<pre>span {flow: a}</pre>

<p>The SPANs are thus taken out of the P, causing the P's spaces to
end up next to each other at the start of flow “a” and collapse
away. Another, maybe more intuitive way is to set the spaces to zero
width in the P, and then back to normal in the SPANs (see
'word-spacing' in [[CSS3TEXT]]):

<pre>
p {word-spacing: -100%}
span {word-spacing: normal}
</pre>

<p>[Editorial note: Remove this last method if percentage values on
'word-spacing' will not be allowed. They currently (March 2015) have
Working Draft status.]
</div>

<div class=example>
<p>This example uses a <em>page-based template</em> to create a
running header that is not at the top of the page, but exactly in the
middle. We need to define our own page template for this, because the
predefined page template of [[CSS3PAGE]] only has margin boxes along
the edge of the page.

<pre>
@page {
  grid:
    "top-half"
    "running-header" 3em
    "bottom-half";
  chains: top-half bottom-half
}
::slot(running-header) {
  content: "My Magazine - page " counter(page) " - February 2013";
  text-align: center;
  <span class=index>vertical-align</span>: middle;
  border: solid;
  margin: 1em 0;
}
</pre>
<div class=figure>
<p><img src="page.png" alt="Image: Example rendering" longdesc="page.desc">

<p class=caption>An example of a page rendered with the above page
template. It has the “running header” with the page number right in
the middle of the page.
</div>
</div>

<!--=================================================================-->

<h3 id=dependencies>Dependencies on other modules</h3>

<p>This CSS3 module depends on the following other CSS3 modules:

<ul>
<li><cite>CSS 2.1</cite> [[!CSS21]]
&ndash; <a href="https://www.w3.org/TR/CSS21/visuren.html#block-boxes"
><dfn>block container,</dfn></a> the property <dfn>content,</dfn> and
the concept of <dfn>stacking context.</dfn>

<li><cite>CSS syntax module</cite> [[!CSS3SYN]] &ndash;
<dfn>interactive</dfn> vs <dfn>non-interactive</dfn> media.

<li><cite>CSS values and Units module</cite> [[!CSS3VAL]] &ndash; <a
href="https://www.w3.org/TR/css3-values#strings">
<dfn><var>&lt;string&gt;</var></dfn></a> and <a
href="https://www.w3.org/TR/css3-values#length-value">
<dfn><var>&lt;length&gt;</var></dfn></a>.

<li><cite>CSS box module</cite> [[!CSS3BOX]] &ndash; properties
<dfn>margin</dfn>, <dfn>margin-right</dfn>, <dfn>margin-left</dfn>,
<dfn>width</dfn>, <dfn>height</dfn>, <dfn>float</dfn>,
<dfn>display</dfn> and <dfn>overflow</dfn>; definitions of the terms
<dfn>intrinsic minimum</dfn> and <dfn>intrinsic preferred</dfn> (as in
the phrases “intrinsic preferred width” and “intrinsic minimum width,”
referring roughly to the width of the content without breaking any
lines and the width with as many line breaks as possible.)

<li><cite>CSS text layout</cite> [[!CSS3-WRITING-MODES]] &ndash;
defines <dfn>block flow direction</dfn> and the
properties <dfn>direction</dfn> and <dfn>writing-mode</dfn>. The
latter defines when an element has a <dfn>vertical writing mode</dfn>
(text lines are vertical) or a <dfn>horizontal writing mode</dfn>
(text lines are horizontal). For brevity, we refer to the former as
a <dfn title="vertical|vertical element" >vertical element</dfn> and
to the latter as a <dfn title="horizontal|horizontal element"
>horizontal element.</dfn>

<li><cite>CSS po
10BC8
sitioning</cite> [[!CSS3POS]] &ndash; properties
<dfn>z-index</dfn>, <dfn>position</dfn>, <dfn>top</dfn>,
<dfn>right</dfn>, <dfn>bottom</dfn>, <dfn>left</dfn>.

<li><cite>CSS fragmentation</cite> [[CSS3-BREAK]] &ndash; properties
<dfn>break-before</dfn>, <dfn>break-after</dfn> and
<dfn>break-inside</dfn>; the definition of <dfn>fragmenter</dfn>.

<li><cite>CSS paged media</cite> [[CSS3PAGE]] &ndash; definition of
<dfn>page area</dfn> (in particular its width &amp; height).

<li><cite>CSS background and borders</cite> [[CSS3BG]] &ndash;
properties <dfn>background</dfn>, <dfn>background-image</dfn>,
<dfn>background-size</dfn>, <dfn>box-decoration-break</dfn>
<span class=issue>and <dfn>box-shadow</dfn>?</span>

<li><cite>CSS tables</cite> [[!CSS3TBL]] &ndash; property
<dfn>vertical-align</dfn>. <span class=issue>[or Box Module?]</span>

<li><cite>CSS Multi-column Layout Module</cite> [[CSS3COL]] &ndash;
property <dfn>columns</dfn>.

<li><cite>CSS Generated and replaced content module level 3</cite>
[[!CSS3GENCON]] &ndash; property <dfn>content</dfn>.

<li><cite>CSS regions</cite> [[!CSS3-REGIONS]] &ndash; at-rule
<dfn>@region</dfn> and the term <dfn>region.</dfn>

<li><cite>CSS Grid Layout</cite> [[!CSS3-GRID-LAYOUT]] &ndash; the
term <dfn>grid container</dfn> and the properties
<dfn>grid-auto-flow</dfn>, <dfn>grid-auto-columns</dfn> and
<dfn>grid-auto-rows</dfn>. (Note: A grid container is an element with
a 'display' of ''grid'' or ''grid-inline''.)

<li><cite>CSS Cascading and Inheritance Level 3</cite>
[[!CSS3CASCADE]] &ndash; keywords <dfn>inherit,</dfn>
<dfn>initial</dfn> and <dfn>unset.</dfn>
</ul>

<p class=issue>Add to the above all the properties that are allowed on
slots (see <a href="#slot-pseudo">“Styling slots: the ''::slot()''
pseudo-element”</a>).

<p>It has non-normative (informative) references to the following other
CSS3 modules:

<ul>
<li><cite>Selectors</cite> [[SELECT]]

<li><cite>CSS Text Module Level 3</cite> [[CSS3TEXT]] &ndash; the
<dfn>word-spacing</dfn> property
</ul>

<p>See <a
href="https://www.w3.org/TR/CSS21/about.html#property-defs" >section&nbsp;1.4.2
of CSS level&nbsp;2</a> [[CSS21]] for the grammar and other notations
that this specification uses in property definitions.

<!--=================================================================-->

<h3 id=values>Values</h3>

<p>This specification follows
the <a href="https://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 ''inherit'', ''initial'', and ''unset'' keywords as their
property value.

<!--=================================================================-->

<h3 id=accessibility>A note about accessibility</h3>

<p><em>(This section is not normative.)</em>

<p>The facilities in this specification allow elements from a document
to be displayed in a visual order that is to a large extent
independent of the order in the document. That may have both positive
and negative effects on accessibility. The positive aspect is that it
allows the content in the document to be kept in logical order, so
that the document is as functional as possible without the style sheet
and on media where the style sheet doesn't apply. A negative aspect is
that a document that has a defined tab order (the order in which
elements get the focus when the tab-key is pressed) will show on the
screen with a tab order unrelated to the visual order. It may be
necessary to use the keyboard control features of the CSS Basic User
Interface module [[CSS3UI]] to ensure that the tab navigation follows
the visual order, or to refrain from positioning semantically related
elements in different parts of a template.

<p>The following two requirements from the Web Content Accessibility
Guidelines (WCAG) 2.0 [[WCAG20]] are particularly relevant. See that
specification for more information.

<blockquote>
<p><a
href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#content-structure-separation-sequence"
><strong>1.3.2 Meaningful Sequence:</strong></a> When the sequence in
which content is presented affects its meaning,
a <a
href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#correct-reading-sequencedef"
>correct reading sequence</a> can
be <a
href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#programmaticallydetermineddef
" >programmatically determined.</a> (Level A)
</blockquote>

<blockquote>
<p><a
href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#navigation-mechanisms-focus-order"
><strong>2.4.3 Focus Order:</strong></a> If
a <a href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#webpagedef"
>Web page</a> can
be <a href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#nav-seqdef"
>navigated sequentially</a> and the navigation sequences affect
meaning or operation, focusable components receive focus in an order
that preserves meaning and operability. (Level A)
</blockquote>

<!--=================================================================-->

<h2>Stack of cards layout</h2>

<p class=issue>This is a placeholder.

<p>A stack is a layout model based on the metaphor of a stack of index
cards: sibling elements are formatted as boxes (<dfn
title="card">“cards”</dfn>) of the same size at the same position with
a <dfn>tab</dfn> sticking out at one side that contains a title for
that card. All tabs are visible (and automatically positioned so they
overlap as little as possible), but only one box is visible at a
time. In an interactive UA, the user can activate a card (e.g., by
clicking on a tab, or by pressing page-down), which displays that card
and hides all the others in the stack. E.g., a rule 'display: stack'
could make an element into a stack of cards (all children become
cards, sized to the largest among them) and 'display: tab' could make
their descendants into titles for the cards. Maybe other properties
determine the style of the tabs and on which side they display (top,
bottom, left or right).

<!--=================================================================-->

<h2>Declaring templates</h2>

<p>Grid templates are declared with the 'grid-template-areas', 'grid-template-rows',
'grid-template-columns' and 'grid' properties, defined below.

<p>An element that has a grid template is called a <dfn>grid
element.</dfn> More precisely: an element is a grid element if (1) it
generates a <span>block container</span> box [[!CSS21]] and (2) at
least one of 'grid-template-areas', 'grid-template-rows' or 'grid-template-columns' has a
value other than the initial value.

<p class=note>Block containers are, informally, elements that can have
block elements as children, such as blocks, inline blocks or table
cells, but not inline elements or replaced elements.

<p>An element's <dfn>grid ancestor</dfn> is the nearest ancestor that
is a <span>grid element.</span>

<p class=note>Note that not all elements have a grid ancestor. E.g.,
the root element has none.

<p>The <dfn>number of columns</dfn> of a grid element is the number
of <var>&lt;col-width&gt;</var> values in 'grid-template-columns' or the number
of columns in 'grid-template-areas', whichever is larger.

<p>The <dfn>number of rows</dfn> is the number
of <var>&lt;row-height&gt;</var> values in 'grid-template-rows' or the number
of rows in 'grid-template-areas', whichever is larger.

<p class=note>Note that a grid element always has at least one row and one
column. Those may, however, have zero size.

<!--=================================================================-->

<h3 id=grid-template >Declaring a template: 'grid-template-areas'</h3>

<p>The 'grid-template-areas' property implicitly assigns a grid to an
element and defines named slots in the grid. It can also designate a
slot as the <em>default slot</em> for content without an explicit
flow.

<table class=propdef>
<tbody>
<tr><th>Name: <td><dfn>grid-template-areas</dfn>
<tr><th><a href="#values" >Value</a>: <td>none | <var>&lt;string&gt;</var>+
<tr><th>Initial: <td>none
<tr><th>Applies to: <td><span>block container</span> elements [[!CSS21]] and <em>grid container</em> elements [[!CSS3-GRID-LAYOUT]]
<tr><th>Inherited: <td>no
<tr><th>Animatable: <td>no
<tr><th>Percentages: <td>N/A
<tr><th>Media: <td>visual
<tr><th>Computed&nbsp;value: <td>specified value
<tr><th>Canonical order: <td><abbr title="follows order of property
value definition">per grammar</abbr>
</table>

<p>Each string consist of one or more asterisks (“*”), identifiers,
periods (“.”), spaces and tabs. Each string represents one row in the
template, each symbol other than a space or tab represents one column
in that row. Spaces and tabs have no meaning. They separate
identifiers and may also be added for readability.

<p>The symbols in the template have the following meaning

<dl>
<dt>&lt;identifier&gt;
<dd>
Named slot for content.

<!--
<p class=issue>Unicode normalization is not yet
defined in css3-values nor anywhere else. E.g., we want é to match é,
no matter if the author typed U+0301 U+0065 or U+00C9.)
-->

<dt>*
<dd>
(Asterisk.) Default slot for content (explained below).

<dt>.
<dd>
(Period.) Blank space.
</dl>

<p>Multiple identical identifiers in adjacent rows or columns form a
single <dfn>slot</dfn> that spans those rows and columns. Ditto for
multiple “*”s. Slot names are case-sensitive, so uppercase and
lowercase letters are considered to be different letters.

<p class=issue>Case-sensitivity should instead be defined by reference
to the definition of identifiers, probably in css3-values.

<p>Non-rectangular slots and multiple slots with the same name are
illegal. A template without any slot name or “*” is illegal. A template
with more than one “*” slot is illegal. Strings with different numbers
of columns (<em>before</em> padding with “.”, see the next paragraph) are
illegal. These errors cause the declaration to be ignored.