csswg-drafts/css-box/Overview.src.html at ef135bbdae38045aef32edc515a9098a80ad6619 · w3c/csswg-drafts · GitHub

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<!doctype html public '-//W3C//DTD HTML 4.01//EN'>

<head profile="
http://microformats.org/profile/h-card
http://microformats.org/profile/hcard
http://microformats.org/profile/rel-license
">

<!-- Note: the h-card profile is for a "microformats 2" vocabulary and
is experimental as of September 2012 -->

<meta content="text/html; charset=utf-8" http-equiv=content-type>
<title>CSS basic box model</title>

<link href="../default.css" rel="stylesheet"
  title="Default" type="text/css">
<style type="text/css">
  ol.stack p {margin-top: 0}
  img.small {max-width: 100%}
  .equiv-table tr {vertical-align: baseline}
  .equiv-table img {max-width: 12em; max-width: 100%}
  .minidiagram {height: 5em; vertical-align: middle}
  dt {display: inline}
  dt + dt::before {content: ", "}
  /* Undo the max-width in the 2012 style for editor's drafts: */
</style>
<link href="http://www.w3.org/StyleSheets/TR/W3C-MO.css" rel="stylesheet"
  title="Default" type="text/css">

<link rel=contents href="#contents">
<link rel=index href="#index">


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

<h1>CSS basic box model</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE: 26 Sep 2006]</h2>
<dl>
  <dt>This version:

    <dd><a href="[VERSION]"
    >http://www.w3.org/TR/[YEAR]/ED-css3-box-[CDATE]</a>

  <dt>Latest version:

    <dd><a href="http://www.w3.org/TR/css3-box"
    >http://www.w3.org/TR/css3-box</a>

  <dt>Previous version:

    <dd><a href="http://www.w3.org/TR/2002/WD-css3-box-20021024"
    >http://www.w3.org/TR/2002/WD-css3-box-20021024</a>

  <dt>Feedback:</dt>

    <dd><a
    href="mailto:www-style@w3.org?subject=%5B[css3-box]%5D%20feedback"
    >www-style@w3.org</a> with subject line &ldquo;[css3-box]
    <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="h-card vcard"><a class="p-name fn u-url url" rel=author
    href="http://www.w3.org/People/Bos/" >Bert Bos</a>, <span
    class="p-org org" >W3C</span>, <a class="u-email email"
    href="mailto:bert@w3.org" >&lt;bert@w3.org&gt;</a>

    <dd class="h-card vcard"><span class="p-name fn" >Anton
    Prowse</span>, <span class="p-org org" >Invited Expert</span>, <a
    class="u-email email" href="mailto:prowse@moonhenge.net"
    >&lt;prowse@moonhenge.net&gt;</a>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>


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

<p><abbr title="Cascading Style Sheets">CSS</abbr> describes the
rendering of documents on various media. When textual documents (e.g.,
HTML) are laid out on visual media (e.g., screen or print), CSS models
their layout as a collection of rectangular boxes containing words, lines,
paragraphs, tables, etc., each with properties such as size, color and
font.

<p>This module describes the basic types of boxes: block, list item,
inline, etc.; and some of their properties, including margins, padding
and width/height. It defines a layout called a “flow,” in which a
series of boxes is laid out one after the other, and it defines
“floating” boxes. Other kinds of layout, such as tables, absolute
positioning, ruby annotations, grid layouts, columns and page boxes,
are described by other modules. Also, the layout of text inside a line
(including the handling of left-to-right and right-to-left scripts) is
defined elsewhere.

<p>In CSS level 3, boxes may contain either horizontal or vertical text.
Different orientations can be mixed.


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

<!--status-->

<p>The CSS Working Group plans to ask for W3C Recommendation status
for this specification only when there are sufficiently many
implementations. See the section <a href="#exit">“CR exit criteria”
</a> for details.

<p class=mtb>The terminology and the order of the sections is
currently in flux. This working draft is published to show the
features that are being discussed in the context of this module, but
it is likely that some of the names of the features will change.

<p>This module should eventually replace corresponding parts
of the revised CSS level 2 specification [[!CSS21]]. But this is an
early draft and any differences to level 2 are most likely
unintentional, unless they concern new features, such as vertical text
or float displacement. <em>Please help us improve the next draft by
pointing out such differences.</em>

<p>Some issues that affect multiple sections in this document:


    <div class=issue>
      <p>Horizontal centering (more precisely: centering in the inline
	direction). The draft contains several suggestions:

      <ol>
	<li>Setting 'margin' to ''fill''. ''Fill'' is similar to
	  ''auto'', but allowed to be negative:

	  <pre>P {margin: 1em fill}</pre>

	<li>Setting a property 'alignment' on the element to center:

	  <pre>P {alignment: center}</pre>

	<li>Setting a property 'child-align' on the parent of the
	  element to center:

	  <pre>DIV {child-align: middle}</pre>

	<li>Rely on grid templates [[CSS3LAYOUT]]. You set a template
	  on the parent or on the element itself. One or the other may
	  be easier.  E.g., to horizontally center the single child of
	  a <span>horizontal</span> DIV):

	  <pre>DIV {grid: ". @ ." }</pre>

	  <p>To horizontally center a P with a template on the P
	    itself requires that the width of the P isn't set with the
	    'width' property, but with the 'grid-columns' property (or
	    the 'grid' shorthand). In the example below the width is
	    set to 'fit-content':

	  <pre>P {grid: * fit-content *  ".  @ ."}</pre>

	  <p>To vertically center the single child of a
	    <span>vertical</span> DIV:

	  <pre>DIV {grid: "@"}
DIV:slot(@) {vertical-align: middle}</pre>

	  <p>To vertically center a P inside a vertical parent by
	    means of a template on the P itself requires that the
	    height of the P isn't set with 'height', but with
	    'grid-rows' (or the 'grid' shorthand). Below, the height
	    is set to 10em:

	  <pre>P {grid: "." *
         "@" 10em
         "." * }</pre>

	  <p>See also <a href="/Style/CSS/Tracker/actions/18"
	    >ACTION-18</a> (especially <a
	    href="http://lists.w3.org/Archives/Public/www-style/2008May/0045.html"
	    >the ideas at end of Bert's comment</a> ), which is
	    continued in <a href="/Style/CSS/Tracker/issues/53"
	    >ISSUE-53.</a>
      </ol>
    </div>

    <div class=issue id=contain-floats>
      <p>Ensuring that the bottom padding (more precisely: the foot
	padding) starts after any preceding and descendant floats.

      <p>The main use case is to make sure an element is at least as
	high as all its floating descendants. I.e., at the end of the
	element, clear the floats that are descendants of the
	element. That may or may not also clear any floats that were
	declared before the element. Some possible approaches:

      <ol>
	<li>Set 'overflow' to anything else than 'visible' (and
	  'height' to 'auto'). This requires no new property, but has
	  the side effect of influencing the scrolling mechanism. It
	  also doesn't clear any floats that occurred before the
	  element and doesn't distinguish left, right or other
	  floats. [2004-02-29 ftf]

	<li>A new property 'clear-after'.

	<li>Use the value ''shrink-wrap'' (or some other name, e.g.,
	  ''contain'') on 'height' (and 'width', in case of vertical
	  text) to make an element into a <em>flow root.</em> This
	  requires no new property, only a new value, but doesn't
	  clear any floats before the element and doesn't distinguish
	  left, right or other floats. [The idea to add the
	  functionality to 'height' is due to IanH 2004-02-29]

	<li>The same, but only allow the new keyword on the
	  'min-height' and 'min-width' properties. [The idea to add
	  the functionality to 'min-height' is due to Fantasai,
	  2012-09]

	<li>A new value ''container'' on 'display'. This doesn't allow
	  a ''list-item'' to clear its floats.
      </ol>
    </div>

    <div class=issue>
      <p>Vertical centering and alignment (more precisely: in the
	block flow direction).

      <p>The use cases are centering a single child vertically in its
	parent, centering all content of an element vertically (e.g.,
	all text on a slide) and aligning only the last child (or the
	last <var>n</var> children) against the bottom of the parent.

      <p>Possible solutions include:

      <ol>
	<li>Extending 'vertical-align' to apply to block elements:

	  <pre>DIV {vertical-align: middle}</pre>

	  <p>This centers or aligns all content, it cannot top-align
	    the first child and bottom-align the last.

	<li>Allowing 'vertical-align' on a new type of box called
	  ''container''. (The main function of ''container'' is to
	  contain its floating descendants.) This wouldn't allow a
	  ''list-item'', a ''table-caption'' or an 'inline-block'' to
	  center its content.')

	<li>Setting 'margin' to ''fill'':

	  <pre>P {margin-top: fill}</pre>

	  <p>This works to center an element in its parent, but not to
	    center the content of an element with only text.

	<li>Relying on grid templates [[CSS3LAYOUT]]. If there are
	  enough elements, that could work:

	  <pre>
&lt;STYLE>
 BODY {grid: "a . b"} /* 3 columns */
 SECTION.a {flow: a; grid: "@" "." "z"} /* 3 rows */
 SECTION.b {flow: b; grid: "@" "." "z"} /* 3 rows */
 P.note {flow: z}
&lt;/STYLE>
&lt;SECTION CLASS=a>
 &lt;H1>...&lt;/H1>
 &lt;P>...
 &lt;P CLASS=note>...
&lt;/SECTION>
&lt;SECTION CLASS=b>
 &lt;H1 ID=b>...&lt;/H1>
 &lt;P>...
 &lt;P CLASS=note>...
&lt;/SECTION>
</pre>

	  <p>And
8096
 assuming 'vertical-align' applies to slots, then that
	    also vertically centers the contents of an element. (This
	    centers horizontally if the element is
	    <span>vertical,</span> because 'vertical-align' is
	    relative to the writing mode.)

	  <pre>P {grid: "@"}
P:slot(@) {vertical-align: center}</pre>

	  <p>But it is not enough to bottom-align the two paragraphs
	    with class “note” in this example:

	  <pre>
&lt;STYLE>
 BODY {grid: "a . b"}
 H1#a, H1#a ~ P {flow: a}
 H1#b, H1#b ~ P {flow: b}
 P.note {/* align to bottom?? */}
&lt;/STYLE>
&lt;H1 ID=>...&lt;/H1>
&lt;P>...
&lt;P CLASS=note>...
&lt;H1 ID=b>...&lt;/H1>
&lt;P>...
&lt;P CLASS=note>...
</pre>

	<li>
	  <p>Use centering in the inline direction after switching the
	    parent to an orthogonal block flow direction. E.g., if the
	    document is <span>horizontal</span> and we want to
	    vertically center the single child of a DIV:

	  <pre>DIV {writing-mode: vertical-rl}
DIV > * {writing-mode: horizontal; width: 100%;
  margin-top: auto; margin-bottom: auto}</pre>

	  <p>This cannot top-align the first child and bottom-align
	    the second.
      </ol>
    </div>


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


<h2>How to read this specification</h2>

<p>All sections are normative, unless stated otherwise.

<div class=example>
<p>Examples look like this and normally start with the word
&ldquo;Example.&rdquo; Examples are not normative.
</div>

<p class=note>Notes look like this and normally start with the word
&ldquo;Note.&rdquo; Notes are not normative.

<p class=issue>Editorial notes look like this. They will be removed before
the document becomes Candidate Recommendation.

<p>Each property is defined in part in the text and in part by a table that
groups together a number of facts about the property, including a regular
expression to restrict its syntax. See <span class=issue>[where?]</span>
for the meaning. The &ldquo;Inherited&rdquo; and &ldquo;Initial&rdquo; rows
in the table are used by the Cascading and Inheritance module [[CSS3CASCADE]]
and &ldquo;Media&rdquo; by the Media Queries specification [[MEDIAQ]].

<p>The specification may refer to the <em>used value</em> and the
<em>computed value</em> of a property. Unless stated explicitly, the
short form “value” means the computed value.


<h2>Dependencies on other modules</h2>

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

<ul>
  <li><cite>CSS Writing Modes Module</cite> [[!CSS3-WRITING-MODES]]
    &ndash; switch between vertical and horizontal text, properties
    <dfn>writing-mode</dfn> and <dfn>direction</dfn>, definitions of the
    terms <dfn>block flow direction</dfn> and <dfn>inline base
    direction.</dfn>
  <li><cite>CSS Syntax Module</cite> [[!CSS3SYN]] &ndash; syntax of
    properties and values
  <li><cite>CSS Values and Units Module</cite> [[!CSS3VAL]] &ndash;
    definitions of value types <dfn><var>&lt;number&gt;</var></dfn>,
    <dfn><var>&lt;percentage&gt;</var></dfn>,
    <dfn><var>&lt;angle&gt;</var></dfn> and
    <dfn><var>&lt;length&gt;</var></dfn>; defines the terms <dfn>computed
    value</dfn> and <dfn>used value.</dfn>
  <li><cite>CSS Text Module</cite> [[!CSS3TEXT]] &ndash; inline layout and
    definition of <dfn>line box.</dfn>
  <li><cite>CSS Generated and Replaced Content module</cite> [[!CSS3GENCON]]
    &ndash; defines the terms <dfn>replaced element</dfn>
    and <dfn>intrinsic</dfn> size (and intrinsic ratio).
  <li><cite>CSS Backgrounds and Borders Module</cite> [[!CSS3BG]] &ndash;
    defines <dfn><var>&lt;bg-position&gt;</var></dfn>
  <li><cite>CSS Basic User Interface Module</cite> [[!CSS3UI]] &ndash;
    defines <dfn>box-sizing</dfn>
  <li><cite>CSS 2D Transforms</cite> [[!CSS3-2D-TRANSFORMS]] &ndash;
    defines <dfn>transform</dfn>
</ul>


<h2>Introduction &amp; definitions</h2>

<p class=issue>Investigate if it is possible to make a pseudo-element
to select anonymous boxes ('::paragraph'). See minutes of 10 Aug 2011.

<p class=note>Note: The model in this specification differs from the
model described in the CSS level 2 specification [[!CSS21]], because
it is generalized to apply also to vertical text. A document using
only features from level 2 renders exactly as described there, (except
possibly for features that are undefined in that specification and
have become defined since).

<p>CSS assumes that the document to lay out is modeled as a <span
class=index title="tree!!of elements|document tree|element
tree">tree</span> of <dfn title="element">elements.</dfn> Each element
has an ordered list of zero or more child elements, with an optional
string of text before the list, in-between the children and after the
list. Each child has one parent, except for the unique element that
has no parent, which is called the <dfn>root element.</dfn>

<p class=note>Note that such a tree can be described in XML or SGML,
but also in other notations and that it is the definition of that
notation, not CSS, that determines the equivalence between the
abstract tree and the concrete notation. E.g., SGML often allows white
space to be added between elements for readability, which does not
result in a string of spaces in the abstract tree.

<p>CSS describes how each element and each string of text is laid out
by transforming the document tree into a set of rectangular boxes,
whose size, position, and stacking level on the <em>canvas</em> depend
on their properties. We say that an element <dfn title="generated
box">generates</dfn> one or more boxes. There are numerous different
types of boxes. <span class=issue>[Or maybe only one? I.e., a
block-level box is just a normal box that happens to be generated by a
block-level element?]</span> A block box is like a paragraph; a line
box is like a single line of text; and inline boxes are like words
inside a line.

<div class="issue">
<p>Define the <dfn title="box tree|tree!!of boxes" >box
tree.</dfn> (Is the space
between words a kind of box as well, or a spacer item? Is the current
selection a box? Is there a defined order among sibling boxes? If so,
are bidi inline boxes ordered in reading order or in visual order?)

<p>[Define the box tree before pagination and then describe how
pagination can break one box into two?]

<!--
<ul>
<li>If the parent element is a list item, the boxes go into the
parent's block box, not the marker box.

<li>If the element is run-in, its boxes go either into a box of its
parent element, or into a box of a later sibling.

<li>The order of sibling boxes is always such that boxes that belong
to elements earlier in the document come before boxes of elements that
come later in the document.

<li>Note that 'position' can put a box into the box of an ancestor
(the one establishing the containing block) rather than a parent.

<li>Note that 'flow' puts boxes into a named flow root.
</ul>
-->
</div>

<p>When the specification says a box <var>A</var> <dfn>contains</dfn>
a box <var>B</var> it means <var>B</var> is a child of <var>A</var> in
the <em>box tree.</em> Typically, box <var>B</var> is geometrically
contained within box <var>A</var> as well, but that need not be the
case, because of, e.g., overflow or negative margins.

<p class=note>Note: The precise rules are below and in other modules,
but in summary, <span class=issue>[write something that helps convey
the idea of a flow, e.g.:]</span> a block-level box contains either other
block-level boxes (e.g., a section containing paragraphs, or a table
containing rows), or it contains line boxes (e.g., a paragraph
containing lines of text). A line box contains inline-level boxes
(e.g., a line with words in different styles). An inline-level box may
contain either text interspersed with more inline-level boxes, or it
may contain a block-level box (e.g., a small table that is rendered
inline).

<div class=figure>
  <p><img src="box-intro.png" alt="Each generated box in the rendering
  has a link to the element that generated it.">

  <p class="caption">Relation between four displayed boxes in the
  rendered document (on the right) and the three corresponding
  elements in the source document on the (left).
</div>

<div class=example>
<p>For example, a fragment of HTML such as
<pre>&lt;ul&gt;
 &lt;li&gt;The first item in the list.
 &lt;li&gt;The second item.
&lt;/ul&gt;</pre>

<p>may result in <span class=issue>[rewrite the following
sentence?]</span> one block-level box for the <code>ul</code> element,
containing two block-level boxes for the two <code>li</code> elements,
each of which has one line box (i.e., one line of text). Both line
boxes contain two inline-level boxes: one that contains the list
bullet and one that contains the text.

<p>Note how the <code>li</code> is transformed into multiple boxes, including
one that contains &ldquo;generated content,&rdquo; viz., the list bullet,
which is not present in the source document.

<p>If the document is rendered in a narrow window, it may be that the
<code>li</code> elements get transformed into even more boxes, because the
text requires multiple lines. And if the document is rendered on paper
or in multiple columns, it
may be that a page break falls in the middle of the <code>ul</code> element,
so that it is not transformed into a single block-level box, but into two
smaller ones, each on a different page or column.
</div>

<p>Each box belongs to exactly one element. It is either <span
title="generated box">generated</span> directly by the element, or it
is an <em>anonymous box,</em> which has been inserted to ensure that
every box and its parent are of mutually compatible types as defined
by the layout rules of CSS. An anonymous box is defined to belong to
whichever element generates the box's closest element-generated
ancestor box. When we need to be precise, we say that the anonymous
box is <dfn>induced,</dfn> rather than generated, by the element it
belongs to.

<p class=note>For example, an anonymous table wrapper box that is
created around a table cell because the table cell element doesn't
have a table element as parent, belongs to the parent element of the
table cell, not to the table cell itself.

<p class=note>An anonymous box is never a <em>principal</em>
box. Except for that, an anonymous box cannot be distinguished by its
property values and characteristics from a box generated by some
hypothetical document element. <span class=issue>Remove this note? Do
we actually use the concept of “principal box” anywhere?</span>

<p>Properties are set on elements [[!CSS3CASCADE]] and influence how
the element is turned into boxes. Every box has the same properties
with the same values as the element to which it belongs, except for
anonymous boxes: inheritable properties on anonymous boxes have the
same value as the element, while non-inheritable properties have the
initial value. The only property that boxes do not have is the
'display' property. <span class=issue>[Is this useful? Or is it better
to say that boxes are just rectangles with a border, a background, a
position, a bunch of other boxes inside, and a reference to the
element they belong to? They don't really have properties then and it
is only a convenience to say the “the box has a font
<var>X.</var>”</span>

<p>Boxes do not have specified values [[!CSS3CASCADE]], but they do
have computed, used and actual values.

<p class=note>Boxes are frequently referred to by their property
values. For example, a “floated box” is a box whose computed value of
'float' is not ''none'', and a “positioned box” is a box whose
computed value of 'position' is not ''static''. However, terms such as
<em>block-level box,</em> <em>block container box</em> and <em>table
box</em> describe characteristics which cannot be deduced solely from
a box's property values.

<div class=figure id=various>
  <p><img src="box.png" alt="Diagram of a typical box, showing the
  content, padding, border and margin areas">

  <p class="caption">The various areas and edges of a typical box
</div>

<p>Boxes have padding, a border and margins (see the <a
href="#various">figure</a>). Different properties determine the
thickness of each of these (which may be zero). The margins of
adjacent boxes are also subject to <em
title="collapse">collapsing</em> (i.e., the actual margin between
adjacent boxes may be less than the sum of the boxes' individual
margins).

<p>Each box has a <dfn>content area</dfn> (a.k.a. <dfn>content
box</dfn>). The rectangle that bounds this area is the <dfn>content
edge.</dfn> Around the content area is the <dfn>padding area</dfn> and
its outside bounds are called the <dfn>padding edge.</dfn> The padding
area and content area together form the <dfn>padding box.</dfn>
Outside the padding is the <dfn>border area</dfn> and the outside
boundary of that area is the <dfn>border edge.</dfn> The border area,
padding area and content area together form the <dfn>border box.</dfn>
Finally, outside the border is the <dfn>margin area</dfn> and its
outer edge is the <dfn>margin edge.</dfn>

<p>When the specification says that the padding or border is <dfn
title="absent|padding!!absent|border!!absent"
>&ldquo;absent&rdquo;</dfn> on some side of the box, that means that
its thickness is zero.

<p><em title="line box">Line boxes</em> cannot have any padding,
border or margin, and therefore their margin edge, border edge,
padding edge and content edge all coincide.

<p class=note>Note that the margin, unlike the border and padding, may have
a negative thickness. That is one way to make adjacent boxes overlap each
other.

<p class=note>Note that the edges always form rectangles, even if
there is a 'border-radius' [[CSS3BG]] or a shape [[CSS3-EXCLUSIONS]].

<p>We say that a box or element is <dfn>horizontal</dfn> if its
'writing-mode' property is ''horizontal-tb'', otherwise it is
<dfn>vertical.</dfn>

<div class=note>
<p>Note that there are theoretically eight possible orientations for
text, but CSS only defines six:

      <table class=equiv-table style="table-layout: fixed; width: 100%">
	<thead>
	  <tr>
	    <th>
	    <th>'writing-mode: horizontal-tb'
	    <th>'writing-mode: vertical-rl'
	    <th>'writing-mode: vertical-lr'
	<tbody>
	  <tr>
	    <th>'direction: ltr'
	    <td>
	      <p>Text is written from left to right and paragraphs
		grow downwards <p><img class=small
		src="horizontal.png" alt="">
	    <td>
	      <p>Text is written top to bottom and paragraphs grow to
		the left <p><img class=small src="vertical.png"
		alt="">
	    <td>
	      <p>Text is written top to bottom and paragraphs grow to
		the right <p><img class=small src="vertical-lr.png"
		alt="">
	  <tr>
	    <th>'direction: rtl'
	    <td>
	      <p>Text is written from right to left and paragraphs
		grow downwards <p><img class=small
		src="horizontal-rtl.png" alt="">
	    <td>
	      <p>Text is written bottom to top and paragraphs grow to
		the left <p><img class=small src="vertical-rtl.png"
		alt="">
	    <td>
	      <p>Text is written bottom to top and paragraphs grow to
		the right <p><img class=small
		src="vertical-lr-rtl.png" alt="">
      </table>

<p>There is no “horizontal-bt.” More information about 'writing-mode'
and 'direction' can be found in [[!CSS3-WRITING-MODES]].
</div>

<p class=mtb>Calculations of the size and position of boxes are
usually very similar for horizontal and vertical boxes, except that
the horizontal and vertical directions are interchanged. To avoid many
almost identical definitions, this specification therefore often uses
abstract terms instead of top, right, bottom, left, width and height:
four terms (head edge, foot edge, start edge and end edge) for the
four edges of a box, four terms (A edge, B edge, C edge and D edge)
for the edges of the children of
2334
 a box and two terms (block dimension
and inline dimension) for the width and height of a box.

<p class=issue>The names head and foot are under discussion. There is
an issue in [[!CSS3-WRITING-MODES]] that suggests some possible names:
“head,” “before,” “before-block,” “foot,” “tail,” “after” and
“after-block.”

<p>The <dfn>head,</dfn> <dfn>foot,</dfn> <dfn>start</dfn> and
<dfn>end</dfn> edge of a box refer to the top, right, bottom and left
edges, depending on the 'writing-mode' and 'direction' properties of
that box, as follows:

    <table class=equiv-table>
      <thead>
	<tr>
	  <th rowspan=2>Value of<br>'writing-mode'
	  <th rowspan=2>Value of<br>'direction'
	  <th colspan=4>Meaning of…
          <th rowspan=2>
	<tr>
	  <th>“head”
	  <th>“end”
	  <th>“foot”
	  <th>“start”
      <tbody>
	<tr><th rowspan=2>''horizontal-tb'' <th>''ltr'' <td>top
	  <td>right <td>bottom <td>left <td><img class=minidiagram
	  src="box-tb-ltr.svg" alt="">

	<tr><!-- horizontal-tb --><th>''rtl'' <td>top <td>left
	  <td>bottom <td>right <td><img class=minidiagram
	  src="box-tb-rtl.svg" alt="">

	<tr><th rowspan=2>''vertical-rl'' <th>''ltr'' <td>right
	  <td>bottom <td>left <td>top <td><img class=minidiagram
	  src="box-rl-ltr.svg" alt="">

	<tr><!-- vertical-rl'' --><th>''rtl'' <td>right <td>top
	  <td>left <td>bottom <td><img class=minidiagram
	  src="box-rl-rtl.svg" alt="">

	<tr><th rowspan=2>''vertical-lr'' <th>''ltr'' <td>left
	  <td>bottom <td>right <td>top <td><img class=minidiagram
	  src="box-lr-ltr.svg" alt="">

	<tr><!-- vertical-lr --><th>''rtl'' <td>left <td>top <td>right
          <td>bottom <td><img class=minidiagram src="box-lr-rtl.svg"
          alt="">
    </table>

<p class=note>For example, the “head padding” by default refers to the
'padding-top' and the “end border” is by default the 'border-right'.

<p>Similarly, the specification sometimes refers to the <dfn>inline
dimension</dfn> and <dfn>block dimension</dfn> of a box, instead of
width and height, as follows:

  <table class=equiv-table>
   <tr>
    <th>'Writing mode'
    <th>Meaning of “inline dimension”
    <th>Meaning of “block dimension”
   <tr><td>''horizontal-tb'' <td>width <td>height
   <tr><td>''vertical-rl'' <td>height <td>width
   <tr><td>''vertical-lr'' <td>height <td>width
  </table>

<p>These correspond to the dimensions in the <em>inline base
direction</em> and the <em>block flow direction,</em> respectively.

<p>An alternative term for the inline dimension is <dfn title="logical
width|width!!logical" >logical width</dfn> and an alternative term for
block dimension is <dfn title="logical height|height!!logical"
>logical height.</dfn> These terms occasionally occur in other CSS
modules.

<p class=note>The block dimension of a box is thus the distance
between the head and foot edges, while the inline dimension of the box
is the distance between the start and end edges.

<p>When boxes are laid out in sequence to form a <em class=index
>flow</em> (see below), their edges play different roles in the
alignment of the boxes within that flow. The orientation of the flow
determines which edge of a box is aligned to which edge of another
box. These roles are independent of the orientation of the box
itself. We distinguish four roles for the edges of a box, called
<dfn>A edge,</dfn> <dfn>B edge,</dfn> <dfn>C edge</dfn> and <dfn>D
edge.</dfn> They depend on the 'writing-mode' and 'direction'
properties of the box's <em class=index >containing block</em>
(defined below), and map to the four sides as follows:

    <table class=equiv-table>
      <thead>
	<tr>
	  <th rowspan=2>'writing-mode' of<br><em>containing block</em>
	  <th rowspan=2>'direction' of<br><em>containing block</em>
          <th colspan=4>Meaning of…
        <tr>
	  <th>“A”
	  <th>“B”
	  <th>“C”
	  <th>“D”
      <tbody>
	<tr><th rowspan=2>''horizontal-tb'' <th>''ltr''
	    <td>top <td>right <td>bottom <td>left
	<tr><!-- horizontal-tb --><th>''rtl''
	    <td>top <td>left <td>bottom <td>right
	<tr><th rowspan=2>''vertical-rl'' <th>''ltr''
	    <td>right <td>bottom <td>left <td>top
	<tr><!-- vertical-rl --><th>''rtl''
	    <td>right <td>top <td>left <td>bottom
	<tr><th rowspan=2>''vertical-lr'' <th>''rtl''
	    <td>left <td>bottom <td>right <td>top
	<tr><!-- vertical-lr --><th>''ltr''
	    <td>left <td>top <td>right <td>bottom
    </table>

<p class=note>For example, block-level boxes in a flow are laid out
such that the A edge of the first box is against the head edge of the
containing block and then the A edge of each subsequent box is put
against the C edge of its predecessor. For typical English text (i.e.,
<em>horizontal</em> text), this means the top edge (= A edge) of the
first paragraph is at the top edge (= head edge) of the document, the
top (A edge) of the second paragraph is against the bottom (C edge) of
the first paragraph, and so on, until the bottom (C edge) of the last
paragraph becomes the bottom (foot edge) of the document.


<h2>The viewport and the canvas</h2>

<p>User agents for continuous media generally offer users a
<dfn>viewport</dfn> (a window or other viewing area on the screen)
through which users consult a document. User agents may change the
document's layout when the viewport is resized (see the <em>initial
containing block</em>).

<p>When the viewport is smaller than the area of the <em>canvas</em>
(see below) on which the document is rendered, the user agent should
offer a scrolling mechanism. There is at most one viewport per canvas,
but user agents may render to more than one canvas (i.e., provide
different views of the same document).

<p>For all media, the term <dfn>canvas</dfn> describes the space where
the formatting structure is rendered. The canvas is infinite for each
dimension of the space.


<h2 id=flows>Flows</h2>

<p>The <dfn>flow</dfn> (sometimes called <dfn>normal flow</dfn>) is
one of the two basic kinds of layout in CSS, together with line layout
[[CSS3LINE]]. It can be used, e.g., to layout a sequence of
paragraphs, headings, lists and floated elements in a single
column. Other CSS modules, such as tables [[CSS3TBL]], positioning
[[CSS3POS]], flex boxes [[CSS3-FLEXBOX]] and grid templates
[[CSS3LAYOUT]], build on the flow model by defining ways to split a
document into more than one flow and positioning and aligning those
flows in various ways on the canvas.

<p>A flow is constructed starting from a <span class=index title="flow
root!!box" >box</span> called the <dfn>flow root</dfn> by the <a
href="#construct-flow">rules below.</a> We say the flow is
<dfn>channeled</dfn> by the flow root.

<p class=issue>Are there more intuitive names than flow root?

<p>The <dfn title="initial flow|flow!!initial" >initial flow</dfn> is
the flow that contains the root of the box tree.

<p>Despite not being a box, the <em>initial containing block</em> is
said to be the flow root of the initial flow.

<p class=issue id=pagination-issue>How do we model pagination? If a
flow is broken over two pages or columns, do we call it a single flow
or two flows? I.e., is the page box the flow root of the flow, or is
the page box something else, more like the viewport, which influences
the layout of a flow but isn't part of it?

<p>Only <em title="block container box" >block container boxes</em>
(defined below) can be flow roots. (Which implies that the children of
a flow root are laid out in a <em>block formatting context.</em>)

<p>Several (combinations of) properties cause an element to generate
boxes that are flow roots. This module defines a few. Other modules
may define more.

<div class=note>
<p>For example, this module defines that any of the following make a
box into a flow root:
<ul>
  <li>The value of 'float' is not 'none'.

  <li>The used value of 'overflow' is not 'visible'.

  <!-- <li>The box is a table wrapper, a table cell, an inline block, or an
  align box. -->

  <!-- <li>The value of 'position' is 'absolute' or 'fixed' (see
  [CSS3POS]). -->

  <li>The box is vertical and its parent is horizontal. <span
  class=issue>[Add link to the relevant section.]</span>

  <li>The box is horizontal and its parent is vertical.

  <!-- <li>The value of 'transform' is not 'none'. -->

  <li>The value of 'child-align' is not 'auto'. <span class=issue>[If
  'child-align' is adopted, rather than, say, 'margin: fill']</span>
</ul>

<p>The table layout module [[CSS3TBL]] defines that a table cell
generates a box that is a flow root; the positioning module
[[CSS3POS]] defines that an absolutely positioned element generates a
flow root; the grid template module [[CSS3LAYOUT]] defines that a
template element generate several flow roots; and the transforms
module [[CSS3-TRANSFORMS]] defines that an element with a transform
generates a flow root
</div>

<p class=mtb id=construct-flow>The flow of a given <em>flow root</em>
is a set of boxes. A box belongs to the flow if all of the following
are true:
<ol>
  <li>The used value of its 'display' is ''block'', ''list-item'' or
  ''table''.

  <li>The used value of its 'float' is ''none''.

  <li>The used value of its 'position' is ''static'' or ''relative''.

  <li>It is either a child of the flow root or a child of a box that
  belongs to the flow.
</ol>

<p class=note>Note that the flow root is itself not part of the flow
it channels and that each flow root channels at most one flow.

<p class=note>Note that the last rule above implies that a flow with
its flow root together are always a connected subset of the box
tree. In other words; together, they form a subtree of the box tree
with possibly some branches removed.

<p class=note>Note that the element tree and the box tree are not
necessarily parallel: the element a box belongs to need not be the
child of the element that generated the box's parent. E.g., run-in
elements may generate boxes that become children of boxes from sibling
elements; and the 'flow' property [[CSS3LAYOUT]] and the 'position'
property [[CSS3POS]] may cause an element to generate a box that
becomes the child of a box from a grandparent or other ancestor
element. E.g., 'DIV {flow: b}' (see [[CSS3LAYOUT]]) makes the DIV
generate boxes that become children of the box generated by slot b in
some ancestor. And 'DIV {position: absolute}' causes the DIV to
generate a box that becomes a child of the box generated by the
ancestor that established the DIV's <em>containing block.</em>

<div class=example>
<p>For example, the fragment
<pre>&lt;div class=sidebar&gt;
 &lt;p&gt;Text in a sidebar.
 &lt;p&gt;Here is quote:
 &lt;blockquote lang=ja&gt;
  &lt;p&gt;...
 &lt;/blockquote&gt;
 &lt;p&gt;Etc. etc.
&lt;/div&gt; </pre>

<p>with the style
<pre>div.sidebar { writing-mode: tb; float: left }
blockquote[lang|=ja] { writing-mode: rl; height: 10em }</pre>

<p>defines two flows:
<ol>
  <li>The <code>div</code> is a flow root, because it floats. Its flow
  consist of the 1st, 2nd and 4th <code>p</code> and the
  <code>blockquote</code>.

  <li>The <code>blockquote</code> is a <em>vertical</em> box inside a
  <em>horizontal</em> parent and it is thus a flow root. Its flow is
  formed by the 3rd <code>p</code>.
</ol>
<p>(The <code>div</code> itself belongs to a third flow, but its flow
</div>

<p class=note>Note that a flow root is not necessarily block-level, it may
be an ''inline-block'', e.g.

<p class=note>Note that a box belongs to at most one flow. (Some boxes
do not belong to any flow, because they are part of a different kind of
layout, such as table layout.)

<p>An element that generates a flow root box is itself also called a
<span class=index title="flow root!!element">flow root.</span>

<p class=note>Note: The terminology in the CSS level 2 specification
is different. A flow root is called “an element that establishes a
<span class=index title="formatting context|new formatting
context">new formatting context.</span>”

<p class=note>A consequence of this terminology is that an
<em>element,</em> unlike a box, can be the flow root of several
flows. E.g., template elements [[CSS3LAYOUT]] generate several flow
root boxes. Depending on what properties are set on a list item and on
its marker pseudo-element, a list item can also generate zero, one or
two flow roots.

<!--
<p class=mtb>The following rules define the position of
<em>block-level</em> boxes relative to the box that is their <em>flow
root.</em> Other sections and other modules describe how other boxes
are laid out. E.g., floating boxes are described further down in this
module and absolutely positioned boxes are described in the Absolute
Positioning module [[CSS3POS]].

<p>Before applying these rules, the width, height and margins of each
box must be computed as described in the sections <a
href="#Calculating">“Calculating widths, heights and margins”</a>
and <a href="#Collapsing">“Collapsing margins.”</a>

<ul>
  <li>
    <p>Consider the first box (in document order) of a set of sibling
    boxes that all belong to the same flow. There are four cases:
    <ol>
      <li>
        <p>If none of its margins <span>collapse</span> with its
        parent's <span>head</span> margin and the box is not
        <span>collapsed through,</span> then the box is placed such
        that the box's margin <span>A edge</span> coincides with the
        parent's <span>head</span> <span>content edge</span>, the
        margin <span>D edge</span> touches the parent's
        <span>start</span> <span>content edge</span> and the margin
        <span>B edge</span> touches the parent's <span>end</span>
        <span>content edge.</span>

	<div class=figure>
	  <p><img src="flow1.png" alt="For example:">