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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head
profile="http://www.w3.org/2006/03/hcard http://microformats.org/wiki/rel-license"><meta
content="text/html; charset=utf-8" http-equiv=content-type>
<title>CSS basic box model</title>
<style type="text/css">
ol.stack p {margin-top: 0}
img.small {max-width: 100%}
</style>
<link href="#contents" rel=contents>
<link href="#index" rel=index>
<link href="../default.css" rel=stylesheet title=Default type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
title=Default type="text/css">
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS basic box model</h1>
<h2 class="no-num no-toc" id=longstatus-date-26-sep-2006>Editor's Draft 16
May 2012</h2>
<dl>
<dt>This version:
<dd><a href="http://www.w3.org/TR/2012/ED-css3-box-20120516/">
http://www.w3.org/TR/2012/ED-css3-box-20120516</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>Editors:
<dd class=vcard><span class=fn>Bert Bos</span> (<span
class=org>W3C</span>) <span class=email>bert@w3.org</span>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2012 <a href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><abbr
title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
href="http://www.ercim.eu/"><abbr
title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
<a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p><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" id=status-of-this-document>Status of this
document</h2>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss3-box%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“css3-box” in the subject, preferably like this:
“[<!---->css3-box<!---->] <em>…summary of comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<p>The 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>This module should eventually replace corresponding parts of
the revised CSS level 2 specification <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. 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>
<h2 class="no-num no-toc" id=table-of-contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#how-to-read-this-specification"><span class=secno>1.
</span>How to read this specification</a>
<li><a href="#dependencies-on-other-modules"><span class=secno>2.
</span>Dependencies on other modules</a>
<li><a href="#introduction-amp-definitions"><span class=secno>3.
</span>Introduction & definitions</a>
<li><a href="#the-viewport-and-the-canvas"><span class=secno>4. </span>The
viewport and the canvas</a>
<li><a href="#aspect-ratios-of-replaced-elements"><span class=secno>5.
</span>Aspect ratios of replaced elements</a>
<li><a href="#basic-flow-layout"><span class=secno>6. </span>Basic flow
layout</a>
<li><a href="#types-of-boxes"><span class=secno>7. </span>Types of
boxes</a>
<ul class=toc>
<li><a href="#display"><span class=secno>7.1. </span>The display
property</a>
<li><a href="#mixing-the-box-model-with-other-formatti"><span
class=secno>7.2. </span>Mixing the box model with other formatting
models</a>
<li><a href="#block-level-boxes-containing-blocks-flow"><span
class=secno>7.3. </span>Block-level boxes, containing blocks, flows and
anonymous boxes</a>
<li><a href="#run-in-boxes"><span class=secno>7.4. </span>Run-in
boxes</a>
<li><a href="#compact-boxes"><span class=secno>7.5. </span>Compact
boxes</a>
</ul>
<li><a href="#the-padding-properties"><span class=secno>8. </span>The
padding properties</a>
<li><a href="#margins"><span class=secno>9. </span>Margins</a>
<ul class=toc>
<li><a href="#the-margin-properties"><span class=secno>9.1. </span>The
margin properties</a>
<li><a href="#collapsing-margins"><span class=secno>9.2.
</span>Collapsing margins</a>
</ul>
<li><a href="#the-width-and-height-properties"><span class=secno>10.
</span>The width and height properties</a>
<li><a href="#min-max"><span class=secno>11. </span>The min-width,
max-width, min-height and max-height properties</a>
<li><a href="#intrinsic"><span class=secno>12. </span>Definition of
intrinsic sizes – high quality</a>
<li><a href="#intrinsic-low"><span class=secno>13. </span>Definition of
intrinsic sizes – low quality</a>
<li><a href="#Calculating"><span class=secno>14. </span>Calculating
widths, heights and margins</a>
<ul class=toc>
<li><a href="#inline-non-replaced"><span class=secno>14.1.
</span>Inline, non-replaced elements</a>
<li><a href="#inline-replaced"><span class=secno>14.2. </span>Inline or
floating, replaced elements</a>
<li><a href="#blockwidth"><span class=secno>14.3. </span>Block-level,
non-replaced elements in normal flow when ‘<code
class=property>overflow</code>’ computes to ‘<code
class=css>visible</code>’</a>
<li><a href="#other-block-level-non-replaced-elements-"><span
class=secno>14.4. </span>Other block-level, non-replaced elements in
normal flow</a>
<li><a href="#inline-block-or-floating-non-replaced-el"><span
c
A90C
lass=secno>14.5. </span>‘<code class=property>Inline-block</code>’
or floating, non-replaced elements</a>
<li><a href="#abs-non-replaced-width"><span class=secno>14.6.
</span>Absolutely positioned, non-replaced elements</a>
<li><a href="#abs-replaced"><span class=secno>14.7. </span>Absolutely
positioned, replaced elements</a>
<li><a href="#block-level-replaced-elements-in-normal-"><span
class=secno>14.8. </span>Block-level, replaced elements in normal
flow</a>
<li><a href="#floating-non-replaced-elements"><span class=secno>14.9.
</span>Floating, non-replaced elements</a>
<li><a href="#root-height"><span class=secno>14.10. </span>Auto heights
for flow roots</a>
</ul>
<li><a href="#the-float-property"><span class=secno>15. </span>The float
property</a>
<ul class=toc>
<li><a href="#introduction-to-floats"><span class=secno>15.1.
</span>Introduction to floats</a>
<li><a href="#rules-for-positioning-floats"><span class=secno>15.2.
</span>Rules for positioning floats</a>
</ul>
<li><a href="#the-clear-property"><span class=secno>16. </span>The clear
property</a>
<li><a href="#the-clear-after-property"><span class=secno>17. </span>The
clear-after property</a>
<li><a href="#overflow0"><span class=secno>18. </span>Overflow</a>
<ul class=toc>
<li><a href="#overflow"><span class=secno>18.1. </span>The overflow,
overflow-x and overflow-y properties</a>
</ul>
<li><a href="#the-visibility-property"><span class=secno>19. </span>The
visibility property</a>
<li><a href="#the-alignment-property"><span class=secno>20. </span>The
alignment property</a>
<li><a href="#the-child-align-property"><span class=secno>21. </span>The
child-align property</a>
<li><a href="#stacking-contexts"><span class=secno>22. </span>Stacking
contexts</a>
<ul class=toc>
<li><a href="#definitions"><span class=secno>22.1.
</span>Definitions</a>
<li><a href="#painting-order"><span class=secno>22.2. </span>Painting
order</a>
<li><a href="#notes"><span class=secno>22.3. </span>Notes</a>
<li><a href="#the-ltnumbergt-value"><span class=secno>22.4. </span>The
<number> value</a>
<li><a href="#auto"><span class=secno>22.5. </span>The auto value</a>
<li><a href="#complex-size"><span class=secno>22.6. </span>Algorithm for
shrink-wrapping replaced elements with complex width/height</a>
</ul>
<li><a href="#the-float-displace-property"><span class=secno>23.
</span>The ‘<code class=property>float-displace</code>’ property</a>
<li><a href="#the-indent-edge-reset-property"><span class=secno>24.
</span>The ‘<code class=property>indent-edge-reset</code>’
property</a>
<li><a href="#conformance-and-profiles"><span class=secno>25.
</span>Conformance and profiles</a>
<li><a href="#exit"><span class=secno>26. </span>CR exit criteria</a>
<li><a href="#acknowledgments"><span class=secno>27.
</span>Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<li class=no-num><a href="#index">Index</a>
<li class=no-num><a href="#property-index">Property index</a>
</ul>
<!--end-toc-->
<h2 id=how-to-read-this-specification><span class=secno>1. </span>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 “Example.”
Examples are not normative.
</div>
<p class=note>Notes look like this and normally start with the word
“Note.” 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 “Inherited” and “Initial” rows in the table
are used by the Cascading and Inheritance module <a href="#CSS3CASCADE"
rel=biblioentry>[CSS3CASCADE]<!--{{CSS3CASCADE}}--></a> and “Media” by
the Media Queries specification <a href="#MEDIAQ"
rel=biblioentry>[MEDIAQ]<!--{{MEDIAQ}}--></a>.
<p>The specification may refer to the <a href="#used-value."><em>used
value</em></a> and the <a href="#computed-value"><em>computed
value</em></a> of a property. Unless stated explicitly, the short form
“value” means the computed value.
<h2 id=dependencies-on-other-modules><span class=secno>2.
</span>Dependencies on other modules</h2>
<p>This CSS module depends on the following other CSS modules:
<ul>
<li><cite>CSS Writing Modes Module</cite> <a href="#CSS3-WRITING-MODES"
rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>
– switch between vertical and horizontal text, properties <dfn
id=writing-mode>writing-mode</dfn> and <dfn id=direction>direction</dfn>,
definitions of the terms <dfn id=block-flow-direction>block flow
direction</dfn> and <dfn id=inline-base-direction.>inline base
direction.</dfn>
<li><cite>CSS Syntax Module</cite> <a href="#CSS3SYN"
rel=biblioentry>[CSS3SYN]<!--{{!CSS3SYN}}--></a> – syntax of properties
and values
<li><cite>CSS Values and Units Module</cite> <a href="#CSS3VAL"
rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a> – definitions of value
types <dfn id=ltnumbergt><var><number></var></dfn>, <dfn
id=ltpercentagegt><var><percentage></var></dfn>, <dfn
id=ltanglegt><var><angle></var></dfn> and <dfn
id=ltlengthgt><var><length></var></dfn>; defines the terms <dfn
id=computed-value>computed value</dfn> and <dfn id=used-value.>used
value.</dfn>
<li><cite>CSS Text Module</cite> <a href="#CSS3TEXT"
rel=biblioentry>[CSS3TEXT]<!--{{!CSS3TEXT}}--></a> – inline layout and
definition of <dfn id=line-box.>line box.</dfn>
<li><cite>CSS Generated and Replaced Content module</cite> <a
href="#CSS3GENCON" rel=biblioentry>[CSS3GENCON]<!--{{!CSS3GENCON}}--></a>
– defines the terms <dfn id=replaced-element>replaced element</dfn> and
<dfn id=intrinsic0>intrinsic</dfn> size (and intrinsic ratio).
<li><cite>CSS Backgrounds and Borders Module</cite> <a href="#CSS3BG"
rel=biblioentry>[CSS3BG]<!--{{!CSS3BG}}--></a> – defines <dfn
id=ltbg-positiongt><var><bg-position></var></dfn>
<li><cite>CSS Basic User Interface Module</cite> <a href="#CSS3UI"
rel=biblioentry>[CSS3UI]<!--{{!CSS3UI}}--></a> – defines <dfn
id=box-sizing>box-sizing</dfn>
<li><cite>CSS 2D Transforms</cite> <a href="#CSS3-2D-TRANSFORMS"
rel=biblioentry>[CSS3-2D-TRANSFORMS]<!--{{!CSS3-2D-TRANSFORMS}}--></a>
– defines <dfn id=transform>transform</dfn>
</ul>
<h2 id=introduction-amp-definitions><span class=secno>3.
</span>Introduction & definitions</h2>
<p class=issue>Investigate if it is possible to make a pseudo-element to
select anonymous boxes (‘<code class=css>::paragraph</code>’). 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 <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, because it is generalized to
apply also to vertical text.
<p>CSS assumes that the document to lay out is modeled as a <span
class=index id=tree
title="tree!!of elements|document tree|element tree">tree</span> of <dfn
id=elements. 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 id=root-element.>root element.</dfn>
<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 <a
href="#canvas"><em>canvas</em></a> depend on their properties. We say that
an element <dfn id=generates title="generated box">generates</dfn> one or
more boxes. There are <a href="#block-level"><em>block-level</em></a>
boxes, <a href="#line-box."><em title="line box">line boxes</em></a> and
<a href="#inline-level"><em title=inline-level>inline-level
boxes.</em></a> A block-level box is like a paragraph. A line box is like
a line of text. Inline-level boxes are like words inside a line.
<p>When the specification says a box <a href="#aprime"><var>A</var></a>
<dfn id=contains>contains</dfn> a box <a href="#bprime"><var>B</var></a>,
then <a href="#bprime"><var>B</var></a> is a box generated by an element
that is a descendant of the element that generated <a
href="#aprime"><var>A</var></a>. Typically, box <a
href="#bprime"><var>B</var></a> is geometrically contained in box <a
href="#aprime"><var>A</var></a> 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, 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 alt="Schematic representation of rel" src=box-intro.png>
<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><ul>
<li>The first item in the list.
<li>The second item.
</ul></pre>
<p>may result in 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 “generated content,” 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, 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.
</div>
<p>Each box belongs to exactly one element. It is either <a
href="#generates" title="generated box">generated</a> directly by the
element, or it is an <a href="#anonymous-box"><em>anonymous box</em></a>
(defined below), which is inserted because the layout rules of CSS require
a box to contain a certain kind of other box, but the boxes generated by
the child elements are not of that kind. The anonymous box is defined to
belong to the parent.
<p class=note>Note: This is mainly important for <span class=index
id=wysiwyg-document-editors>WYSIWYG document editors</span>: if the user
selects a box on the screen in order to set a property, the editor knows
which element to set the property on.
<p>Properties are set on elements and influence how the element is turned
into boxes, but in this specification we refer interchangeably to “the
<var>P</var> property of an element” and “the <var>P</var> property of
a box” (both of which actually mean “<em>the value</em> of property
<var>P</var> of…”), unless it is important to distinguish the box and
the element, e.g., because the element has several boxes and they don't
all have the same value for the property.
<div class=figure id=various>
<p><img
alt="Diagram of a typical box, showing the content, padding, border and margin areas"
src=box.png>
<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 are also subject to <a
href="#collapse."><em title=collapse>collapsing.</em></a>
<p>Each box has a <dfn id=content-area>content area</dfn> (a.k.a. <dfn
id=content-box>content box</dfn>). The rectangle that bounds this area is
the <dfn id=content-edge.>content edge.</dfn> Around the content area is
the <dfn id=padding-area>padding area</dfn> and its outside bounds are
called the <dfn id=padding-edge.>padding edge.</dfn> The padding area and
content area together form the <dfn id=padding-box.>padding box.</dfn>
Outside the padding is the <dfn id=border-area>border area</dfn> and the
outside boundary of that area is the <dfn id=border-edge.>border
edge.</dfn> The border area, padding area and content area together form
the <dfn id=border-box.>border box.</dfn> Finally, outside the border is
the <dfn id=margin-area>margin area</dfn> and its outer edge is the <dfn
id=margin-edge.>margin edge.</dfn>
<p>When the specification says that the padding or border is “absent”
on some side of the box, that actually means that its thickness is zero.
<p><a href="#line-box."><em title="line box">Line boxes</em></a> 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 ‘<code class=property>border-radius</code>’ <a href="#CSS3BG"
rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a> or a shape <a
href="#CSS3-EXCLUSIONS"
rel=biblioentry>[CSS3-EXCLUSIONS]<!--{{CSS3-EXCLUSIONS}}--></a>.
<p>A box or element is <dfn id=horizontal>horizontal</dfn> if its ‘<a
href="#writing-mode"><code class=property>writing-mode</code></a>’
property is ‘<code class=css>horizontal-tb</code>’, otherwise it is
<dfn id=vertical.>vertical.</dfn> The <dfn
id=orientation>orientation</dfn> of a box or element is the pair of values
of its ‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’ and ‘<a
href="#direction"><code class=property>direction</code></a>’
<div class=note>
<p>Note that there are theoretically eight possible orientations, but CSS
only defines six:
<table class=equiv-table>
<thead>
<tr>
<th>
<th>‘<code class=css>writing-mode: horizontal-tb</code>’
<th>‘<code class=css>writing-mode: vertical-rl</code>’
<th>‘<code class=css>writing-mode: vertical-lr</code>’
<tbody>
<tr>
<th>‘<code class=css>direction: ltr</code>’
<td>
<p>Text is written from left to right and paragraphs grow downwards
<p><img alt="" class=small src=horizontal.png>
<td>
<p>Text is written top to bottom and paragraphs grow to the left
<p><img alt="" class=small src=vertical-rl.png>
<td>
<p>Text is written top to bottom and paragraphs grow to the right
<p><img alt="" class=small src=vertical-lr.png>
<tr>
<th>‘<code class=css>direction: rtl</code>’
<td>
<p>Text is written from right to left and paragraphs grow downwards
<td>
<p>Text is written bottom to top and paragraphs grow to the left
<td>
<p>Text is written bottom to top and paragraphs grow to the right
</table>
<p>There is no “horizontal-bt.”
</div>
<p>This specification sometimes refers to abstract edges <dfn
id=head>head,</dfn> <dfn id=end>end,</dfn> <dfn id=tail>tail</dfn> and
<dfn id=start>start,</dfn> which are relative to the orientation of a box.
They map to top, right, bottom and left as follows:
<table class=equiv-table>
<thead>
<tr>
<th>Value of ‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’
<th>Value of ‘<a href="#direction"><code
class=property>direction</code></a>’
<th>Meaning of “head”
<th>Meaning of “end”
<th>Meaning of “tail”
<th>Meaning of “start”
<tbody>
<tr>
<th>‘<code class=css>horizontal-tb</code>’
<th>‘<code class=css>ltr</code>’
<td>top
<td>right
<td>bottom
<td>left
<tr>
<th>‘<code class=css>horizontal-tb</code>’
<th>‘<code class=css>rtl</code>’
<td>top
<td>left
<td>bottom
<td>right
<tr>
<th>‘<code class=css>vertical-rl</code>’
<th>‘<code class=css>ltr</code>’
<td>right
<td>bottom
<td>left
<td>top
<tr>
<th>‘<code class=css>vertical-rl</code>’
<th>‘<code class=css>rtl</code>’
<td>right
<td>top
<td>left
<td>bottom
<tr>
<th>‘<code class=css>vertical-lr</code>’
<th>‘<code class=css>rtl</code>’
<td>left
<td>bottom
<td>right
<td>top
<tr>
<th>‘<code class=css>vertical-lr</code>’
<th>‘<code class=css>ltr</code>’
<td>left
<td>top
<td>right
<td>bottom
</table>
<p>For example, the “head padding” by default refers to the ‘<a
href="#padding-top"><code class=property>padding-top</code></a>’ and the
“end border” is by default the ‘<code
class=property>border-right</code>’.
<div class=issue>
<p>[Alternative terminology:]
<p>This specification sometimes refers to abstract edges <dfn
id=a>A,</dfn> <dfn id=b>B,</dfn> <dfn id=c>C</dfn> and <dfn id=d>D,</dfn>
which are relative to the orientation of a box. They map to top, right,
bottom and left as follows:
<table class=equiv-table>
<thead>
<tr>
<th>Value of ‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’
<th>Value of ‘<a href="#direction"><code
class=property>direction</code></a>’
<th>Meaning of “A”
<th>Meaning of “B”
<th>Meaning of “C”
<th>Meaning of “D”
<tbody>
<tr>
<th>‘<code class=css>horizontal-tb</code>’
<th>‘<code class=css>ltr</code>’
<td>top
<td>right
<td>bottom
<td>left
<tr>
<th>‘<code class=css>horizontal-tb</code>’
<th>‘<code class=css>rtl</code>’
<td>top
<td>left
<td>bottom
<td>right
<tr>
<th>‘<code class=css>vertical-rl</code>’
<th>‘<code class=css>ltr</code>’
<td>right
<td>bottom
<td>left
<td>top
<tr>
<th>‘<code class=css>vertical-rl</code>’
<th>‘<code class=css>rtl</code>’
<td>right
<td>top
<td>left
<td>bottom
<tr>
<th>‘<code class=css>vertical-lr</code>’
<th>‘<code class=css>rtl</code>’
<td>left
<td>bottom
<td>right
<td>top
<tr>
<th>‘<code class=css>vertical-lr</code>’
<th>‘<code class=css>ltr</code>’
<td>left
<td>top
<td>right
<td>bottom
</table>
<p>For example, the “A padding” by default refers to the ‘<a
href="#padding-top"><code class=property>padding-top</code></a>’ and
the “B border” is by default the ‘<code
class=property>border-right</code>’.
</div>
<p>Similarly, the specification sometimes refers to the <dfn id=inline
title="inline dimension">inline</dfn> and <dfn id=block-flow
title="block flow dimension">block flow</dfn> dimensions of a box, instead
of width and height, as follows:
<table class=equiv-table>
<tbody>
<tr>
<th>Writing mode
<th>Meaning of “inline dimension”
<th>Meaning of “block flow dimension”
<tr>
<td><a href="#horizontal">horizontal</a>
<td>width
<td>height
<tr>
<td><a href="#vertical.">vertical</a>
<td>height
<td>width
</table>
<p>These correspond to the dimensions in the <a
href="#inline-base-direction."><em>inline base direction</em></a> and the
<a href="#block-flow-direction"><em>block flow direction,</em></a>
respectively.
<div class=issue>
<p>[Alternative terminology:]
<p>Similarly, the specification sometimes refers to the <dfn
id=b-d>B-D</dfn> and <dfn id=a-c>A-C</dfn> dimensions of a box, instead
of width and height, as follows:
<table class=equiv-table>
<tbody>
<tr>
<th>Writing mode
<th>Meaning of “B-D”
<th>Meaning of “A-C”
<tr>
<td><a href="#horizontal">horizontal</a>
<td>width
<td>height
<tr>
<td><a href="#vertical.">vertical</a>
<td>height
<td>width
</table>
<p>These correspond to the dimensions in the <a
href="#inline-base-direction."><em>inline base direction</em></a> and the
<a href="#block-flow-direction"><em>block flow direction,</em></a>
respectively.
</div>
<p>Finally, the <dfn id=headside>headside,</dfn> <dfn
id=endside>endside,</dfn> <dfn id=tailside>tailside</dfn> and <dfn
id=startside>startside</dfn> of a box are defined as the sides that
correspond to the <a href="#head"><em>head,</em></a> <a
href="#end"><em>end,</em></a> <a href="#tail"><em>tail</em></a> and <a
href="#start"><em>start,</em></a> of the <a
href="#containing-block"><em>containing block</em></a> of the box.
<div class=example>
<p>For example, if a box has a ‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’ of ‘<code
class=css>horizontal-tb</code>’, then its top content edge acts as its
<a href="#head"><em>head</em></a> content edge, and the <a
href="#hea
52BE
dside"><em>headside</em></a> edges of its children are
therefore their top edges, independent of what their own ‘<a
href="#writing-mode"><code class=property>writing-mode</code></a>’ is.
<div class=figure>
<p><img
alt="[Figure: the “head content edge” of the child box the right edge, but the“headside content edge” is the top.]"
src=headside>
<p class=caption>The second child box is a vertical box and has its
“head” content edge on the right. But its “headside” content
edge is at the top.
</div>
</div>
<div class=issue>
<p>[Alternative terminology:]
<p>Finally, the <dfn id=aprime>A′,</dfn> <dfn id=bprime>B′,</dfn> <dfn
id=cprime>C′</dfn> and <dfn id=dprime>D′</dfn> sides of a box are
defined as the sides that correspond to the <a
href="#aprime"><em>A,</em></a> <a href="#bprime"><em>B,</em></a> <a
href="#cprime"><em>C</em></a> and <a href="#dprime"><em>D</em></a> sides
of the <a href="#containing-block"><em>containing block</em></a> of the
box.
<div class=example>
<p>For example, if a box has a ‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’ of ‘<code
class=css>horizontal-tb</code>’, then its top content edge acts as its
<a href="#aprime"><em>A</em></a> content edge, and the <a
href="#aprime"><em>A′</em></a> edges of its children are therefore
their top edges, independent of what their own ‘<a
href="#writing-mode"><code class=property>writing-mode</code></a>’ is.
<div class=figure>
<p><img
alt="[Figure: the “A content edge” of the child box the right edge, but the “A′ content edge” is the top.]"
src=headside>
<p class=caption>The second child box is a vertical box and has its
“A” content edge on the right. But its “A′” content edge is
at the top.
</div>
</div>
<!-- /figure --></div>
<!-- /example -->
<!-- /issue -->
<p class=issue>Check terms. At Cambridge ftf Aug 2008 we chose: Property
name: block-flow with values tb | lr | rl | bt. Descriptive terms:
horizontal mode (= tb & bt), vertical mode (= lr & rl). The May
2012 Writing Modes module now has ‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’ with values ‘<code
class=css>horizontal-tb</code>’, ‘<code
class=css>vertical-rl</code>’ and ‘<code
class=css>vertical-lr</code>’ (and no ‘<code class=css>bt</code>’
anymore).
<h2 id=the-viewport-and-the-canvas><span class=secno>4. </span>The viewport
and the canvas</h2>
<p>User agents for continuous media generally offer users a <dfn
id=viewport>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 <a
href="#initial-containing-block."><em>initial containing block</em></a>).
<p>When the viewport is smaller than the area of the <a
href="#canvas"><em>canvas</em></a> (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 id=canvas>canvas</dfn> describes the space
where the formatting structure is render
4296
ed. The canvas is infinite for
each dimension of the space.
<h2 id=aspect-ratios-of-replaced-elements><span class=secno>5.
</span>Aspect ratios of replaced elements</h2>
<p>For the purposes of the width and height calculations below, CSS
distinguishes four kinds of <a href="#replaced-element"><em class=index
id=replaced-elements title="replaced element">replaced elements</em></a>:
<dl>
<dt>Type 1: replaced elements with both <em class=index
id=intrinsic-width>intrinsic width</em> and <em class=index
id=intrinsic-height.>intrinsic height.</em>
<dd>These are replaced elements that are meant to be displayed with a
certain fixed factor (the <em>intrinsic ratio</em>) between their width
and height and also have a default size. Different formats may have
different ways of specifying the size, e.g., a width and a ratio, or a