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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head
profile="http://dublincore.org/documents/2008/08/04/dc-html/ 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="http://purl.org/dc/terms/" rel=schema.dcterms>
<link href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=dcterms.rights>
<meta content="CSS basic box model" name=dcterms.title>
<meta content=text name=dcterms.type>
<meta content=2012-11-01 name=dcterms.issued>
<meta content="Bert Bos" name=dcterms.creator>
<meta content="Anton Prowse" name=dcterms.creator>
<meta content=W3C name=dcterms.publisher>
<meta content="http://www.w3.org/TR/2012/ED-css3-box-20121101/"
name=dcterms.identifier>
<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}
/* Undo the max-width in the 2012 style for editor's drafts: */
body {max-width: none}
</style>
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
title=Default type="text/css">
<link href="#contents" rel=contents>
<link href="#index" rel=index>
<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 1
November 2012</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2012/ED-css3-box-20121101/">http://www.w3.org/TR/2012/ED-css3-box-20121101</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="h-card vcard"><a class="p-name fn u-url url"
href="http://www.w3.org/People/Bos/" rel=author>Bert Bos</a>, <span
class="p-org org">W3C</span>, <a class="u-email email"
href="mailto:bert@w3.org"><bert@w3.org></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"><prowse@moonhenge.net></a>
</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="#flows"><span class=secno>5. </span>Flows</a>
<li><a href="#types-of-boxes"><span class=secno>6. </span>Types of
boxes</a>
<ul class=toc>
<li><a href="#display"><span class=secno>6.1. </span>The display
property</a>
<li><a href="#mixing-the-box-model-with-other-formatti"><span
class=secno>6.2. </span>Mixing the box model with other formatting
models</a>
<li><a href="#block-level-boxes-containing-blocks-flow"><span
class=secno>6.3. </span>Block-level boxes, containing blocks, flows and
anonymous boxes</a>
<li><a href="#run-in-boxes"><span class=secno>6.4. </span>Run-in
boxes</a>
<li><a href="#compact-boxes"><span class=secno>6.5. </span>Compact
boxes</a>
</ul>
<li><a href="#the-padding-properties"><span class=secno>7. </span>The
padding properties</a>
<li><a href="#margins"><span class=secno>8. </span>Margins</a>
<ul class=toc>
<li><a href="#the-margin-properties"><span class=secno>8.1. </span>The
margin properties</a>
</ul>
<li><a href="#the-width-and-height-properties"><span class=secno>9.
</span>The width and height properties</a>
<li><a href="#min-max"><span class=secno>10. </span>The min-width,
max-width, min-height and max-height properties</a>
<li><a href="#intrinsic"><span class=secno>11. </span>Definition of
min-content/max-content</a>
<li><a href="#aspect-ratios-of-replaced-elements"><span class=secno>12.
</span>Aspect ratios of replaced elements</a>
<li><a href="#block-level-formatting"><span class=secno>13.
</span>Block-level formatting</a>
<ul class=toc>
<li><a href="#collapsing-margins"><span class=secno>13.1.
</span>Collapsing margins</a>
</ul>
<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
class=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. 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 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 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 <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 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 id=box-tree. 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 ‘<code class=property>position</code>’ can put a box
into the box of an ancestor (the one establishing the containing block)
rather than a parent.
<li>Note that ‘<a href="#flow0"><code class=property>flow</code></a>’
puts boxes into a named flow root.
</ul>
</div>
<p>When the specification says a box <var>A</var> <dfn
id=contains>contains</dfn> a box <var>B</var> it means <var>B</var> is a
child of <var>A</var> in the <a href="#box-tree."><em>box tree.</em></a>
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 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 <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 “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 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 <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>
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 id=induced>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 <a href="#CSS3CASCADE"
rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a> 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 ‘<a href="#display0"><code
class=property>display</code></a>’ property. <span
class=issue>[Actually, 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. But it is
convenient to say the “the box has a font <var>X</var>” rather than
the verbose “the element that generated the box has a font
<var>X</var>.”</span>
<p>Boxes do not have specified values <a href="#CSS3CASCADE"
rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a>, 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 ‘<a
href="#float"><code class=property>float</code></a>’ is not ‘<code
class=css>none</code>’, and a “positioned box” is a box whose
computed value of ‘<code class=property>position</code>’ is not
‘<code class=css>static</code>’. However, terms such as
<em>block-level box,</em> <a href="#block-container-box"><em>block
container box</em></a> 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
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 of adjacent boxes are
also subject to <a href="#collapse."><em
title=collapse>collapsing</em></a> (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 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 <dfn
id=ldquoabsentrdquo
title="absent|padding!!absent|border!!absent">“absent”</dfn> on some
side of the box, that 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>We say that 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>
<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>‘<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.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
<p><img alt="" class=small src=horizontal-rtl.png>
<td>
<p>Text is written bottom to top and paragraphs grow to the left
<p><img alt="" class=small src=vertical-rtl.png>
<td>
<p>Text is written bottom to top and paragraphs grow to the right
<p><img alt="" class=small src=vertical-lr-rtl.png>
</table>
<p>There is no “horizontal-bt.” More information about ‘<a
href="#writing-mode"><code class=property>writing-mode</code></a>’ and
‘<a href="#direction"><code class=property>direction</code></a>’ can
be found in <a href="#CSS3-WRITING-MODES"
rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>.
</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 a box and two terms (block dimension and inline dimension) for
the width and height of a box.
<p>The <dfn id=head>head,</dfn> <dfn id=foot>foot,</dfn> <dfn
id=start>start</dfn> and <dfn id=end>end</dfn> edge of a box refer to the
top, right, bottom and left edges, depending on the ‘<a
href="#writing-mode"><code class=property>writing-mode</code></a>’ and
‘<a href="#direction"><code class=property>direction</code></a>’
properties of that box, as follows:
<table class=equiv-table>
<thead>
<tr>
<th rowspan=2>Value of<br>
‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’
<th rowspan=2>Value of<br>
‘<a href="#direction"><code class=property>direction</code></a>’
<th colspan=4>Meaning of…
<th rowspan=2>
<tr>
<th>“head”
<th>“end”
<th>“foot”
<th>“start”
<tbody>
<tr>
<th rowspan=2>‘<code class=css>horizontal-tb</code>’
<th>‘<code class=css>ltr</code>’
<td>top
<td>right
<td>bottom
<td>left
<td><img alt="" class=minidiagram src=box-tb-ltr.svg>
<tr><!-- horizontal-tb -->
<th>‘<code class=css>rtl</code>’
<td>top
<td>left
<td>bottom
<td>right
<td><img alt="" class=minidiagram src=box-tb-rtl.svg>
<tr>
<th rowspan=2>‘<code class=css>vertical-rl</code>’
<th>‘<code class=css>ltr</code>’
<td>right
<td>bottom
<td>left
<td>top
<td><img alt="" class=minidiagram src=box-rl-ltr.svg>
<tr><!-- vertical-rl'' -->
<th>‘<code class=css>rtl</code>’
<td>right
<td>top
<td>left
<td>bottom
<td><img alt="" class=minidiagram src=box-rl-rtl.svg>
<tr>
<th rowspan=2>‘<code class=css>vertical-lr</code>’
<th>‘<code class=css>ltr</code>’
<td>left
<td>bottom
<td>right
<td>top
<td><img alt="" class=minidiagram src=box-lr-ltr.svg>
<tr><!-- vertical-lr -->
<th>‘<code class=css>rtl</code>’
<td>left
<td>top
<td>right
<td>bottom
<td><img alt="" class=minidiagram src=box-lr-rtl.svg>
</table>
<p class=note>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>’.
<p>Similarly, the specification sometimes refers to the <dfn
id=inline-dimension>inline dimension</dfn> and <dfn
id=block-dimension>block dimension</dfn> of a box, instead of width and
height, as follows:
<table class=equiv-table>
<tbody>
<tr>
<th>‘<code class=css>Writing mode</code>’
<th>Meaning of “inline dimension”
<th>Meaning of “block dimension”
<tr>
<td>‘<code class=css>horizontal-tb</code>’
<td>width
<td>height
<tr>
<td>‘<code class=css>vertical-rl</code>’
<td>height
<td>width
<tr>
<td>‘<code class=css>vertical-lr</code>’
<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.
<p>An alternative term for the inline dimension is <dfn id=logical-width
title="logical width|width!!logical">logical width</dfn> and an
alternative term for block dimension is <dfn id=logical-height.
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 <a href="#flow0"><em
class=index id=flow>flow</em></a> (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
id=a-edge>A edge,</dfn> <dfn id=b-edge>B edge,</dfn> <dfn id=c-edge>C
edge</dfn> and <dfn id=d-edge.>D edge.</dfn> They depend on the ‘<a
href="#writing-mode"><code class=property>writing-mode</code></a>’ and
‘<a href="#direction"><code class=property>direction</code></a>’
properties of the box's <a href="#containing-block0"><em class=index
id=containing-block>containing block</em></a> (defined below), and map to
the four sides as follows:
<table class=equiv-table>
<thead>
<tr>
<th rowspan=2>‘<a href="#writing-mode"><code
class=property>writing-mode</code></a>’ of<br>
<a href="#containing-block0"><em>containing block</em></a>
<th rowspan=2>‘<a href="#direction"><code
class=property>direction</code></a>’ of<br>
<a href="#containing-block0"><em>containing block</em></a>
<th colspan=4>Meaning of…
<tr>
<th>“A”
<th>“B”
<th>“C”
<th>“D”
<tbody>
<tr>
<th rowspan=2>‘<code class=css>horizontal-tb</code>’
<th>‘<code class=css>ltr</code>’
<td>top
<td>right
<td>bottom
<td>left
<tr><!-- horizontal-tb -->
<th>‘<code class=css>rtl</code>’
<td>top
<td>left
<td>bottom
<td>right
<tr>
<th rowspan=2>‘<code class=css>vertical-rl</code>’
<th>‘<code class=css>ltr</code>’
<td>right
<td>bottom
<td>left
<td>top
<tr><!-- vertical-rl -->
<th>‘<code class=css>rtl</code>’
<td>right
<td>top
<td>left
<td>bottom
<tr>
<th rowspan=2>‘<code class=css>vertical-lr</code>’
<th>‘<code class=css>rtl</code>’
<td>left
<td>bottom
<td>right
<td>top
<tr><!-- vertical-lr -->
<th>‘<code class=css>ltr</code>’
<td>left
<td>top
<td>right