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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang=en>
<head
profile="http://dublincore.org/documents/2008/08/04/dc-html/ http://www.w3.org/2006/03/hcard">
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Positioned Layout Module Level 3</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 Positioned Layout Module Level 3" name=dcterms.title>
<meta content=text name=dcterms.type>
<meta content=2013-11-11 name=dcterms.date>
<meta content="Rossen Atanassov" name=dcterms.creator>
<meta content="Arron Eicholz" name=dcterms.creator>
<meta content=W3C name=dcterms.publisher>
<meta content="http://dev.w3.org/csswg/css3-positioning/"
name=dcterms.identifier>
<link href=default.css rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<style type="text/css">
.cb-example-table
{
border: 1px solid;
border-collapse: collapse;
font-size: smaller;
text-align: center;
margin: 0 2em;
}
.cb-example-table td, .cb-example-table th
{
border: 1px solid;
padding: 0 3px;
}
code span, td span
{
white-space: nowrap;
}
.relationship-table
{
border: 1px solid;
border-collapse: collapse;
font-size: smaller;
margin: 0 6em;
}
.relationship-table th
{
text-align: center;
white-space:nowrap;
}
.relationship-table td, .relationship-table th
{
border: 1px solid;
padding: 0 3px;
}
h2 dfn, h3 dfn
{
font: inherit;
}
table.lpr
{
border-collapse: collapse;
}
.lpr td, .lpr th
{
border: 1px solid;
text-align: center;
}
.lpr col
{
background: rgb(221, 238, 255);
border-right: 2px solid;
font-weight: bold;
padding: 0 5px;
}
.lpr div
{
font-weight: bold;
-moz-transform: rotate(-90deg) translatex(-0.5em);
-ms-transform: rotate(-90deg) translatex(-0.5em);
-o-transform: rotate(-90deg) translatex(-0.5em);
-webkit-transform: rotate(-90deg) translatex(-0.5em);
transform: rotate(-90deg) translatex(-0.5em);
width: 1.5em;
}
.lpr th:first-child
{
border: none;
background: white;
}
.lpr tbody td:first-child
{
font-weight: normal;
}
.lpr thead td
{
background: rgb(238, 238, 238);
font-weight: bold;
padding: 0 5px;
vertical-align: bottom;
white-space: nowrap;
}
.lpr thead tr:nth-child(2)
{
border-bottom: 2px solid;
}
</style>
<body>
<div class=head> <!--begin-logo-->
<p id=x><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 Positioned Layout Module Level 3</h1>
<h2 class="no-num no-toc">Editor's Draft 11 November 2013</h2>
<dl>
<dt>This version:
<dd><a
href="http://dev.w3.org/csswg/css3-positioning/">http://dev.w3.org/csswg/css3-positioning/</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-positioning/">http://www.w3.org/TR/css3-positioning/</a>
<dt>Editor's draft:
<dd><a
href="http://dev.w3.org/csswg/css3-positioning/">http://dev.w3.org/csswg/css3-positioning/</a>
<dt>Previous version:
<dd><a
href="http://www.w3.org/TR/2012/WD-css3-positioning-20120207/">http://www.w3.org/TR/2012/WD-css3-positioning-20120207/</a>
<dt>Issues List
<dd><a
href="http://wiki.csswg.org/spec/css3-positioning/">http://wiki.csswg.org/spec/css3-positioning/</a>
<dt>Feedback:
<dd><a
href="mailto:www-style@w3.org?subject=%5Bcss-position%5D%20feedback">www-style@w3.org</a>
with subject line “<kbd>[css-position] <var>… message topic
…</var></kbd>” (<a
href="http://lists.w3.org/Archives/Public/www-style/"
rel=discussion>archives</a>)
<dt>Editors:
<dd class=vcard><span class=fn>Rossen Atanassov</span>, <span
class=org>Microsoft Corporation</span>
<dd class=vcard><span class=fn>Arron Eicholz</span>, <span
class=org>Microsoft Corporation</span>
</dl>
<!--begin-c
63B3
opyright-->
<p class=copyright id=copyright-copy-year-2000-w3creg-mit-erci><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2013 <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>, <a
href="http://ev.buaa.edu.cn/">Beihang</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 id=css-is-a-language-for-describing-the-ren> CSS is a language for
describing the rendering of structured documents (such as HTML and XML) on
screen, on paper, in speech, etc. This module contains the features of CSS
level 3 relating to positioning and stacking of elements. It includes and
extends the functionality of CSS level 2 <a href="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS level 1
<a href="#ref-CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>. The main
extensions compared to level 2 are the ability to position elements based
on CSS Region boxes, and the ability to specify a different containing
blocks for elements.
<p id=other-kinds-of-layout-such-as-tables-flo> Other kinds of layout, such
as tables, "floating" boxes, ruby annotations, grid layouts, columns and
basic handling of normal "flow" content, are described in other modules.
Also, the layout of text inside each line is defined elsewhere.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p id=this-is-a-public-copy-of-the-editors-dra>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 id=the-archived-public-mailing-list-www-sty>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-positioning%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-positioning” in the subject, preferably like this:
“[<!---->css3-positioning<!---->] <em>…summary of comment…</em>”
<p id=this-document-was-produced-by-the-css-wo>This document was produced
by the <a href="http://www.w3.org/Style/CSS/members">CSS Working Group</a>
(part of the <a href="http://www.w3.org/Style/">Style Activity</a>).
<p id=this-document-was-produced-by-a-group-op>This document was produced
by a group operating under the <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
2004 W3C Patent Policy</a>. W3C maintains a <a
href="http://www.w3.org/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="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
6 of the W3C Patent Policy</a>.</p>
<!--end-status-->
<!--<p>The following features are at risk:</p>-->
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li id=introduction-1.1.-module-interactions-1.><a href="#intro"><span
class=secno>1. </span>Introduction</a>
<ul class=toc>
<li id=module-interactions-><a href="#placement"><span class=secno>1.1.
</span>Module Interactions</a>
<li id=values-><a href="#values"><span class=secno>1.2.
</span>Values</a>
</ul>
<li id=the-viewport-><a href="#viewport"><span class=secno>2. </span>The
Viewport</a>
<li id=containing-blocks-3.1.-definition-of-con><a
href="#containing-block"><span class=secno>3. </span>Containing
Blocks</a>
<ul class=toc>
<li id=definition-of-containing-block-><a
href="#def-containing-blocks"><span class=secno>3.1. </span>Definition
of containing block</a>
</ul>
<li id=normal-flow-><a href="#normal-flow"><span class=secno>4.
</span>Normal Flow</a>
<li id=floats-><a href="#floats"><span class=secno>5. </span>Floats</a>
<li id=positioning-schemes-6.1.-relative-positi><a
href="#positioning-schemes"><span class=secno>6. </span>Positioning
schemes</a>
<ul class=toc>
<li id=relative-positioning-><a href="#relative-positioning"><span
class=secno>6.1. </span>Relative positioning</a>
<li id=sticky-positioning-><a href="#sticky-positioning"><span
class=secno>6.2. </span><span class=index-def id=sticky-positioning0
title="sticky positioning|sticky position|stickily positioned">Sticky
positioning</span></a>
<li id=absolute-positioning-><a href="#absolute-positioning"><span
class=secno>6.3. </span><span class=index-def id=absolute-positioning1
title="absolute positioning|absolute position|absolutely positioned">Absolute
positioning</span></a>
<li id=center-positioning-><a href="#center-positioning"><span
class=secno>6.4. </span><span class=index-def id=center-positioning0
title="center positioning|center position|center positioned">Center
positioning</span></a>
<li id=page-positioning-><a href="#page-positioning"><span
class=secno>6.5. </span><span class=index-def id=page-positioning0
title="page positioning|page position|page positioned">Page
positioning</span></a>
<li id=fixed-positioning-><a href="#fixed-positioning"><span
class=secno>6.6. </span><span class=index-def id=fixed-positioning0
title="fixed positioning|fixed position|fixed positioned">Fixed
positioning</span></a>
<li id=choosing-a-positioning-scheme-position-p><a
href="#position-property"><span class=secno>6.7. </span>Choosing a
positioning scheme: ‘<code class=property>position</code>’
property</a>
<li id=box-offsets-top-right-bottom-left-><a
href="#box-offsets-trbl"><span class=secno>6.8. </span>Box offsets:
‘<code class=property>top</code>’, ‘<code
class=property>right</code>’, ‘<code
class=property>bottom</code>’, ‘<code
class=property>left</code>’</a>
<li id=logical-box-offsets-offset-before-offset><a
href="#logical-box-offsets-beaso"><span class=secno>6.9. </span>Logical
box offsets: ‘<code class=property>offset-before</code>’, ‘<code
class=property>offset-end</code>’, ‘<code
class=property>offset-after</code>’, ‘<code
class=property>offset-start</code>’, and ‘<code
class=property>offset</code>’</a>
</ul>
<li id=choosing-a-different-containing-block-fo><a
href="#position-root"><span class=secno>7. </span>Choosing a different
containing block for positioning</a>
<ul class=toc>
<li id=attachment-properties-attachment-type-at><a
href="#attachment-properties"><span class=secno>7.1. </span>Attachment
properties: ‘<code class=property>attachment-type</code>’, ‘<code
class=property>attachment-top</code>’, ‘<code
class=property>attachment-right</code>’, ‘<code
class=property>attachment-bottom</code>’, ‘<code
class=property>attachment-left</code>’, and ‘<code
class=property>attachment</code>’</a>
</ul>
<li id=sizing-and-positioning-details-8.1.-the-><a
href="#size-and-position-details"><span class=secno>8. </span>Sizing and
positioning details</a>
<ul class=toc>
<li id=the-width-of-absolute-page-or-fixed-posi><a
href="#abs-non-replaced-width"><span class=secno>8.1. </span>The width
of absolute, page or fixed positioned, non-replaced elements</a>
<li id=the-width-of-a-center-positioned-non-rep><a
href="#center-non-replaced-width"><span class=secno>8.2. </span>The
width of a center positioned, non-replaced elements</a>
<li id=the-width-of-absolute-page-or-fixed-posi0><a
href="#abs-replaced-width"><span class=secno>8.3. </span>The width of
absolute, page or fixed positioned, replaced elements</a>
<li id=the-width-of-a-center-positioned-replace><a
href="#center-replaced-width"><span class=secno>8.4. </span>The width
of a center positioned, replaced elements</a>
<li id=the-height-of-absolute-page-or-fixed-pos><a
href="#abs-non-replaced-height"><span class=secno>8.5. </span>The
height of absolute, page or fixed positioned, non-replaced elements</a>
<li id=the-height-of-a-center-positioned-non-re><a
href="#center-non-replaced-height"><span class=secno>8.6. </span>The
height of a center positioned, non-replaced elements</a>
<li id=the-height-of-absolute-page-or-fixed-pos0><a
href="#abs-replaced-height"><span class=secno>8.7. </span>The height of
absolute, page or fixed positioned, replaced elements</a>
<li id=the-height-of-a-center-positioned-replac><a
href="#center-replaced-height"><span class=secno>8.8. </span>The height
of a center positioned, replaced elements</a>
<li id=auto-heights-for-block-formatting-contex><a
href="#root-height"><span class=secno>8.9. </span>‘<code
class=property>Auto</code>’ heights for block formatting context
roots</a>
</ul>
<li id=relationships-between-display-position-a><a
href="#dis-pos-flo"><span class=secno>9. </span>Relationships between
‘<code class=property>display</code>’, ‘<code
class=property>position</code>’, and ‘<code
class=property>float</code>’</a>
<li id=comparison-of-normal-flow-floats-and-pos><a
href="#comparison"><span class=secno>10. </span>Comparison of normal
flow, floats, and positioning</a>
<ul class=toc>
<li id=normal-flow-0><a href="#comp-normal-flow"><span class=secno>10.1.
</span>Normal flow</a>
<li id=relative-positioning-0><a href="#comp-relpos"><span
class=secno>10.2. </span>Relative positioning</a>
<li id=floating-a-box-><a href="#comp-floating"><span class=secno>10.3.
</span>Floating a box</a>
<li id=absolute-positioning-0><a href="#comp-abspos"><span
class=secno>10.4. </span>Absolute positioning</a>
<li id=page-positioning-0><a href="#comp-pagepos"><span
class=secno>10.5. </span>Page positioning</a>
</ul>
<li id=clipping-the-clip-property-><a href="#clip-property"><span
class=secno>11. </span>Clipping: the ‘<code
class=property>clip</code>’ property</a>
<li id=layered-presentation-><a href="#layered-presentation"><span
class=secno>12. </span>Layered presentation</a>
<li id=detailed-stacking-context-13.1.-definiti><a
href="#det-stacking-context"><span class=secno>13. </span>Detailed
stacking context</a>
<ul class=toc>
<li id=definitions-><a href="#definitions-stack"><span class=secno>13.1.
</span>Definitions</a>
<li id=painting-order-><a href="#painting-order"><span class=secno>13.2.
</span>Painting order</a>
<li id=notes-><a href="#stacking-notes"><span class=secno>13.3.
</span>Notes</a>
</ul>
<li id=conformance-14.1.-document-conventions-1><a
href="#conformance"><span class=secno>14. </span>Conformance</a>
<ul class=toc>
<li id=document-conventions-><a href="#conve
F3AF
ntions"><span
class=secno>14.1. </span>Document Conventions</a>
<li id=conformance-classes-><a href="#conformance-classes"><span
class=secno>14.2. </span>Conformance Classes</a>
<li id=partial-implementations-><a href="#partial"><span
class=secno>14.3. </span>Partial Implementations</a>
<li id=experimental-implementations-><a href="#experimental"><span
class=secno>14.4. </span>Experimental Implementations</a>
<li id=non-experimental-implementations-><a href="#testing"><span
class=secno>14.5. </span>Non-Experimental Implementations</a>
<li id=cr-exit-criteria-><a href="#cr-exit-criteria"><span
class=secno>14.6. </span>CR Exit Criteria</a>
</ul>
<li class=no-num id=acknowledgments-><a
href="#acknowledgments">Acknowledgments</a>
<li class=no-num id=references-normative-references-other-re><a
href="#references">References</a>
<ul class=toc>
<li class=no-num id=normative-references-><a
href="#normative-references">Normative references</a>
<li class=no-num id=other-references-><a href="#other-references">Other
references</a>
</ul>
<li class=no-num id=index-><a href="#index">Index</a>
<li class=no-num id=property-index0><a href="#property-index">Property
index</a>
</ul>
<!--end-toc-->
<h2 id=intro><span class=secno>1. </span>Introduction</h2>
<p id=this-section-is-not-normative.><em>This section is not
normative.</em>
<p id=css-assumes-the-document-layout-is-model> CSS assumes the document
layout is modeled as a tree of elements. The unique element that has no
parent is called the root element. This module describes how any of the
elements from the tree of elements can be arranged independent of document
order (i.e. taken out of "flow"). With a positioned element the element
may be placed anywhere within the content not specifically respecting the
tree of elements order.
<p id=in-css21-the-visual-formatting-model-exp> In <a href="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, the visual formatting model
explained how each element in the document tree generates zero or more
boxes according to the box model. This module further explains and extends
the positioning scheme. The layout of these boxes is governed by:
<ul>
<li id=box-dimensions-and-type.><a
href="http://www.w3.org/TR/CSS2/box.html#box-dimensions">box
dimensions</a> and <a
href="http://www.w3.org/TR/CSS2/visuren.html#box-gen">type</a>.
<li id=positioning-scheme-normal-flow-float-and>positioning scheme (<a
href="#normal-flow0">normal flow</a>, float, and absolute positioning).
<li id=relationships-between-elements-in-the-do>relationships between
elements in the <a
href="http://www.w3.org/TR/CSS2/conform.html#doctree">document tree</a>.
<li id=external-information-e.g.-viewport-size->external information
(e.g., viewport size, intrinsic dimensions of images, etc.).
</ul>
<p id=the-properties-defined-in-this-module-ap>The properties defined in
this module apply to both continuous media and paged media.
<h3 id=placement><span class=secno>1.1. </span>Module Interactions</h3>
<p id=this-module-replaced-and-extends-the-pos>This module replaced and
extends the positioning scheme features defined in <a href="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections:
<ul>
<li id=the-viewport><a
href="http://www.w3.org/TR/CSS2/visuren.html#viewport">9.1.1 The
viewport</a>
<li id=containing-blocks><a
href="http://www.w3.org/TR/CSS2/visuren.html#containing-block">9.1.2
Containing blocks</a>
<li id=positioning-schemes0><a
href="http://www.w3.org/TR/CSS2/visuren.html#positioning-scheme">9.3
Positioning schemes</a>
<li id=relative-positioning0><a
href="http://www.w3.org/TR/CSS2/visuren.html#relative-positioning">9.4.3
Relative positioning</a>
<li id=absolute-positioning0><a
href="http://www.w3.org/TR/CSS2/visuren.html#absolute-positioning">9.6
Absolute positioning</a>
<li id=relationships-between-display-position-a0><a
href="http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">9.7
Relationships between ‘<code class=property>display</code>’, ‘<code
class=property>position</code>’, and ‘<code
class=property>float</code>’</a>
<li id=comparison-of-normal-flow-floats-and-abs><a
href="http://www.w3.org/TR/CSS2/visuren.html#comparison">9.8 Comparison
of normal flow, floats, and absolute positioning</a>
<li id=layered-presentation0><a
href="http://www.w3.org/TR/CSS2/visuren.html#layers">9.9 Layered
presentation</a>
<li id=definition-of-containing-block><a
href="http://www.w3.org/TR/CSS2/visudet.html#containing-block-details">10.1
Definition of "containing block"</a>
<li id=absolutely-positioned-non-replaced-eleme><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">10.3.7
Absolutely positioned, non-replaced elements</a>
<li id=absolutely-positioned-replaced-elements><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-width">10.3.8
Absolutely positioned, replaced elements</a>
<li id=absolutely-positioned-non-replaced-eleme0><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">10.6.4
Absolutely positioned, non-replaced elements</a>
<li id=absolutely-positioned-replaced-elements0><a
href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-height">10.6.5
Absolutely positioned replaced elements</a>
<li id=auto-heights-for-block-formatting-contex0><a
href="http://www.w3.org/TR/CSS2/visudet.html#root-height">10.6.7 ‘<code
class=css>Auto</code>’ heights for block formatting context roots</a>
<li id=clipping-the-clip-property><a
href="http://www.w3.org/TR/CSS2/visufx.html#clipping">11.1.2 Clipping:
the ‘<code class=property>clip</code>’ property</a>
<li id=appendix-e.-elaborate-description-of-sta><a
href="http://www.w3.org/TR/CSS2/zindex.html">Appendix E. Elaborate
description of Stacking Contexts</a>
</ul>
<!-- End section" Module Interactions -->
<h3 id=values><span class=secno>1.2. </span>Values</h3>
<p id=this-specification-follows-the-css-prope>This specification follows
the CSS property definition conventions from <a href="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<p id=value-types-not-defined-in-this-specific> Value types not defined in
this specification are defined in CSS Level 2 Revision 1 <a
href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
modules may expand the definitions of these value types: for example <a
href="#ref-CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>, when
combined with this module, expands the definition of the <length>
value type as used in this specification.
<p id=in-addition-to-the-property-specific-val> In addition to the
property-specific values listed in their definitions, all properties
defined in this specification also accept the <a
href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
keyword as their property value. For readability it has not been repeated
explicitly.</p>
<!-- End section: Values -->
<!-- End section Introduction -->
<h2 id=viewport><span class=secno>2. </span>The <dfn id=viewport0
title=viewport>Viewport</dfn></h2>
<p id=user-agents-for-continuous-media-general> User agents for <a
href="http://www.w3.org/TR/CSS2/media.html#continuous-media-group">continuous
media</a> generally offer users a viewport (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
<span>initial containing block</span>).
<p id=when-the-viewport-is-smaller-than-the-ar> When the viewport is
smaller than the area of the canvas on which the document is rendered, the
user agent may offer a scrolling mechanism. There is at most one viewport
per <a href="http://www.w3.org/TR/CSS2/intro.html#canvas">canvas</a>, but
user agents may render to more than one canvas (i.e., provide different
views of the same document).</p>
<!-- End section: The Viewport -->
<h2 id=containing-block><span class=secno>3. </span>Containing Blocks</h2>
<p id=in-css-many-box-positions-and-sizes-are-> In CSS, many box positions
and sizes are calculated with respect to the edges of a rectangular box
called a containing block. In general, generated boxes act as containing
blocks for descendant boxes; we say that a box "establishes" the
containing block for its descendants. The phrase "a box's containing
block" means "the containing block in which the box lives," not the one it
generates.
<p id=each-box-is-given-a-position-with-respec> Each box is given a
position with respect to its containing block, but it is not confined by
this containing block; it may <a
href="http://www.w3.org/TR/CSS2/visufx.html#overflow">overflow</a>.
<h3 id=def-containing-blocks><span class=secno>3.1. </span>Definition of
<dfn id=containing-block0>containing block</dfn></h3>
<p id=the-position-and-size-of-an-element39s-b> The position and size of an
element's box(es) are sometimes computed relative to a certain rectangle,
called the containing block of the element. The containing block of a <a
href="#static" title="'position'!!''static''">‘<code
class=css>static</code>’</a> or <a href="#relative"
title="'position'!!''relative''">‘<code class=css>relative</code>’</a>
element is defined in the Box Model <a href="#ref-CSS3BOX"
rel=biblioentry>[CSS3BOX]<!--{{!CSS3BOX}}--></a>. The containing block of
a <a href="#sticky" title="'position'!!'sticky'">‘<code
class=property>sticky</code>’</a> element is the same as for a <a
href="#relative" title="'position'!!''relative''">‘<code
class=css>relative</code>’</a> element. For <a href="#fixed"
title="'position'!!''fixed''">‘<code class=css>fixed</code>’</a>, <a
href="#absolute" title="'position'!!''absolute''">‘<code
class=css>absolute</code>’</a>, <a href="#center"
title="'position'!!''center''">‘<code class=css>center</code>’</a> and
<a href="#page" title="'position'!!''page''">‘<code
class=css>page</code>’</a>, it is defined as follows:
<ol>
<li id=if-the-element-has-positioned-fixed-the-> If the element has <a
href="#fixed" title="'position'!!''fixed''">‘<code
class=css>positioned: fixed</code>’</a>, the containing block is
established by the <a href="#viewport0">viewport</a> in the case of
continuous media or the page area in the case of paged media.
<li id=if-the-element-has-position-absolute-the> If the element has <a
href="#absolute" title="'position'!!''absolute''">‘<code
class=css>position: absolute</code>’</a>, the containing block is
established by the nearest ancestor with a ‘<a href="#position"><code
class=property>position</code></a>’ other than <a href="#static"
title="'position'!!''static''">‘<code class=css>static</code>’</a>,
in the following way:
<ol>
<li id=in-the-case-that-the-ancestor-is-block-l> In the case that the
ancestor is block-level, the containing block is formed by the padding
edge of the ancestor.
<li id=in-the-case-that-the-ancestor-is-inline-> In the case that the
ancestor is inline-level, the containing block depends on the ‘<code
class=property>direction</code>’ property of the ancestor:
<ol>
<li id=if-the-direction-is-ltr-the-top-and-left> If the ‘<code
class=property>direction</code>’ is ‘<code
class=css>ltr</code>’, the top and left of the containing block are
the top and left content edges of the first box generated by the
ancestor, and the bottom and right are the bottom and right content
edges of the last box of the ancestor.
<li id=if-the-direction-is-rtl-the-top-and-righ> If the ‘<code
class=property>direction</code>’ is ‘<code
class=css>rtl</code>’, the top and right are the top and right
edges of the first box generated by the ancestor, and the bottom and
left are the bottom and left content edges of the last box of the
ancestor.
<p class=note id=note-in-some-cases-when-a-line-wraps-it-> Note, in
some cases when a line wraps it may seem as if the left and right
positions are swapped.
</ol>
</ol>
<li id=i-the-element-has-position-center-the-co> I the element has <a
href="#center" title="'position'!!''center''">‘<code
class=css>position: center</code>’</a>, the containing block is
established by the nearest ancestor with a ‘<a href="#position"><code
class=property>position</code></a>’ other than <a href="#static"
title="'position'!!''static''">‘<code class=css>static</code>’</a>,
in the following way:
<ol>
<li id=in-the-case-that-the-ancestor-is-block-l0> In the case that the
ancestor is block-level, the containing block is formed by the padding
edge of the ancestor.
<li id=in-the-case-that-the-ancestor-is-inline-0> In the case that the
ancestor is inline-level, the containing block depends on the content
edges of the boxes generated by the ancestor. The top most, right most,
bottom most and left most content edges of all the boxes generated by
the ancestor determine the edges for the containing block.
</ol>
<li id=if-the-element-has-position-page-and-the> If the element has <a
href="#page" title="'position'!!''page''">‘<code class=css>position:
page</code>’</a> and the element is within paged media or within a
rerion, the containing block is the <span>initial containing
block</span>. This is the <a href="#viewport0">viewport</a> or the page
area when in paged media. In the case of CSS Regions <a
href="#ref-CSS3-REGIONS"
rel=biblioentry>[CSS3-REGIONS]<!--{{!CSS3-REGIONS}}--></a> this is the
individual region. When a page positioned element is not in paged media
or a region the containing block is determined per the absolute
positioning model.
<li id=if-there-is-no-such-ancestor-the-contain>If there is no such
ancestor, the containing block is the <span>initial containing
block</span>.
</ol>
<p id=in-paged-media-an-absolutely-positioned-> In paged media, an
absolutely positioned element is positioned relative to its containing
block ignoring any page breaks (as if the document were continuous). The
element may subsequently be broken over several pages.
<p id=for-absolutely-positioned-content-that-r> For absolutely positioned
content that resolves to a position on a page other than the page being
laid out (the current page), or resolves to a position on the current page
which has already been rendered for printing, printers may place the
content:
<ul>
<li id=on-the-current-page>on the current page,
<li id=on-a-subsequent-page-or>on a subsequent page, or
<li id=may-omit-it-altogether.>may omit it altogether.
</ul>
<p class=note id=note-a-block-level-element-that-is-split> Note, a
block-level element that is split over several pages may have a different
width on each page and that there may be device-specific limits.
<div class=example>
<p id=with-no-positioning-the-containing-block>With no positioning, the
containing blocks (C.B.) in the following document:
<pre><code class=html>
<!DOCTYPE html>
<html>
<head>
<title>Illustration of containing blocks</title>
</head>
<body id="body">
<div id="div1">
<p id="p1">This is text in the first paragraph...</p>
<p id="p2">This is text <em id="em1"> in the
<strong id="strong1">second</strong> paragraph.</em></p>
</div>
</body>
</html>
</code></pre>
<p id=are-established-as-follows>are established as follows:
<table class=cb-example-table>
<tbody>
<tr>
<th>For box generated by
<th>C.B. is established by
<tr>
<td>html
<td>initial C.B. (UA-dependent)
<tr>
<td>body
<td>html
<tr>
<td>div1
<td>body
<tr>
<td>p1
<td>div1
<tr>
<td>p2
<td>div1
<tr>
<td>em1
<td>p2
<tr>
<td>strong1
<td>p2
</table>
<p id=if-we-position-div1>If we position "div1":
<pre><code class=css>
#div1 { position: absolute; left: 50px; top: 50px }
</code></pre>
<p id=its-containing-block-is-no-longer-body-i> its containing block is no
longer "body"; it becomes the <span>initial containing block</span>
(since there are no other positioned ancestor boxes).
<p id=if-we-position-em1-as-well>If we position "em1" as well:
<pre><code class=css>
#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
</code></pre>
<p id=the-table-of-containing-blocks-becomes>the table of containing
blocks becomes:
<table class=cb-example-table>
<tbody>
<tr>
<th>For box generated by
<th>C.B. is established by
<tr>
<td>html
<td>initial C.B. (UA-dependent)
<tr>
<td>body
<td>html
<tr>
<td>div1
<td>initial C.B.
<tr>
<td>p1
<td>div1
<tr>
<td>p2
<td>div1
<tr>
<td>em1
<td>div1
<tr>
<td>strong1
<td>em1
</table>
<p id=by-positioning-em1-its-containing-block-> By positioning "em1", its
containing block becomes the nearest positioned ancestor box (i.e., that
generated by "div1").
</div>
<!-- End section: Containing blocks -->
<h2 id=normal-flow><span class=secno>4. </span><dfn id=normal-flow0
title="normal flow">Normal Flow</dfn></h2>
<p id=boxes-in-the-normal-flow-belong-to-a-for> Boxes in the normal flow
belong to a formatting context, which may be block or inline, but not both
simultaneously. See the CSS Basic Box Model module <a href="#ref-CSS3BOX"
rel=biblioentry>[CSS3BOX]<!--{{!CSS3BOX}}--></a> for further details about
<a href="http://www.w3.org/TR/CSS2/visuren.html#normal-flow">normal
flow</a>.</p>
<!-- End section: Normal Flow -->
<h2 id=floats><span class=secno>5. </span><dfn id=floats0
title="float|floated box|floating box">Floats</dfn></h2>
<p id=a-float-is-a-box-that-is-positioned-with> A float is a box that is
positioned within content, often left or right on the current line. The
most interesting characteristic of a float (or "floated" or "floating"
box) is that inline-level content may flow along its side (or be
prohibited from doing so by the ‘<code class=property>clear</code>’
property) or around the floated box. See the CSS 2.1 <a href="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a><!--CSS Floats module [[!CSS3FLOATS]]-->
for details about <a
href="http://www.w3.org/TR/CSS2/visuren.html#floats">floating boxes</a>.</p>
<!-- End section: Floats -->
<h2 id=positioning-schemes><span class=secno>6. </span><dfn
id=positioning-schemes1 title="positioning schemes">Positioning
schemes</dfn></h2>
<p id=in-css-a-box-may-be-laid-out-according-t>In CSS, a box may be laid
out according to three positioning schemes:
<ol>
<li id=normal-flow-in-css-normal-flow-includes-> Normal flow<br>
In CSS, <a href="#normal-flow0">normal flow</a> includes <a
href="http://www.w3.org/TR/CSS2/visuren.html#block-formatting">block
formatting</a> of block-level boxes, <a
href="http://www.w3.org/TR/CSS2/visuren.html#inline-formatting">inline
formatting</a> of inline-level boxes, and <a
href="#relative-positioning1" title="relative positioning">relative</a>
and <span title="sticky positioning">sticky</span> positioning of
block-level and inline-level boxes.
<li id=floats-in-the-float-model-a-box-is-first> Floats<br>
In the <a href="#floats0" title=floats>float</a> model, a box is first
laid out according to the <a href="#normal-flow0">normal flow</a>, then
taken out of the flow and positioned, typically to the left or right.
Content may flow along the side of a <a href="#floats0"
title=floats>float</a>.
<li id=absolute-positioning-in-the-absolute-pos> Absolute positioning<br>
In the absolute positioning model, a box is removed from the <a
href="#normal-flow0">normal flow</a> entirely (it has no impact on later
siblings) and assigned a position with respect to a <a
href="#containing-block0">containing block</a>.
</ol>
<p id=an-element-is-called-out-of-flow-if-it-i> An element is called
<em>out-of-flow</em> if it is <a href="#floats0" title=floats>floated</a>,
absolutely positioned, or is the root element. An element is called
<em>in-flow</em> if it is not out-of-flow. The <em>flow of an element</em>
A is the set consisting of A and all in-flow elements whose nearest
out-of-flow ancestor is A.
<h3 id=relative-positioning><span class=secno>6.1. </span><dfn
id=relative-positioning1
title="relative positioning|relative position|relatively positioned">Relative
positioning</dfn></h3>
<p id=once-a-box-has-been-laid-out-according-t> Once a box has been laid
out according to the <a href="#normal-flow0">normal flow</a> or <a
href="#floats0" title=floats>floated</a>, it may be offset relative to
this position. This is called relative positioning. Offsetting a box (B1)
in this way has no effect on the box (B2) that follows: B2 is given a
position as if B1 were not offset and B2 is not re-positioned after B1's
offset is applied. This implies that relative positioning may cause boxes
to overlap. However, if relative positioning causes an ‘<code
class=css>overflow: auto</code>’ or ‘<code class=css>overflow:
scroll</code>’ box to have overflow, the user agent must allow the user
to access this content (at its offset position), which, through the
creation of a scrolling mechanism, may affect layout.
<p id=a-relatively-positioned-box-keeps-its-no> A relatively positioned box
keeps its <a href="#normal-flow0">normal flow</a> size, including line
breaks and the space originally reserved for it.
<p id=a-relatively-positioned-box-establishes-> A relatively positioned box
establishes a new a new <a href="#containing-block0">containing block</a>
for absolutely positioned descendants. (This is a common use of relatively
positioned boxes.) The section on <a href="#containing-block0"
title="containing block">containing blocks</a> explains when a relatively
positioned box establishes a new <a href="#containing-block0">containing
block</a>.
<p id=for-relatively-positioned-elements-left-> For relatively positioned
elements, ‘<a href="#left"><code class=property>left</code></a>’ and
‘<a href="#right"><code class=property>right</code></a>’ move the
box(es) horizontally, without changing their size. ‘<a
href="#left"><code class=property>Left</code></a>’ moves the boxes to
the right, and ‘<a href="#right"><code
class=property>right</code></a>’ moves them to the left. Since boxes are