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<!doctype html>
<html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<title>CSS Positioned Layout Module Level 3</title>
<link href="../default.css" rel="stylesheet" type="text/css">
<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
<style>
body {
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
background-attachment: fixed;
color: black;
font-family: sans-serif;
margin: 0 auto;
max-width: 50em;
padding: 2em 1em 2em 70px;
}
:link { color: #00C; background: transparent }
:visited { color: #609; background: transparent }
a[href]:active { color: #C00; background: transparent }
a[href]:hover { background: #ffa }
a[href] img { border-style: none }
h1, h2, h3, h4, h5, h6 { text-align: left }
h1, h2, h3 { color: #005A9C; }
h1 { font: 170% sans-serif }
h2 { font: 140% sans-serif }
h3 { font: 120% sans-serif }
h4 { font: bold 100% sans-serif }
h5 { font: italic 100% sans-serif }
h6 { font: small-caps 100% sans-serif }
.hide { display: none }
div.head { margin-bottom: 1em }
div.head h1 { margin-top: 2em; clear: both }
div.head table { margin-left: 2em; margin-top: 2em }
p.copyright { font-size: small }
p.copyright small { font-size: small }
pre { margin-left: 2em }
dt { font-weight: bold }
ul.toc, ol.toc {
list-style: none;
}
</style>
<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>
</head>
<body class="h-entry">
<div class="head">
<p data-fill-with="logo"><a class="logo" href="http://www.w3.org/">
<img alt="W3C" height="48" src="https://www.w3.org/Icons/w3c_home" width="72">
</a>
</p>
<h1 class="p-name no-ref" id="title">CSS Positioned Layout Module Level 3</h1>
<h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft,
<time class="dt-updated" datetime="2015-01-14">14 January 2015</time></span></h2>
<div data-fill-with="spec-metadata">
<dl>
<dt>This version:</dt>
<dd><a class="u-url" href="http://dev.w3.org/csswg/css-position/">http://dev.w3.org/csswg/css-position/</a></dd>
<dt>Latest version:</dt>
<dd><a href="http://www.w3.org/TR/css3-positioning/">http://www.w3.org/TR/css3-positioning/</a></dd>
<dt>Previous Versions:</dt>
<dd><a href="http://www.w3.org/TR/2012/WD-css3-positioning-20120207/" rel="previous">http://www.w3.org/TR/2012/WD-css3-positioning-20120207/</a></dd>
<dt>Feedback:</dt>
<dd><span><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>)</span></dd>
<dt>Issue Tracking:</dt>
<dd><a href="#issues-index">Inline In Spec</a></dd>
<dt class="editor">Editors:</dt>
<dd class="editor">
<div class="p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ratan@microsoft.com">Rossen Atanassov</a> (<span class="p-org org">Microsoft</span>)</div>
</dd>
<dd class="editor">
<div class="p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:arronei@microsoft.com">Arron Eicholz</a> (<span class="p-org org">Microsoft</span>)</div>
</dd>
<dt>Issues list:</dt>
<dd><span><a href="http://wiki.csswg.org/spec/css-position/">in Wiki</a></span></dd>
</dl>
</div>
<div data-fill-with="warning"></div>
<p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2014 <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>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
<div class="p-summary" data-fill-with="abstract">
<p>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 data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a>), which builds on CSS level 1 <a data-biblio-type="informative" data-link-type="biblio" href="#biblio-css1" title="CSS1">[CSS1]</a>. The main extensions compared to level 2 are the ability to position elements based on CSS Region boxes, the ability to specify different containing blocks for elements and sticky positioning. </p>
<p>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.</p>
<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.</div>
<h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
<div data-fill-with="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>
<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=%5Bcss-position%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 “css-position” in the subject,
preferably like this:
“[css-position] <em>…summary of comment…</em>”
</p>
<p>
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>
<p>
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>
<p>
This document is governed by the <a href="http://www.w3.org/2014/Process-20140801/">1 August 2014 W3C Process Document</a>.
</p></div>
<div data-fill-with="at-risk"></div>
<h2 class="no-num no-toc no-ref heading settled" id="contents"><span class="content">Table of Contents</span></h2>
<div data-fill-with="table-of-contents" role="navigation">
<ul class="toc" role="directory">
<li><a href="#intro"><span class="secno">1</span> <span class="content">
Introduction</span></a>
<ul class="toc">
<li><a href="#placement"><span class="secno">1.1</span> <span class="content">
Module Interactions</span></a></li>
<li><a href="#values"><span class="secno">1.2</span> <span class="content">
Values</span></a></li>
</ul>
</li>
<li><a href="#vp"><span class="secno">2</span> <span class="content">
The <span data-dfn-type="dfn" data-noexport="" title="viewport">Viewport</span></span></a></li>
<li><a href="#cb"><span class="secno">3</span> <span class="content">
Containing Blocks</span></a>
<ul class="toc">
<li><a href="#def-cb"><span class="secno">3.1</span> <span class="content">
Definition of <span data-dfn-type="dfn" data-noexport="" title="containing block">containing block</span></span></a></li>
</ul>
</li>
<li><a href="#nf"><span class="secno">4</span> <span class="content">
<span data-dfn-type="dfn" data-noexport="" title="normal flow">Normal Flow</span></span></a></li>
<li><a href="#floats"><span class="secno">5</span> <span class="content">
<span data-dfn-type="dfn" data-noexport="" title="float|floated box|floating box">Floats</span></span></a></li>
<li><a href="#pos-sch"><span class="secno">6</span> <span class="content">
<span data-dfn-type="dfn" data-noexport="" title="positioning schemes">Positioning schemes</span></span></a>
<ul class="toc">
<li><a href="#rel-pos"><span class="secno">6.1</span> <span class="content">
Relative positioning</span></a></li>
<li><a href="#sticky-pos"><span class="secno">6.2</span> <span class="content">
Sticky positioning</span></a></li>
<li><a href="#abs-pos"><span class="secno">6.3</span> <span class="content">
Absolute positioning</span></a></li>
<li><a href="#center-pos"><span class="secno">6.4</span> <span class="content">
Center positioning</span></a></li>
<li><a href="#page-positioning"><span class="secno">6.5</span> <span class="content">
Page positioning</span></a></li>
<li><a href="#fixed-pos"><span class="secno">6.6</span> <span class="content">
Fixed positioning</span></a></li>
<li><a href="#position-property"><span class="secno">6.7</span> <span class="content">
Choosing a positioning scheme: <span class="property" data-link-type="propdesc" title="position">position</span> property</span></a></li>
<li><a href="#box-offsets-trbl"><span class="secno">6.8</span> <span class="content">
Box offsets: <span class="property" data-link-type="propdesc" title="top">top</span>, <span class="property" data-link-type="propdesc" title="right">right</span>, <span class="property" data-link-type="propdesc" title="bottom">bottom</span>, <span class="property" data-link-type="propdesc" title="left">left</span></span></a></li>
<li><a href="#logical-box-offsets-beaso"><span class="secno">6.9</span> <span class="content">
Logical box offsets: <span class="property" data-link-type="propdesc" title="offset-before">offset-before</span>, <span class="property" data-link-type="propdesc" title="offset-end">offset-end</span>, <span class="property" data-link-type="propdesc" title="offset-after">offset-after</span> and <span class="property" data-link-type="propdesc" title="offset-start">offset-start</span></span></a></li>
</ul>
</li>
<li><a href="#position-root"><span class="secno">7</span> <span class="content">
Choosing a different containing block for positioning</span></a>
<ul class="toc">
<li><a href="#attachment-properties"><span class="secno">7.1</span> <span class="content">
Attachment properties: <span class="property" data-link-type="propdesc" title="attachment-type">attachment-type</span>, <span class="property" data-link-type="propdesc" title="attachment-top">attachment-top</span>, <span class="property" data-link-type="propdesc" title="attachment-right">attachment-right</span>, <span class="property" data-link-type="propdesc" title="attachment-bottom">attachment-bottom</span>, <span class="property" data-link-type="propdesc" title="attachment-left">attachment-left</span>, and <span class="property" data-link-type="propdesc" title="attachment">attachment</span></span></a></li>
</ul>
</li>
<li><a href="#size-and-position-details"><span class="secno">8</span> <span class="content">
Sizing and positioning details</span></a>
<ul class="toc">
<li><a href="#abs-non-replaced-width"><span class="secno">8.1</span> <span class="content">
The width of absolute, page or fixed positioned, non-replaced elements</span></a></li>
<li><a href="#center-non-replaced-width"><span class="secno">8.2</span> <span class="content">
The width of a center positioned, non-replaced elements</span></a></li>
<li><a href="#abs-replaced-width"><span class="secno">8.3</span> <span class="content">
The width of absolute, page or fixed positioned, replaced elements</span></a></li>
<li><a href="#center-replaced-width"><span class="secno">8.4</span> <span class="content">
The width of a center positioned, replaced elements</span></a></li>
<li><a href="#abs-non-replaced-height"><span class="secno">8.5</span> <span class="content">
The height of absolute, page or fixed positioned, non-replaced elements</span></a></li>
<li><a href="#center-non-replaced-height"><span class="secno">8.6</span> <span class="content">
The height of a center positioned, non-replaced elements</span></a></li>
<li><a href="#abs-replaced-height"><span class="secno">8.7</span> <span class="content">
The height of absolute, page or fixed positioned, replaced elements</span></a></li>
<li><a href="#center-replaced-height"><span class="secno">8.8</span> <span class="content">
The height of a center positioned, replaced elements</span></a></li>
<li><a href="#root-height"><span class="secno">8.9</span> <span class="content">
Auto heights for block formatting context roots</span></a></li>
</ul>
</li>
<li><a href="#dis-pos-flo"><span class="secno">9</span> <span class="content">
Relationships between <span class="property" data-link-type="propdesc" title="display">display</span>, <span class="property" data-link-type="propdesc" title="position">position</span>, and <span class="property" data-link-type="propdesc" title="float">float</span></span></a></li>
<li><a href="#comparison"><span class="secno">10</span> <span class="content">
Comparison of normal flow, floats, and positioning</span></a>
<ul class="toc">
<li><a href="#comp-normal-flow"><span class="secno">10.1</span> <span class="content">
Normal flow</span></a></li>
<li><a href="#comp-relpos"><span class="secno">10.2</span> <span class="content">
Relative positioning</span></a></li>
<li><a href="#comp-floating"><span class="secno">10.3</span> <span class="content">
Floating a box</span></a></li>
<li><a href="#comp-abspos"><span class="secno">10.4</span> <span class="content">
Absolute positioning</span></a></li>
<li><a href="#comp-pagepos"><span class="secno">10.5</span> <span class="content">
Page positioning</span></a></li>
</ul>
</li>
<li><a href="#layered-presentation"><span class="secno">11</span> <span class="content">
Layered presentation</span></a></li>
<li><a href="#det-stacking-context"><span class="secno">12</span> <span class="content">
Detailed stacking context</span></a>
<ul class="toc">
<li><a href="#definitions-stack"><span class="secno">12.1</span> <span class="content">
Definitions</span></a></li>
<li><a href="#painting-order"><span class="secno">12.2</span> <span class="content">
Painting order</span></a></li>
<li><a href="#stacking-notes"><span class="secno">12.3</span> <span class="content">
Notes</span></a></li>
</ul>
</li>
<li><a href="#ack"><span class="secno">13</span> <span class="content">
Acknowledgments</span></a></li>
<li><a href="#conformance"><span class="secno"></span> <span class="content">
Conformance</span></a>
<ul class="toc">
<li><a href="#conventions"><span class="secno"></span> <span class="content">
Document conventions</span></a></li>
<li><a href="#conformance-classes"><span class="secno"></span> <span class="content">
Conformance classes</span></a></li>
<li><a href="#partial"><span class="secno"></span> <span class="content">
Partial implementations</span></a></li>
<li><a href="#experimental"><span class="secno"></span> <span class="content">
Experimental implementations</span></a></li>
<li><a href="#testing"><span class="secno"></span> <span class="content">
Non-experimental implementations</span></a></li>
</ul>
</li>
<li><a href="#references"><span class="secno"></span> <span class="content">References</span></a>
<ul class="toc">
<li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a></li>
<li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a></li>
</ul>
</li>
<li><a href="#index"><span class="secno"></span> <span class="content">Index</span></a></li>
<li><a href="#property-index"><span class="secno"></span> <span class="content">Property Index</span></a></li>
<li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a></li>
</ul></div>
<main>
<h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">
Introduction</span><a class="self-link" href="#intro"></a></h2>
<em>This section is not normative.</em>
<p>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>
<p>In <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="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:</p>
<ul>
<li>
<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>
<li>
positioning scheme (<a data-link-type="dfn" href="#normal-flow">normal flow</a>, float, and absolute positioning).
</li>
<li>
relationships between elements in the
<a href="http://www.w3.org/TR/CSS2/conform.html#doctree">document tree</a>.
</li>
<li>
external information (e.g., viewport size, intrinsic dimensions of images, etc.).
</li>
</ul>
<p>The properties defined in this module apply to both continuous media and paged
media.</p>
<h3 class="heading settled" data-level="1.1" id="placement"><span class="secno">1.1. </span><span class="content">
Module Interactions</span><a class="self-link" href="#placement"></a></h3>
<p>This module replaced and extends the positioning scheme features defined in
<a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a> sections:</p>
<ul>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#viewport">9.1.1 The viewport</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#containing-block">9.1.2 Containing blocks</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#positioning-scheme">9.3 Positioning schemes</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#relative-positioning">9.4.3 Relative positioning</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#absolute-positioning">9.6 Absolute positioning</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">9.7 Relationships between display, position, and float</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#comparison">9.8 Comparison of normal flow, floats, and absolute positioning</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visuren.html#layers">9.9 Layered presentation</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visudet.html#containing-block-details">10.1 Definition of "containing block"</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-width">10.3.7 Absolutely positioned, non-replaced elements</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-width">10.3.8 Absolutely positioned, replaced elements</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height">10.6.4 Absolutely positioned, non-replaced elements</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visudet.html#abs-replaced-height">10.6.5 Absolutely positioned replaced elements</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/visudet.html#root-height">10.6.7 Auto heights for block formatting context roots</a>
</li>
<li>
<a href="http://www.w3.org/TR/CSS2/zindex.html">Appendix E. Elaborate description of Stacking Contexts</a>
</li>
</ul>
<h3 class="heading settled" data-level="1.2" id="values"><span class="secno">1.2. </span><span class="content">
Values</span><a class="self-link" href="#values"></a></h3>
<p>This specification follows the CSS property definition conventions from <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a>.</p>
<p>Value types not defined in this specification are defined in CSS Level 2
Revision 1 <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a>. Other CSS modules may expand the definitions of these
value types: for example <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css3val" title="CSS3VAL">[CSS3VAL]</a>, when combined with this module, expands
the definition of the <length> value type as used in this specification.</p>
<p>In addition to the property-specific values listed in their definitions, all
properties defined in this specification also accept the
<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>
<h2 class="heading settled" data-level="2" id="vp"><span class="secno">2. </span><span class="content">
The <dfn data-dfn-type="dfn" data-noexport="" id="viewport" title="viewport">Viewport<a class="self-link" href="#viewport"></a></dfn></span><a class="self-link" href="#vp"></a></h2>
<p>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>
<p>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>
<h2 class="heading settled" data-level="3" id="cb"><span class="secno">3. </span><span class="content">
Containing Blocks</span><a class="self-link" href="#cb"></a></h2>
<p>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>
<p>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>.</p>
<h3 class="heading settled" data-level="3.1" id="def-cb"><span class="secno">3.1. </span><span class="content">
Definition of <dfn data-dfn-type="dfn" data-noexport="" id="containing-block">containing block<a class="self-link" href="#containing-block"></a></dfn></span><a class="self-link" href="#def-cb"></a></h3>
<p>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 <span title="'position'!!<fake-maybe-placeholder>static</fake-maybe-placeholder>"><span class="css">static</span></span> or
<span title="'position'!!<fake-maybe-placeholder>relative</fake-maybe-placeholder>"><span class="css">relative</span></span> element is defined
in the Box Model <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css3box" title="CSS3BOX">[CSS3BOX]</a>. The containing block of a
<span title="'position'!!<fake-maybe-placeholder>sticky</fake-maybe-placeholder>"><span class="css">sticky</span></span> element is the same as for a
<span title="'position'!!<fake-maybe-placeholder>relative</fake-maybe-placeholder>"><span class="css">relative</span></span> element. For
<span title="'position'!!<fake-maybe-placeholder>fixed</fake-maybe-placeholder>"><a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-backgrounds-3/#fixed">fixed</a></span>,
<span title="'position'!!<fake-maybe-placeholder>absolute</fake-maybe-placeholder>"><span class="css">absolute</span></span>,
<span title="'position'!!<fake-maybe-placeholder>center</fake-maybe-placeholder>"><a class="css" data-link-type="maybe" href="http://www.w3.org/TR/css3-2d-transforms/#valuedef-center0">center</a></span> and
<span title="'position'!!<fake-maybe-placeholder>page</fake-maybe-placeholder>"><a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-multicol-1/#valuedef-page">page</a></span>, it is defined as follows:</p>
<ol>
<li>
If the element has <span title="'position'!!<fake-maybe-placeholder>fixed</fake-maybe-placeholder>">'positioned: fixed'</span>, the containing block is established by the
<a data-link-type="dfn" href="#viewport">viewport</a> in the case of continuous media or the page area in
the case of paged media.
</li>
<li>
If the element has <span title="'position'!!<fake-maybe-placeholder>absolute</fake-maybe-placeholder>">'position: absolute'</span>,
the containing block is established by the nearest ancestor with a <a class="property" data-link-type="propdesc" href="#propdef-position">position</a>
other than <span title="'position'!!<fake-maybe-placeholder>static</fake-maybe-placeholder>"><span class="css">static</span></span>, in the following way:
<ol>
<li>
In the case that the ancestor is block-level, the containing block is formed
by the padding edge of the ancestor.
</li>
<li>
In the case that the ancestor is inline-level, the containing block depends
on the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-writing-modes-3/#propdef-direction">direction</a> property of the ancestor:
<ol>
<li>
If the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-writing-modes-3/#propdef-direction">direction</a> is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-writing-modes-3/#valdef-direction-ltr">ltr</a>, 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>
<li>
If the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-writing-modes-3/#propdef-direction">direction</a> is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-writing-modes-3/#valdef-direction-rtl">rtl</a>, 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" role="note">
Note, in some cases when a line wraps it may seem as if the left
and right positions are swapped.
</p>
</li>
</ol>
</li>
</ol>
</li>
<li>
I the element has <span title="'position'!!<fake-maybe-placeholder>center</fake-maybe-placeholder>">'position: center'</span>, the containing block is established
by the nearest ancestor with a <a class="property" data-link-type="propdesc" href="#propdef-position">position</a> other than <span title="'position'!!<fake-maybe-placeholder>static</fake-maybe-placeholder>"><span class="css">static</span></span>, in the following way:
<ol>
<li>
In the case that the ancestor is block-level, the containing block is formed
by the padding edge of the ancestor.
</li>
<li>
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.
</li>
</ol>
</li>
<li>
If the element has <span title="'position'!!<fake-maybe-placeholder>page</fake-maybe-placeholder>">'position: page'</span>
and the element is within paged media or within a rerion, the containing
block is the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-display-3/#initial-containing-block">initial containing block</a>. This is the <a data-link-type="dfn" href="#viewport">viewport</a>
or the page area when in paged media. In the case of CSS Regions
<a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css3-regions" title="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>
<li>
If there is no such ancestor, the containing block is the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-display-3/#initial-containing-block" title="initial containing block">initial
containing block</a>.
</li>
</ol>
<p>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>
<p>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:</p>
<ul>
<li>on the current page,</li>
<li>on a subsequent page, or</li>
<li>may omit it altogether.</li>
</ul>
<p class="note" role="note">
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.
</p>
<div class="example">
<p>With no positioning, the containing blocks (C.B.) in the following document:</p>
<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>are established as follows:</p>
<table class="cb-example-table">
<tbody>
<tr>
<th>For box generated by </th>
<th>C.B. is established by</th>
</tr>
<tr>
<td>html</td>
<td>initial C.B. (UA-dependent)</td>
</tr>
<tr>
<td>body</td>
<td>html</td>
</tr>
<tr>
<td>div1</td>
<td>body</td>
</tr>
<tr>
<td>p1</td>
<td>div1</td>
</tr>
<tr>
<td>p2</td>
<td>div1</td>
</tr>
<tr>
<td>em1</td>
<td>p2</td>
</tr>
<tr>
<td>strong1</td>
<td>p2</td>
</tr>
</tbody></table>
<p>If we position "div1":</p>
<pre><code>#div1 { position: absolute; left: 50px; top: 50px }
</code></pre>
<p>its containing block is no longer "body"; it becomes the <span>initial containing block</span>
(since there are no other positioned ancestor boxes).</p>
<p>If we position "em1" as well:</p>
<pre><code>#div1 { position: absolute; left: 50px; top: 50px }
#em1 { position: absolute; left: 100px; top: 100px }
</code></pre>
<p>the table of containing blocks becomes:</p>
<table class="cb-example-table">
<tbody>
<tr>
<th>For box generated by</th>
<th>C.B. is established by</th>
</tr>
<tr>
<td>html</td>
<td>initial C.B. (UA-dependent)</td>
</tr>
<tr>
<td>body</td>
<td>html</td>
</tr>
<tr>
<td>div1</td>
<td>initial C.B.</td>
</tr>
<tr>
<td>p1</td>
<td>div1</td>
</tr>
<tr>
<td>p2</td>
<td>div1</td>
</tr>
<tr>
<td>em1</td>
<td>div1</td>
</tr>
<tr>
<td>strong1</td>
<td>em1</td>
</tr>
</tbody>
</table>
<p>By positioning "em1", its containing block becomes the nearest positioned ancestor box
(i.e., that generated by "div1").</p>
</div>
<h2 class="heading settled" data-level="4" id="nf"><span class="secno">4. </span><span class="content">
<dfn data-dfn-type="dfn" data-noexport="" id="normal-flow" title="normal flow">Normal Flow<a class="self-link" href="#normal-flow"></a></dfn></span><a class="self-link" href="#nf"></a></h2>
<p>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 data-biblio-type="normative" data-link-type="biblio" href="#biblio-css3box" title="CSS3BOX">[CSS3BOX]</a> for further details about <a href="http://www.w3.org/TR/CSS2/visuren.html#normal-flow">normal flow</a>.</p>
<h2 class="heading settled" data-level="5" id="floats"><span class="secno">5. </span><span class="content">
<dfn data-dfn-type="dfn" data-noexport="" id="float" title="float|floated box|floating box">Floats<a class="self-link" href="#float"></a></dfn></span><a class="self-link" href="#floats"></a></h2>
<p>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 <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/visuren.html#propdef-clear">clear</a> property) or around the floated box.
See the CSS 2.1 <a data-biblio-type="normative" data-link-type="biblio" href="#biblio-css21" title="CSS21">[CSS21]</a> for details
about <a href="http://www.w3.org/TR/CSS2/visuren.html#floats">floating boxes</a>.</p>
<h2 class="heading settled" data-level="6" id="pos-sch"><span class="secno">6. </span><span class="content">
<dfn data-dfn-type="dfn" data-noexport="" id="positioning-schemes" title="positioning schemes">Positioning schemes<a class="self-link" href="#positioning-schemes"></a></dfn></span><a class="self-link" href="#pos-sch"></a></h2>
<p>In CSS, a box may be laid out according to three positioning schemes:</p>
<ol>
<li>
Normal flow<br>
In CSS, <a data-link-type="dfn" href="#normal-flow">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 <span title="relative positioning">relative</span> and <span title="sticky positioning">sticky</span> positioning of block-level and inline-level boxes.
</li>
<li>
Floats<br>
In the <span title="floats">float</span> model, a box is first laid out according to the <a data-link-type="dfn" href="#normal-flow">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 <span title="floats">float</span>.
</li>
<li>
Absolute positioning<br>
In the absolute positioning model, a box is removed from the <span>normal
flow</span> entirely (it has no impact on later siblings) and assigned a
position with respect to a <a data-link-type="dfn" href="#containing-block">containing block</a>.
</li>
</ol>
<p>An element is called <em>out-of-flow</em> if it is <span title="floats">floated</span>, 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.</p>
<h3 class="heading settled" data-level="6.1" id="rel-pos"><span class="secno">6.1. </span><span class="content">
Relative positioning</span><a class="self-link" href="#rel-pos"></a></h3>
<p>Once a box has been laid out according to the <a data-link-type="dfn" href="#normal-flow">normal flow</a> or
<span title="floats">floated</span>, 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 'overflow: auto' or 'overflow: scroll' 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>
<p>A relatively positioned box keeps its <a data-link-type="dfn" href="#normal-flow">normal flow</a> size, including
line breaks and the space originally reserved for it.</p>
<p>A relatively positioned box establishes a new a new <a data-link-type="dfn" href="#containing-block">containing block</a> for
absolutely positioned descendants. (This is a common use of relatively positioned
boxes.) The section on <span title="containing block">containing blocks</span>
explains when a relatively positioned box establishes a new <a data-link-type="dfn" href="#containing-block">containing block</a>.</p>
<p>For relatively positioned elements, <a class="property" data-link-type="propdesc" href="#propdef-left">left</a> and <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> move the box(es) horizontally,
without changing their size. <a class="property" data-link-type="propdesc" href="#propdef-left">Left</a> moves the boxes to the right, and <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> moves
them to the left. Since boxes are not split or stretched as a result of <a class="property" data-link-type="propdesc" href="#propdef-left">left</a> or
<a class="property" data-link-type="propdesc" href="#propdef-right">right</a>, the used values are always: left = -right.</p>
<p>If both <a class="property" data-link-type="propdesc" href="#propdef-left">left</a> and <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> are <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a> (their initial values), the used values
are <span class="css">0</span> (i.e., the boxes stay in their original position).</p>
<p>If <a class="property" data-link-type="propdesc" href="#propdef-left">left</a> is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a>, its used value is minus the value of <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> (i.e., the
boxes move to the left by the value of <a class="property" data-link-type="propdesc" href="#propdef-right">right</a>).</p>
<p>If <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> is specified as <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a>, its used value is minus the value of <a class="property" data-link-type="propdesc" href="#propdef-left">left</a>.</p>
<p>If neither <a class="property" data-link-type="propdesc" href="#propdef-left">left</a> nor <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a>, the position is over-constrained, and
one of them has to be ignored. If the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-writing-modes-3/#propdef-direction">direction</a> property of the <a data-link-type="dfn" href="#containing-block" title="containing block">containing
block</a> is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-writing-modes-3/#valdef-direction-ltr">ltr</a>, the value of <a class="property" data-link-type="propdesc" href="#propdef-left">left</a> wins and <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> becomes -<a class="property" data-link-type="propdesc" href="#propdef-left">left</a>.
If <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-writing-modes-3/#propdef-direction">direction</a> of the containing block is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-writing-modes-3/#valdef-direction-rtl">rtl</a>, <a class="property" data-link-type="propdesc" href="#propdef-right">right</a> wins and <a class="property" data-link-type="propdesc" href="#propdef-left">left</a> is
ignored.</p>
<div class="example">
<p>The following three rules are equivalent:</p>
<pre><code>div.a8 { position: relative; direction: ltr; left: -1em; right: auto }
div.a8 { position: relative; direction: ltr; left: auto; right: 1em }
div.a8 { position: relative; direction: ltr; left: -1em; right: 5em }
</code></pre>
</div>
<p>The <a class="property" data-link-type="propdesc" href="#propdef-top">top</a> and <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a> properties move relatively positioned element(s) up or
down without changing their size. <a class="property" data-link-type="propdesc" href="#propdef-top">Top</a> moves the boxes down, and <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a> moves
them up. Since boxes are not split or stretched as a result of <a class="property" data-link-type="propdesc" href="#propdef-top">top</a> or <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a>,
the used values are always: top = -bottom.</p>
<p>If <a class="property" data-link-type="propdesc" href="#propdef-top">top</a> and <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a> are <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a>, their used values are both <span class="css">0</span>.</p>
<p>If one of them, <a class="property" data-link-type="propdesc" href="#propdef-top">top</a> or <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a>, is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a>, the <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a> value becomes the
negative of the other.</p>
<p>If neither <a class="property" data-link-type="propdesc" href="#propdef-top">top</a> and <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a> is <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-align-3/#valdef-justify-content-align-content-auto">auto</a>, <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a> is ignored (i.e., the used
value of <a class="property" data-link-type="propdesc" href="#propdef-bottom">bottom</a> will be minus the value of <a class="property" data-link-type="propdesc" href="#propdef-top">top</a>).</p>
<p class="note" role="note">
Note, Although relative positioning could be used as a form of superscripting
and subscripting, the line height is not automatically adjusted to take the