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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<head>
<title>Visual formatting model</title>
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<hr class="navbar">
<h1><a name="q0">9 Visual formatting model</a></h1>
<div class="subtoc">
<p><strong>Contents</strong>
<ul class="toc">
<li class="tocline2"><a href="visuren.html#visual-model-intro" class="tocxref">9.1 Introduction to the visual formatting model</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#viewport" class="tocxref">9.1.1 The viewport</a>
<li class="tocline3"><a href="visuren.html#containing-block" class="tocxref">9.1.2 <span class="index-def" title="containing block"> Containing blocks</span></a>
</ul>
<li class="tocline2"><a href="visuren.html#box-gen" class="tocxref">9.2 Controlling box generation</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#block-boxes" class="tocxref">9.2.1 Block-level elements and block boxes</a>
<ul class="toc">
<li class="tocline4"><a href="visuren.html#anonymous-block-level" class="tocxref">9.2.1.1 Anonymous block boxes</a>
</ul>
<li class="tocline3"><a href="visuren.html#inline-boxes" class="tocxref">9.2.2 Inline-level elements and inline boxes</a>
<ul class="toc">
<li class="tocline4"><a href="visuren.html#anonymous" class="tocxref">9.2.2.1 Anonymous inline boxes</a>
</ul>
<li class="tocline3"><a href="visuren.html#run-in" class="tocxref">9.2.3 Run-in boxes</a>
<li class="tocline3"><a href="visuren.html#display-prop" class="tocxref">9.2.4 The <span class="propinst-display">'display'</span> property</a>
</ul>
<li class="tocline2"><a href="visuren.html#positioning-scheme" class="tocxref">9.3 Positioning schemes</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#choose-position" class="tocxref">9.3.1 Choosing a positioning scheme: <span class="propinst-position">'position'</span> property</a>
<li class="tocline3"><a href="visuren.html#position-props" class="tocxref">9.3.2 Box offsets: <span class="propinst-top">'top'</span>, <span class="propinst-right">'right'</span>, <span class="propinst-bottom">'bottom'</span>, <span class="propinst-left">'left'</span></a>
</ul>
<li class="tocline2"><a href="visuren.html#normal-flow" class="tocxref">9.4 Normal flow</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#block-formatting" class="tocxref">9.4.1 Block formatting contexts</a>
<li class="tocline3"><a href="visuren.html#inline-formatting" class="tocxref">9.4.2 Inline formatting context</a>
<li class="tocline3"><a href="visuren.html#relative-positioning" class="tocxref">9.4.3 Relative positioning</a>
</ul>
<li class="tocline2"><a href="visuren.html#floats" class="tocxref">9.5 Floats</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#float-position" class="tocxref">9.5.1 Positioning the float: the <span class="propinst-float">'float'</span> property</a>
<li class="tocline3"><a href="visuren.html#flow-control" class="tocxref">9.5.2 Controlling flow next to floats: the <span class="propinst-clear">'clear'</span> property</a>
</ul>
<li class="tocline2"><a href="visuren.html#absolute-positioning" class="tocxref">9.6 Absolute positioning</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#fixed-positioning" class="tocxref">9.6.1 Fixed positioning</a>
</ul>
<li class="tocline2"><a href="visuren.html#dis-pos-flo" class="tocxref">9.7 Relationships between 'display', 'position', and 'float'</a>
<li class="tocline2"><a href="visuren.html#comparison" class="tocxref">9.8 Comparison of normal flow, floats, and absolute positioning</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#comp-normal-flow" class="tocxref">9.8.1 Normal flow</a>
<li class="tocline3"><a href="visuren.html#comp-relpos" class="tocxref">9.8.2 Relative positioning</a>
<li class="tocline3"><a href="visuren.html#comp-float" class="tocxref">9.8.3 Floating a box</a>
<li class="tocline3"><a href="visuren.html#comp-abspos" class="tocxref">9.8.4 Absolute positioning</a>
</ul>
<li class="tocline2"><a href="visuren.html#layers" class="tocxref">9.9 Layered presentation</a>
<ul class="toc">
<li class="tocline3"><a href="visuren.html#z-index" class="tocxref">9.9.1 Specifying the stack level: the <span class="propinst-z-index">'z-index'</span> property</a>
</ul>
<li class="tocline2"><a href="visuren.html#direction" class="tocxref">9.10 Text direction: the <span class="propinst-direction">'direction'</span> and <span class="propinst-unicode-bidi">'unicode-bidi'</span> properties</a>
</ul>
</div>
<h2>9.1 <a name="visual-model-intro">Introduction to the visual formatting model</a></h2>
<p>This chapter and the next describe the <a name="x0"><span class="index-def"
title="visual formatting model">visual formatting model</span></a>: how user
agents process the <a href="conform.html#doctree">document tree</a>
for visual <a href="media.html">media</a>.
</p>
<p>In the visual formatting model, each element in the document tree
generates zero or more boxes according to the <a href="box.html">box
model</a>. The layout of these boxes is governed by:</p>
<ul>
<li><a href="box.html#box-dimensions">box dimensions</a> and <a
href="#box-gen">type</a>.
</li><li><a href="#positioning-scheme">positioning scheme</a>
(normal flow, float, and absolute positioning).
</li><li>relationships between elements
in the <a href="conform.html#doctree">document tree.</a>
</li><li>external information (e.g., viewport size, <a
href="conform.html#intrinsic">intrinsic</a> dimensions of images, etc.).
</li></ul>
<p>The properties defined in this chapter and the next apply to both
<a href="media.html#continuous-media-group">continuous media</a> and
<a href="media.html#paged-media-group">paged media</a>. <span class="paged">
However, the
meanings of the <a href="box.html#margin-properties">margin
properties</a> vary when applied to paged media (see the <a
href="page.html#page-margins">page model</a> for details).
</span>
</p>
<p>The visual formatting model does not specify all aspects of
formatting (e.g., it does not specify a letter-spacing algorithm). <a
href="conform.html#conformance">Conforming user agents</a> may behave
differently for those formatting issues not covered by this
specification.
</p>
<h3>9.1.1 <a name="viewport">The viewport</a></h3>
<p>User agents for <a
href="media.html#continuous-media-group">continuous media</a>
generally offer users a <a name="x1"><span class="index-def" title="viewport">
<dfn>viewport</dfn></span></a> (a window or other
viewing area on the screen) through which users consult a
document. User agents may change the document's layout when the
viewport is resized (see the <a
href="visudet.html#containing-block-details">initial containing block</a>).
</p>
<p>
When the viewport is smaller than the area of the canvas on which
the document is rendered, the user agent should offer a scrolling
mechanism.
There is at most
one viewport per <a href="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>
<h3>9.1.2 <span class="index-def" title="containing block">
<a name="containing-block">Containing blocks</a></span>
</h3>
<p>In CSS 2.1, many box positions and sizes are calculated with respect
to the edges
of a rectangular box called a <a name="x3"><span class="index-def"
title="containing block"><dfn>containing block</dfn></span></a>. 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="visufx.html#overflow">overflow</a>.
</p>
<p>The <a href="visudet.html#containing-block-details">details</a> of
how a containing block's dimensions are calculated are described in
the <a href="visudet.html">next chapter</a>.
</p>
<h2>9.2 <a name="box-gen">Controlling box generation</a></h2>
<p>The following sections describe the types of boxes that may be
generated in CSS 2.1. A box's type affects, in part, its behavior in the
visual formatting model. The <a href="visuren.html#propdef-display" class="noxref"><span
class="propinst-display">'display'</span></a> property, described below,
specifies a box's type.
</p>
<h3>9.2.1 <a name="block-boxes">Block-level elements and block boxes</a></h3>
<p><span class="index-def" title="block-level
element"><a name="block-level"><dfn>Block-level elements</dfn></a></span> are those elements of the source document that are formatted visually as
blocks (e.g., paragraphs). Several values of the <a href="visuren.html#propdef-display" class="noxref"><span
class="propinst-display">'display'</span></a> property make an element
block-level: 'block', 'list-item', and 'run-in' (part of the
time; see <a href="#run-in">run-in boxes</a>), and 'table'.
</p>
<p>Block-level elements (except for display 'table' elements, which are described in a later chapter) generate a <span class="index-def"
title="principal block box"><a name="principal-box"><dfn>principal
block box</dfn></a></span> that contains either only <span class="index-def"
title="block box"><a name="block-box"><dfn title="block box">block
boxes</dfn></a></span> or only <span class="index-def"
title="inline box"><a name="x7" href="#inline-box">inline boxes</a></span>. The principal block box establishes the <a
href="#containing-block">containing block</a> for descendant boxes and
generated content and is also the box involved in any positioning
scheme. Principal block boxes participate in a <a
href="#block-formatting">block formatting context</a>.
</p>
<p>Some block-level elements generate additional boxes outside of the
principal box: 'list-item' elements. These additional boxes are
placed with respect to the principal box.
</p>
<h4>9.2.1.1 <a name="anonymous-block-level">Anonymous block boxes</a></h4>
<p>In a document like this:
</p>
<pre><code class="html">
<DIV>
Some text
<P>More text
</DIV>
</code></pre>
<p>
(and assuming the DIV and the P both have 'display: block'), the
DIV appears to have both inline content and block content. To make it
easier to define the formatting, we assume that there is an <em><a name="x8"><span
class="index-def" title="anonymous">anonymous block box</span></a></em>
around "Some text".
</p>
<div class="figure">
<p><img src="images/anon-block.png" alt="diagram showing the three
boxes for the example above"><SPAN class="dlink"> <A name="img-anon-block" href="images/longdesc/anon-block-desc.html" title="Long description for anonymous block box">[D]</A></SPAN>
</p><p class="caption">Diagram showing the
three boxes, of which one is anonymous, for the example above.
</p></div>
<p>In other words: if a block box (such as that generated for the DIV
above) has another block box or run-in box inside it (such as the P
above), then we force it to have <em>only</em> block boxes and run-in
boxes inside it.
</p>
<p>
When an inline box contains a block box, the inline box (and its inline ancestors within the same line box) are broken around the block. The line boxes before the break and after the break are enclosed in anonymous boxes, and the block box becomes a sibling of those anonymous boxes.
When such an inline box is affected by relative positioning,
the relative positioning also affects the block box.
</p>
<div class="example"><P style="display:none">Example(s):</P>
<p>This model would apply in the following example if the following
rules:</p>
<pre><code class="css">
p { display: inline }
span { display: block }
</code></pre>
<p>were used with this HTML document:</p>
<pre class="html-example"><code class="html">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HEAD>
<TITLE>Anonymous text interrupted by a block</TITLE>
</HEAD>
<BODY>
<P>
<em>This is anonymous text before the SPAN.</em>
<SPAN>This is the content of SPAN.</SPAN>
<em>This is anonymous text after the SPAN.</em>
</P>
</BODY>
</code></pre>
<p>The P element contains a chunk (C1) of anonymous text followed
by a block-level element followed by another chunk (C2) of anonymous
text. The resulting boxes would be an anonymous block box around the P,
containing an anonymous block box around C1, the SPAN block box, and
another anonymous block box around C2.
</p></div>
<p>The properties of anonymous boxes are inherited from the
enclosing non-anonymous box (e.g., in the example just below the subsection heading "Anonymous block boxes", the one for DIV).
Non-inherited properties have their initial value. For example,
the font of the anonymous box is inherited from the DIV, but the
margins will be 0.
</p>
<p>
Properties set on elements that cause anonymous block boxes to be
generated still apply to the boxes and content of that element. For
example, if a border had been set on the P element in the above
example, the border would be drawn around C1 (open at the end of the
line) and C2 (open at the start of the line).
</p>
<p>
Some user agents have implemented borders on inlines containing
blocks in other ways, e.g., by wrapping such nested blocks inside
"anonymous line boxes" and thus drawing inline borders around such
boxes. As CSS1 and CSS2 did not define this behavior, CSS1-only and
CSS2-only user agents may implement this alternative model and still
claim conformance to this part of CSS 2.1. This does not apply to UAs
developed after this specification was released.
</p>
<h3>9.2.2 <a name="inline-boxes">Inline-level elements and inline boxes</a></h3>
<p><span class="index-def" title="inline-level element">
<a name="inline-level"><dfn>Inline-level
elements</dfn></a></span> are those elements of the source document that
do not form new blocks of content; the content is distributed in lines
(e.g., emphasized pieces of text
within a paragraph, inline images,
etc.). Several values of the <a href="visuren.html#propdef-display" class="noxref"><span
class="propinst-display">'display'</span></a> property make an element
inline: 'inline', 'inline-table', 'inline-block'
and 'run-in' (part of the time; see <a href="#run-in">run-in boxes</a>).
Inline-level elements generate <a name="inline-box"><dfn>inline
boxes</dfn></a>.
</p>
<h4>9.2.2.1 <a name="anonymous">Anonymous inline boxes</a></h4>
<p>In a document with HTML markup like this:</p>
<div class="html-example">
<pre><code class="html">
<p>Some <em>emphasized</em> text</p>
</code></pre>
</div>
<p>the <code><p></code> generates a block box, with several inline boxes inside
it. The box for "emphasized" is an inline box generated by an inline
element (<code><em></code>), but the other boxes ("Some" and "text") are inline boxes generated by a block-level element (<code><p></code>). The latter are called <a name="x10"><span
class="index-def" title="anonymous inline boxes">anonymous inline
boxes</span></a>, because they do not have an associated inline-level element.
</p>
<p>Such anonymous inline boxes inherit inheritable properties from
their block parent box. Non-inherited properties have their initial
value. In the example, the color of the anonymous inline boxes is
inherited from the P, but the background is transparent.
</p>
<p>
White space content that would subsequently be collapsed away according to the <a href="text.html#propdef-white-space" class="noxref"><span class="propinst-white-space">'white-space'</span></a> property does not generate any anonymous inline boxes.
</p>
<p>If it is clear from the context which type of anonymous box is
meant, both anonymous inline boxes and anonymous block boxes are
simply called anonymous boxes in this specification.
</p>
<p>There are more types of anonymous boxes that arise when formatting
<a href="tables.html#anonymous-boxes">tables</a>.
</p>
<h3>9.2.3 <a name="run-in">Run-in boxes</a></h3>
<p>A <a name="x11"><span class="index-def" title="run-in element"><dfn>run-in
element</dfn></span></a> (or pseudo-element) <var>A</var> behaves as
follows:
<ol>
<li>If <var>A</var> has any <a
href="conform.html#child">children</a> that <a
href="#inhibit-run-in">inhibit run-in behavior</a> (see below), then
<var>A</var> is rendered as if it had 'display: block'.
<li>Let <var>B</var> be the first of <var>A</var>'s following <a
href="conform.html#sibling">siblings</a> that is neither floating
nor <a href="visuren.html#absolutely-positioned">absolutely
positioned nor has 'display: none'. If <var>B</var> exists and has a
specified value for 'display' of 'block' or 'list-item' and is not
<a href="conform.html#replaced-element">replaced,</a> then
<var>A</var> is rendered as an 'inline' element at the start of
<var>B</var>'s <a href="visuren.html#principal-box">principal
box.</a> Note: <var>A</var> is rendered before <var>B</var>'s
':before' pseudo-element, if any. See <a
href="generate.html#before-after-content">12.1.</a>
<li>Otherwise, <var>A</var> is rendered as if it had 'display:
block'.
</ol>
<p>In the above, "siblings" and "children" include both normal
elements and :before/:after pseudo-elements.
<p id="inhibit-run-in">An element or pseudo-element <var>C</var>
<dfn>inhibits run-in behavior</dfn> if one of the following is
true. (Note that the definition is recursive.)
<ol>
6D50
<li><var>C</var> is not floating and not <a
href="visuren.html#absolutely-positioned">absolutely positioned</a>
and the computed value of its 'display' is one of 'block',
'list-item', 'table' or 'run-in'.
<li><var>C</var> has a computed value for 'display' of 'inline' and
it has one or more children that inhibit run-in behavior. (Where
"children" includes both normal elements and :before/:after
pseudo-elements.)
</ol>
<p>A 'run-in' box is useful for run-in headers, as
in this example:
</p>
<div class="html-example">
<pre><code class="html">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>A run-in box example</TITLE>
<STYLE type="text/css">
H3 { display: run-in }
</STYLE>
</HEAD>
<BODY>
<H3>A run-in heading.</H3>
<P>And a paragraph of text that
follows it.
</BODY>
</HTML>
</code></pre>
</div>
<p>This example might be formatted as:</p>
<pre>
<b>A run-in heading.</b> And a
paragraph of text that
follows it.
</pre>
<p>Despite appearing visually part of the following block box,
a run-in element still inherits properties from its parent
in the source tree.
It is undefined in CSS 2.1 if a run-in inherits from a
':first-line' pseudo-element.
</p>
<p>Please consult the section on
<a href="generate.html#before-after-content">generated content</a>
for information about how run-in boxes interact with generated
content.
</p>
<h3>9.2.4 <a name="display-prop">The</a> <a href="visuren.html#propdef-display" class="noxref"><span
class="propinst-display">'display'</span></a> property</h3>
<div class="propdef">
<dl><dt>
<span class="index-def" title="'display'"><a name="propdef-display" class="propdef-title"><strong>'display'</strong></a></span>
<dd>
<table class="propinfo" cellspacing=0 cellpadding=0>
<tr valign=baseline><td><em>Value:</em> <td>inline | block | list-item | run-in | inline-block |
table | inline-table | table-row-group | table-header-group |
table-footer-group | table-row | table-column-group | table-column |
table-cell | table-caption | none | <a href="cascade.html#value-def-inherit" class="noxref"><span class="value-inst-inherit">inherit</span></a>
<tr valign=baseline><td><em>Initial:</em> <td>inline
<tr valign=baseline><td><em>Applies to:</em> <td>all elements
<tr valign=baseline><td><em>Inherited:</em> <td>no
<tr valign=baseline><td><em>Percentages:</em> <td>N/A
<tr valign=baseline><td><em>Media:</em> <td><a href="media.html#all-media-group" class="noxref">all</a>
<tr valign=baseline><td><em>Computed value:</em> <td>see text
</table>
</dl>
</div>
<p>The values of this property have the following meanings:</p>
<dl>
<dt><span class="index-def" title="'block', definition of"><a
name="value-def-block"><strong>block</strong></a></span>
</dt>
<dd>This value causes an element to generate a block box.
</dd>
<dt><span class="index-def" title="'inline-block', definition of"><a
name="value-def-inline-block"><strong>inline-block</strong></a></span>
</dt>
<dd>This value causes an element to generate a block box, which itself
is flowed as a single inline box, similar to a replaced element.
The inside of an inline-block is formatted as a block box, and the
element itself is formatted as an inline replaced element.
</dd>
<dt><span class="index-def" title="'inline', definition of"><a
name="value-def-inline"><strong>inline</strong></a></span>
</dt>
<dd>This value causes an element to generate one or more inline boxes.
</dd>
<dt><span class="index-def" title="'list-item', definition of"><a
name="value-def-list-item"><strong>list-item</strong></a></span>
</dt>
<dd>This value causes an element (e.g., LI in HTML) to generate a
9E70
principal block box and a list-item inline box. For information about
lists and examples of list formatting, please consult the section on
<a href="generate.html#lists">lists</a>.
</dd>
<dt><strong>none</strong></dt>
<dd><a name="x17"><span class="index-def" title="'none'::as display value">This
value</span></a> causes an element to not appear in the <a
href="intro.html#formatting-structure">formatting structure</a> (i.e.,
in visual media the element generates no boxes and has no effect on
layout). Descendant elements do not generate any boxes either; the
element and its content are removed from the formatting structure
entirely. This behavior <strong>cannot</strong> be overridden by
setting the <a href="visuren.html#propdef-display" class="noxref"><span class="propinst-display">'display'</span></a> property
on the descendants.
<p>Please note that a display of 'none' does not create an invisible
box; it creates no box at all. CSS includes mechanisms that enable an
element to generate boxes in the formatting structure that affect
formatting but are not visible themselves. Please consult the section
on <a href="visufx.html#visibility">visibility</a> for details.
</p>
</dd>
<dt><span class="index-def" title="'run-in', definition of"><a
name="value-def-run-in"><strong>run-in</strong></a></span>
</dt>
<dd>This value creates either block or
inline boxes, depending on context.
Properties apply to run-in boxes based on their
final status (inline-level or block-level).
</dd>
<dt>
<a href="tables.html#value-def-table" class="noxref"><span class="value-inst-table">table</span></a>,
<a href="tables.html#value-def-inline-table" class="noxref"><span class="value-inst-inline-table">inline-table</span></a>,
<a href="tables.html#value-def-table-row-group" class="noxref"><span class="value-inst-table-row-group">table-row-group</span></a>,
<a href="tables.html#value-def-table-column" class="noxref"><span class="value-inst-table-column">table-column</span></a>,
<a href="tables.html#value-def-table-column-group" class="noxref"><span class="value-inst-table-column-group">table-column-group</span></a>,
<a href="tables.html#value-def-table-header-group" class="noxref"><span class="value-inst-table-header-group">table-header-group</span></a>,
<a href="tables.html#value-def-table-footer-group" class="noxref"><span class="value-inst-table-footer-group">table-footer-group</span></a>,
<a href="tables.html#value-def-table-row" class="noxref"><span class="value-inst-table-row">table-row</span></a>,
<a href="tables.html#value-def-table-cell" class="noxref"><span class="value-inst-table-cell">table-cell</span></a>, and
<a href="tables.html#value-def-table-caption" class="noxref"><span class="value-inst-table-caption">table-caption</span></a>
</dt>
<dd>These values cause an element to behave like a table element
(subject to restrictions described in the chapter on <a
href="tables.html">tables</a>).
</dd>
</dl>
<p>The computed value is the same as the specified value, except for
positioned and floating elements (see <a
href="#dis-pos-flo">Relationships between 'display', 'position', and
'float'</a>) and for the root element.
For the root element, the computed value is changed as described in
the section on the <a href="#dis-pos-flo">relationships between 'display', 'position', and 'float'</a>.
</p>
<p>Note that although the <a href="about.html#initial-value">initial
value</a> of <a href="visuren.html#propdef-display" class="noxref"><span class="propinst-display">'display'</span></a> is
'inline', rules in the user agent's <a
href="cascade.html#default-style-sheet">default style sheet</a> may <a
href="cascade.html">override</a> this value. See the <a
href="sample.html">sample style sheet</a> for HTML 4 in the
appendix.
</p>
<div class="example"><P style="display:none">Example(s):</P>
<p>Here are some examples of the <a href="visuren.html#propdef-display" class="noxref"><span
class="propinst-display">'display'</span></a> property:
</p>
<pre><code class="css">
p { display: block }
em { display: inline }
li { display: list-item }
img { display: none } /* Do Not display images */
</code></pre>
</div>
<h2>9.3 <a name="positioning-scheme">Positioning schemes</a></h2>
<p>In CSS 2.1, a box may be laid out according to three <a name="x19"><span
class="index-def" title="positioning scheme"><dfn>positioning
schemes:</dfn></span></a></p>
<ol>
<li><a href="#normal-flow">Normal flow</a>. In CSS 2.1, normal
flow includes <a href="#block-formatting">block formatting</a>
of <a href="#block-box">block</a> boxes,
<a href="#inline-formatting">inline formatting</a>
of <a href="#inline-box">inline </a> boxes, <a
href="#relative-positioning">relative positioning</a> of
block or inline boxes, and positioning of
<a href="#run-in">run-in</a> boxes.
</li>
<li><a href="#floats">Floats</a>. In the float model,
a box is first laid out according to the normal flow, then
taken out of the flow and shifted
to the left or right as far as possible. Content may
flow along the side of a float.
</li>
<li><a href="#absolute-positioning">Absolute positioning</a>.
In the absolute positioning model, a box is removed from
the normal flow entirely (it has no impact on later siblings)
and assigned a position with respect to a containing block.
</li>
</ol>
<div class="note">
<em><strong>Note.</strong>
CSS 2.1's positioning schemes help authors make their documents
more accessible by allowing them to avoid mark-up tricks
(e.g., invisible images) used for layout effects.
</em>
</div>
<h3>9.3.1 <a name="choose-position">Choosing a positioning scheme:</a> <a href="visuren.html#propdef-position" class="noxref"><span
class="propinst-position">'position'</span></a> property</h3>
<p>The <a href="visuren.html#propdef-position" class="noxref"><span class="propinst-position">'position'</span></a> and <a href="visuren.html#propdef-float" class="noxref"><span
class="propinst-float">'float'</span></a> properties determine which
of the CSS 2.1 positioning algorithms is used to calculate
the position of a box.
</p>
<div class="propdef">
<dl><dt>
<span class="index-def" title="'position'"><a name="propdef-position" class="propdef-title"><strong>'position'</strong></a></span>
<dd>
<table class="propinfo" cellspacing=0 cellpadding=0>
<tr valign=baseline><td><em>Value:</em> <td>static | relative | absolute | fixed | <a href="cascade.html#value-def-inherit" class="noxref"><span class="value-inst-inherit">inherit</span></a>
<tr valign=baseline><td><em>Initial:</em> <td>static
<tr valign=baseline><td><em>Applies to:</em> <td>all elements
<tr valign=baseline><td><em>Inherited:</em> <td>no
<tr valign=baseline><td><em>Percentages:</em> <td>N/A
<tr valign=baseline><td><em>Media:</em> <td><a href="media.html#visual-media-group" class="noxref">visual</a>
<tr valign=baseline><td><em>Computed value:</em> <td>as specified
</table>
</dl>
</div>
<p>The values of this property have the following meanings:</p>
<dl>
<dt><strong>static</strong></dt>
<dd>The box is a normal box, laid out according to the <a
href="#normal-flow">normal flow</a>. The
<a href="visuren.html#propdef-top" class="noxref"><span class="propinst-top">'top'</span></a>,
<a href="visuren.html#propdef-right" class="noxref"><span class="propinst-right">'right'</span></a>,
<a href="visuren.html#propdef-bottom" class="noxref"><span class="propinst-bottom">'bottom'</span></a>,
and <a href="visuren.html#propdef-left" class="noxref"><span class="propinst-left">'left'</span></a>
properties do not apply.
</dd>
<dt><strong>relative</strong></dt>
<dd>The box's position is calculated according to the <a
href="#normal-flow">normal flow</a> (this is called the position in
normal flow). Then the box is offset <a
href="#relative-positioning">relative</a> to its normal position. When
a box B is relatively positioned, the position of the following box is
calculated as though B were not offset. The effect of 'position:relative' on table-row-group, table-header-group, table-footer-group, table-row, table-column-group, table-column, table-cell, and table-caption elements is undefined.
</dd>
<dt><strong>absolute</strong></dt>
<dd> The box's position (and possibly size) is specified
with the <a href="visuren.html#propdef-top" class="noxref"><span class="propinst-top">'top'</span></a>,
<a href="visuren.html#propdef-right" class="noxref"><span class="propinst-right">'right'</span></a>,
<a href="visuren.html#propdef-bottom" class="noxref"><span class="propinst-bottom">'bottom'</span></a>, and
<a href="visuren.html#propdef-left" class="noxref"><span class="propinst-left">'left'</span></a>
properties.
These properties specify offsets with respect to the box's
<a href="#containing-block">containing block</a>. Absolutely
positioned boxes are taken out of the normal flow. This means
they have no impact on the layout of later siblings. Also,
though <a href="#absolutely-positioned">absolutely positioned</a>
boxes have margins, they
do not <a href="box.html#collapsing-margins">collapse</a>
with any other margins.
</dd>
<dt><strong>fixed</strong></dt>
<dd> The box's position is calculated according to the 'absolute'
model, but in addition, the box is <a
href="#fixed-positioning">fixed</a> with respect to some reference.
As with the 'absolute' model, the box's margins do not collapse with any other margins.
In the case of handheld, projection, screen, tty, and tv media types,
the box is fixed with respect to the <a href="#viewport">viewport</a>
and does not move when
scrolled. In the case of the print media type, the box is rendered on every page, and is fixed with respect to the page box, even if the page is seen through a <a href="#viewport">viewport</a>
(in the case of a print-preview, for example). For other media
types, the presentation is undefined.
Authors may wish to specify 'fixed' in a
media-dependent way. For instance, an author may want a box to remain
at the top of the <a href="#viewport">viewport</a> on the screen, but
not at the top of each printed page. The two specifications may be
separated by using an <a href="media.html#at-media-rule">@media
rule</a>, as in:
<div class="example"><P style="display:none">Example(s):</P>
<pre><code class="css">
@media screen {
h1#first { position: fixed }
}
@media print {
h1#first { position: static }
}
</code></pre>
</div>
<p>UAs must not paginate the content of fixed boxes. <span
class="note">Note that UAs may print invisible content in other
ways. See <a href="page.html#outside-page-box">"Content outside the
page box"</a> in chapter 13.</span>
</dd>
</dl>
<p>
User agents may treat position as 'static' on the root element.
</p>
<h3>9.3.2 <a name="position-props">Box offsets</a>: <a href="visuren.html#propdef-top" class="noxref"><span
class="propinst-top">'top'</span></a>, <a href="visuren.html#propdef-right" class="noxref"><span
class="propinst-right">'right'</span></a>, <a href="visuren.html#propdef-bottom" class="noxref"><span
class="propinst-bottom">'bottom'</span></a>, <a href="visuren.html#propdef-left" class="noxref"><span
class="propinst-left">'left'</span></a></h3>
<p>An element is said to be <span class="index-def" title="positioned
element/box"><a name="positioned-element"><dfn>positioned</dfn></a></span>
if its <a href="visuren.html#propdef-position" class="noxref"><span class="propinst-position">'position'</span></a> property has
a value other than 'static'. Positioned elements generate
positioned boxes, laid out according to four properties:</p>
<div class="propdef">
<dl><dt>
<span class="index-def" title="'top'"><a name="propdef-top" class="propdef-title"><strong>'top'</strong></a></span>
<dd>
<table class="propinfo" cellspacing=0 cellpadding=0>
<tr valign=baseline><td><em>Value:</em>&nb
8096
sp; <td><a href="syndata.html#value-def-length" class="noxref"><span class="value-inst-length"><length></span></a> | <a href="syndata.html#value-def-percentage" class="noxref"><span class="value-inst-percentage"><percentage></span></a> | auto | <a href="cascade.html#value-def-inherit" class="noxref"><span class="value-inst-inherit">inherit</span></a>
<tr valign=baseline><td><em>Initial:</em> <td>auto
<tr valign=baseline><td><em>Applies to:</em> <td>positioned elements
<tr valign=baseline><td><em>Inherited:</em> <td>no
<tr valign=baseline><td><em>Percentages:</em> <td>refer to height of containing block
<tr valign=baseline><td><em>Media:</em> <td><a href="media.html#visual-media-group" class="noxref">visual</a>
<tr valign=baseline><td><em>Computed value:</em> <td>for 'position:relative', see section
<a href="visuren.html#relative-positioning">Relative
Positioning.</a> For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise, 'auto'.
</table>
</dl>
</div>
<p>This property specifies how far an <a
href="#absolutely-positioned">absolutely positioned</a> box's top
margin edge is offset below the top edge of the box's <a
href="#containing-block">containing block</a>. For relatively
positioned boxes, the offset is with respect to the top edges of the
box itself (i.e., the box is given a position in the normal flow, then
offset from that position according to these properties).
</p>
<div class="propdef">
<dl><dt>
<span class="index-def" title="'right'"><a name="propdef-right" class="propdef-title"><strong>'right'</strong></a></span>
<dd>
<table class="propinfo" cellspacing=0 cellpadding=0>
<tr valign=baseline><td><em>Value:</em> <td><a href="syndata.html#value-def-length" class="noxref"><span class="value-inst-length"><length></span></a> | <a href="syndata.html#value-def-percentage" class="noxref"><span class="value-inst-percentage"><percentage></span></a> | auto | <a href="cascade.html#value-def-inherit" class="noxref"><span class="value-inst-inherit">inherit</span></a>
<tr valign=baseline><td><em>Initial:</em> <td>auto
<tr valign=baseline><td><em>Applies to:</em> <td>positioned elements
<tr valign=baseline><td><em>Inherited:</em> <td>no
<tr valign=baseline><td><em>Percentages:</em> <td>refer to width of containing block
<tr valign=baseline><td><em>Media:</em> <td><a href="media.html#visual-media-group" class="noxref">visual</a>
<tr valign=baseline><td><em>Computed value:</em> <td>for 'position:relative', see section
<a href="visuren.html#relative-positioning">Relative
Positioning.</a> For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise,
'auto'.
</table>
</dl>
</div>
<p>Like 'top', but specifies how far a box's right margin edge is
offset to the left of the right edge of the box's <a
href="#containing-block">containing block</a>. For relatively
positioned boxes, the offset is with respect to the right edge of the
box itself.
</p>
<div class="propdef">
<dl><dt>
<span class="index-def" title="'bottom'"><a name="propdef-bottom" class="propdef-title"><strong>'bottom'</strong></a></span>
<dd>
<table class="propinfo" cellspacing=0 cellpadding=0>
<tr valign=baseline><td><em>Value:</em> <td><a href="syndata.html#value-def-length" class="noxref"><span class="value-inst-length"><length></span></a> | <a href="syndata.html#value-def-percentage" class="noxref"><span class="value-inst-percentage"><percentage></span></a> | auto | <a href="cascade.html#value-def-inherit" class="noxref"><span class="value-inst-inherit">inherit</span></a>
<tr valign=baseline><td><em>Initial:</em> <td>auto
<tr valign=baseline><td><em>Applies to:</em> <td>positioned elements
<tr valign=baseline><td><em>Inherited:</em> <td>no
<tr valign=baseline><td><em>Percentages:</em> <td>refer to height of containing block
<tr valign=baseline><td><em>Media:</em> <td><a href="media.html#visual-media-group" class="noxref">visual</a>
<tr valign=baseline><td><em>Computed value:</em> <td>for 'position:relative', see section
<a href="visuren.html#relative-positioning">Relative
Positioning.</a> For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise, 'auto'.
</table>
</dl>
</div>
<p>Like 'top', but specifies how far a box's bottom margin edge is
offset above the bottom of the box's <a
href="#containing-block">containing block</a>. For relatively
positioned boxes, the offset is with respect to the bottom edge of the
box itself.
</p>
<div class="propdef">
<dl><dt>
<span class="index-def" title="'left'"><a name="propdef-left" class="propdef-title"><strong>'left'</strong></a></span>
<dd>
<table class="propinfo" cellspacing=0 cellpadding=0>
<tr valign=baseline><td><em>Value:</em> <td><a href="syndata.html#value-def-length" class="noxref"><span class="value-inst-length"><length></span></a> | <a href="syndata.html#value-def-percentage" class="noxref"><span class="value-inst-percentage"><percentage></span></a> | auto | <a href="cascade.html#value-def-inherit" class="noxref"><span class="value-inst-inherit">inherit</span></a>
<tr valign=baseline><td><em>Initial:</em> <td>auto
<tr valign=baseline><td><em>Applies to:</em> <td>positioned elements
<tr valign=baseline><td><em>Inherited:</em> <td>no
<tr valign=baseline><td><em>Percentages:</em> <td>refer to width of containing block
<tr valign=baseline><td><em>Media:</em> <td><a href="media.html#visual-media-group" class="noxref">visual</a>
<tr valign=baseline><td><em>Computed value:</em> <td>for 'position:relative', see section
<a href="visuren.html#relative-positioning">Relative
Positioning.</a> For 'position:static', 'auto'. Otherwise: if
specified as a length, the corresponding absolute length; if
specified as a percentage, the specified value; otherwise,
'auto'.
</table>
</dl>
</div>
<p>Like 'top', but specifies how far a box's left margin edge is
offset to the right of the left edge of the box's <a
href="#containing-block">containing block</a>. For relatively
positioned boxes, the offset is with respect to the left edge of the
box itself.
</p>
<p>The values for the four properties have the following meanings:</p>
<dl>
<dt><strong><a href="syndata.html#value-def-length" class="noxref"><span class="value-inst-length"><length></span></a></strong></dt>
<dd>The offset is a fixed distance from the reference edge. Negative values are allowed.
</dd>
<dt><strong><a href="syndata.html#value-def-percentage" class="noxref"><span class="value-inst-percentage"><percentage></span></a></strong></dt>
<dd>The offset is a percentage of the containing block's width (for <a href="visuren.html#propdef-left" class="noxref"><span class="propinst-left">'left'</span></a> or <a href="visuren.html#propdef-right" class="noxref"><span
class="propinst-right">'right'</span></a>) or height (for <a href="visuren.html#propdef-top" class="noxref"><span
class="propinst-top">'top'</span></a> and <a href="visuren.html#propdef-bottom" class="noxref"><span
class="propinst-bottom">'bottom'</span></a>). Negative values are allowed.
</dd>
<dt><strong>auto</strong></dt>
<dd>For non-replaced elements, the effect of this value
depends on which of related properties have the value 'auto' as
well. See the sections on the
<a href="visudet.html#abs-non-replaced-width">width</a>
and <a href="visudet.html#abs-non-replaced-height">height</a>
of <a href="#absolutely-positioned">absolutely positioned</a>,
non-replaced elements for details. For replaced elements, the
effect of this value depends only on the intrinsic dimensions of the
replaced content. See the sections on the <a
href="visudet.html#abs-replaced-width">width</a> and <a
href="visudet.html#abs-replaced-height">height</a> of absolutely
positioned, replaced elements for details.
</dd>
</dl>
<h2>9.4 <a name="normal-flow">Normal flow</a></h2>
<p>Boxes in the normal flow belong to a <a name="x26"><span class="index-def"
title="formatting context">formatting context</span></a>, which may be
block or inline, but not both simultaneously. <a
href="#block-box">Block</a> boxes participate in a <a
href="#block-formatting">block formatting</a> context. <a
href="#inline-box">Inline boxes</a> participate in an <a
href="#inline-formatting">inline formatting</a> context.
</p>
<h3>9.4.1 <a name="block-formatting">Block formatting contexts</a></h3>
<p>Floats, absolutely positioned elements, inline-blocks, table-cells,
table-captions, and elements with 'overflow' other than 'visible'
(except when that value has been propagated to the viewport) establish
new block formatting contexts.</p>
<p>In a block formatting context, boxes are laid out one after the
other, vertically, beginning at the top of a containing block. The
vertical distance between two sibling boxes is determined by the <a href="box.html#propdef-margin" class="noxref"><span
class="propinst-margin">'margin'</span></a> properties. Vertical margins
between adjacent block boxes in a block formatting context <a
href="box.html#collapsing-margins">collapse</a>.
</p>
<p>
In a block formatting context, each box's left outer edge touches the
left edge of the containing block (for right-to-left formatting, right
edges touch). This is true even in the presence of floats (although a
box's <em>line boxes</em> may shrink due to the floats), unless the box
establishes a new block formatting context (in which case the box itself
<a href="#bfc-next-to-float"><em>may</em> become narrower</a> due to
the floats).
</p>
<p>For information about page breaks in paged media, please consult
the section on <a href="page.html#allowed-page-breaks">allowed
page breaks</a>.
</p>
<h3>9.4.2 <a name="inline-formatting">Inline formatting context</a></h3>
<p>In an inline formatting context, boxes are laid out horizontally,
one after the other, beginning at the top of a containing
block. Horizontal margins, borders, and padding are respected between
these boxes. The boxes may be aligned vertically in different ways: their
bottoms or tops may be aligned, or the baselines of text within them
may be aligned. The rectangular area that contains the boxes that form
a line is called a <span class="index-def" title="line box"><a
name="line-box"><dfn>line box</dfn></a></span>.
</p>
<p>The width of a line box is determined by a <a
href="#containing-block">containing block</a> and the presence of floats.
The height of a line
box is determined by the rules given in the section on <a
href="visudet.html#line-height">line height calculations</a>.
</p>
<p>A line box is always tall enough for all of the boxes it contains.
However, it may be taller than the tallest box it contains
(if, for example, boxes are aligned so that baselines line up).
When the height of a box B is less than the height of the line box containing it,
the vertical alignment of B within the line box is determined by
the <a href="visudet.html#propdef-vertical-align" class="noxref"><span class="propinst-vertical-align">'vertical-align'</span></a> property.
When several inline boxes cannot fit horizontally within a single
line box, they are distributed among two or more vertically-stacked
line boxes. Thus, a paragraph is a vertical stack of line boxes. Line
boxes are stacked with no vertical separation and they never overlap.
</p>
<p>In general, the left edge of a line box touches the left edge
of its containing block and the right edge touches the right edge of
its containing block. However, floating boxes may come between the
containing block edge and the line box edge. Thus, although line
boxes in the same inline formatting context generally have the same
width (that of the containing block), they may vary in width if
available horizontal space is reduced due to <a
href="#floats">floats</a>. Line boxes in the same inline formatting
context generally vary in height (e.g., one line might contain a tall
image while the others contain only text).
</p>
<p>When the total width of the inline boxes on a line is less than the
width of the line box containing them, their horizontal distribution
within the line box is determined by the <a href="text.html#propdef-text-align" class="noxref"><span
class="propinst-text-align">'text-align'</span></a> property. If that
property has the value 'justify', the user agent may stretch spaces
and words in inline boxes (except for inline-table and inline-block
boxes) as well.
</p>
<p>
When an inline box exceeds the width of a line box, it is split into several boxes and these boxes are distributed across several line boxes. If an inline box cannot be split (e.g., if the inline box contains a single character, or language specific word breaking rules disallow a break within the inline box, or if the inline box is affected by a white-space value of nowrap or pre), then the inline box overflows the line box.
</p>
<p>
When an inline box is split, margins,
borders, and padding have no visual effect where the split occurs (or
at any split, when there are several).
</p>
<p>Inline boxes may also be split into several boxes <em>within the
same line box</em> due to <a href="#direction">bidirectional text
processing</a>.
</p>
<p>Line boxes that contain no text, no preserved white space, no
inline elements with non-zero margins, padding, or borders, and no
other in-flow content (such as images, inline blocks or inline
tables), and do not end with a line feed must be treated as zero-height
line boxes. For the purposes of margin collapsing, this line box must
be ignored.
</p>
<div class="html-example"><p>
Here is an example of inline box construction. The following paragraph
(created by the HTML block-level element P) contains anonymous text
interspersed with the elements EM and STRONG:
</p>
<pre><code class="html">
<P>Several <EM>emphasized words</EM> appear
<STRONG>in this</STRONG> sentence, dear.</P>
</code></pre>
<p>The P element generates a block box that contains five inline
boxes, three of which are anonymous:</p>
<ul>
<li>Anonymous: "Several"</li>
<li>EM: "emphasized words"</li>
<li>Anonymous: "appear"</li>
<li>STRONG: "in this"</li>
<li>Anonymous: "sentence, dear."</li>
</ul>
<p>To format the paragraph, the user agent flows the five boxes into
line boxes. In this example, the box generated for the P element
establishes the containing block for the line boxes. If the containing
block is sufficiently wide, all the inline boxes will fit into a
single line box:
</p>
<pre>
Several <em>emphasized words</em> appear <strong>in this</strong> sentence, dear.
</pre>