csswg-drafts/css2/visuren.src at 59dc7c2436ba4193d224899283f65dba786f406e · xfq/csswg-drafts · GitHub

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<!-- $Id: visuren.src,v 2.122 2005-09-13 19:16:22 bbos Exp $ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Visual formatting model</title>
<meta name="editor" lang="tr" content="Tantek &Ccedil;elik">
<!-- Changed by: Tantek Celik, 2005-06-04 -->
<style type="text/css">
.current,.proposed,span.delcurrent { background:#feb }
ins.proposed,span.insproposed,li.insproposed { background:#bfb }
del.proposed,span.delproposed { background:#fbb }
span.insproposed,li.insproposed { text-decoration:underline }
span.delproposed,span.delcurrent { text-decoration:line-through }
body>del,body>ins {display:block}
</style>
</head>
<body>
<h1>Visual formatting model</h1>

<h2>Introduction to the visual formatting model</h2>

<p>This chapter and the next describe the <span class="index-def"
title="visual formatting model">visual formatting model</span>: 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>The viewport</h3>

<p>User agents for <a
href="media.html#continuous-media-group">continuous media</a>
generally offer users a <span class="index-def" title="viewport"><a
name="viewport"><dfn>viewport</dfn></a></span> (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><span class="index-def" title="containing block">
<a name="containing-block">Containing blocks</a></span>
</h3>

<p>In CSS&nbsp;2.1, many box positions and sizes are calculated with respect
to the edges
of a rectangular box called a <span class="index-def"
title="containing block"><dfn>containing block</dfn></span>.  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><a name="box-gen">Controlling box generation</a></h2>

<p>The following sections describe the types of boxes that may be
generated in CSS&nbsp;2.1. A box's type affects, in part, its behavior in the
visual formatting model. The <span
class="propinst-display">'display'</span> property, described below,
specifies a box's type.
</p>
<h3>Block-level elements and block boxes</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 <span
class="propinst-display">'display'</span> 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>block
box</dfn>es</a></span> or only <span class="index-def"
title="inline box"><a 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><a name="anonymous-block-level">Anonymous block boxes</a></h4>

<p>In a document like this:
</p>
<pre><code class="html">
&lt;DIV&gt;
  Some text
  &lt;P&gt;More text
&lt;/DIV&gt;
</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><span
class="index-def" title="anonymous">anonymous block box</span></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">
</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.
</p>

<div class="example">
<p>This model would apply in the following example if the following
rules:</p>

<pre><code class="css">
body { display: inline }
p    { display: block }
</code></pre>

<p>were used with this HTML document:</p>

<pre class="html-example"><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HEAD&gt;
&lt;TITLE&gt;Anonymous text interrupted by a block&lt;/TITLE&gt;
&lt;/HEAD&gt;
&lt;BODY&gt;
<em>This is anonymous text before the P.</em>
&lt;P&gt;This is the content of P.&lt;/P&gt;
<em>This is anonymous text after the P.</em>
&lt;/BODY&gt;
</code></pre>

<p>The BODY 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 BODY,
containing an anonymous block box around C1, the P 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 BODY 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&nbsp;2.1. This does not apply to UAs
developed after this specification was released.
</p>
<h3>Inline-level elements and inline boxes</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 <span
class="propinst-display">'display'</span> property make an element
inline: 'inline', 'inline-table',
 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><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">
&lt;p&gt;Some &lt;em&gt;emphasized&lt;/em&gt; text&lt;/p&gt;
</code></pre>
</div>

<p>The <code>&lt;p&gt;</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>&lt;em&gt;</code>), but the other boxes ("Some" and "text") are inline boxes generated by a block-level element (<code>&lt;p&gt;</code>). The latter are called <span
class="index-def" title="anonymous inline boxes">anonymous inline
boxes</span>, because they don't 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>
Whitespace content that would subsequently be collapsed away according to the <span class="propinst-white-space">'white-space'</span> 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><a name="run-in">Run-in boxes</a></h3>

<p>A <span class="index-def" title="run-in box"><dfn>run-in
box</dfn></span> behaves as follows:</p>

<ol>
<li>
If the run-in box contains a <a href="#block-box">block</a>
box, the run-in box becomes a block box.
</li>
<li>If a sibling <a href="#block-box">block</a>
box (that does not float and is not
<a href="#absolutely-positioned">absolutely positioned</a>) follows the run-in
box, the run-in box becomes the first inline box of the block box.
<!-- it may also be an anonymous block -->
A run-in cannot run in to a block that already starts with a
run-in or that itself is a run-in.
</li>
<li>Otherwise, the run-in box becomes a block box.</li>
</ol>
<!-- there can never be two run-ins in the same block -->
<!-- the intended effect is that the run-in starts a line and doesn't
end one -->

<p>A 'run-in' box is useful for run-in headers, as
in this example:
</p>
<div class="html-example">
<pre><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;A run-in box example&lt;/TITLE&gt;
    &lt;STYLE type="text/css"&gt;
      H3 { display: run-in }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;H3&gt;A run-in heading.&lt;/H3&gt;
    &lt;P&gt;And a paragraph of text that
       follows it.
  &lt;/BODY&gt;
&lt;/HTML&gt;
</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.
</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><a name="display-prop">The</a> <span
class="propinst-display">'display'</span> property</h3>

<!-- #include src=properties/display.srb -->

<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>
&l
F882
t;/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
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><span class="index-def" title="'none'::as display value">This
value</span> causes an element to generate <b>no</b> boxes in the <a
href="intro.html#formatting-structure">formatting structure</a> (i.e.,
the element has no effect on layout).  Descendant elements do not
generate any boxes either; this behavior <strong>cannot</strong> be
overridden by setting the <span
class="propinst-display">'display'</span> 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>
<span class="value-inst-table">table</span>,
<span class="value-inst-inline-table">inline-table</span>,
<span class="value-inst-table-row-group">table-row-group</span>,
<span class="value-inst-table-column">table-column</span>,
<span class="value-inst-table-column-group">table-column-group</span>,
<span class="value-inst-table-header-group">table-header-group</span>,
<span class="value-inst-table-footer-group">table-footer-group</span>,
<span class="value-inst-table-row">table-row</span>,
<span class="value-inst-table-cell">table-cell</span>, and
<span class="value-inst-table-caption">table-caption</span>
</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 as follows: 'inline-table' and 'table' become
'table', 'none' stays 'none', everything else becomes 'block'.
-->
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 <span class="propinst-display">'display'</span> 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>Here are some examples of the <span
class="propinst-display">'display'</span> property:
</p>
<pre><code class="css">
p   { display: block }
em  { display: inline }
li  { display: list-item }
img { display: none }      /* Don't display images */
</code></pre>
</div>


<h2><a name="positioning-scheme">Positioning schemes</a></h2>

<p>In CSS&nbsp;2.1, a box may be laid out according to three <span
class="index-def" title="positioning scheme"><dfn>positioning
schemes:</dfn></span></p>

<ol>
<li><a href="#normal-flow">Normal flow</a>. In CSS&nbsp;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&nbsp;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><a name="choose-position">Choosing a positioning scheme:</a> <span
class="propinst-position">'position'</span> property</h3>

<p>The <span class="propinst-position">'position'</span> and <span
class="propinst-float">'float'</span> properties determine which
of the CSS&nbsp;2.1 positioning algorithms is used to calculate
the position of a box.
</p>

<!-- #include src=properties/position.srb -->

<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
<span class="propinst-top">'top'</span>,
<span class="propinst-right">'right'</span>,
<span class="propinst-bottom">'bottom'</span>,
and <span class="propinst-left">'left'</span>
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 <span class="propinst-top">'top'</span>,
<span class="propinst-right">'right'</span>,
<span class="propinst-bottom">'bottom'</span>, and
<span class="propinst-left">'left'</span>
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 doesn't 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">
<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&nbsp;13.</span>
</dd>
</dl>
<p>
User agents may treat position as 'static' on the root element.
</p>

<h3><a name="position-props">Box offsets</a>: <span
class="propinst-top">'top'</span>, <span
class="propinst-right">'right'</span>, <span
class="propinst-bottom">'bottom'</span>, <span
class="propinst-left">'left'</span></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 <span class="propinst-position">'position'</span> property has
a value other than 'static'. Positioned elements generate
positioned boxes, laid out according to four properties:</p>

<!-- #include src=properties/top.srb -->

<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).
<span class="note">
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the <em>padding</em> edge of that element.
</p>

<!-- #include src=properties/right.srb -->

<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.
<span class="note">
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the <em>padding</em> edge of that element.
</span>
</p>

<!-- #include src=properties/bottom.srb -->

<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.
<span class="note">
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the <em>padding</em> edge of that element.
</span>
</p>

<!-- #include src=properties/left.srb -->

<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.
<span class="note">
Note: For absolutely positioned elements whose containing block is based on a block-level element, this property is an offset from the <em>padding</em> edge of that element.
</span>
</p>

<p>The values for the four properties have the following meanings:</p>

<dl>
<dt><strong><span class="value-inst-length">&lt;length&gt;</span></strong></dt>
<dd>The offset is a fixed distance from the reference edge. Negative values are allowed.
</dd>
<dt><strong><span class="value-inst-percentage">&lt;percentage&gt;</span></strong></dt>

<dd>The offset is a percentage of the containing block's width (for <span class="propinst-left">'left'</span> or <span
class="propinst-right">'right'</span>) or height (for <span
class="propinst-top">'top'</span> and <span
class="propinst-bottom">'bottom'</span>). 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><a name="normal-flow">Normal flow</a></h2>

<p>Boxes in the normal flow belong to a <span class="index-def"
title="formatting context">formatting context</span>, 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 id="block-formatting">Block formatting contexts</h3>

<p>Floats, absolutely positioned elements, inline-blocks, table-cells, and elements with 'overflow' other than 'visible' 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 <span
class="propinst-margin">'margin'</span> 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
<em>may</em> become narrower due to the floats).
</p>

<p>For information about page breaks in paged media, please consult
the section on <a href
9E70
="page.html#allowed-page-breaks">allowed
page breaks</a>.
</p>

<h3><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 'vertical-align' 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 <span
class="propinst-text-align">'text-align'</span> property.  If that
property has the value 'justify', the user agent may stretch the
inline 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>
<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">
&lt;P&gt;Several &lt;EM&gt;emphasized words&lt;/EM&gt; appear
&lt;STRONG&gt;in this&lt;/STRONG&gt; sentence, dear.&lt;/P&gt;
</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>

<p>If not, the inline boxes will be split up and distributed across
several line boxes. The previous paragraph might be split as follows:
</p>
<pre>
Several <em>emphasized words</em> appear
<strong>in this</strong> sentence, dear.
</pre>

or like this:

<pre>
Several <em>emphasized</em>
<em>words</em> appear <strong>in this</strong>
sentence, dear.
</pre>

</div>

<p>In the previous example, the EM box was split into two EM boxes
(call them "split1" and "split2"). Margins, borders,
padding, or text decorations have no visible effect after split1 or
before split2.
</p>
<div class="html-example"><p>
Consider the following example:
</p>
<pre><code class="html">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Example of inline flow on several lines&lt;/TITLE&gt;
    &lt;STYLE type="text/css"&gt;
      EM {
        padding: 2px;
        margin: 1em;
        border-width: medium;
        border-style: dashed;
        line-height: 2.4em;
      }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;P&gt;Several &lt;EM&gt;emphasized words&lt;/EM&gt; appear here.&lt;/P&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</code></pre>

<p>Depending on the width of the P, the boxes may be distributed as
follows:</p>

<div class="figure">
<p><img src="./images/inline-layout.png"
     alt="Image illustrating the effect of line breaking on the
display of margins, borders, and padding."></p>
</div>

<ul>
<li> The margin is inserted before "emphasized" and after "words". </li>

<li> The padding is inserted before, above, and below
"emphasized" and after, above, and below "words". A
dashed border is rendered on three sides in each case.</li>
</ul>
</div>

<h3><a name="relative-positioning">Relative positioning</a></h3>

<p>Once a box has been laid out according to the <a
href="#normal-flow">normal flow</a> or floated, it may be shifted relative to
this position. This is called <span class="index-def" title="relative
positioning"><dfn>relative positioning</dfn></span>. 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' box to have
overflow, the UA must allow the user to access this content, which,
through the creation of scrollbars, may affect layout.
</p>
<p>A relatively positioned box keeps its normal flow size, including
line breaks and the space originally reserved for it. The section on
<a href="#containing-block">containing blocks</a> explains when a
relatively positioned box establishes a new containing block.
</p>
<p>For relatively positioned elements, 'left' and 'right' move the
box(es) horizontally, without changing their size. 'left' moves the
boxes to the right, and 'right' moves them to the left. Since boxes
are not split or stretched as a result of 'left' or 'right', the
computed values are always: left = -right.
</p>
<p>If both 'left' and 'right' are 'auto' (their initial values), the
computed values are '0' (i.e., the boxes stay in their original
position).
</p>
<p>If 'left' is 'auto', its computed value is minus the value of 'right'
(i.e., the boxes move to the left by the value of 'right').
</p>
<p>If 'right' is specified as 'auto', its computed value is minus the
value of 'left'.
</p>
<p>If neither 'left' nor 'right' is 'auto', the position is
over-constrained, and one of them has to be ignored. If the 'direction' property of the containing block is 'ltr, the value of 'left' wins and 'right'
becomes -'left'. If 'direction' of the containing block is 'rtl', 'right' wins and 'left' is ignored.
</p>
<div class="example">
<p><strong>Example.</strong> The following three rules are equivalent:
</p>
<pre><code class="css">
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 'top' and 'bottom' properties move relatively positioned
element(s) up or down without changing their size.  'top' moves the
boxes down, and 'bottom' moves them up. Since boxes
are not split or stretched as a result of 'top' or 'bottom', the
computed values are always: top = -bottom.
If both are 'auto', their computed values are both '0'. If one of them is
'auto', it becomes the negative of the other. If neither is 'auto',
'bottom' is ignored (i.e., the computed value of 'bottom' will be
minus the value of 'top').
</p>

<p class="note">
Note.
Dynamic movement of relatively positioned boxes can produce
animation effects in scripting environments (see also the <span
class="propinst-visibility">'visibility'</span> property).
Although relative positioning may be used as a form of superscripting and
subscripting, the line height is not automatically adjusted to take the
positioning into consideration. See the description of <a
href="visudet.html#line-height">line height calculations</a> for more
information.
</p>
<p>Examples of relative positioning are provided in the section <a
href="#comparison">comparing normal flow, floats, and absolute
positioning</a>.
</p>
<h2><a name="floats">Floats</a></h2>

<p>A float is a box that is shifted to the left or right on the
current line. The most interesting characteristic of a float (or
"floated" or "floating" box) is that content may flow along its side
(or be prohibited from doing so by the <span
class="propinst-clear">'clear'</span> property). Content flows down
the right side of a left-floated box and down the left side of a
right-floated box. The following is an introduction to float
positioning and content flow; the exact <a
href="#float-rules">rules</a> governing float behavior are given in
the description of the <span class="propinst-float">'float'</span>
property.
</p>
<p>
A floated box is
shifted to the left or right until its outer edge touches the
containing block edge or the outer edge of another float.
If there's a line box, the top of the floated box is aligned with
the top of the current line box.
</p>
<p>
If there isn't enough horizontal room for the
float, it is shifted downward until either it fits or there are no
more floats present.
</p>
<p>Since a float is not in the flow, non-positioned block boxes
created before and after the float box flow vertically as if the float
didn't exist. However, line boxes created next to the float are
shortened to make room for margin box of the float.  If a shortened line box
is too small to contain any further content, then it is shifted downward until
either it fits or there are no more floats present.
Any content in the current line before a floated box is reflowed
in the first available line on the other side of the float.
In other words, if inline boxes are placed
on the line before a left float is encountered that fits in the remaining line box space,
the left float is placed on that line, aligned with the top of the line box, and then the inline boxes already on the line are moved accordingly to the right of the float (the right being the other side of the left float) and vice versa for rtl and right floats.
</p>
<p>
The margin box of a table, a block-level replaced element, or an element in the normal flow that establishes a new <a href="visuren.html#block-formatting">block formatting context</a>
(such as an element with 'overflow' other than 'visible')
must not overlap any floats in the same block formatting context as
the element itself. If necessary, implementations should clear the
said element by placing it below any preceding floats, but may place
it adjacent to such floats if there is sufficient space.
</p>
<div class="example">
<p><strong>Example.</strong>
In the following document fragment, the containing block is too narrow
to contain the content next to the float,
so the content gets moved to below the floats
where it is aligned in the line box according to the text-align
property.
</p>
<pre><code class="css">
p { width: 10em; border: solid aqua; }
span { float: left; width: 5em; height: 5em; border: solid blue; }
</code>

...

<code class="html">
&lt;p&gt;
  &lt;span&gt; &lt;/span&gt;
  Supercalifragilisticexpialidocious
&lt;/p&gt;
</code></pre>
<p>This fragment might look like this:</p>
<div class="figure"><p>
<img style="display:block" src="images/supercal.png"
 alt="Image illustrating the effect of an unbreakable piece of content
 being reflowed to just after a float which left insufficient room next to it