csswg-drafts/css2/visuren.src at 2a2478ec02ebfe5cc0129f0fd7692fee99a96c37 · w3c/csswg-drafts · GitHub

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html lang="en">
<!-- $Id: visuren.src,v 2.56 1998-04-30 06:06:09 ijacobs Exp $ -->
<HEAD>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<TITLE>Visual formatting model</TITLE>
</HEAD>
<BODY>
<H1 align="center">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>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><a href="#positioning-scheme">positioning scheme</a>
(normal flow, float, and absolute).
<li>relationships between elements
in the <a href="conform.html#doctree">document tree.</a>
<li>external information (e.g., viewport size, intrinsic
dimensions of images, etc.).
</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>. 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-model">page model</a> for details).

<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.

<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="#initial-containing-block">initial containing block</a>).  When
the viewport is smaller than the document's initial containing block,
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).


<H3><span class="index-def" title="containing block">
<a name="containing-block">Containing blocks</a></span>
</H3>

<P>In CSS2, 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>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>The root of the <a href="conform.html#doctree">document tree</a>
generates a box that serves as the <span class="index-def"
title="initial containing block|containing block::initial"><a
name="initial-containing-block"><dfn>initial containing
block</dfn></a></span> for subsequent layout.

<P>The width of the initial containing block may be specified with the
<span class="propinst-width">'width'</span> property for the root
element. If this property has the value 'auto', the user agent
supplies the initial width (e.g., the user agent uses the current
width of the <a href="#viewport">viewport</a>).

<P>The height of the initial containing block may be specified with
the <span class="propinst-height">'height'</span> property for the
root element. If this property has the value 'auto', the containing
block height will grow to accommodate the document's content.

<P>The initial containing block cannot be positioned or floated (i.e.,
user agents <a href="syndata.html#ignore">ignore</a> the <span
class="propinst-position">'position'</span> <span
class="propinst-float">'float'</span> properties for the root
element).

<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>.

<H2><a name="box-gen">Controlling box generation</a></h2>

<P>The following sections describe the types of boxes that may be
generated in CSS2. 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.

<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></
77EC
a></span> are those elements of
the source document language 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', 'compact' and 'run-in' (part of the
time; see <a href="#compact">compact</a> and <a href="#run-in">run-in boxes</a>),
and 'table'.

<P>Block-level elements generate a <span class="index-def"
title="principal block box"><a name="principal-box"><dfn>principal
block box</dfn></a></span> that only contains <span class="index-def"
title="block box"><a name="block-box"><dfn>block
boxes</dfn></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.  Sibling principal block boxes participate in a <a
href="#block-formatting">block formatting context</a>.

<P>Some block-level elements generate additional boxes outside of the
principal box: 'list-item' elements and those with markers. These
additional boxes follow the principal box when it is
<a href="#positioned-element">positioned</a>.

<h4><a name="anonymous-block-level">Anonymous block boxes</a></h4>

<p>In a document like this:

<pre>
&lt;DIV>
  Some text
  &lt;P>Some more text
&lt;/DIV>
</pre>

(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".

<div class="figure">
<p><img src="images/anon-block.gif" alt="diagram showing the three
boxes for the example above"> <p class="caption">Diagram showing the
three boxes, of which one is anonymous, for the example above
</div>

<p>In other words: if a block box (such as that generated for
the DIV above) has another block box inside it (such as the P
above), then we force it to have <em>only</em> block boxes
inside it, by wrapping any inline boxes in an anonymous block box.

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

<PRE>
/* Note: HTML UAs may not respect these rules */
BODY { display: inline }
P    { display: block }
</PRE>

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

<PRE class="html-example">
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//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;/&gt;
<em>This is anonymous text after the P.</em>
&lt;/BODY&gt;
</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 for BODY,
containing an anonymous block box around C1, the P block box, and
another anonymous block box around C2.
</div>

<p>The properties of anonymous boxes are inherited from the
enclosing non-anonymous box (in the example: 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.

<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',
'compact' and 'run-in' (part of the time; see <a
href="#compact">compact</a> and <a href="#run-in">run-in boxes</a>).
Inline-level elements generate <a name="inline-box">inline boxes</a>.

<P>Inline boxes may participate in several formatting contexts:</p>

<ul>
<li>Within a block box, an inline boxes participate in an <a
href="#inline-formatting">inline formatting context</a>.
<li>A <a href="#compact">compact</a> inline box
is given a position in the margin of a block box.
<li><a href="generate.html#markers">Marker</a> inline boxes are
also given positions outside of a block box.
</ul>

<h4><a name="anonymous">Anonymous inline boxes</a></h4>

<p>In a document like this:

<pre>
&lt;P>Some &lt;EM>emphasized&lt;/em> text
</pre>

<p>The P generates a block box, with several inline boxes inside
it. The box for "emphasized" is an inline box generated by an inline
element (EM), but the other boxes ("Some" and "text") are inline boxes
generated by a block-level element (P). 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>Such anonymous inline boxes inherit inheritable properties from
their block parent box. Non-inherited properties have their
initial value.

<p>In the example, the color of the anonymous initial boxes is
inherited from the P, but the background is transparent.

<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 his specification.

<p>There are more types of anonymous boxes in the <a
href="tables.html#anonymous-boxes">table context</a>.

<H3><a name="compact">Compact boxes</a></h3>

<P>A <span class="index-def" title="compact box"><a name="compact"><dfn>compact
box</dfn></a></span> behaves as follows:</p>

<ul>
<li>If a <a href="#block-box">block</a> box (that does not
float and is not absolutely positioned)
follows the compact box, the compact box is
formatted like a one-line inline box.
The resulting box width is compared to one of the side margins of
the block box.
The choice of left or right margin is determined
by the <span class="propinst-direction">'direction'</span> specified
for the element producing the <a href="#containing-block">
containing block</a> for the compact box and following box.
If the inline box width is less
than or equal to the margin, the inline box is given
a position in the margin as described immediately below.

<li>Otherwise, the compact box becomes a block box.
</ul>

<P>The compact box is given a position in the margin as follows: it is
outside (to the left or right) of the first <a href="#line-box">line
box</a> of the block, but it affects the calculation of that <a
href="visudet.html#box-height">line box's height</a>. The <span
class="propinst-vertical-align">'vertical-align'</span> property of
the compact box determines the vertical position of the compact box
relative to that line box. The horizontal position of the compact box
is always in the margin of the block box.

<P>An element that cannot be formatted on one line cannot be placed in
the margin of the following block. For example, a 'compact' element in
HTML that contains a BR element will always be formatted as a
block box (assuming the default style for BR, which inserts a
newline). For placing multi-line texts in the margin, the <span
class="propinst-float">'float'</span> property is often more appropriate.

<div class="html-example">
<p>The following example illustrates a compact box.

<pre>
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;A compact box example&lt;/TITLE&gt;
    &lt;STYLE type="text/css"&gt;
      DT { display: compact }
      DD { margin-left: 4em }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;DL&gt;
      &lt;DT&gt;Short
        &lt;DD&gt;&lt;P&gt;Description goes here.
      &lt;DT&gt;too long for the margin
        &lt;DD&gt;&lt;P&gt;Description goes here.
    &lt;/DL&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</pre>
</div>

<P>This example might be formatted as:

<pre>
<b>short</b>    Description goes here

<b>too long for the margin</b>
         Description goes here
</pre>

<p>The <span class="propinst-text-align">'text-align'</span> property
can be used to align the compact element inside the margin: against
the left edge of the margin ('left'), against the right edge
('right'), or centered in the margin ('center'). The value 'justify'
doesn't apply, and is handled as either 'left' or 'right', depending
on the <span class="propinst-direction">'direction'</span> of the
block-level element in whose margin the compact element is
formatted. ('left' if the direction is 'ltr', 'right' if it is 'rtl'.)

<P>Please consult the section on
<a href="generate.html#compact-run-in-gen">generated content</a>
for information about how compact boxes interact with generated
content.

<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>

<ul>
<li>If a <a href
133E
="#block-box">block</a>
box (that does not float and is not
absolutely positioned) follows the run-in
box, the run-in box becomes the first inline box of the block box.
<li>Otherwise, the run-in box becomes a block box.
</ul>

<p>A 'run-in' box is useful for run-in headers, as
in this example:

<div class="html-example">
<P>
<pre>
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//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;/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;
</pre>
</div>

<p>This example might be formatted as:

<pre>
  <b>A run-in heading.</b> And a
  paragraph of text that
  follows it.
</pre>

<p>The properties of the run-in element are inherited from its
parent in the source tree, not from the block box it visually
becomes part of.

<P>Please consult the section on
<a href="generate.html#compact-run-in-gen">generated content</a>
for information about how run-in boxes interact with generated
content.

<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>

<dd>This value causes an element to generate a
principal block box.

<dt><span class="index-def" title="'inline', definition of"><a
name="value-def-inline"><strong>inline</strong></a></span>

<dd>This value causes an element to generate one or more inline boxes.

<dt><span class="index-def" title="'list-item', definition of"><a
name="value-def-list-item"><strong>list-item</strong></a></span>
<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>.

<dt><span class="index-def" title="marker', definition of"><a
name="value-def-marker"><strong>marker</strong></a></span>
<dd>This value declares <a href="generate.html">generated content</a>
before or after a box to be a marker. This value should only be
used with <a href="generate.html#before-after-content">:before and :after
pseudo-elements</a> attached to block-level elements.
In other cases, this value is interpreted as 'inline'.
Please consult the section on <a
href="generate.html#markers">markers</a> for more information.

<dt><strong>none</strong>

<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.

<dt><span class="index-def" title="'run-in', definition of"><a
name="value-def-run-in"><strong>run-in</strong></a></span>
and <span class="index-def" title="'compact', definition of"><a
name="value-def-compact"><strong>compact</strong></a></span>
<dd>These values create either block or
inline boxes, depending on context.
Properties apply to run-in and compact boxes based on their
final status (inline-level or block-level).  For example, the <span
class="propinst-white-space">'white-space'</span> property only
applies if the box becomes a block box.

<dt><span class="index-inst" title="'table'"><a
name="value-inst-table"><strong>table</strong></a></span>, <span
class="index-inst" title="'inline-table'"><a
name="value-inst-inline-table"><strong>inline-table</strong></a></span>, <span
class="index-inst" title="'table-row-group'"><a
name="value-inst-table-row-group"><strong>table-row-group</strong></a></span>,
<span class="index-inst" title="'table-column-group'"><a
name="value-inst-table-column-group"><strong>table-column-group</strong></a></span>,
<span class="index-inst" title="'table-header-group'"><a name="value-inst-table-header-group"><strong>table-header-group</strong></a></span>,
<span class="index-inst" title="'table-footer-group'"><a name="value-inst-table-footer-group"><strong>table-footer-group</strong></a></span>,
<span class="index-inst" title="'table-row'"><a
name="value-inst-table-row"><strong><strong>table-row</strong></strong></a></span>, <span
class="index-inst" title="'table-cell'"><a
name="value-inst-table-cell"><strong><strong>table-cell</strong></strong></a></span>, and <span
class="index-inst" title="'table-caption'"><a
name="value-inst-table-caption"><strong>table-caption</strong></a></span>
<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>).
</dl>

<P>Note that although the <a href="cascade.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 4.0 in the
appendix.

<div class="example"><P>
<P>Here are some examples of the <span
class="propinst-display">'display'</span> property:

<PRE>
P   { display: block }
EM  { display: inline }
LI  { display: list-item }
IMG { display: none }      /* Don't display images */
</PRE>
</div>

<P><span class="index-inst" title="conformance"><a
href="conform.html#conformance">Conforming</a></span> HTML user agents
may <span class="index-inst" title="ignore"><a
href="syndata.html#ignore">ignore</a></span> the <span
class="propinst-display">'display'</span> property when specified in
author and user style sheets but must specify a value for it in the <a
href="cascade.html#default-style-sheet">default style sheet</a>.

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

<P>In CSS2, 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 CSS2, 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="#compact">compact</a> and <a href="#run-in">run-in</a> boxes.
<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><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.
</ol>

<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 CSS2 positioning algorithms is used to calculate
the coordinates of a box.

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

<P>The values of this property have the following meanings:</P>

<dl>
<dt><strong>static</strong>

<dd>The box is a normal box, laid out according to the <a
href="#normal-flow">normal flow</a>. The <span
class="propinst-left">'left'</span> and <span
class="propinst-top">'top'</span> properties do not apply.
<strong>Note.</strong>The word
"static" may seem confusing&#8212;'fixed' is arguably more static than
'static'&#8212;but 'static' was chosen because the expectation is that it
will most often be used in scripts that "animate" elements by
modifying <span class="propinst-top">'top'</span>, <span
class="propinst-left">'left'</span>, <span
class="propinst-bottom">'bottom'</span>, and/or <span
class="propinst-right">'right'</span>.

<dt><strong>relative</strong>

<dd>The box coordinates are calculated according to the <a
href="#normal-flow">normal flow</a>, 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.

<dt><strong>absolute</strong>
<dd> The box coordinates (and possibly size) are specified
with the <span class="propinst-left">'left'</span>,
<span class="propinst-right">'right'</span>,
<span class="propinst-top">'top'</span>,
and <span class="propinst-bottom">'bottom'</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 absolutely positioned boxes have margins, they
do not <a href="box.html#collapsing-margins">collapse</a>
with any other margins.

<dt><strong>fixed</strong>
<dd> The box coordinates are calculated according to the 'absolute'
model, but in addition, the box is <a
href="#fixed-positioning">fixed</a> with respect to some reference. In
the case of <a href="media.html#continuous-media-group">continuous
media</a>, the box is fixed with respect to the <a
href="#viewport">viewport</A> (and doesn't move when scrolled).  In
the case of <a href="media.html#paged-media-group">paged media</a>,
the box is fixed with respect to the page, even if that page is seen
through a <a href="#viewport">viewport</A> (in the case of a
print-preview, for example).  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>
<PRE>
@media screen {
  H1#first { position: fixed }
}
@media print {
  H1#first { position: static }
}
</PRE>
</div>
</dl>

<P>An element is said to be <span class="index-def" title="positioned
element"><a name="positioned-element"><dfn>positioned</dfn></a></span>
if its <span class="propinst-position">'position'</span> property has
a value other than 'static'. A positioned element generates
"positioned boxes." A positioned box's containing block must be
established by a positioned box (see the <a
href="visudet.html#containing-block-details">containing block
details</a>).

<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>The position of a positioned box is established by four properties:

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

<P>This property specifies how far a box's top content edge is offset below
the top edge of the box's <a href="#containing-block">containing block</a>.

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

<P>This property specifies how far a box's right content edge is offset
to the left of the right edge of the box's <a
href="#containing-block">containing block</a>.

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

<P>This property specifies how far a box's bottom content edge is offset
above the bottom of the box's <a href="#containing-block">containing
block</a>.

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

<P>This property specifies how far a box's left content edge is offset
to the right of the left edge of the box's <a
href="#containing-block">containing block</a>.

<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>
<dd>The offset is a fixed distance from the reference edge.

<dt><strong><span class="value-inst-percentage">&lt;percentage&gt;</span></strong>

<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>). For 'top'
and 'bottom', if the height of the
containing block is not specified explicitly
(i.e., it depends on content height), the percentage value
is interpreted like 'auto'.
<dt><strong>auto</strong>
<dd>The effect of this value
depends on which of related properties have the value 'auto' as
well. See the sections
<a href="visudet.html#abs-non-replaced-width">"Absolutely positioned,
non-replaced elements"</a> and
<a href="visudet.html#abs-height">"Height of absolutely positioned
boxes"</a> for details.
</dl>

<P>For absolutely positioned boxes, the offsets are with respect to
the box's <a href="#containing-block">containing block</a>. For
relatively positioned boxes, the offsets are with respect to the outer
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).

<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.

<h3><a name="block-formatting">Block formatting context</a></h3>

<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>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>content</em> area may shrink due to the floats).

<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>.

<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. They 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>. The margin between the
first inline box in a line box and the near line box edge is
respected; the same holds for the last inline box and the opposite
line box edge.

<P>The width of a line box is determined by a <A
HREF="#containing-block">containing block</A>.  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>. 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 <span
class="propinst-vertical-align">'vertical-align'</span> property.

<P>When several inline boxes cannot fit 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>In left-to-right formatting, the left edge of a line box generally
touches the left edge of its containing block (right edges touch in
right-to-left formatting). 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>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>Since an inline box may not exceed the width of a line box, long
inline boxes are split into several boxes and these boxes distributed
across several line boxes. When an inline box is split, margins,
borders, and padding have no visual effect where the split occurs.
Formatting of margins, borders, and padding may not be fully defined if
the split occurs within a bidirectional embedding.

<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>.

<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:

<PRE>
&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;
</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>EM: "emphasized words"
<li>Anonymous: "appear"
<li>STRONG: "in this"
<li>Anonymous: "sentence, dear."
</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:

<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:

<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.

<div class="html-example"><P>
Consider the following example:

<PRE>
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//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;
</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.gif"
     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> 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.
</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>, 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.

<!-- The expression "normal flow shape" is confusing since,
in terms of line boxes, the shape is always rectangular -IJ -->

<p>Relatively positioned boxes keep their normal flow size, including
line breaks and the space originally reserved for them. A relatively
positioned box establishes a new a new <a
href="#containing-block">containing block</a> for normal
flow children and positioned descendants.

<P>A relatively positioned box is generated when the <span
class="propinst-position">'position'</span> property for an element
has the value 'relative'.  The offset is specified by the <span
class="propinst-top">'top'</span>, <span
class="propinst-bottom">'bottom'</span>, <span
class="propinst-left">'left'</span>, and <span
class="propinst-right">'right'</span> properties.

<P>Dynamic movement of relatively positioned boxes can produce
animation effects in scripting environments (see also the <span
class="propinst-visibility">'visibility'</span> property). Relative
positioning may also be used as a general form of superscripting and
subscripting except that 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>Examples of relative positioning are provided in the section <a
href="#comparison">comparing normal, relative, floating, and
absolute positioning</a>.

<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 floated 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>A floated box must have an explicit width (assigned via the <span
class="propinst-width">'width'</span> property, or its intrinsic width
in the case of replaced elements). Any floated box becomes a <a
href="#block-box">block box</a> that is shifted to the left or right
until its outer edge touches the containing block edge or the outer
edge of another float. The top of the floated box is aligned with the
top of the current line box (or bottom of the preceding block box if
no line box exists). If there isn't enough horizontal room on the
current line for the float, it is shifted downward, line by line,
until a line has room for it.

<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 the floated box.  Any content in the
current line before a floated box is reflowed in the first available
line on the other side of the float.

<P>Several floats may be adjacent, and this model also applies to
adjacent floats in the same line.

<div class="example"><P>
The following rule floats all IMG boxes with
<samp>class="icon"</samp> to the left (and
sets the left margin to '0'):</p>

<PRE>
IMG.icon {
  float: left;
  margin-left: 0;
}
</PRE>
</div>


<div class="html-example"><P>
Consider the following HTML source and style sheet:</p>

<PRE>
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Float example&lt;/TITLE&gt;
    &lt;STYLE type="text/css"&gt;
      IMG { float: left }
      BODY, P, IMG { margin: 2em }
    &lt;/STYLE&gt;
  &lt;/HEAD&
4871
gt;
  &lt;BODY&gt;
    &lt;P&gt;&lt;IMG src=img.gif alt="This image will illustrate floats"&gt;
       Some sample text that has no other...
  &lt;/BODY&gt;
&lt;/HTML&gt;
</PRE>

<P>The IMG box is floated to the left. The content that follows is
formatted to the right of the float, starting on the same line as the
float. The line boxes to the right of the float are shortened due to
the float's presence, but resume their "normal" width (that of the
containing block established by the P element) after the float.  This
document might be formatted as:</P>

<div class="figure">
<P><img src="./images/floateg.gif"
     alt="Image illustrating how floating boxes interact with
margins."></p>
</div>

<P>Formatting would have been exactly the same if the document had
been:

<PRE>
  &lt;BODY&gt;