csswg-drafts/css21/visuren.src at 8cc9f8bc31a1a897a97a1d5bb8cfb9116660daba · w3c/csswg-drafts · GitHub

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<!-- $Id: visuren.src,v 2.195 2011-10-18 19:25:28 bbos Exp $ -->
<head>
<title>Visual formatting model</title>
<!--script src="http://www.w3c-test.org/css/harness/annotate.js#CSS21_DEV" type="text/javascript" defer></script-->
<style type="
77EC
;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}
blockquote.math {display: table}
blockquote.math p {display: table-row}
blockquote.math span {display: table-cell; padding-right: 0.3em}
blockquote.math span:first-child {text-align: right}
</style>
</head>
<body>
<h1>Visual formatting model</h1>

<h2><a name="visual-model-intro">Introduction to the visual formatting model</a></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><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 <span class="index-def" title="viewport">
<dfn>viewport</dfn></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><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). The following values of the <span
class="propinst-display">'display'</span> property make an element
block-level: 'block', 'list-item', and 'table'.
</p>

<p><span class="index-def" title="block-level box"><dfn>Block-level
boxes</dfn></span> are boxes that participate in
a <a href="#block-formatting">block formatting context.</a> Each
block-level element generates a <span class="index-def"
title="principal block-level
box"><a name="principal-box"><dfn>principal block-level
box</dfn></a></span> that contains descendant boxes and generated
content and is also the box involved in any positioning scheme.

Some block-level elements may generate additional boxes in addition
to the
principal box: 'list-item' elements. These additional boxes are
placed with respect to the principal box.

<p>Except for table boxes, which are described in a later chapter, and
replaced elements, a block-level box is also a block container box.
A <span class="index-def" title="block container box"><a
name="block-container-box"><dfn >block
container box</dfn></a></span> either
contains only block-level boxes or establishes an inline formatting
context and thus contains only inline-level boxes. Not all block
container boxes are block-level boxes: non-replaced inline blocks
and non-replaced table cells are block containers but not block-level
boxes. Block-level boxes that are also block containers are
called <span class="index-def" title="block box"><dfn>block
boxes</dfn></span>.

<p>The three terms "block-level box," "block container box," and
"block box" are sometimes abbreviated as "block" where unambiguous.

<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 container box (such as that generated
for the DIV above) has a block-level box inside it (such as the P
above), then we force it to have <em>only</em> block-level boxes
inside it.

<p>When an inline box contains an in-flow block-level box, the inline box
(and its inline ancestors within the same line box) are broken around
the block-level box (and any block-level siblings that are consecutive
or separated only by collapsible whitespace and/or out-of-flow
elements), splitting the inline box into two boxes (even if either
side is empty), one on each side of the block-level box(es). The line
boxes before the break and after the
break are enclosed in anonymous block boxes, and the block-level box
becomes a sibling of those anonymous boxes. When such an inline box is
affected by relative positioning, any resulting translation also
affects the block-level box contained in the inline box.

<div class="example">
<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">
&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;
&lt;P&gt;
<em>This is anonymous text before the SPAN.</em>
&lt;SPAN&gt;This is the content of SPAN.&lt;/SPAN&gt;
<em>This is anonymous text after the SPAN.</em>
&lt;/P&gt;
&lt;/BODY&gt;
</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 a block box representing the BODY,
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&nbsp;2.1. This does not apply to UAs
developed after this specification was released.
</p>

<p>Anonymous block boxes are ignored when resolving percentage values
that would refer to it: the closest non-anonymous ancestor box is used
instead. For example, if the child of the anonymous block box inside
the DIV above needs to know the height of its containing block to
resolve a percentage height, then it will use the height of the
containing block formed by the DIV, not of the anonymous block box.

<h3><a name="inline-boxes">Inline-level elements and inline boxes</a></h3>

<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.). The following values of the <span
class="propinst-display">'display'</span> property make an element
inline-level: 'inline', 'inline-table', and 'inline-block'.

Inline-level elements generate <span class="index-def"
title="inline-level box"><dfn>inline-level
boxes</dfn></span>, which are boxes that participate in an inline
formatting context.

<p>An <span class="index-def" title="inline box"><a
name="inline-box"><dfn>inline
box</dfn></a></span> is one that is both inline-level and whose
contents participate in its containing inline formatting context. A
non-replaced element with a 'display' value of 'inline' generates an
inline box.
Inline-level boxes that are not inline boxes (such as replaced
inline-level elements, inline-block elements, and inline-table
elements) are called <span class="index-def" title="atomic
inline-level box"><dfn>atomic inline-level boxes</dfn></span> because
they participate in their inline formatting context as a single opaque
box.

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

<p>Any text that is directly contained inside a block container element (not
inside an inline element) must be treated as an anonymous inline
element.

<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 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 <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>[This section exists so that the section numbers are the same as in
previous drafts. <span class="index-def" title="run-in">'Display:
run-in'</span> is now defined in CSS level&nbsp;3 (see <a
href="/TR/css3-box">CSS basic box model</a>).]

<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>
</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 an inline-level block
container.
The inside of an inline-block is formatted as a block box, and the
element itself is formatted as an atomic inline-level box.
</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 marker 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 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 <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="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 }      /* Do not 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 block-level boxes,
<a href="#inline-formatting">inline formatting</a>
of inline-level boxes, and <a
href="#relative-positioning">relative positioning</a> of
block-level and inline-level 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>

<p>An element is called <dfn><span class="index-def">out of
flow</span></dfn> if it is floated, absolutely positioned, or is the
root element. An element is called <dfn><span
class="index-def">in-flow</span></dfn> if it is not out-of-flow. The
<dfn><span class="index-def">flow of an element</span></dfn>
<var>A</var> is the set consisting of <var>A</var> and all in-flow
elements whose nearest out-of-flow ancestor is <var>A</var>.

<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">norma
6D50
l 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 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">
<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).
</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.
</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.
</p>


<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><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-level">Block-level</a> boxes participate in a <a
href="#block-formatting">block formatting</a> context.  <a
href="#inline-level">Inline-level boxes</a> participate in an <a
href="#inline-formatting">inline formatting</a> context.
</p>
<h3><a name="block-formatting">Block formatting contexts</a></h3>

<p>Floats, absolutely positioned elements, block containers (such as
inline-blocks, table-cells, and table-captions) that are not block
boxes, and block boxes with 'overflow' other than 'visible'
(except when that value has been propagated to the viewport) establish
new block formatting contexts for their contents.</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-level 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><a name="inline-formatting">Inline formatting contexts</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 <span class="propinst-vertical-align">'vertical-align'</span> property.
When several inline-level 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 (except as specified
elsewhere) 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-level 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 spaces
and words in inline boxes (but not 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 id="phantom-line-box">Line boxes are created as needed to hold
inline-level content within an inline formatting context. Line boxes
that contain no text, no <a
href="text.html#white-space-prop">preserved white space,</a> no inline
elements with non-zero margins, padding, or borders, and no other <a
blocks or inline tables), and do not end with a preserved newline must
be treated as zero-height line boxes for the purposes of determining
the positions of any elements inside of them, and must be treated as
not existing for any other purpose.
</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
(B1) in this way has no effect on the box (B2) that follows: B2 is
given a position as if B1 were not offset and B2 is not re-positioned
after B1's offset is applied. This implies that relative positioning
may cause boxes to overlap.
However, if relative positioning causes an 'overflow:auto' or
'overflow:scroll' box to have
overflow, the UA must allow the user to access this content (at its offset
position), 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
used values are always: left = -right.
</p>
<p>If both 'left' and 'right' are 'auto' (their initial values), the
used values are '0' (i.e., the boxes stay in their original
position).
</p>
<p>If 'left' is 'auto', its used 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 used 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
used values are always: top = -bottom.
If both are 'auto', their used 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 used 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 is a line box, the outer top of the floated box is aligned with
the top of the current line box.
</p>
<p>
If there is not 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
did not exist. However,
the current and subsequent
line boxes created next to the float are
shortened
as necessary
to make room for the margin box of the float.

<p>A line box is next to a float when there exists a vertical position
that satisfies all of these four conditions: (a) at or below the top
of the line box, (b) at or above the bottom of the line box, (c) below
the top margin edge of the float, and (d) above the bottom margin edge
of the float.

<p class=note>Note: this means that floats with zero outer height or
negative outer height do not shorten line boxes.

<p>If a shortened line box is too small to contain any content, then
the line box is shifted downward (and its width recomputed) until
either some content fits or there are no more floats present.
Any content in the current line before a floated box is reflowed
in the same line on the other side of the float.
In other words, if inline-level 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-level 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 id="bfc-next-to-float">
The border 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 the margin box of 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.
They may even make the border box of said element narrower than
defined by <a href="visudet.html#blockwidth">section&nbsp;10.3.3.</a>
CSS2 does not define when a UA may put said element next to the float
or by how much said element may become narrower.
</p>
<div class="example">
<p><strong>Example.</strong>