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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
<title>CSS Flexible Box Layout Module</title>
<link href="../default.css" rel=stylesheet type="text/css">
<style type="text/css">
p { margin: .5em 0; }
dd > p { text-indent: 0; }
1F85
a > b { font-weight: normal; color: #800; }
</style>
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED" rel=stylesheet
type="text/css">
<!-- TODO: change before build -->
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1 id=head-box-flexible>CSS Flexible Box Layout Module</h1>
<h2 class="no-num no-toc" id=w3c-working>Editor's Draft, 15 June 2011</h2>
<dl>
<dt>This version:
<dd><!-- <a href="http://www.w3.org/TR/2011/WD-css3-flexbox-20110615/">http://www.w3.org/TR/2011/WD-css3-flexbox-20110615/</a></dd> -->
<a
href="http://dev.w3.org/csswg/css3-flexbox/">http://dev.w3.org/csswg/css3-flexbox/</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-flexbox/">http://www.w3.org/TR/css3-flexbox/</a>
<dt>Previous version:
<dd><a
href="http://www.w3.org/TR/2009/WD-css3-flexbox-20090723/">http://www.w3.org/TR/2009/WD-css3-flexbox-20090723/</a>
<dt>Editors:
<dd>Tab Atkins Jr. <a
href="mailto:jackalmage@gmail.com">jackalmage@gmail.com</a>
<dd>Alex Mogilevsky <a
href="mailto:alexmog@microsoft.com">alexmog@microsoft.com</a>
<dd>L. David Baron, Mozilla Corporation, <a
href="mailto:dbaron@dbaron.org">dbaron@dbaron.org</a>
<dt>Authors and former editors:
<dd>Neil Deakin, Mozilla Corporation, <a
href="mailto:enndeakin@gmail.com">enndeakin@gmail.com</a>
<dd>Ian Hickson, formerly of Opera Software, <a
href="mailto:ian@hixie.ch">ian@hixie.ch</a>
<dd>David Hyatt, Apple, <a
href="mailto:hyatt@apple.com">hyatt@apple.com</a>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2011 <a
href="http://www.w3.org/"><acronym title="World Wide Web
Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and
Mathematics">ERCIM</acronym></a>, <a
href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>The draft describes a CSS box model optimized for interface design. It
provides an additional layout system alongside the ones already in CSS. <a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> In this new box
model, the children of a box are laid out either horizontally or
vertically, and unused space can be assigned to a particular child or
distributed among the children by assignment of "flex" to the children
that should expand. Nesting of these boxes (horizontal inside vertical, or
vertical inside horizontal) can be used to build layouts in two
dimensions. This model is based on the box model in the XUL user-interface
language used for the user interface of many Mozilla-based applications
(such as Firefox).
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss3-flexbox%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“css3-flexbox” in the subject, preferably like this:
“[<!---->css3-flexbox<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<h2 class="no-num no-toc" id=table>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#overview"><span class=secno>1. </span> Overview</a>
<li><a href="#display-flexbox"><span class=secno>2. </span> New values for
‘<code class=property>display</code>’ property</a>
<li><a href="#flex-direction"><span class=secno>3. </span> Flexbox
Direction: the ‘<code class=property>flex-direction</code>’
property</a>
<li><a href="#flex-order"><span class=secno>4. </span> Display Order: the
‘<code class=property>flex-order</code>’ property</a>
<li><a href="#flexibility"><span class=secno>5. </span> Flexibility</a>
<ul class=toc>
<li><a href="#resolving-flexible-lengths"><span class=secno>5.1. </span>
Resolving Flexible Lengths</a>
<li><a href="#fraction-unit"><span class=secno>5.2. </span> The
‘<code class=css>fr</code>’ unit</a>
<li><a href="#flex-function"><span class=secno>5.3. </span> The
‘<code class=css>flex()</code>’ function</a>
</ul>
<li><a href="#flex-pack"><span class=secno>6. </span> The ‘<code
class=property>flex-pack</code>’ property</a>
<li><a href="#flex-align"><span class=secno>7. </span> The ‘<code
class=property>flex-align</code>’ property</a>
<li><a href="#free-space-algorithm"><span class=secno>8. </span> Free
Space Calculation and Distribution</a>
<ul class=toc>
<li><a href="#initial-computation"><span class=secno>8.1. </span>
Initial Computation</a>
<li><a href="#first-distribution-round"><span class=secno>8.2. </span>
First Distribution Round</a>
<li><a href="#second-distribution-round"><span class=secno>8.3. </span>
Second Distribution Round</a>
<li><a href="#intermediate-computation"><span class=secno>8.4. </span>
Intermediate Computation</a>
<li><a href="#third-distribution-round"><span class=secno>8.5. </span>
Third Distribution Round</a>
<li><a href="#free-space-allocation-algorithm"><span class=secno>8.6.
</span> Free Space Allocation Algorithm</a>
</ul>
<li><a href="#layout-interface"><span class=secno>9. </span> Interface
With Other Layout Algorithms</a>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<ul class=toc>
<li class=no-num><a href="#normative">Normative references</a>
</ul>
<li class=no-num><a href="#index">Index</a>
<li class=no-num><a href="#property">Property index</a>
</ul>
<!--end-toc-->
<!-- ====================================================================== -->
<h2 id=overview><span class=secno>1. </span> Overview</h2>
<p>CSS 2.1 defined four layout modes, algorithms which determine the size
and position of boxes based on their relationships with their sibling and
ancestor boxes: block layout, designed for laying out documents and simple
applications, inline layout, designed for laying out text, table layout,
designed for laying out information in a tabular format, and positioned
layout, designed for very explicit positioning without much regard for
other elements in the document. This module introduces a new layout mode,
flexbox layout, which is designed for laying out more complex applications
and webpages.
<p>Flexbox layout is superficially similar to block layout. It lacks many
of the more complex text or document-formatting properties that can be
used in block layout, such as ‘<code
class=property>float</code>’ and ‘<code
class=property>columns</code>’, but in return it gains more simple
and powerful tools for aligning its contents in ways that webapps and
complex web pages often need. The contents of a flexbox can be laid out in
any direction, can have their order swapped around dynamically, and can
"flex" their sizes and positions to respond to the available space.
<p>Similar to other layout modes such as table layout, a flexbox acts like
a block when placed into other layout modes. Inline flexboxes can also be
declared, which act like inline-blocks.
<p>Block-level and atomic inline-level children of the flexbox are referred
to as <dfn id=flexbox-item>flexbox item</dfn>s in this specification.
Inline children are wrapped in anonymous blocks, and these anonymous
blocks are also <a href="#flexbox-item"><i>flexbox item</i></a>s.
<div class=example>
<p>For example:</p>
<pre class=html>
<p style="width: 400px; display: flexbox;">
<button style="width: 200px;">Child 1</button>
<button style="width: 100px;">Child 2</button>
</p>
</pre>
<p>In this example, the first button is 200 pixels wide and the second
button is 100 pixels wide. As the flexbox is 400 pixels wide, there is
extra space left over which is placed after the two buttons but inside
the flexbox. The placement of this extra space may be modified using the
various flex properties described in the next sections. Neither the
height of the buttons nor the height of the flexbox are specified, so the
height is computed intrinsically from the content. In this case, the
buttons will be the height needed to display the label and button border,
and the flexbox will also be this height plus any margins around the
buttons.</p>
</div>
<!-- ====================================================================== -->
<h2 id=display-flexbox><span class=secno>2. </span> New values for
‘<code class=property>display</code>’ property</h2>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td>display
<tr>
<th>New value:
<td>flexbox | inline-flexbox
</table>
<p class=issue>ISSUE: The proliferation of "inline-*" display values is
untenable and restrictive. Table cells should be able to use the flexbox
layout mode for their contents, for example, rather than being forced to
use block layout. It's expected that this will be fixed by splitting the
‘<code class=property>display</code>’ property into
subproperties controlling how the element formats its contents
(‘<code class=property>display-inside</code>’) and how it
reacts to its surroundings (‘<code
class=property>display-outside</code>’). Once that occurs, this
section will instead describe a single new ‘<code
class=property>display-inside</code>’ value that triggers flexbox
layout.
<p>You can declare that an element is a flexbox, and thus should use
flexbox layout, by setting the ‘<code
class=property>display</code>’ property on the element to the value
‘<code class=css>flexbox</code>’.
<p>The special behaviors of a flexbox, such as flexing and reordering
children, only apply to <a href="#flexbox-item"><i>flexbox item</i></a>s.
All block-level or atomic inline-level children of the flexbox are <a
href="#flexbox-item"><i>flexbox item</i></a>s. Contiguous runs of
non-replaced inline children are wrapped in an anonymous block-level box,
which is itself a <a href="#flexbox-item"><i>flexbox item</i></a>.
<p class=note>Note that out-of-flow elements like absolutely positioned
elements leave behind a ‘<code
class=property>placeholder</code>’ inline element in their original
position in the document <span class=issue>(ISSUE: This will be defined in
Positioned Layout.)</span> Floats are also out-of-flow, but children of
flexboxes can't float. <span class=issue>(ISSUE: Should this restriction
exist, or should I just wrap floats in the anonymous boxes like other
inlines?)</span> It is recommended that authors avoid creating inline
flexbox children, as the anonymous block that is generated can't be
targetted and controlled by the various flexbox properties.
<div class=issue>
<p>ISSUE: Need to reword how flexbox items are determined. The algorithm
clearly operates on the box tree, as it creates anonymous boxes, but it
needs to operate kinda on elements. In particular, an inline child
element that contains a block should be wrapped in a single flexbox item,
rather than wrapping the pre-block and post-block parts in separate
anonymous blocks and then the block inside being a flexbot item on its
own.</p>
<p>Whatever wording is used here should be shared with the table-fixup
algorithm, because it's essentially the identical problem. Perhaps we can
have it operate on the element-tree and create anonymous pseudo-elements,
similar to how ::outside would work? Pseudoelements don't interfere with
normal selectors, so this would let us insert wrappers that work
correctly without screwing up the way selectors work.</p>
</div>
<p>A flexbox item creates a new BFC. The margins of a flexbox item do not
collapse with any other margin. Flexboxes "shrinkwrap" their contents by
default (when their ‘<code class=property>width</code>’ or
‘<code class=property>height</code>’ properties are
‘<code class=css>auto</code>’), similar to tables or floats.
<p>Setting ‘<code class=css>display:flexbox</code>’ on an
element forces it to use a new layout algorithm, and so some properties
that were designed with the assumption of block layout don't make sense in
a flexbox context. In particular:
<ul>
<li>all of the ‘<code class=css>column-*</code>’ properties in
the Multicol module compute to their initial values on a flexbox
(‘<code class=property>break-before</code>’, ‘<code
class=property>break-inside</code>’, and ‘<code
class=property>break-after</code>’ are still valid on a flexbox).
<li>‘<code class=property>float</code>’ and ‘<code
class=property>clear</code>’ compute to their initial values on a
flexbox item
<li>‘<code class=property>vertical-align</code>’ has no effect
on a flexbox item
<li class=issue>[Others?]
</ul>
<!-- ====================================================================== -->
<h2 id=flex-direction><span class=secno>3. </span> Flexbox Direction: the
‘<a href="#flex-direction0"><code
class=property>flex-direction</code></a>’ property</h2>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=flex-direction0>flex-direction</dfn>
<tr>
<th>Values:
<td>lr | rl | tb | bt | inline | inline-reverse | block | block-reverse
<tr>
<th>Initial:
<td>inline
<tr>
<th>Applies To:
<td>flexboxes
<tr>
<th>Inherited:
<td>no
<tr>
<th>Computed Value:
<td>lr | rl | tb | bt (See text)
<tr>
<th>Media:
<td>visual
</table>
<p>The ‘<code class=property>flexbox-direction</code>’ property
specifies how <a href="#flexbox-item"><i>flexbox item</i></a>s are placed
in the flexbox. A value of ‘<code class=css>lr</code>’ means
that the first flexbox item is placed against the left edge of the
flexbox, with later children placed immediately to the right of the
preceding flexbox item. A value of ‘<code class=css>rl</code>’
is the same, but with the initial flexbox item placed against the right
edge of the flexbox and later children placed to the left. Both of these
values mean that the flexbox is a <dfn id=horizontal-flexbox>horizontal
flexbox</dfn>. A value of ‘<code class=css>tb</code>’ means
that the first flexbox child is placed against the top edge of the
flexbox, with later children placed immediately below it, while
‘<code class=css>bt</code>’ means the first child is placed
against the bottom edge of the flexbox, with later children placed
immediately above it. Both of these values mean that the flexbox is a <dfn
id=vertical-flexbox>vertical flexbox</dfn>.
<p>A value of ‘<code class=css>inline</code>’ computes to one
of ‘<code class=css>lr</code>’, ‘<code
class=css>rl</code>’, ‘<code class=css>tb</code>’, or
‘<code class=css>bt</code>’, depending on the direction and
block-flow of the flexbox: if the start side of the flexbox is the left
side, ‘<code class=css>inline</code>’ must compute to
‘<code class=css>lr</code>’; similarly, if the start side of
the flexbox is the right, top, or bottom side, ‘<code
class=css>inline</code>’ must compute to ‘<code
class=css>rl</code>’, ‘<code class=css>tb</code>’, or
‘<code class=css>bt</code>’ respectively. ‘<code
class=css>inline-reverse</code>’, ‘<code
class=css>block</code>’, and ‘<code
class=css>block-reverse</code>’ determine their computed value
identically to ‘<code class=css>inline</code>’, but use the
location of the end side, before side, and after side of the flexbox,
respectively.
<div class=example>
<p>A vertical flexbox can be used to emulate the functionality of HTML's
‘<code class=css><center></code>’ element:</p>
<pre>
<div>
<span>foo foo foo foo</span>
<span>bar bar<br>bar bar</span>
<span>foo foo foo foo foo foo foo foo foo foo foo foo</span>
</div>
<style>
div {
display: flexbox;
flex-direction: tb;
width: 200px;
}
span {
margin: 0 auto;
}
</style>
</pre>
<p><img alt="" src="images/basic-vertical-flexbox.png"></p>
</div>
<p>In the rest of this specification, the direction in which new <a
href="#flexbox-item"><i>flexbox item</i></a>s are laid out will be
referred to as the <dfn id=measure-direction>measure direction</dfn> (if a
precise direction is required) or the <dfn id=measure-axis>measure
axis</dfn> (if only the axis orientiation is required).
<p>The axis perpendicular to the ‘<a href="#measure-axis"><code
class=css>measure axis</code></a>’ will be refered to as the <dfn
id=length-axis>length axis</dfn>, with the <dfn id=length-direction>length
direction</dfn> being either a line extending from the ‘<code
class=css>before</code>’ side of the flexbox to the ‘<code
class=css>after</code>’ side, or from the ‘<a
href="#flex-pack-start"><code class=css>start</code></a>’ side of
the flexbox to the ‘<a href="#flex-pack-end"><code
class=css>end</code></a>’ side, whichever would produce a direction
parallel to the ‘<a href="#length-axis"><code class=css>length
axis</code></a>’.
<p>Similarly, the <dfn id=inner-measure>inner measure</dfn> is the width of
the flexbox item in a horizontal flexbox, or the height in a vertical
flexbox. The <dfn id=inner-length>inner length</dfn> is the height of the
flexbox item in a horizontal flexbox, or the width in a vertical flexbox.</p>
<!-- ====================================================================== -->
<h2 id=flex-order><span class=secno>4. </span> Display Order: the ‘<a
href="#flex-order0"><code class=property>flex-order</code></a>’
property</h2>
<p><a href="#flexbox-item"><i>flexbox item</i></a>s are, by default,
displayed and laid out in the same order as they appear in the source
document. The ‘<a href="#flex-order0"><code
class=property>flex-order</code></a>’ property may be used to change
this ordering.
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=flex-order0>flex-order</dfn>
<tr>
<th>Value:
<td><integer>
<tr>
<th>Initial:
<td>1
<tr>
<th>Applies to:
<td><a href="#flexbox-item"><i>flexbox item</i></a>s
<tr>
<th>Inherited:
<td>no
<tr>
<th>Computed value:
<td>specified value
<tr>
<th>Media:
<td>visual
</table>
<p class=issue>Maybe name this ‘<code
class=property>flex-index</code>’ instead, to be consistent with
‘<code class=property>z-index</code>’.
<p><a href="#flexbox-item"><i>flexbox item</i></a>s may be assigned to
ordinal groups using the ‘<a href="#flex-order0"><code
class=property>flex-order</code></a>’ property. This property is an
integer with an initial value of 1.
<p>Ordinal groups control the order in which <a
href="#flexbox-item"><i>flexbox item</i></a>s appear. A flexbox will lay
out its children starting from the lowest numbered ordinal group and going
up. Elements with the same ordinal group are laid out in the order they
appear in the source document.
<div class=example>
<p>This example shows how ordinal groups might be used.</p>
<pre><code>
div { display: flexbox; }
#span1 { flex-order: 2; }
#span3 { flex-order: 2; }
#span4 { flex-order: 1; }
<div>
<span id="span1">span1</span>
<span id="span2">span2</span>
<span id="span3">span3</span>
<span id="span4">span4</span>
</div></code></pre>
<p>The first ordinal group, 1, contains span2 and span4. As span2 does not
specify an ordinal group, it will default to 1. The elements will be
displayed in document order, so span2 will be displayed before span4. The
second ordinal group, 2, contains the remaining two spans. The resulting
display order will be:</p>
<pre><code>span2 span4 span1 span3</code></pre>
</div>
<!-- ====================================================================== -->
<h2 id=flexibility><span class=secno>5. </span> Flexibility</h2>
<p>The defining aspect of flexbox layout is the ability to make various
lengths of the <a href="#flexbox-item"><i>flexbox item</i></a>s flexible.
The ‘<code class=property>width</code>’, ‘<code
class=property>height</code>’, ‘<code
class=property>padding</code>’, and ‘<code
class=property>margin</code>’ properties of a <a
href="#flexbox-item"><i>flexbox item</i></a> can all be made flexible.
Paddings and margins are made flexible by setting their value to
‘<code class=css>auto</code>’, while the width and height of a
box can be controlled more precisely with the ‘<code
class=css>fr</code>’ unit or the ‘<code
class=css>flex()</code>’ function, both defined below.
<div class=example>
<p class=issue>TODO: Examples!</p>
</div>
<h3 id=resolving-flexible-lengths><span class=secno>5.1. </span> Resolving
Flexible Lengths</h3>
<p>CSS resolves a <i>flexible length</i> into a definite length by first
collecting all the lengths, flexible or inflexible, that will share some
space. For example, for a horizontal flexbox, the lengths of the left and
right margins, left and right borders, left and right paddings, and widths
of all <a href="#flexbox-item"><i>flexbox item</i></a>s share the width of
the flexbox itself. Conversely, each <a href="#flexbox-item"><i>flexbox
item</i></a>‘<code class=css>s vertical margins, borders, padding,
and height individually share the height of the flexbox.</code>
<div class=figure>
<p class="caption issue">TODO: Diagram showing the relevant lengths in
each axis.</p>
</div>
<p>CSS then sums the <dfn id=preflex-size>preflex size</dfn> of the lengths
in each set. The <a href="#preflex-size"><i>preflex size</i></a> of an
inflexible length is just the length itself. The <a
href="#preflex-size"><i>preflex size</i></a> of a flexible length is its
<i>preferred size</i>. If the sum of all the <a href="#preflex-size"><i
title="preflex size">preflex sizes</i></a> is less than the available
width/height of the flexbox, then the difference is split up among all the
<i title="flexible length">flexible lengths</i> with <dfn
id=positive-flexibility>positive flexibility</dfn>, with the space divvied
up proportionally to the flexibility of each length. If the sum is greater
than the available width/height, then all the <i title="flexible
length">flexible lengths</i> with <dfn id=negative-flexibility>negative
flexibility</dfn> shrink in proportion to their flexibility to try and
make the sum equal the available width/height.
<p>The ’flex-pack' and ‘<a href="#flex-align0"><code
class=property>flex-align</code></a>’ properties offer more complex
control over free-space distribution. The precise details of how free
space is determined and assigned to flexible lengths is detailed in a
later chapter.
<h3 id=fraction-unit><span class=secno>5.2. </span> The ‘<code
class=css>fr</code>’ unit</h3>
<p>The ‘<code class=css>fr</code>’ unit, defined in the Values
and Units spec <a href="#CSS3VAL"
rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>, can be used to specify a
flexible length. When the ‘<code class=css>fr</code>’ unit is
used, the <i>preferred size</i> of the length is ‘<code
class=css>0</code>’, the <a href="#positive-flexibility"><i>positive
flexibility</i></a> is equal to the numeric part of the value, and the <a
href="#negative-flexibility"><i>negative flexibility</i></a> is
‘<code class=css>0</code>’.
<h3 id=flex-function><span class=secno>5.3. </span> The ‘<code
class=css>flex()</code>’ function</h3>
<p>The ‘<code class=css>flex()</code>’ function is used to
specify the parameters of a <i>flexible length</i>: the <a
href="#positive-flexibility"><i title="positive
flexibility">positive</i></a> and <a
href="#negative-flexibility"><i>negative flexibility</i></a>, and the
<i>preferred size</i>. The syntax of the ‘<code
class=css>flex()</code>’ function is roughly (see following prose
for a precise description):
<pre
class=prod>flex( [ <a href="#positive-flexibility"><i title="positive flexibility"><pos-flex></i></a> || <!--
--><a href="#negative-flexibility"><i title="negative flexibility"><neg-flex></i></a> || <!--
--><i title="preferred size"><preferred-size></i> ] )</pre>
<dl>
<dt>If the ‘<code class=css>flex()</code>’ function contains a
single value:
<dd>
<ul>
<li>If the value is a non-negative number, the <a
href="#positive-flexibility"><i>positive flexibility</i></a> is set to
that value, the <a href="#negative-flexibility"><i>negative
flexibility</i></a> is set to ‘<code class=css>0</code>’,
and the <i>preferred size</i> is set to ‘<code
class=css>auto</code>’.
<li>Otherwise, if the value is a <length>, a <percentage>, or a
valid keyword for ‘<code class=property>width</code>’ or
‘<code class=property>height</code>’, the <i>preferred
size</i> is set to that value, the <a
href="#positive-flexibility"><i>positive flexibility</i></a> is set to
1, and the <a href="#negative-flexibility"><i>negative
flexibility</i></a> is set to ‘<code class=css>0</code>’.
<li>Otherwise, the ‘<code class=css>flex()</code>’ function
is invalid.
</ul>
<dt>If the ‘<code class=css>flex()</code>’ function contains
two values:
<dd>
<ul>
<li>If both values are non-negative numbers, the <a
href="#positive-flexibility"><i>positive flexibility</i></a> is set to
the first value, the <a href="#negative-flexibility"><i>negative
flexibility</i></a> is set to the second value, and the <i>preferred
size</i> is set to ‘<code class=css>auto</code>’.
<li>Otherwise, if one value is a non-negative number and the other is a
<length>, a <percentage>, or a valid keyword for ‘<code
class=property>width</code>’ or ‘<code
class=property>height</code>’, the <a
href="#positive-flexibility"><i>positive flexibility</i></a> is set to
the number, the <a href="#negative-flexibility"><i>negative
flexibility</i></a> is set to ‘<code class=css>0</code>’,
and the <i>preferred size</i> is set to the <length> or
<percentage>.
<li>Otherwise, the ‘<code class=css>flex()</code>’ function
is invalid.
</ul>
<dt>If the ‘<code class=css>flex()</code>’ function contains
three values:
<dd>
<ul>
<li>If the first two values are non-negative numbers and the third value
is ‘<code class=css>0</code>’, the <a
href="#positive-flexibility"><i>positive flexibility</i></a> is set to
the first value, the <a href="#negative-flexibility"><i>negative
flexibility</i></a> is set to the second value, and the <i>preferred
size</i> is set to ‘<code class=css>0px</code>’.
<li>Otherwise, If two of the values are non-negative numbers, and the
other is a <length> (with a unit suffix), a <percentage>, or a
valid keyword for ‘<code class=property>width</code>’ or
‘<code class=property>height</code>’, the <a
href="#positive-flexibility"><i>positive flexibility</i></a> is set to
the first number, the <a href="#negative-flexibility"><i>negative
flexibility</i></a> is set to the second number, and the <i>preferred
size</i> is set to the other value.
<li>Otherwise, the ‘<code class=css>flex()</code>’ function
is invalid.
</ul>
</dl>
<p class=note>Note that, while ‘<code class=css>0</code>’
<length>s are normally allowed to be specified without a unit suffix,
the definition of the ‘<code class=css>flex()</code>’ function
implicitly disallows this. Allowing a unitless ‘<code
class=css>0</code>’ length would cause ambiguity in some cases; even
in the unambiguous cases, the flexible length could instead be written
using the ‘<code class=css>fr</code>’
2A21
unit, which is shorter
and simpler.</p>
<!-- ====================================================================== -->
<h2 id=flex-pack><span class=secno>6. </span> The ‘<a
href="#flex-pack0"><code class=property>flex-pack</code></a>’
property</h2>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=flex-pack0>flex-pack</dfn>
<tr>
<th>Value:
<td>start | end | center | justify
<tr>
<th>Initial:
<td>start
<tr>
<th>Applies to:
<td>flexboxes
<tr>
<th>Inherited:
<td>no
<tr>
<th>Computed Value:
<td>specified value
<tr>
<th>Media:
<td>visual
</table>
<p>Between and around the margins of <a href="#flexbox-item"><i>flexbox
item</i></a>s there are additional flexible lengths, called <dfn
id=packing-space>packing space</dfn>. Packing space can absorb leftover
free space in a flexbox if there aren't any other <i>flexible length</i>s,
or if all the <i>flexible length</i>s have reached their maximum size. The
‘<a href="#flex-pack0"><code
class=property>flex-pack</code></a>’ property defines the
flexibility of these packing spaces:
<dl>
<dt><dfn id=flex-pack-start>start</dfn>
<dd><a href="#flexbox-item"><i>flexbox item</i></a>s are packed toward the
start of the flexbox. The last packing space (between the margin of the
last flexbox item and the edge of the flexbox) must absorb all leftover
free space; all other packing spaces must have a length of ‘<code
class=css>0</code>’.
<dt><dfn id=flex-pack-end>end</dfn>
<dd><a href="#flexbox-item"><i>flexbox item</i></a>s are packed toward the
end of the flexbox. The first packing space (between the edge of the
flexbox and the margin of the first flexbox item ) must absorb all
leftover free space; all other packing spaces must have a length of
‘<code class=css>0</code>’.
<dt><dfn id=flex-pack-center>center</dfn>
<dd><a href="#flexbox-item"><i>flexbox item</i></a>s are packed toward the
center of the flexbox. The first and last packing spaces must equally
split all leftover free space; all other packing spaces must have a
length of ‘<code class=css>0</code>’.
<dt><dfn id=flex-pack-justify>justify</dfn>
<dd><a href="#flexbox-item"><i>flexbox item</i></a>s are evenly
distributed through the flexbox. The first and last packing spaces must
have a length of ‘<code class=css>0</code>’; all other
packing spaces must equally split all leftover free space.
</dl>
<div class=figure>
<p class=caption><span class=issue>TODO: Provide a diagram showing packing
spaces.</span></p>
</div>
<div class=example>
<p class=issue>TODO: Examples showing the four values.</p>
</div>
<!-- ====================================================================== -->
<!-- Aborted new introduction to flexboxes
<p>A flexbox has two axes, which are treated differently by the space distribute algorithm: the <i>measure axis</i>, which is the axis along which the flexbox grows as you add more children, and the <i>length axis</i>, which is the perpendicular axis.</p>
<img alt="In a horizontal flexbox, the measure axis is horizontal, while the length axis is vertical.">
<p>The measure axis is more interesting, as there are more places where things can potentially flex. As such, there are more controls for specifying how to distribute free space in this axis: the 'width' or 'height' property starts things off, modified by the 'flex' property; the 'margin' property gives individual control over spacing between <i>flexbox item</i>s; and the 'flex-pack' property gives a more flexbox-global level of control over the between-children spacing.</p>
<p>In general terms, determining where to allocate free space in the measure axis starts by determining just how much free space there is. All the non-flexible lengths, and the preferred sizes of all the flexible widths or heights, are subtracted from the total width or height of the flexbox. If a positive length is left over, this is split up among all the flexible lengths according to their flexibility. Flexible margins automatically have a flexibility of 1, while widths or heights have their flexibility specified by the 'flex-grow' property.</p>
<div class=example>
<p>Three boxes, all with a preferred size of 0, will end up with sizes exactly proportional to their flex-grow values.</p>
<img alt="">
</div>
<div class=example>
<p>If the three boxes have different preferred sizes, then they'll split the free space proportionately, but their final sizes won't be exactly proportional.</p>
<img alt="">
</div>
<div class=example>
<p>Margins can also participate in flex calculations. They have a preferred size of 0, and a flexibility of 1.</p>
<img alt="">
</div>
<p>If none of the lengths are flexible, or if all the flexible widths or height have reached their maximum size (if 'max-width' or 'max-height' properties are in effect), then any leftover space is distributed according to the 'flex-pack' property between the margins of the <i>flexbox item</i>s.</p>
<div class=example>
<p>If the flexbox is small enough, then the flexible child can absorb all the space...</p>
<img>
<p>...but if it gets large enough, there'll be extra space left over for 'flex-pack' to distribute.</p>
<img>
</div>
<p>If the original calculation of how much free space was left gave a negative length (that is, if the preferred sizes of all the <i>flexbox item</i>s sum to a larger length than the width or height of the flexbox), then flexible margins get set to 0, 'flex-pack' has no effect, and <i>flexbox item</i>s with a non-zero 'flex-shrink' value split the responsibility for shrinking between themselves proportionally. Just like when they grow, their size starts from the preferred size, then combines that with the proportional share of (negative) space.</p>
<div class=example>
<p>When the children are too wide, the shrinkable elements reduce their size to fit...</p>
<img>
<p>...but if they all reach their minimum size, they just overflow.</p>
</div>
[write the length axis section]
-->
<!-- ====================================================================== -->
<h2 id=flex-align><span class=secno>7. </span> The ‘<a
href="#flex-align0"><code class=property>flex-align</code></a>’
property</h2>
<table class=propdef>
<tbody>
<tr>
<th>Name:
<td><dfn id=flex-align0>flex-align</dfn>
<tr>
<th>Value:
<td>auto | baseline
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>flexboxes
<tr>
<th>Inherited:
<td>no
<tr>
<th>Computed Value:
<td>specified value
<tr>
<th>Media:
<td>visual
</table>
<p>By default, laying out <a href="#flexbox-item"><i>flexbox item</i></a>s
in the <a href="#length-axis"><i>length axis</i></a> is a simple matter of
determining free space and then splitting it between any <i>flexible
length</i>s in that axis. This is not always sufficient, however; in
particular, it is often useful to align <a href="#flexbox-item"><i>flexbox
item</i></a>s by their baseline. The ‘<a href="#flex-align0"><code
class=property>flex-align</code></a>’ property changes the way free
space is allocated in the <a href="#length-axis"><i>length axis</i></a>.
<dl>
<dt>auto
<dd>Allocate free space in the <a href="#length-axis"><i>length
axis</i></a> as normal, by distributing it proportionally between
<i>flexible length</i>s in that axis.
<dt>baseline
<dd>
<p>Align all <a href="#flexbox-item"><i>flexbox item</i></a>s so that
their baselines line up, then distribute free space above and below the
content.</p>
<p>This only has an effect on <a href="#flexbox-item"><i>flexbox
item</i></a>s with a horizontal baseline in a horizontal flexbox, or <a
href="#flexbox-item"><i>flexbox item</i></a>s with a vertical baseline
in a vertical flexbox. Otherwise, alignment for that flexbox item
proceeds as if ‘<code class=css>flex-align:auto</code>’ had
been specified.</p>
</dl>
<p>The precise effects that ‘<a href="#flex-align0"><code
class=property>flex-align</code></a>’ has on alignment and free
space distribution are detailed in a later chapter.</p>
<!-- ====================================================================== -->
<h2 id=free-space-algorithm><span class=secno>8. </span> Free Space
Calculation and Distribution</h2>
<p>The following algorithms detail precisely how to determine the
dimensions and positions of a flexbox and its contents, and how to resolve
flexible lengths used on <a href="#flexbox-item"><i>flexbox item</i></a>s
into definite lengths.
<p>The algorithm described here is designed to be clear, but not
necessarily efficient. Implementations may use different algorithms, but
they must generate the same results as this algorithm.
<p>This section defines several terms to clarify the operation of the
algorithms described within:
<dl>
<dt><dfn id=relevant-length>relevant length</dfn>
<dd>The width, height, margin, border, or padding of a flexbox item. Also,
the <a href="#packing-space"><i>packing space</i></a>s of a flexbox.
<dt><dfn id=flex-tuple>flex tuple</dfn>
<dd>A 5-tuple of information about a flexible length, containing a minimum
size, a maximum size, a preferred size, a positive flexibility, and a
negative flexibility.
<dt><dfn id=total-free-space>total free space</dfn>
<dd>The inner width or height of the flexbox (depending on the
distribution round), which will be used to calculate the sizes of the <a
href="#flexbox-item"><i>flexbox item</i></a>s.
<dt><dfn id=available-free-space>available free space</dfn>
<dd>The <a href="#total-free-space"><i>total free space</i></a> minus the
sum of the preferred sizes of all the relevant lengths, which is used to
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