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<!DOCTYPE html>
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>CSS Flexible Box Layout Module</title>
<link rel="stylesheet" type="text/css" href="http://dev.w3.org/csswg/default.css">
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-ED">
<style>
code.one-line { white-space: pre; }
.code-and-figure {
display: table;
margin: 0 auto;
border-spacing: 1em;
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display: table-cell;
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:not(code) var { font-style: normal; color: #005A9C; }
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ol[start="0"] > li,
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</style>
</head><body>
<div class="head">
<!--logo-->
<h1 id="head-box-flexible">CSS Flexible Box Layout Module</h1>
<h2 class="no-num no-toc" id="w3c-working">[LONGSTATUS], [DATE]</h2>
<dl>
<dt>This version:
<!--<dd><a href="[VERSION]">http://www.w3.org/TR/[YEAR]/WD-css3-flexbox-[CDATE]/</a>-->
<dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
<dt>Latest version:
<!--<dd><a href="[LATEST]">[LATEST]</a>-->
<dd><a href="http://www.w3.org/TR/css3-flexbox/">http://www.w3.org/TR/css3-flexbox/</a>
<dt>Editor's Draft:
<dd><a href="http://dev.w3.org/csswg/css3-flexbox/">http://dev.w3.org/csswg/css3-flexbox/</a>
<dt>Previous versions:
<dd><a href="http://www.w3.org/TR/2012/WD-css3-flexbox-20120322/">http://www.w3.org/TR/2012/WD-css3-flexbox-20120322/</a>
<dd><a href="http://www.w3.org/TR/2011/WD-css3-flexbox-20111129/">http://www.w3.org/TR/2011/WD-css3-flexbox-20111129/</a>
<dd><a href="http://www.w3.org/TR/2011/WD-css3-flexbox-20110322/">http://www.w3.org/TR/2011/WD-css3-flexbox-20110322/</a>
<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>Issues List:
<dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&component=Flexbox&resolution=---">Bugzilla Bugs for Flexbox</a>
<dt>Discussion:
<dd><a href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a> with subject line "<code>[css3-flexbox] …message topic…</code>"
<dt>Editors:
<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a>, Google Inc.
<dd>Alex Mogilevsky, Microsoft Corporation, <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, formerly of Netscape Corporation, <a href="mailto:hyatt@apple.com">hyatt@apple.com</a>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id="abstract">
Abstract</h2>
<p>
The specification describes a CSS box model optimized for user interface design.
In the flex layout model,
the children of a flex container can be laid out in any direction,
and can "flex" their sizes,
either growing to fill unused space
or shrinking to avoid overflowing the parent.
Both horizontal and vertical alignment of the children can be easily manipulated.
Nesting of these boxes (horizontal inside vertical, or vertical inside horizontal) can be used to build layouts in two dimensions.
<h2 class="no-num no-toc" id="status">Status of this document</h2>
<!--status-->
<h2 class="no-num no-toc" id="table">Table of contents</h2>
<!--toc-->
<h2 id="intro">
Introduction</h2>
<p><em>This section is not normative.</em>
<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; 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, flex layout, which is designed for laying out more complex applications and webpages.
<p>Flex 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 'float' and 'columns', but in return it gains more simple and powerful tools for aligning its contents in ways that webapps and complex web pages often need.
<p>The contents of a flex container can be laid out in any direction (left, right, down, or even up!), can have their order swapped around dynamically (i.e., display order is independent of source order), and can "flex" their sizes and positions to respond to the available space. If a flex container is <i>multi-line</i>, the flex items flow in two dimensions, wrapping into separate lines in a fashion similar to how text is wrapped into multiple lines.
<div class="example">
<p>For example, the following HTML snippet uses a flex container to create a toolbar with icons. The flex container is horizontal, and the children's widths don't fill the flex container's width, so the additional space is distributed around and between the children. As the flex container grows (perhaps because the user is viewing the page on a wider screen), the children spread out evenly and automatically:
<pre>
<ul>
<li><button><img src='new.svg' alt="New"></button></li>
<li><button><img src='upload.svg' alt="Upload"></button></li>
<li><button><img src='save.svg' alt="Save"></button></li>
<li><button><img src='trash.svg' alt="trash"></button></li>
</ul>
<style>
ul { display: flex; flex-pack: distribute; }
/* Irrelevant styling for this example removed. */
</style></pre>
<div class='figure'>
<img src="images/toolbar-example.svg" width=400 height=140 alt>
<p class='caption'>Example rendering of the above code snippet, at two different flex container widths.
</div>
</div>
<h3 id="placement">
Module interactions</h3>
<p>This module extends the definition of the 'display' property.
<h3 id="values">
Values</h3>
<p>This specification follows the <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property definition conventions</a> from [[!CSS21]]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [[!CSS21]]. Other CSS modules may expand the definitions of these value types: for example [[CSS3VALUES]], when combined with this module, expands the definition of the <length> value type as used in this specification.
<p>In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">''inherit''</a> keyword as their property value. For readability it has not been repeated explicitly.
<h2 id='box-model'>
Flex Layout Box Model</h2>
<p>An element with ''display:flex'' or ''display:inline-flex'' is a <dfn>flex container</dfn>. Children of a flex container are called <i>flex items</i> and are laid out using the flex layout model.
<p>Unlike block layout, which is normally biased towards laying things out vertically, and inline layout, which is normally biased toward laying things out horizontally, the flex layout algorithm is agnostic as to the direction the flex container happens to be laid out in. To make it easier to talk about flex layout in a general way, we will define several direction-agnostic terms here to make the rest of the spec easier to read and understand.
<div class='figure'>
<img src='images/flex-direction-terms.svg' width=510 height=300 alt>
<p class='caption'>An illustration of the various directions and sizing terms used in this specification, respectively for ''row'' and ''column'' flex containers.
</div>
<p>
The <dfn>main axis</dfn> of a flex container is the axis along which <i>flex items</i> are laid out.
The <i>flex items</i> are ordered
such that they start on the <dfn>main-start</dfn> side of the flex container,
and go toward the <dfn>main-end</dfn> side.
A <i>flex item's</i> width or height,
whichever is in the <i>main dimension</i>,
is the item's <dfn>main size</dfn>.
The <i>flex item's</i> <dfn>main size property</dfn> is
either the 'width' or 'height' property,
whichever is in the <i>main dimension</i>.
<p>
The axis perpendicular to the <i>main axis</i> is called the <dfn>cross axis</dfn>,
and similarly has <dfn>cross-start</dfn> and <dfn>cross-end</dfn> sides defined.
The width or height of a <i>flex item</i>,
whichever is in the <i>cross dimension</i>,
is the item's <dfn>cross size</dfn>,
and similarly the <dfn>cross size property</dfn> is
whichever of 'width' or 'height' that is in the <i>cross dimension</i>.
<p>
The contents of a flex container can be easily and powerfully manipulated with a handful of properties.
Most significantly,
<i>flex items</i> can "flex" their <i>main size</i> by using the 'flex' property.
This "flexing" allows the items to get bigger or smaller based on the available space in the page.
If there is leftover space in the flex container after all of the <i>flex items</i> have finished flexing,
the items can be aligned, centered, or distributed with the 'flex-pack' property.
<i>Flex items</i> can also be completely rearranged within the flex container with the 'flex-order' property.
<p>
In the <i>cross axis</i>,
<i>flex items</i> can either "stretch" to fill the available space
or be aligned within the space with the 'flex-align' property.
If a flex container is <i>multi-line</i>,
new lines are added in the <i>cross-end</i> direction,
and can similarly be aligned, centered, or distributed
within the flex container with the 'flex-line-pack' property.
<h2 id='display-flex'>
Flex Containers: the ''flex'' and ''inline-flex'' 'display' values</h2>
<table class="propdef">
<tr>
<th>Name:
<td>display
<tr>
<th><a href="#values">New value</a>:
<td><dfn id='display-flex'>flex</dfn> | <dfn id='display-inline-flex'>inline-flex</dfn>
</table>
<p>
An element whose computed 'display' is either ''flex'' or ''inline-flex''
is a flex container,
and establishes a new <dfn>flex formatting context</dfn> for its contents.
This is the same as establishing a block formatting context,
except that flex layout is used instead of block layout:
floats do not intrude into the flex container,
and the flex container's margins do not collapse with the margins of its contents.
Additionally,
each of the <i>flex items</i> establishes a new formatting context for its contents.
<p>
The ''flex'' value makes the flex container a block-level element.
The ''inline-flex'' value makes the flex container an atomic inline-level element.
<p>
Flex containers are not block containers,
and so some properties that were designed with the assumption of block layout don't apply in the context of flex layout.
In particular:
<ul>
<li>
all of the 'column-*' properties in the Multicol module have no effect on a flex container.
<li>
'float' and 'clear' have no effect on a <i>flex item</i>.
Using 'float' on an element still causes that element's 'display' property to compute to ''block'',
and thus 'float' may affect whether an element becomes a flex item
or is wrapped in one.
(See <a href="http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">CSS 2.1 Chapter 9.7</a>.)
<li>
'vertical-align' has no effect on a flex item.
</ul>
<p>
If an element's specified value for 'display' is ''inline-flex''
and the element is floated or absolutely positioned,
the computed value of 'display' is ''flex''.
The table in <a href="http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">CSS 2.1 Chapter 9.7</a> is thus amended
to contain an additional row,
with ''inline-flex'' in the "Specified Value" column
and ''flex'' in the "Computed Value" column.
<p>
The baseline of a flex container is determined as follows:
<ul>
<li>
If any of the <i>flex items</i> on the flex container's first line
(after reordering with 'flex-order')
have a computed value of ''baseline'' for 'flex-item-align',
the flex container's baseline is the baseline of those <i>flex items</i>.
<li>
Otherwise, if the flex container has at least one <i>flex item</i>,
and the baseline of its first <i>flex item</i>
(after reordering with 'flex-order')
is parallel to the flex container's <i>main-axis</i>,
the flex container's baseline is the baseline of its first <i>flex item</i>.
<li>
Otherwise, the flex container's baseline is the "after" edge of its content box.
</ul>
<h2 id='flex-items'>
Flex Items</h2>
<p>
The flex layout algorithm operates on boxes generated by <dfn title="flex item|flex item's" id="flex-item">flex items</dfn>.
Each of the following becomes a <i>flex item</i>:
<ol>
<li>
A block-level child of a flex container
<li>
An atomic inline-level child of a flex container
<li>
An HTML
<code><img></code>,
<code><canvas></code>,
<code><svg></code>,
<code><math></code>,
<code><audio></code>,
<code><video></code>,
<code><iframe></code>,
<code><object></code>,
<code><embed></code>,
<code><applet></code>,
<code><progress></code>,
<code><meter></code>,
<code><input></code>,
<code><button></code>,
<code><select></code>,
or <code><textarea></code> element.
<span class='note'>Note: In other words, any element that is "intended to be" a replaced element, even if it becomes non-replaced (like an <code><object></code> that doesn't load and is replaced by its contents).</span>
<li>
An anonymous block wrapped around a contiguous run of non-replaced inline child elements.
However, if the anonymous block would contain solely an anonymous inline containing only whitespace,
the box is not generated,
as if it had ''display:none''.
</ol>
<p>
Some values of 'display' trigger the generation of anonymous boxes.
For example, a ''table-cell'' child of a block container is fixed up by generating ''table'' and ''table-row'' elements around it.
This fixup must occur <em>before</em> a flex container's contents are checked to see if it's necessary to generate anonymous flex items.
<p>
The computed value for 'display' for elements that are <i>flex items</i>
must be determined by applying the table in <a href="http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo">CSS 2.1 Chapter 9.7</a>.
<span class='note'>This is expected to be superseded by a future specification that defines a new 'display' value for flex items.</span>
<div class="example">
<p>Examples of flex items:
<pre>
<div style="display:flex">
<!-- flex item: block-level child -->
<div id="item1">block</div>
<!-- not a flex item, because it's out-of-flow -->
<!-- however, the placeholder it leaves behind is -->
<div id="not-an-item-A" style="position: absolute;">block</div>
<!-- flex item: block-level child -->
<div id="item3" style="display:table">table</div>
<!-- flex item: anonymous table wrapped around table-cell -->
<div id="item4" style="display:table-cell">table-cell</div>
<!-- flex item: anonymous block box around inline content -->
anonymous item 5
<!-- flex item: block-level child -->
<div id="item6">block</div>
<!-- flex item: anonymous block around inline content -->
item 7
<span>
item 7
<div id="not-an-item-B">item 7</div>
item 7
</span>
<!-- flex item: block-level replaced element -->
<iframe id="item8" style="display:block;"></iframe>
<!-- flex item: inline-level replaced element -->
<img id="item9">
<!-- flex item: atomic inline-level element -->
<button id="item10">button</button>
<!-- flex item: inline-table -->
<div id="item11" style="display:inline-table">table</div>
<!-- flex item: floated inline, which changes to a block -->
<span id="item12" style="float: left;">span</span>
</div></pre>
<p>
Notice that block element "not-an-item-B" is not a flex item,
because it's not a child element of the flex container,
even though the inline <span> is eventually broken around it by Block Layout.
Similarly,
the block element "not-an-item-A" is not a flex item,
because absolutely positioned children have special treatment.
</div>
<p class='note'>
Future display types may generate anonymous containers (e.g. ruby) or otherwise mangle the box tree (e.g. run-ins).
It is intended that flex item determination run after these operations.
<h3 id=flex-item-margins>
Flex Item Margins</h3>
<p>
The margins of adjacent <i>flex items</i> do not collapse.
Auto margins absorb extra space in that dimension and can be used for alignment
and to push adjacent flex items apart;
see <a href="#auto-margins">Aligning with ''auto'' margins</a>.
<h3 id='abspos-flex-items'>
Out-of-flow Items</h3>
<p>
Absolutely positioned children of a flex container are not themselves <i>flex items</i>,
but they leave behind "placeholders" in their normal position in the box tree.
These placeholders are atomic inline boxes with a width, height, and flex of ''0'',
becoming zero-sized flex items that otherwise interact normally with the flex layout algorithm.
<p>
The <i>static position</i> of an absolutely positioned child of a flex container
(the position when the 'top'/'right'/'bottom'/'left' properties are ''auto''), then,
is the final position of its corresponding placeholder, after flex layout has been performed.
<p class='note'>
Note: In most cases,
this means that absolutely-positioned items have no effect on flex layout.
However, when the flex container has ''flex-pack:justify'' or ''flex-pack:distribute'',
each placeholder flex item will cause there to be two packing spaces
where there would otherwise be only one,
possibly resulting in increased space between two "real" items.
<h3 id='visibility-collapse'>
Collapsed Items</h3>
<p>
Specifying ''visibility:collapse'' on a flex item
causes it to become a <dfn title="collapsed flex item|collapsed">collapsed flex item</dfn>,
producing an effect similar to ''visibility:collapse'' on a table-row or table-column:
the collapsed element is removed from rendering entirely,
but leaves behind a "strut" that keeps the flex line's cross-size stable.
Thus, if a flex container has only one flex line,
dynamically collapsing or uncollapsing items
is guaranteed to have no effect on the flex container's <i>cross size</i>
and won't cause the rest of the page's layout to "wobble".
Flex line wrapping <em>is</em> re-done after collapsing, however,
so the cross-size of a flex container with multiple lines might or might not change.
<p>
Though collapsed flex items aren't rendered,
they do appear in the <a href="http://www.w3.org/TR/CSS21/intro.html#formatting-structure">formatting struture</a>.
Therefore, unlike on ''display:none'' items [[!CSS21]],
effects that depend on an element appearing in the formatting structure
(like incrementing counters or running animations and transitions)
still operate on collapsed items.
<p>
To compute the size of the strut, flex layout is first performed with all items uncollapsed,
and then re-run with each collapsed item replaced by a strut that maintains
the original cross-size of the item's original line.
See the <a href="#layout-algorithm">Flex Layout Algorithm</a>
for the normative definition of how ''visibility:collapse''
interacts with flex layout.
<p class='note'>
Note that using ''visibility:collapse'' on any flex items
will cause the flex layout algorithm to repeat partway through,
re-running the most expensive steps.
It's recommended that authors continue to use ''display:none'' to hide items
if the items will not be dynamically collapsed and uncollapsed,
as that is more efficient for the layout engine.
<h3 id='min-size-auto'>
Implied Minimum Size of Flex Items</h3>
<p>
To provide a more reasonable default minimum size for flex containers,
this specification introduces a new <a href="#min-auto">''auto''</a> value as the initial value
of the 'min-width' and 'min-height' properties
defined in CSS 2.1. [[!CSS21]]
<table class="propdef">
<tr>
<th>Name:
<td>min-width, min-height
<tr>
<th><a href="#values">New value</a>:
<td>auto
<tr>
<th>New Initial Value:
<td><strong title="min-width/height:auto">auto</strong>
<tr>
<th>New Computed Value:
<td>the percentage as specified or the absolute length or a keyword
</table>
<dl>
<dt><dfn title="min-width/height:auto" id='min-auto'>auto</dfn>
<dd>
On a flex item,
this keyword indicates a minimum size of the min-content size.
<p class='note'>
It is intended that this will compute to the ''min-content'' keyword
when the specification defining it (<a href="http://www.w3.org/TR/css3-writing-modes/#intrinsic-sizing">Writing Modes Appendix D</a>) is sufficiently mature.
<p>
On any other element,
this keyword computes to ''0''
(unless otherwise defined by a future specification).
</dl>
<h2>
Ordering and Orientation</h2>
<p>
The contents of a flex container can be laid out in any direction and in any order.
This allows an author to trivially achieve effects that would previously have required complex or fragile methods,
such as hacks using the 'float' and 'clear' properties.
This functionality is exposed through the 'flex-direction', 'flex-wrap', and 'flex-order' properties.
<h3 id='flex-direction-property'>
Flex Flow Direction: the 'flex-direction' property</h3>
<table class=propdef>
<tr>
<th>Name:</th>
<td><dfn>flex-direction</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>row | row-reverse | column | column-reverse
<tr>
<th>Initial:
<td>row
<tr>
<th>Applies To:
<td>flex containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Computed Value:
<td>specified value
<tr>
<th>Media:
<td>visual
<tr>
<th>Animatable:
<td>no
<tr>
<th>Canonical Order:
<td><abbr title="follows order of property value definition">per grammar</abbr>
</table>
<p>
The 'flex-direction' property specifies how <i>flex items</i> are placed in the flex container,
by setting the direction of the flex container's <i>main axis</i>.
This determines the direction that flex items are laid out in.
<dl>
<dt><dfn id='flex-flow-row'>''row''</dfn>
<dd>
The flex container's <i>main axis</i> has the same orientation as the inline axis of the current <a href="http://www.w3.org/TR/css3-writing-modes/#writing-mode">writing mode</a>
(the primary direction that text is laid out in).
The <i>main-start</i> and <i>main-end</i> directions are equivalent to the <a href="http://www.w3.org/TR/css3-writing-modes/#start">start</a> and <a href="http://www.w3.org/TR/css3-writing-modes/#end">end</a> directions, respectively,
of the current <a href="http://www.w3.org/TR/css3-writing-modes/#writing-mode">writing mode</a>.
<dt><dfn id='flex-flow-row-reverse'>''row-reverse''</dfn>
<dd>
Same as ''row'',
except the <i>main-start</i> and <i>main-end</i> directions are swapped.
<dt><dfn id='flex-flow-column'>''column''</dfn>
<dd>
The flex container's <i>main axis</i> has the same orientation as the block axis of the current <a href="http://www.w3.org/TR/css3-writing-modes/#writing-mode">writing mode</a>
(the primary direction that blocks are laid out in).
The <i>main-start</i> and <i>main-end</i> directions are equivalent to the <a href="http://www.w3.org/TR/css3-writing-modes/#before">before</a> and <a href="http://www.w3.org/TR/css3-writing-modes/#after">after</a> directions, respectively,
of the current <a href="http://www.w3.org/TR/css3-writing-modes/#writing-mode">writing mode</a>.
<dt><dfn id='flex-flow-column-reverse'>''column-reverse''</dfn>
<dd>
Same as ''column'',
except the <i>main-start</i> and <i>main-end</i> directions are swapped.
</dl>
<h3 id='flex-wrap-property'>
Flex Line Wrapping: the 'flex-wrap' property</h3>
<table class=propdef>
<tr>
<th>Name:</th>
<td><dfn>flex-wrap</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>nowrap | wrap | wrap-reverse
<tr>
<th>Initial:
<td>nowrap
<tr>
<th>Applies To:
<td>flex containers
<tr>
<th>Inherited:
<td>no
<tr>
<th>Computed Value:
<td>specified value
<tr>
<th>Media:
<td>visual
<tr>
<th>Animatable:
<td>no
<tr>
<th>Canonical Order:
<td><abbr title="follows order of property value definition">per grammar</abbr>
</table>
<p>
The 'flex-wrap' property controls whether the flex container is <i>single-line</i> or <i>multi-line</i>,
and the direction of the <i>cross-axis</i>,
which determines the direction new lines are stacked in.
<dl>
<dt><dfn id='flex-flow-nowrap'>''nowrap''</dfn>
<dd>
The flex container is <i>single-line</i>.
The <i>cross-start</i> direction is equivalent to either
the <a href="http://www.w3.org/TR/css3-writing-modes/#start">start</a>
or <a href="http://www.w3.org/TR/css3-writing-modes/#before">before</a>
direction of the current <a href="http://dev.w3.org/csswg/css3-writing-modes/#writing-mode">writing mode</a>,
whichever is in the <i>cross axis</i>,
and the <i>cross-end</i> direction is the opposite direction of <i>cross-start</i>.
<dt><dfn id='flex-flow-wrap'>''wrap''</dfn>
<dd>
The flex container is <i>multi-line</i>.
The <i>cross-start</i> direction is equivalent to either
the <a href="http://www.w3.org/TR/css3-writing-modes/#start">start</a>
or <a href="http://www.w3.org/TR/css3-writing-modes/#before">before</a>
direction of the current <a href="http://dev.w3.org/csswg/css3-writing-modes/#writing-mode">writing mode</a>,
whichever is in the <i>cross axis</i>,
and the <i>cross-end</i> direction is the opposite direction of <i>cross-start</i>.
<dt><dfn id='flex-flow-wrap-reverse'>''wrap-reverse''</dfn>
<dd>
Same as ''wrap'',
except the <i>cross-start</i> and <i>cross-end</i> directions are swapped.
</dl>
<h3 id='flex-flow-property'>
Flex Direction and Wrap: the 'flex-flow' shorthand</h3>
<table class=propdef>
<tr>
<th>Name:</th>
<td><dfn>flex-flow</dfn>
<tr>
<th><a href="#values">Value</a>:
<td><'flex-direction'> || <'flex-wrap'>
<tr>
<th>Initial:
<td>see individual properties
<tr>
<th>Applies To:
<td>flex containers
<tr>
<th>Inherited:
<td>see individual properties
<tr>
<th>Computed Value:
<td>see individual properties
<tr>
<th>Media:
<td>visual
<tr>
<th>Animatable:
<td>no
<tr>
<th>Canonical Order:
<td><abbr title="follows order of property value definition">per grammar</abbr>
</table>
<p>The 'flex-flow' property is a shorthand for setting the 'flex-direction' and 'flex-wrap' properties together.
<div class="example">
<p>Some examples of valid flows:
<table style="margin: 0 auto; vertical-align: middle; border-spacing: 2em 1em;">
<tr>
<td><pre>
div { flex-flow: row; }
/* Initial value. Main-axis is
inline, no wrap. */</pre>
<td><img src='images/flex-flow1.svg' width=240 height=80 alt>
<tr>
<td><pre>
div { flex-flow: column wrap; }
/* Main-axis is block-direction and lines
wrap in the inline direction. For an
English page, the main-axis is top-to-bottom
and lines wrap to the right. */</pre>
<td><img src='images/flex-flow2.svg' width=170 height=200 alt>
<tr>
<td><pre>
div { writing-mode: vertical-rl;
flex-flow: column wrap-reverse; }
/* Main-axis is block direction (right to
left). New lines wrap upwards. */</pre>
<td><img src='images/flex-flow3.svg' width=200 height=170 alt>
</table>
</div>
<h3 id='flex-order-property'>
Display Order: the 'flex-order' property</h3>
<p><i>Flex items</i> are, by default, displayed and laid out in the same order as they appear in the source document. The 'flex-order' property may be used to change this ordering.
<table class=propdef>
<tr>
<th>Name:
<td><dfn>flex-order</dfn>
<tr>
<th><a href="#values">Value</a>:
<td><number>
<tr>
<th>Initial:
<td>0
<tr>
<th>Applies to:
<td><i>flex items</i></td>
<tr>
<th>Inherited:
<td>no
<tr>
<th>Computed value:
<td>specified value
<tr>
<th>Media:
<td>visual
<tr>
<th>Animatable:
<td>yes
<tr>
<th>Canonical Order:
<td><abbr title="follows order of property value definition">per grammar</abbr>
</table>
<p>
The 'flex-order' property assigns <i>flex items</i> to ordinal groups.
<p>
Ordinal groups control the order in which <i>flex items</i> appear.
A flex container will lay out its content starting from the lowest numbered ordinal group and going up.
Items with the same ordinal group are laid out in the order they appear in the source document.
'flex-order' has no effect on stacking/layering;
elements must still be drawn over/under each other based on document order, 'z-index', and other relevant means.
<div class='example'>
<p>The following figure shows a simple tabbed interface, where the tab for the active pane is always in front:
<div class='figure'><img src="images/flex-order-example.png" alt></div>
<p>This could be implemented with the following CSS (showing only the relevant code):
<pre>
.tabs {
display: flex;
}
.tabs > .current {
flex-order: -1; /* Lower than the default of 0 */
}</pre>
</div>
<div class='example'>
<p>
Many web pages have a similar shape in the markup,
with a header on top,
a footer on bottom,
and then a content area and one or two additional columns in the middle.
Generally,
it's desirable that the content come first in the page's source code,
before the additional columns.
However, this makes many common designs,
such as simply having the additional columns on the left and the content area on the right,
difficult to achieve.
This has been addressed in many ways over the years,
often going by the name "Holy Grail Layout" when there are two additional columns.
'flex-order' makes this trivial.
For example, take the following sketch of a page's code and desired layout:
<div class='code-and-figure'>
<div><pre>
<!DOCTYPE html>
<header>...</header>
<div id='main'>
<article>...</article>
<nav>...</nav>
<aside>...</aside>
</div>
<footer>...</footer></pre></div>
<div><img src="images/flex-order-page.svg" width=400 height=360 alt="In this page the header is at the top and the footer at the bottom, but the article is in the center, flanked by the nav on the right and the aside on the left."></div>
</div>
<p>
This layout can be easily achieved with flex layout:
<pre>
#main { display: flex; }
#main > article { flex:1; flex-order: 2; }
#main > nav { width: 200px; flex-order: 1; }
#main > aside { width: 200px; flex-order: 3; }</pre>
<p>
As an added bonus,
the columns will all be <a href="#flex-align-stretch">equal-height</a> by default,
and the main content will be as wide as necessary to fill the screen.
Additionally,
this can then be combined with media queries to switch to an all-vertical layout on narrow screens:
<pre>
@media all and (max-width: 600px) {
/* Too narrow to support three columns */
#main { flex-flow: column; }
#main > article, #main > nav, #main > aside {
/* Return them to document order */
flex-order: 0; width: auto;
}
}</pre>
<p><small>(Further use of multiline flex containers to achieve even more intelligent wrapping left as an exercise for the reader.)</small>
</div>
<h2 id='flex-lines'>
Flex Lines</h2>
<p>
A flex container can be either <i>single-line</i> or <i>multi-line</i>,
depending on the 'flex-wrap' property:
<ul>
<li>
A <dfn>single-line</dfn> flex container lays out all of its children in a single line,
even if that would cause its contents to overflow.
<li>
A <dfn>multi-line</dfn> flex container breaks its <i>flex items</i> across multiple lines to avoid overflowing,
similar to how text is broken onto a new line when it gets too wide to fit on the existing line.
When additional lines are created,
they are stacked in the flex container along the <i>cross axis</i>
according to the 'flex-wrap' property.
Every line contains at least one <i>flex item</i>,
unless the flex container itself is completely empty.
</ul>
<p>
Once content is broken into lines,
each line is laid out independently;
flexible lengths and the 'flex-pack' and 'flex-item-align' properties only consider the items on a single line at a time.
<p>
When a flex container has multiple lines, the <i>cross size</i> of each line
is the minimum size necessary to contain the <i>flex items</i> on the line
(after aligment due to 'flex-item-align'),
and the lines are aligned within the flex container with the 'flex-line-pack' property.
When a flex container (even a <i>multi-line</i> one) has only one line,
the <i>cross size</i> of the line is the <i>cross size</i> of the flex container,
and 'flex-line-pack' has no effect.
The <i>main size</i> of a line is always the same as the <i>main size</i> of the flex container's content box.
<div class=example>
<p>
This example shows four buttons that do not fit horizontally.
<pre><style>
#div1 {
display: flex;
flex-flow: row wrap;
width: 300px;
}
button {
flex:80px 1;
}
<style>
<div id="div1">
<button id="button1">Elephant</button>
<button id="button2">Tiger</button>
<button id="button3">Antelope</button>
<button id="button4">Wildebeest</button>
</div></pre>
<p>
The buttons are first set to their preferred widths,
in this case 80 pixels.
This will allow the first three buttons to fit in 240 pixels with 60 pixels left over of remaining space.
Because the 'flex-flow' property specifies a multi-line flex container
(due to the ''wrap'' keyword appearing in its value),
the flex container will create an additional line to contain the last button.
<p>
Flexibility is applied to each element,
separately for each line.
The first line has 60 pixels of remaining space
and all of the buttons have the same flexibility,
so each of the three buttons on that line will receive 20 pixels of extra width,
ending up 100px wide.
The remaining button is on a line of its own
and will stretch to the entire width of the line, or 300 pixels.
<div style="width:300px; border:medium solid green; overflow: hidden;">
<button style="width:100px; float: left;">Elephant</button>
<button style="width:100px; float: left;">Tiger</button>
<button style="width:100px; float: left;">Antelope</button>
<button style="width:300px; float: left;">Wildebeest</button>
</div>
<p>
If the box were resized, the buttons would rearrange onto different lines as necessary.
<p>
If the style rules in the example above were changed to the following:
<pre>
#div1 {
display: flex;
flex-flow: row wrap;
flex-pack: center;
width: 300px;
}
button {
flex:80px 1;
max-width: 90px;
}</pre>
<p>
Similar to the previous example,
the first three buttons will fit on the first line,
and the last button will wrap onto a new line.
However, when the buttons attempt to flex they can only grow to 90px each,
due to their 'max-width' property.
This leaves 30px of free space on the first line
and 210px of free space on the second line.
Because 'flex-pack' is set to ''center'',
the buttons will be centered on each line,
with the free space split equally on either side.
<div style="width:300px; border:medium solid green; text-align:center;">
<button style="width:90px; float: left; margin-left: 15px;">Elephant</button>
<button style="width:90px; float: left;">Tiger</button>
<button style="width:90px; float: left;">Antelope</button>
<button style="width:90px">Wildebeest</button>
</div>
</div>
<h2 id='flexibility'>
Flexibility</h2>
<p>
The defining aspect of flex layout is the ability to make the <i>flex items</i> "flex",
altering their width or height to fill the available space.
This is done with the 'flex' property.
A flex container distributes free space to its items proportional to their <i>positive flex ratio</i>,
or shrinks them to prevent overflow proportional to their <i>negative flex ratio</i>.
<h3 id='flex-property'>
The 'flex' Shorthand</h3>
<table class=propdef>
<tr>
<th>Name:
<td><dfn>flex</dfn>
<tr>
<th><a href="#values">Value</a>:
<td>none | [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ]
<tr>
<th>Initial:
<td>see individual properties
<tr>
<th>Applies to:
<td>flex items
<tr>
<th>Inherited:
<td>see individual properties
<tr>
<th>Computed Value:
<td>see individual properties
<tr>
<th>Media:
<td>visual
<tr>
<th>Animatable:
<td>see individual properties
<tr>
<th>Canonical Order:
<td>per grammar
</table>
<p>
The 'flex' property specifies the components of a <dfn title="flexible length|flexible length's">flexible length</dfn>:
the <i title="positive flex ratio">positive</i>
and <i>negative flex ratio</i>,
and the <i>flex basis</i>.
When an element is a <i>flex item</i>,
'flex' is consulted <em>instead of</em> the <i>main size property</i>
to determine the <i>main size</i> of the element.
If an element is not a <i>flex item</i>,
'flex' has no effect.
<p>
The <var><'flex-basis'></var> component sets the <dfn>flex basis</dfn>,