css3-flexbox/Overview.src.html
author Alex Mogilevsky <alexmog@microsoft.com>
Fri, 09 Mar 2012 12:45:39 -0800
changeset 4708 166bd5c52fbd
parent 4689 6426ac85f969
child 4724 c515c530ea37
permissions -rw-r--r--
changed initial value for 'flex-line-pack' to 'stretch'
     1 <!DOCTYPE html>
     2 <html lang="en">
     3 <head>
     4 	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
     5 	<title>CSS Flexible Box Layout Module</title>
     6 	<link rel="stylesheet" type="text/css" href="http://dev.w3.org/csswg/default.css">
     7 	<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-ED">
     8 	<style>
     9 	code.one-line { white-space: pre; }
    10 	.code-and-figure {
    11 		display: table;
    12 		margin: 0 auto;
    13 		border-spacing: 1em;
    14 	}
    15 	.code-and-figure > div {
    16 		display: table-cell;
    17 		vertical-align: middle;
    18 	}
    19 	</style>
    20 </head><body>
    21 
    22 <div class="head">
    23 	<!--logo-->
    24 
    25 	<h1 id="head-box-flexible">CSS Flexible Box Layout Module</h1>
    26 	<h2 class="no-num no-toc" id="w3c-working">[LONGSTATUS], [DATE]</h2>
    27 
    28 	<dl>
    29 		<dt>This version:
    30 		<!--<dd><a href="[VERSION]">[VERSION]</a>-->
    31 		<dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
    32 
    33 		<dt>Latest version:
    34 		<!--<dd><a href="[LATEST]">[LATEST]</a>-->
    35 		<dd><a href="http://www.w3.org/TR/css3-flexbox/">http://www.w3.org/TR/css3-flexbox/</a>
    36 
    37 		<dt>Editor's Draft:
    38 		<dd><a href="http://dev.w3.org/csswg/css3-flexbox/">http://dev.w3.org/csswg/css3-flexbox/</a>
    39 
    40 		<dt>Previous versions:
    41 		<dd><a href="http://www.w3.org/TR/2011/WD-css3-flexbox-20111129/">http://www.w3.org/TR/2011/WD-css3-flexbox-20111129/</a>
    42 		<dd><a href="http://www.w3.org/TR/2011/WD-css3-flexbox-20110322/">http://www.w3.org/TR/2011/WD-css3-flexbox-20110322/</a>
    43 		<dd><a href="http://www.w3.org/TR/2009/WD-css3-flexbox-20090723/">http://www.w3.org/TR/2009/WD-css3-flexbox-20090723/</a>
    44 
    45 		<dt>Issues List:</dt>
    46         <dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&component=Flexbox&resolution=---">Bugzilla Bugs for Flexbox</a></dd>
    47 
    48 		<dt>Discussion:</dt>
    49 		<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>"
    50 
    51 		<dt>Editors:
    52 		<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a>, Google Inc.</dd>
    53 		<dd>Alex Mogilevsky, Microsoft Corporation, <a href="mailto:alexmog@microsoft.com">alexmog@microsoft.com</a>
    54 		<dd>L. David Baron, Mozilla Corporation, <a href="mailto:dbaron@dbaron.org">dbaron@dbaron.org</a>
    55 
    56 		<dt>Authors and former editors:
    57 		<dd>Neil Deakin, Mozilla Corporation, <a href="mailto:enndeakin@gmail.com">enndeakin@gmail.com</a>
    58 		<dd>Ian Hickson, formerly of Opera Software, <a href="mailto:ian@hixie.ch">ian@hixie.ch</a>
    59 		<dd>David Hyatt, formerly of Netscape Corporation, <a href="mailto:hyatt@apple.com">hyatt@apple.com</a>
    60 	</dl>
    61 	<!--copyright-->
    62 
    63 	<hr title="Separator for header">
    64 </div>
    65 
    66 <h2 class="no-num no-toc" id="abstract">
    67 Abstract</h2>
    68 
    69 	<p>The specification describes a CSS box model optimized for user interface design. In the flexbox layout model, the children of a flexbox 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.</p>
    70 
    71 <h2 class="no-num no-toc" id="status">Status of this document</h2>
    72 <!--status-->
    73 
    74 <h2 class="no-num no-toc" id="table">Table of contents</h2>
    75 <!--toc-->
    76 
    77 
    78 <h2 id="intro">
    79 Introduction</h2>
    80 
    81 	<p><em>This section is not normative.</em>
    82 
    83 	<p>CSS 2.1 defined four layout modes &mdash; 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, flexbox layout, which is designed for laying out more complex applications and webpages.</p>
    84 
    85 	<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 '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>
    86 
    87     <p>The contents of a flexbox 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 flexbox is <i>multi-line</i>, the flexbox items flow in two dimensions, wrapping into separate lines in a fashion similar to how text is wrapped into multiple lines.</p>
    88 
    89 	<div class="example">
    90 		<p>For example, the following HTML snippet uses flexbox to create a toolbar with icons.  The flexbox is horizontal, and the children's widths don't fill the flexbox's width, so the additional space is distributed around and between the children.  As the flexbox grows (perhaps because the user is viewing the page on a wider screen), the children spread out evenly and automatically:</p>
    91 
    92 		<pre>
    93 &lt;ul>
    94   &lt;li>&lt;button>&lt;img src='new.svg' alt="New">&lt;/button>&lt;/li>
    95   &lt;li>&lt;button>&lt;img src='upload.svg' alt="Upload">&lt;/button>&lt;/li>
    96   &lt;li>&lt;button>&lt;img src='save.svg' alt="Save">&lt;/button>&lt;/li>
    97   &lt;li>&lt;button>&lt;img src='trash.svg' alt="trash">&lt;/button>&lt;/li>
    98 &lt;/ul>
    99 &lt;style>
   100 ul { display: flexbox; flex-pack: distribute; }
   101 /* Irrelevant styling for this example removed. */
   102 &lt;/style></pre>
   103 
   104 		<div class='figure'>
   105 			<img src="images/toolbar-example.svg" width=400 height=140>
   106 			<p class='caption'>Example rendering of the above code snippet, at two different flexbox widths.</p>
   107 		</div>
   108 	</div>
   109 
   110 <h3 id="placement">
   111 Module interactions</h3>
   112 
   113 	<p>This module extends the definition of the 'display' property.
   114 
   115 <h3 id="values">
   116 Values</h3>
   117 
   118 	<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 [[CSS3COLOR]], when combined with this module, expands the definition of the &lt;color&gt; value type as used in this specification.</p>
   119 
   120 	<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.
   121 
   122 <h2 id='box-model'>
   123 The Flexbox Box Model</h2>
   124 
   125 	<p>An element with ''display:flexbox'' or ''display:inline-flexbox'' is a <dfn>flexbox</dfn>.  Children of a flexbox are called <dfn title="flexbox item|flexbox items|flexbox item's" id="flexbox-item">flexbox items</dfn> and are laid out using the flexbox box model.</p>
   126 
   127 	<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 flexbox layout algorithm is agnostic as to the direction the flexbox happens to be laid out in.  To make it easier to talk about flexbox 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.</p>
   128 
   129 	<div class='figure'>
   130 		<img src='images/flex-direction-terms.svg' width=510 height=300>
   131 		<p class='caption'>An illustration of the various directions and sizing terms used in this specification, respectively for ''row'' and ''column'' flexboxes.</p>
   132 	</div>
   133 
   134 	<p>The <dfn>main axis</dfn> of a flexbox is the axis on which <i>flexbox items</i> are laid out along.  The <i>flexbox items</i> are ordered such that they start on the <dfn>main-start</dfn> side of the flexbox, and go toward the <dfn>main-end</dfn> side.  A <i>flexbox item's</i> width or height, whichever is in the <i>main axis</i>, is the item's <dfn>main size</dfn>.  The <i>flexbox item's</i> <dfn>main size property</dfn> is either the 'width' or 'height' property, whichever is in the <i>main axis</i>.</p>
   135 
   136 	<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> directions and sides defined.  The width or height of a <i>flexbox item</i>, whichever is in the <i>cross axis</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 axis</i>.</p>
   137 
   138 	<p>The contents of a flexbox can be easily and powerfully manipulated with a handful of properties.  Most significantly, <i>flexbox 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 flexbox after all of the <i>flexbox items</i> have finished flexing, the items can be aligned, centered, or distributed with the 'flex-pack' property.  <i>Flexbox items</i> can also be completely rearranged within the flexbox with the 'flex-order' property.</p>
   139 
   140 	<p>In the <i>cross axis</i>, <i>flexbox items</i> can either "flex" to fill the available space or be aligned within the space with the 'flex-align' property.  If a flexbox 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 flexbox with the 'flex-line-pack' property.</p>
   141 
   142 
   143 <h2 id='display-flexbox'>
   144 New values for 'display' property</h2>
   145 
   146 	<table class="propdef">
   147 		<tr>
   148 			<th>Name:
   149 			<td>display
   150 		<tr>
   151 			<th><a href="http://dev.w3.org/csswg/css-module/#values">New value</a>:
   152 			<td>flexbox | inline-flexbox
   153 	</table>
   154 
   155 	<p>You can declare that an element is a flexbox, and thus should use flexbox layout for its contents, by setting the 'display' property on the element to the value 'flexbox' or 'inline-flexbox'.</p>
   156 
   157 	<p>The ''flexbox'' value makes the flexbox a block-level element.  The ''inline-flexbox'' value makes the flexbox an inline-level element.  When it is necessary to distinguish them, this specification will refer to the former kind of flexbox as a <dfn id="block-flexbox" title="block flexbox|block flexboxes">block flexbox</dfn> and the latter type as an <dfn id="inline-flexbox" title="inline flexbox|inline flexboxes">inline flexbox</dfn>.</p>
   158 
   159 	<p>Flexboxes 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:</p>
   160 
   161 	<ul>
   162 		<li>all of the 'column-*' properties in the Multicol module have no effect on a flexbox.</li>
   163 
   164 		<li>'float' and 'clear' have no effect on a flexbox item.  Using 'float' on an element still causes that element's 'display' property to compute to ''block'', as normal, because that occurs before flexbox items are determined (the algorithm for wrapping children of a flexbox into <i>flexbox items</i> needs the computed value of 'display').</li>
   165 
   166 		<li>'vertical-align' has no effect on the alignment of a flexbox item in the flexbox</li>
   167 	</ul>
   168 
   169 	<p>If an element's specified value for 'display' is ''inline-flexbox'' and the element is floated or absolutely positioned, the computed value of 'display' must be ''flexbox''.</p>
   170 
   171 	<p>A flexbox establishes a new <dfn>flexbox formatting context</dfn> for its contents.  This is similar to a block formatting context root: floats do not intrude into the flexbox, and the flexbox's margins do not collapse with the margins of its contents.  Additionally, each of the <i>flexbox items</i> establish a new formatting context for its contents.</p>
   172 
   173 <h2 id='flex-items'>
   174 Flexbox Items</h2>
   175 
   176 	<p>Flexbox layout algorithm operates on <i>flexbox items</i>, which are boxes that satisfy at least one of the following criteria:</p>
   177 
   178 	<ol>
   179 		<li>Immediate block-level children of flexbox</li>
   180 
   181 		<li>Atomic inline-level children of flexbox</li>
   182 
   183 		<li>Contiguous run of non-replaced inline children, wrapped into an anonymous box</li>
   184 	</ol>
   185 
   186 	<p>Some values of 'display' trigger "fixup" to ensure a sensible tree structure.  For example, a lone ''table-cell'' box is fixed up by generating ''table'', ''table-row-group'', and ''table-row'' boxes around it.  This fixup must occur <em>before</em> a flexbox's contents are checked to see if it's necessary to generate anonymous flexbox items.</p>
   187 
   188 	<p class='note'>In the future, other kinds of fixup such as ''display:run-in'' or ''display:ruby'' should also run before flexbox fixup.</p>
   189 
   190 	<div class="example">
   191 		<p>Examples of flexbox items:</p>
   192 		<pre>
   193 &lt;div style="display:flexbox">
   194 
   195     &lt;!-- flexbox item: block-level child -->
   196     &lt;div id="item1">block&lt;/div>
   197 
   198     &lt;!-- not a flexbox item, because it's out-of-flow -->
   199     &lt;div id="not-an-item1.5" style="position: absolute;">block&lt;/div>
   200     
   201     &lt;!-- flexbox item: block-level child -->
   202     &lt;div id="item2" style="display:table">table&lt;/div>
   203     
   204     &lt;!-- flexbox item: anonymous table wrapped around table-cell -->
   205     &lt;div id="item3" style="display:table-cell">table-cell&lt;/div> 
   206 
   207     &lt;!-- flexbox item: anonymous block around inline content -->
   208     anonymous item 4
   209 
   210     &lt;!-- flexbox item: block-level child -->
   211     &lt;div id="item5">block&lt;/div>
   212 
   213     &lt;!-- flexbox item: anonymous block around inline content -->
   214     anonymous item 6.1
   215     &lt;span id="item6.1">
   216         text 6.2
   217         &lt;div id="not-an-item6.3">block&lt;/div>
   218         text 6.4
   219     &lt;/span>
   220 
   221     &lt;!-- flexbox item: block-level replaced element -->
   222     &lt;iframe id="item7"></iframe>
   223 
   224     &lt;!-- flexbox item: inline-level replaced element -->
   225     &lt;img id="item7" style="display:inline">
   226 
   227     &lt;!-- flexbox item: inline-level replaced element -->
   228     &lt;button id="item8">button&lt;/button>
   229 
   230     &lt;!-- flexbox item: inline-table -->
   231     &lt;div id="item9" style="display:inline-table">table&lt;/div>
   232 
   233     &lt;!-- flexbox item: floated inline, which changes to a block -->
   234     &lt;span id="item10" style="float: left;">span&lt;/span>
   235 &lt;/div></pre>
   236 
   237 		<p>Notice that block element "not-an-item6.3" is not a separate flexbox item, because it is contained inside an inline element which is being wrapped into an anonymous flex item.  Similarly, the block element "not-an-item1.5" is not a flexbox item, because it's absolutely positioned and thus out of flow.</p>
   238 	</div>
   239 
   240 <h3 id='abspos-flexbox-items'>
   241 Absolutely Positioned Flexbox Children</h3>
   242 
   243 	<p>Absolutely positioned children of a flexbox are not <i>flexbox items</i>, but they leave behind "placeholders" in their normal position in the box tree.  These placeholders are anonymous inline boxes with a width and height of ''0px'', and they interact normally with the flexbox layout algorithm.  In particular, they'll trigger the creation an anonymous flexbox item wrapper boxes, or join neighboring inline elements in <em>their</em> anonymous flexbox item wrapper boxes.</p>
   244 
   245 	<p>The "static position" of an absolutely positioned child of a flexbox (the position when the 'top'/'right'/'bottom'/'left' properties are ''auto''), then, is the final position of their corresponding placeholder, after flexbox layout has been performed.</p>
   246 
   247 	<p class='note'>Note: In most cases, this means that absolutely positioned items will have no effect on flexbox layout, even if they force the generation of an anonymous flexbox item wrapper, because those wrapper items will also collapse to zero size and have no effect.  The only exception is when the flexbox has ''flex-pack:justify'', in which case the anonymous flexbox item will cause there to be two packing spaces where there would otherwise be only one, which will appear as a double-size space between two "real" items.</p>
   248 
   249 
   250 <h2 id='multi-line'>
   251 Multi-line Flexbox</h2>
   252 
   253 	<p>A flexbox can be either <i>single-line</i> or <i>multi-line</i>, depending on the 'flex-wrap' property.  A <dfn>single-line</dfn> flexbox lays out all of its children in a single line, even if that would cause the flexbox to overflow its bounds.  A <dfn>multi-line</dfn> flexbox breaks its <i>flexbox 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.  Every line contains at least one <i>flexbox item</i>, unless the flexbox itself is completely empty.</p>
   254 
   255 	<p>When additional lines are created, they are stacked in the flexbox in the <i>cross axis</i>.  Each line is completely independent; flexible lengths and the 'flex-pack' and 'flex-align' properties only consider the items on a single line at a time.  The <i>main size</i> of a line is the same as the <i>main size</i> of the flexbox's content box.  The <i>cross size</i> of a line depends on whether the flexbox is <i>single-line</i> or <i>multi-line</i>: the <i>cross size</i> of the sole line in a <i>single-line</i> flexbox is the same as the <i>cross size</i> of the flexbox's content box, while the <i>cross size</i> of a line in a <i>multi-line</i> flexbox is the minimum size necessary to contain the <i>flexbox items</i> on the line, after aligning them with 'flex-align'.  The lines themselves are then aligned within a flexbox with the 'flex-line-pack' property.</p>
   256 
   257 	<div class=example>
   258 		<p>This example shows four buttons that do not fit horizontally.</p>
   259 
   260 		<pre>&lt;style>
   261 #div1 {
   262 	display: flexbox;
   263 	flex-flow: row wrap;
   264 	width: 300px;
   265 }
   266 button {
   267 	flex:80px 1;
   268 }
   269 &lt;style>
   270 
   271 &lt;div id="div1">
   272 	&lt;button id="button1">Elephant&lt;/button>
   273 	&lt;button id="button2">Tiger&lt;/button>
   274 	&lt;button id="button3">Antelope&lt;/button>
   275 	&lt;button id="button4">Wildebeest&lt;/button>
   276 &lt;/div></pre>
   277 
   278 		<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 flexbox (due to the ''wrap'' keyword appearing in its value), the flexbox will create an additional line to contain the last button.</p>
   279 
   280 		<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.</p>
   281 
   282 		<div style="width:300px; border:medium solid green; overflow: hidden;">
   283 			<button style="width:100px; float: left;">Elephant</button>
   284 			<button style="width:100px; float: left;">Tiger</button>
   285 			<button style="width:100px; float: left;">Antelope</button>
   286 			<button style="width:300px; float: left;">Wildebeest</button>
   287 		</div>
   288 
   289 		<p>If the box was resized, the buttons may rearrange onto different lines as necessary.</p>
   290 		
   291 		<p>If the style rules in the example above were changed to the following:</p>
   292 
   293 		<pre>
   294 #div1 {
   295 	display: flexbox;
   296 	flex-flow: row wrap;
   297 	flex-pack: center;
   298 	width: 300px;
   299 }
   300 button {
   301 	flex:80px 1;
   302 	max-width: 90px;
   303 }</pre>
   304 
   305 		<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.</p>
   306 
   307 		<div style="width:300px; border:medium solid green; text-align:center;">
   308 			<button style="width:90px; float: left; margin-left: 15px;">Elephant</button>
   309 			<button style="width:90px; float: left;">Tiger</button>
   310 			<button style="width:90px; float: left;">Antelope</button>
   311 			<button style="width:90px">Wildebeest</button>
   312 		</div>
   313 	</div>
   314 
   315 <h2>
   316 Ordering and Orientation</h2>
   317 
   318 	<p>The first level of flexbox functionality is the ability to lay out a flexbox's contents 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 using the 'float' property to lay out a horizontal navigation bar (which then requires further effort with the 'clear' property or others to make the elements interact nicely with the rest of the page).  This functionality is exposed through the 'flex-flow' and 'flex-order' properties.</p>
   319 
   320 <h3 id='flex-direction'>
   321 Flexbox Flow Direction: the 'flex-direction' property</h3>
   322 
   323 	<table class=propdef>
   324 		<tr>
   325 			<th>Name:</th>
   326 			<td><dfn>flex-direction</dfn>
   327 		<tr>
   328 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   329 			<td>row | row-reverse | column | column-reverse
   330 		<tr>
   331 			<th>Initial:
   332 			<td>row
   333 		<tr>
   334 			<th>Applies To:
   335 			<td>flexboxes
   336 		<tr>
   337 			<th>Inherited:
   338 			<td>no
   339 		<tr>
   340 			<th>Computed Value:
   341 			<td>specified value
   342 		<tr>
   343 			<th>Media:
   344 			<td>visual
   345 		<tr>
   346 			<th>Animatable:
   347 			<td>no
   348 	</table>
   349 
   350 	<p>The 'flex-direction' property specifies how <i>flexbox items</i> are placed in the flexbox, by setting the direction of the flexbox's <i>main axis</i>.  This affects the direction that flexbox items are laid out in, and the meaning of the 'flex-pack' property.</p>
   351 
   352 	<dl>
   353 		<dt><dfn id='flex-flow-row'>row</dfn></dt>
   354 		<dd>The flexbox's <i>main axis</i> has the same orientation as the inline axis of the current writing mode (the direction that text is laid out in).  The <i>main-start</i> and <i>main-end</i> directions are equivalent to the "start" and "end" directions, respectively, of the current writing mode.</dd>
   355 
   356 		<dt><dfn id='flex-flow-row-reverse'>row-reverse</dfn></dt>
   357 		<dd>Same as <i>row</i>, except the <i>main-start</i> and <i>main-end</i> directions are swapped.</dd>
   358 
   359 		<dt><dfn id='flex-flow-column'>column</dfn></dt>
   360 		<dd>The flexbox's <i>main axis</i> has the same orientation as the block axis of the current writing mode (the direction that blocks are laid out in).  The <i>main-start</i> and <i>main-end</i> directions are equivalent to the "before" and "after" directions, respectively, of the current writing mode.</dd>
   361 
   362 		<dt><dfn id='flex-flow-column-reverse'>column-reverse</dfn></dt>
   363 		<dd>Same as <i>column</i>, except the <i>main-start</i> and <i>main-end</i> directions are swapped.</dd>
   364 	</dl>
   365 
   366 <h3 id='flex-wrap'>
   367 Flexbox Wrapping: the 'flex-wrap' property</h3>
   368 
   369 	<table class=propdef>
   370 		<tr>
   371 			<th>Name:</th>
   372 			<td><dfn>flex-wrap</dfn>
   373 		<tr>
   374 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   375 			<td>nowrap | wrap | wrap-reverse
   376 		<tr>
   377 			<th>Initial:
   378 			<td>nowrap
   379 		<tr>
   380 			<th>Applies To:
   381 			<td>flexboxes
   382 		<tr>
   383 			<th>Inherited:
   384 			<td>no
   385 		<tr>
   386 			<th>Computed Value:
   387 			<td>specified value
   388 		<tr>
   389 			<th>Media:
   390 			<td>visual
   391 		<tr>
   392 			<th>Animatable:
   393 			<td>no
   394 	</table>
   395 
   396 	<p>The 'flex-wrap' property controls whether the flexbox is <i>single-line</i> or <i>multi-line</i>, and the direction of the <i>cross axis</i>, which affects the direction new lines are stacked in and the meaning of the 'flex-align', 'flex-item-align', and 'flex-line-pack' properties.</p>
   397 
   398 	<dl>
   399 		<dt>nowrap</dt>
   400 		<dd>The flexbox is <i>single-line</i>.  The <i>cross-start</i> direction is equivalent to either the "start" or "before" direction of the current writing mode, whichever is in the <i>cross-axis</i>, and the <i>cross-end</i> direction is the opposite direction of <i>cross-start</i>.
   401 			</dd>
   402 
   403 		<dt><dfn id='flex-flow-wrap'>wrap</dfn></dt>
   404 		<dd>The flexbox is <i>multi-line</i>.  The <i>cross-start</i> direction is equivalent to either the "start" or "before" direction of the current writing mode, whichever is in the <i>cross-axis</i>, and the <i>cross-end</i> direction is the opposite direction of <i>cross-start</i>.</dd>
   405 
   406 		<dt><dfn id='flex-flow-wrap-reverse'>wrap-reverse</dfn></dt>
   407 		<dd>Same as <i>wrap</i>, except the <i>cross-start</i> and <i>cross-end</i> directions are swapped.</dd>
   408 	</dl>
   409 
   410 <h3 id='flex-flow'>
   411 Flex Direction and Wrap: the 'flex-flow' shorthand</h3>
   412 
   413 	<table class=propdef>
   414 		<tr>
   415 			<th>Name:</th>
   416 			<td><dfn>flex-flow</dfn>
   417 		<tr>
   418 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   419 			<td>&lt;&apos;flex-direction&apos;> || &lt;&apos;flex-wrap&apos;>
   420 		<tr>
   421 			<th>Initial:
   422 			<td>see individual properties
   423 		<tr>
   424 			<th>Applies To:
   425 			<td>flexboxes
   426 		<tr>
   427 			<th>Inherited:
   428 			<td>see individual properties
   429 		<tr>
   430 			<th>Computed Value:
   431 			<td>see individual properties
   432 		<tr>
   433 			<th>Media:
   434 			<td>visual
   435 		<tr>
   436 			<th>Animatable:
   437 			<td>no
   438 	</table>
   439 
   440 	<p>The 'flex-flow' property is a shorthand for setting the 'flex-direction' and 'flex-wrap' properties together.</p>
   441 
   442 	<div class="example">
   443 		<p>Some examples of valid flows:</p>
   444 
   445 		<table style="margin: 0 auto; vertical-align: middle; border-spacing: 2em 1em;">
   446 			<tr>
   447 				<td><pre>
   448 div { flex-flow: row; }
   449 /* Initial value. Main axis is 
   450    inline, no wrap. */</pre>
   451 				<td><img src='images/flex-flow1.svg' width=240 height=80>
   452 			<tr>
   453 				<td><pre>
   454 div { flex-flow: column wrap; }
   455 /* Main axis is block-direction and lines
   456    wrap in the inline direction.  For an 
   457    English page, the main axis is top-to-bottom
   458    and lines wrap to the right. */</pre>
   459 				<td><img src='images/flex-flow2.svg' width=170 height=200>
   460 			<tr>
   461 				<td><pre>
   462 div { writing-mode: vertical-rl;
   463       flex-flow: column wrap-reverse; }
   464 /* Main axis is block direction (right to 
   465    left). New lines wrap upwards. */</pre>
   466 				<td><img src='images/flex-flow3.svg' width=200 height=170>
   467 		</table>
   468 	</div>
   469 
   470 
   471 <h3 id='flex-order'>
   472 Display Order: the 'flex-order' property</h3>
   473 
   474 	<p><i>Flexbox 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.</p>
   475 
   476 	<table class=propdef>
   477 		<tr>
   478 			<th>Name:
   479 			<td><dfn>flex-order</dfn>
   480 		<tr>
   481 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   482 			<td>&lt;number>
   483 		<tr>
   484 			<th>Initial:
   485 			<td>0
   486 		<tr>
   487 			<th>Applies to:
   488 			<td><i>flexbox items</i></td>
   489 		<tr>
   490 			<th>Inherited:
   491 			<td>no
   492 		<tr>
   493 			<th>Computed value:
   494 			<td>specified value
   495 		<tr>
   496 			<th>Media:
   497 			<td>visual
   498 		<tr>
   499 			<th>Animatable:
   500 			<td>yes
   501 	</table>
   502 
   503 	<p>The 'flex-order' property assigns <i>flexbox items</i> to ordinal groups.</p>
   504 
   505 	<p>Ordinal groups control the order in which <i>flexbox items</i> appear.  A flexbox 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.</p>
   506 
   507 	<div class='example'>
   508 		<p>The following figure shows a simple tabbed interface, where the tab for the active pane is always in front:</p>
   509 
   510 		<div class='figure'><img src="images/flex-order-example.png"></div>
   511 
   512 		<p>This could be implemented with the following CSS (showing only the flexbox-relevant code):</p>
   513 
   514 		<pre>
   515 .tabs {
   516 	display: flexbox;
   517 }
   518 .tabs > .current {
   519 	flex-order: -1; /* Lower than the default of 0 */
   520 }</pre>
   521 
   522 	</div>
   523 
   524 	<div class='example'>
   525 		<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:</p>
   526 
   527 		<div class='code-and-figure'>
   528 			<div><pre>
   529 &lt;!DOCTYPE html>
   530 &lt;header>...&lt;/header>
   531 &lt;div id='main'>
   532    &lt;article>...&lt;/article>
   533    &lt;nav>...&lt;/nav>
   534    &lt;aside>...&lt;/aside>
   535 &lt;/div>
   536 &lt;footer>...&lt;/footer></pre></div>
   537 			<div><img src="images/flex-order-page.svg" width=400 height=360></div>
   538 		</div>
   539 
   540 		<p>This layout can be easily achieved with Flexbox:</p>
   541 
   542 		<pre>
   543 #main { display: flexbox; }
   544 #main > article { flex:1;         flex-order: 2; }
   545 #main > nav     { width: 200px;   flex-order: 1; }
   546 #main > aside   { width: 200px;   flex-order: 3; }</pre>
   547 		
   548 		<p>As an added bonus, the columns will all be equal-height 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:</p>
   549 
   550 		<pre>
   551 @media all and (max-width: 600px) {
   552 	/* Too narrow to support three columns */
   553 	#main { flex-flow: column; }
   554 	#main > article, #main > nav, #main > aside {
   555 		/* Return them to document order */
   556 		flex-order: 0; width: auto;
   557 	}
   558 }</pre>
   559 
   560 		<p><small>(Further use of multiline flexboxes to achieve even more intelligent wrapping left as an exercise for the reader.)</small></p>
   561 	</div>
   562 
   563 
   564 <h2 id='flexibility'>
   565 Flexibility: the 'flex' property</h2>
   566 
   567 	<p>The defining aspect of flexbox layout is the ability to make the <i>flexbox items</i> "flex", altering their width or height to fill the available space. This is done by using a 'flex' property. A flexbox distributes free space to its items proportional to their positive flexibility, or shrinks them to prevent overflow proportional to their negative flexibility.</p>
   568 
   569 	<table class=propdef>
   570 		<tr>
   571 			<th>Name:
   572 			<td><dfn>flex</dfn>
   573 		<tr>
   574 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   575 			<td>[ [ &lt;pos-flex> &lt;neg-flex>? ] || &lt;preferred-size> ] | none 
   576 		<tr>
   577 			<th>Initial:
   578 			<td>none
   579 		<tr>
   580 			<th>Applies to:
   581 			<td>flexbox items
   582 		<tr>
   583 			<th>Inherited:
   584 			<td>no
   585 		<tr>
   586 			<th>Computed Value:
   587 			<td>specified value
   588 		<tr>
   589 			<th>Media:
   590 			<td>visual
   591 		<tr>
   592 			<th>Animatable:
   593 			<td>yes
   594 	</table>
   595 
   596 	<p>The 'flex' property specifies the parameters of a <dfn title="flexible length|flexible lengths|flexible length's">flexible length</dfn>: the <dfn id="positive-flexibility" title="positive flexibility">positive</dfn> and <dfn>negative flexibility</dfn>, and the <dfn>preferred size</dfn>.</p>
   597 
   598 	<p><code>&lt;pos-flex></code> and <code>&lt;neg-flex></code> are non-negative <code>&lt;number>s</code>. The <code>&lt;pos-flex></code> component sets the <i>positive flexibility</i>; if omitted, the <i>positive flexibility</i> defaults to ''1''. The <code>&lt;neg-flex></code> component sets the <i>negative flexibility</i>; if omitted, it defaults to ''0''.  </p>
   599 <p> The <code>&lt;preferred-size></code> component sets the preferred size. It can be set to any value that would be valid in the 'width' or 'height' property, 
   600 	except values that are not applicable to a component value (of values 
   601 	defined in CSS2.1, only &#39;inherit&#39; is not valid for <code>&lt;preferred-size></code>; in the future other values may be introduced that are not applicable as length component). 
   602 If omitted, the preferred size defaults to ''0px''. If set to 'auto', the value of 'width' or 'height' (whichever is in parallel to main axis) is used as preferred size.</p>
   603 <p> &nbsp;If the <code>&lt;preferred-size></code> is ''0'', it <strong>must</strong> be specified with a unit (like ''0px'') to avoid ambiguity; unitless zero will either be interpreted as as one of the flexibilities, or is a syntax error.</p>
   604 
   605 	<p>The keyword 'none' is equivalent to "0 0 auto". </p>
   606 
   607 	
   608 	<p>A ''&lt;flex>'' value is transitionable, by transitioning the <i>preferred size</i>, <i>positive flexibility</i>, and <i>negative flexibility</i> independently.  ''&lt;flex>'' can also transition to and from a &lt;length>, by treating the length as if it were a flexible length with a positive and negative flexibility of zero and a preferred size of the length.</p>
   609 
   610 	<div class='example'>
   611 		<p>Flexibility allows elements to respond directly to the available space, 
   612 		optionally taking into account size of content:</p>
   613 
   614 		<pre>
   615 &lt;!DOCTYPE html>
   616 &lt;style>
   617 	div { display:flexbox; outline:1px solid silver; }
   618 	p { flex:1 auto; margin:1em; background:gold; }
   619 &lt;/style&gt;
   620 &lt;div&gt;
   621 	&lt;p&gt;&quot;flexing&quot;&lt;/p&gt;
   622 	&lt;p&gt;allows the items to get bigger&lt;/p&gt;
   623 	&lt;p&gt;or&lt;/p&gt;
   624 	&lt;p&gt;smaller&lt;/p&gt;
   625 &lt;/div&gt;
   626 </pre>
   627 
   628 <img src="images/wp7zxxyu.gif" alt="items with equal flexibility extend by equal amount">
   629 
   630 </div> 
   631 	
   632 <h2 id='alignment'>
   633 Alignment</h2>
   634 
   635 	<p>After a flexbox's contents have finished their flexing and dimensions of 
   636 	margin boxes of all flexbox items are finalized, they can be aligned in both the <i>main axis</i> with 'flex-pack' and the <i>cross axis</i> with 'flex-align' and 'flex-item-align'.  These properties make many common types of alignment trivial, including some things that were very difficult in CSS 2.1, like horizontal and vertical centering.</p>
   637 
   638 
   639 <h3 id='flex-pack'>
   640 Axis Alignment: the 'flex-pack' property</h3>
   641 
   642 	<table class=propdef>
   643 		<tr>
   644 			<th>Name:
   645 			<td><dfn>flex-pack</dfn>
   646 		<tr>
   647 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   648 			<td>start | end | center | justify | distribute
   649 		<tr>
   650 			<th>Initial:
   651 			<td>start
   652 		<tr>
   653 			<th>Applies to:
   654 			<td>flexboxes
   655 		<tr>
   656 			<th>Inherited:
   657 			<td>no
   658 		<tr>
   659 			<th>Computed Value:
   660 			<td>specified value
   661 		<tr>
   662 			<th>Media:
   663 			<td>visual
   664 		<tr>
   665 			<th>Animatable:
   666 			<td>no
   667 	</table>
   668 
   669 	<p>The 'flex-pack' property aligns <i>flexbox items</i> in the <i>main axis</i> of the current line of the flexbox.  This is done <em title=''>after</em> any flexible lengths 
   670 	and any auto margins have been resolved.  Typically it helps distribute extra free space leftover when either all the <i>flexbox items</i> on a line are inflexible, or are flexible but have reach their maximum size, but it also exerts some control over the alignment of items when they overflow the line.</p>
   671 
   672 	<dl>
   673 		<dt><dfn id='flex-pack-start'>start</dfn></dt>
   674 		<dd><i>Flexbox items</i> are packed toward the start of the line.  The <i>main-start</i> margin edge of the first <i>flexbox item</i> on the line is placed flush with the <i>main-start</i> edge of the line, and each subsequent <i>flexbox item</i> is placed flush with the preceding item.</dd>
   675 
   676 		<dt><dfn id='flex-pack-end'>end</dfn></dt>
   677 		<dd><i>Flexbox items</i> are packed toward the end of the line.  The <i>main-end</i> margin edge of the last <i>flexbox item</i> is placed flush with the <i>main-end</i> edge of the line, and each preceding <i>flexbox item</i> is placed flush with the subsequent item.</dd>
   678 
   679 		<dt><dfn id='flex-pack-center'>center</dfn></dt>
   680 		<dd><i>Flexbox items</i> are packed toward the center of the line.  The <i>flexbox items</i> on the line are placed flush with each other and aligned in the center of the line, with equal amounts of empty space between the <i>main-start</i> edge of the line and the first item on the line and between the <i>main-end</i> edge of the line and the last item on the line.  (If the leftover free-space is negative, the <i>flexbox items</i> will overflow equally in both directions.)</dd>
   681 
   682 		<dt><dfn id='flex-pack-justify'>justify</dfn></dt>
   683 		<dd><i>Flexbox items</i> are evenly distributed in the line.  If the leftover free-space is negative or there is only a single <i>flexbox item</i> on the line, this value is identical to ''start''.  Otherwise, the <i>main-start</i> margin edge of the first <i>flexbox item</i> on the line is placed flush with the <i>main-start</i> edge of the line, the <i>main-end</i> margin edge of the last <i>flexbox item</i> on the line is placed flush with the <i>main-end</i> edge of the line, and the remaining <i>flexbox items</i> on the line are distributed so that the empty space between any two adjacent items is the same.</dd>
   684 
   685 		<dt><dfn id='flex-pack-distribute'>distribute</dfn></dt>
   686 		<dd><i>Flexbox items</i> are evenly distributed in the line, with half-size spaces on either end.  If the leftover free-space is negative or there is only a single <i>flexbox item</i> on the line, this value is identical to ''center''.  Otherwise, the <i>flexbox items</i> on the line are distributed such that the empty space between any two adjacent <i>flexbox items</i> on the line is the same, and the empty space before the first and after the last <i>flexbox items</i> on the line are half the size of the other empty spaces.</dd>
   687 	</dl>
   688 
   689 	<div class="figure">
   690 		<img src="images/flex-pack.svg" width=504 height=270>
   691 		<p class='caption'>An illustration of the five 'flex-pack' keywords and their effects on a flexbox with three colored items.</p>
   692 	</div>
   693 
   694 
   695 <h3 id='flex-align'>
   696 Cross Axis Alignment: the 'flex-align' and 'flex-item-align' properties</h3>
   697 
   698 	<table class=propdef>
   699 		<tr>
   700 			<th>Name:
   701 			<td><dfn>flex-align</dfn>
   702 		<tr>
   703 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   704 			<td>start | end | center | baseline | stretch
   705 		<tr>
   706 			<th>Initial:
   707 			<td>stretch
   708 		<tr>
   709 			<th>Applies to:
   710 			<td>flexboxes
   711 		<tr>
   712 			<th>Inherited:
   713 			<td>no
   714 		<tr>
   715 			<th>Computed Value:
   716 			<td>specified value
   717 		<tr>
   718 			<th>Media:
   719 			<td>visual
   720 		<tr>
   721 			<th>Animatable:
   722 			<td>no
   723 	</table>
   724 
   725 	<table class=propdef>
   726 		<tr>
   727 			<th>Name:
   728 			<td><dfn>flex-item-align</dfn>
   729 		<tr>
   730 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   731 			<td>auto | start | end | center | baseline | stretch
   732 		<tr>
   733 			<th>Initial:
   734 			<td>auto
   735 		<tr>
   736 			<th>Applies to:
   737 			<td><i>flexbox items</i>
   738 		<tr>
   739 			<th>Inherited:
   740 			<td>no
   741 		<tr>
   742 			<th>Computed Value:
   743 			<td>''auto'' computes to parent's 'flex-align'; otherwise as specified
   744 		<tr>
   745 			<th>Media:
   746 			<td>visual
   747 		<tr>
   748 			<th>Animatable:
   749 			<td>no
   750 	</table>
   751 
   752 	<p><i>Flexbox items</i> can be aligned in the <i>cross axis</i> of the current line of the flexbox, similar to 'flex-pack' but in the perpendicular direction.  'flex-align' sets the default alignment for all of the flexbox's <i title="flexbox items">items</i>, including anonymous <i>flexbox items</i>.  'flex-item-align' allows this default alignment to be overridden for individual <i>flexbox items</i> (for anonymous flexbox items, 'flex-item-align' always matches the value of 'flex-align' on their associated flexbox).</p>
   753 
   754 	<p>A value of <dfn id="flex-item-align-auto">''auto''</dfn> for 'flex-item-align' computes to the value of 'flex-align' on the <i>flexbox item's</i> flexbox.  The alignments are defined as:</p>
   755 
   756 	<dl>
   757 		<dt><dfn id="flex-align-start">start</dfn></dt>
   758 		<dd>The <i>cross-start</i> margin edge of the <i>flexbox item</i> is placed flush with the <i>cross-start</i> edge of the line.</dd>
   759 
   760 		<dt><dfn id="flex-align-end">end</dfn></dt>
   761 		<dd>The <i>cross-end</i> margin edge of the <i>flexbox item</i> is placed flush with the <i>cross-end</i> edge of the line.</dd>
   762 
   763 		<dt><dfn id="flex-align-center">center</dfn></dt>
   764 		<dd>The <i>flexbox item's</i> margin box is centered in the <i>cross axis</i> within the line.  (If the <i>cross size</i> of the flexbox is less than that of the <i>flexbox item</i>, it will overflow equally in both directions.)</dd>
   765 
   766 		<dt><dfn id="flex-align-baseline">baseline</dfn></dt>
   767 		<dd>
   768 			<p>If the <i>flexbox item's</i> inline axis is the same as the <i>cross axis</i>, this value is identical to ''start''.</p>
   769 
   770 			<p>Otherwise, all <i>flexbox items</i> on the line with an alignment of ''baseline'' that don't run afoul of the previous paragraph are aligned such that their baselines align, and the item with the largest distance between its baseline and its <i>cross-start</i> margin edge is placed flush against the <i>cross-start</i> edge of the line.</p>
   771 		</dd>
   772 			  
   773 		<dt><dfn id="flex-align-stretch">stretch</dfn></dt>
   774 		<dd>
   775 			<p>If the <i>cross size property</i> of the <i>flexbox item</i> is ''auto'', it resolves to the length necessary to make the <i>cross size</i> of the item's margin box the same size as the line, while still respecting 'min/max-width/height' constraints as normal.</p>
   776 
   777 			<p>The <i>cross-start</i> margin edge of the <i>flexbox item</i> is placed flush with the <i>cross-start</i> edge of the line.</p>
   778 		</dd>
   779 	</dl>
   780 
   781 	<div class='figure'>
   782 		<img src="images/flex-align.svg" width=508 height=377>
   783 		<p class='caption'>An illustration of the five 'flex-align' keywords and their effects on a flexbox with four colored items.</p>
   784 	</div>
   785 
   786 	<div class=example>   
   787 		<p>By using a vertical flexbox and 'flex-align', we can achieve behavior very close to HTML's <code>&lt;center></code> element:</p>
   788 		
   789 		<div class='code-and-figure'>
   790 			<div><pre>
   791 &lt;div>
   792   &lt;p>foo foo foo foo&lt;/p>
   793   &lt;p>bar bar&lt;br>bar bar&lt;/p>
   794   &lt;p>foo foo foo foo foo foo foo
   795      foo foo foo foo foo&lt;/p>
   796 &lt;/div>
   797 &lt;style>
   798   div {
   799     display: flexbox;
   800     flex-flow: column;
   801     flex-align: center;
   802     width: 200px;
   803   }
   804 &lt;/style></pre></div>
   805 			<div><p><img alt="" src="images/basic-vertical-flexbox.png"></p></div>
   806 		</div>
   807 	</div>
   808 
   809 
   810 <h3 id='auto-margins'>
   811 Resolving &#39;auto&#39; margins</h3>
   812 <p>
   813 Margins on flexbox items can be set to &#39;auto&#39;, with effect very similar to auto 
   814 margins in normal flow. </p>
   815 <ul>
   816 	<li>Through calculations of preferred sizes and flexible lengths, auto 
   817 	margins are considered to be zero.</li>
   818 	<li>&nbsp;Immediately <em>before</em> pack and align steps, if there are any 
   819 	auto margins on items in the direction of alignment and there is positive 
   820 	free space, the free space is distributed equally to the auto margins. </li>
   821 	<li>If free space was distributed to auto margins, the following pack or align 
   822 	step has no effect.</li>
   823 </ul>
   824 <div class="example">
   825 <p>Auto margins can be used for simple alignment or for fine control.</p>
   826 	<p>Note that auto margins work consistently in both dimensions, so a simple 
   827 	markup like this</p>
   828 	<pre>div { 	display:flexbox;
   829 	width:4em; height:4em; background:silver;
   830 }
   831 p { margin:auto; }
   832 &lt;div&gt;&lt;p&gt;OK&lt;/p&gt;&lt;/div&gt;</pre>
   833 	<p>will center the single child:
   834 	<img alt="image of square OK button" height="82" src="images/OK.png" width="82"></p>
   835 	<p>And this</p>
   836 
   837 <pre>div {	display:flexbox;
   838 	width:calc(100% - 4em); height:calc(100% - 4em);
   839 	border: 1em solid blue; border-radius:50%;
   840 	margin:auto;
   841 }
   842 div#demo { width:9em; height:9em; }</pre>
   843 <pre>&lt;div id=&quot;demo&quot;&gt;&lt;div&gt;&lt;div&gt;&lt;/div&gt;&lt;/div&gt;&lt;/div&gt;</pre>
   844 <p>will produce nested centered boxes:
   845 <img alt="concentric blue circles" height="191" src="images/target.png" width="210"></p>
   846 </div>
   847 
   848 <h3 id='flex-line-pack'>
   849 'flex-line-pack' property</h3>
   850 
   851 	<table class=propdef>
   852 		<tr>
   853 			<th>Name:
   854 			<td><dfn>flex-line-pack</dfn>
   855 		<tr>
   856 			<th><a href="http://dev.w3.org/csswg/css-module/#values">Value</a>:
   857 			<td>start | end | center | justify | distribute | stretch
   858 		<tr>
   859 			<th>Initial:
   860 			<td>stretch
   861 		<tr>
   862 			<th>Applies to:
   863 			<td><i>multi-line</i> flexboxes
   864 		<tr>
   865 			<th>Inherited:
   866 			<td>no
   867 		<tr>
   868 			<th>Computed Value:
   869 			<td>specified value
   870 		<tr>
   871 			<th>Media:
   872 			<td>visual
   873 		<tr>
   874 			<th>Animatable:
   875 			<td>no
   876 	</table>
   877 
   878 	<p>The 'flex-line-pack' property aligns a flexbox's lines within the flexbox when there is extra space in the <i>cross axis</i>, similar to how 'flex-pack' aligns individual items within the <i>main axis</i>:</p>
   879 
   880 	<dl>
   881 		<dt><dfn id='flex-line-pack-start'>start</dfn></dt>
   882 		<dd>Lines are packed toward the start of the flexbox.  The <i>cross-start</i> edge of the first line in the flexbox is placed flush with the <i>cross-start</i> edge of the flexbox, and each subsequent line is placed flush with the preceding line.</dd>
   883 
   884 		<dt><dfn id='flex-line-pack-end'>end</dfn></dt>
   885 		<dd>Lines are packed toward the end of the flexbox.  The <i>cross-end</i> edge of the last line is placed flush with the <i>cross-end</i> edge of the flexbox, and each preceding line is placed flush with the subsequent line.</dd>
   886 
   887 		<dt><dfn id='flex-line-pack-center'>center</dfn></dt>
   888 		<dd>Lines are packed toward the center of the flexbox.  The lines in the flexbox are placed flush with each other and aligned in the center of the flexbox, with equal amounts of empty space between the <i>cross-start</i> content edge of the flexbox and the first line in the flexbox and between the <i>cross-end</i> content edge of the flexbox and the last line in the flexbox.  (If the leftover free-space is negative, the lines will overflow equally in both directions.)</dd>
   889 
   890 		<dt><dfn id='flex-line-pack-justify'>justify</dfn></dt>
   891 		<dd>Lines are evenly distributed in the flexbox.  If the leftover free-space is negative or there is only a single line in the flexbox, this value is identical to ''start''.  Otherwise, the <i>cross-start</i> edge of the first line in the flexbox is placed flush with the <i>cross-start</i> content edge of the flexbox, the <i>cross-end</i> edge of the last line in the flexbox is placed flush with the <i>cross-end</i> content edge of the flexbox, and the remaining lines in the flexbox are distributed so that the empty space between any two adjacent lines is the same.</dd>
   892 
   893 		<dt><dfn id='flex-line-pack-distribute'>distribute</dfn></dt>
   894 		<dd>Lines are evenly distributed in the flexbox, with half-size spaces on either end.  If the leftover free-space is negative or there is only a single line in the flexbox, this value is identical to ''center''.  Otherwise, the lines in the flexbox are distributed such that the empty space between any two adjacent lines is the same, and the empty space before the first and after the last lines in the flexbox are half the size of the other empty spaces.</dd>
   895 
   896 		<dt><dfn id='flex-line-pack-stretch'>stretch</dfn></dt>
   897 		<dd>Lines stretch to take up the remaining space.  If the leftover free-space is negative, this value is identical to ''start''.  Otherwise, the free-space is split equally between all of the lines, increasing their cross size.
   898 	</dl>
   899 
   900 	<p class='note'>Note: Only <i>multi-line</i> flexboxes ever have free space in the <i>cross axis</i> for lines to be aligned in, because in a <i>single-line</i> flexbox the sole line automatically stretches to fill the space.</p>
   901 
   902 	<p class="issue">TODO: examples</p>
   903 
   904 
   905 <h2 id='layout-algorithm'>
   906 Flexbox Layout Algorithm</h2>
   907 
   908 	<p>This section contains normative algorithms detailing the exact layout behavior of a flexbox and its contents.  The algorithms here were designed to optimize readability and theoretical simplicity, and may not necessarily be the most efficient.  Implementations may use whatever actual algorithms they wish, but must produce the same results as the algorithms described here.</p>
   909 
   910 	<div class=note>
   911 		<p>This note will outline the general structure of the layout algorithm, before I go into the ugly details below, to aid in reading the relatively-long and complex algorithm.</p>
   912 
   913 		<ol>
   914 			<li value=0>Generate anonymous flexbox items as necessary.</li>
   915 
   916 			<li>Reorder flexbox items according to 'flex-order'.</li>
   917 
   918 			<li>Find the hypothetical main size of every flexbox item.</li>
   919 
   920 			<li>Linebreak the flexbox, if it's multi-line.</li>
   921 
   922 			<li>Based on the hypothetical sizes of the items, find the real main size of the flexbox.</li>
   923 
   924 			<li>Resolve any flexible lengths.  All items now have a real main size.</li>
   925 
   926 			<li>Update layout for the items based on their real main sizes, and determine their hypothetical cross size.</li>
   927 
   928 			<li>Find the real cross size of the flexbox and its lines.</li>
   929 
   930 			<li>Align the lines, per 'flex-line-pack'.</li>
   931 
   932 			<li>Align in the cross axis, per 'flex-align'.</li>
   933 
   934 			<li>Update layout for the items based on their real cross sizes.</li>
   935 
   936 			<li>Align in the main axis, per 'flex-pack'.</li>
   937 		</ol>
   938 	</div>
   939 
   940 	<p>This section is mainly intended for implementors.  Authors writing web pages should generally be served well by the individual property descriptions, and do not need to read this section unless they have a deep-seated urge to understand arcane details of CSS layout.</p>
   941 
   942 	<p>To lay out a flexbox and its contents, follow these steps:</p>
   943 
   944 	<ol>
   945 		<li value=0>Generate anonymous flexbox items around runs of contiguous inline content in the flexbox, as described in the <a href="#flex-items">Flexbox Items</a> section.</li>
   946 
   947 		<li>Re-order the flexbox items according to their 'flex-order'.  The items with the lowest (most negative) 'flex-order' values are first in the ordering.  If multiple items share a 'flex-order' value, they're ordered by document order.  This affects the order in which the flexbox items generate boxes in the box-tree, and how the rest of this algorithm deals with the items.</li>
   948 
   949 		<li>
   950 			Determine the hypothetical main size of items:
   951 
   952 			<ol>
   953 				<li>If item has a definite preferred size, it is the hypothetical main size. </li>
   954 
   955 				<li>
   956 					<p>Otherwise layout the flexbox items using the shrink-to-fit algorithm.</p>
   957 
   958 					<p>Calculation of the shrink-to-fit width is similar to calculating the width of a table cell using the automatic table layout algorithm. Roughly: calculate the preferred width by formatting the content without breaking lines other than where explicit line breaks occur, and also calculate the preferred minimum width, e.g., by trying all possible line breaks. This module doesn&#39;t define the exact algorithm, but it is expected that the shrink-to-fit size is calculated the same way as it is for floats, except in this case it is not affected by available space.</p>
   959 				
   960 					<div class="issue"><p>TODO: define shrink-to-fit. It works 
   961 						differently in row/column flexboxes:</p>
   962 						<ul>
   963 							<li>In horizontal, STF is max-content. </li>
   964 							<li>In vertical, cross-axis 
   965 					alignment has to happen first, then main-axis STF size can 
   966 					be calculated. In multi-line vertical (tricky case), auto 
   967 							sizes on cross axis are resolved as if it was 
   968 							single-line.</li>
   969 						</ul>
   970 					</div>
   971 				</li>
   972 			</ol>
   973 
   974 			<p><em>Do not apply min/max-width/height constraints to the preferred size of flexible lengths - those constraints are handled elsewhere in this algorithm, and doing so will produce incorrect results. </em></p>
   975 		</li>
   976 
   977 		<li>
   978 			<p>If the flexbox is single-line, collect all the flexbox items into a single flexbox line.</p>
   979 		</li>
   980 
   981 		<li>
   982 			<p>If the flexbox is multi-line, group the flexbox items into multiple lines:</p>
   983 
   984 			<ol>
   985 				<li>
   986 					<p>Determine the maximum line length, based on the main size of the flexbox. The maximum line length is main size of the flexbox's content box, constrained by the min and max main size constraints of the flexbox.  If the main size depends on the flexbox's content, then:</p>
   987 
   988 					<ul>
   989 						<li>for ''min-content'', the maximum line length is the flexbox's minimum main size, constrained by the min and max main size constraints of the flexbox.</li>
   990 
   991 						<li>for ''max-content'', the maximum line length is infinity, constrained by the min and max main size constraints of the flexbox.</li>
   992 
   993 						<li>for ''fit-content'', the maximum line length is the greater of the flexbox's min size constraint and the smaller of the flexbox's max size constraint and the available space.</li>
   994 					</ul>
   995 				</li>
   996 
   997 				<li>Collect as many consecutive flexbox items as possible, starting from the first item, while keeping the sum of their main size smaller than the flexbox's available space.  If a flexbox item is sized with a flexible length, then for the purpose of this step, clamp the item's size between its minimum and maximum main sizes.  The items so collected form a single flexbox line.</li>
   998 
   999 				<li>Repeat the previous step, starting each time from the first flexbox item not yet collected into a flexbox line, until all flexbox items have been collected into flexbox lines.</li>
  1000 			</ol>
  1001 		</li>
  1002 
  1003 		<li>Find the actual main size of the flexbox.  If the flexbox's main size doesn't rely on its contents, its actual main size is calculated per the appropriate rules.  Otherwise, its main size is the length of its longest line, calculated by summing the main sizes of the margin boxes of each flexbox item in the line, constrained by the flexbox's min and max main size constraints. 
  1004 		If any margins are set to &#39;auto&#39;, consider them to be set to zero until 
  1005 		resolved in a separate step.</li>
  1006 
  1007 		<li>For each flexbox line, 
  1008 		<ol>
  1009 			<li><i>Resolve the flexible lengths</i> of the items contained within it.  All flexbox items now have a final main size.  Update each item's hypothetical cross size based on this main size.</li>
  1010 
  1011 			<li>Calculate the leftover free-space by subtracting the sum of the 
  1012 			margin-box main sizes of the items on the line from the main size of the flexbox's content box.</li>
  1013 			<li>Resolve &#39;auto&#39; margins on main axis:<ol>
  1014 				<li>If leftover free-space is positive and any items on this 
  1015 				line have at least one main-axis margin set to &#39;auto&#39;, 
  1016 				distribute the leftover free-space equally to these margins.</li>
  1017 				<li>If leftover free-space is negative, any main-axis &#39;auto&#39; 
  1018 				margins on items in this line are set to zero.</li>
  1019 			</ol>
  1020 			</li>
  1021 			<li>
  1022 			Align the items along the main axis per 'flex-pack'.
  1023 			</li>
  1024 		</ol>
  1025 		</li>
  1026 
  1027 		<li>Calculate the cross size of each flexbox line. For each flexbox 
  1028 		line:
  1029 			<ol>
  1030 				<li>If the flexbox is single-line and has a definite cross size, the single flexbox line's cross size is the cross size of the flexbox's content box.  End this step of the algorithm.</li>
  1031 
  1032 				<li>If main axis is parallel to inline axis, collect all the flexbox items with a 'flex-item-align' of 'baseline'. Find the maximum of the distances from their baseline to the cross-start edge of their margin box, and the maximum of the distances from their baseline to the cross-end edge of their margin box. Sum these two values.</li>
  1033 
  1034 				<li>For remaining flexbox items, find the maximum of the cross sizes of their margin boxes.
  1035 
  1036 				<li>The cross size of the flexbox line is the larger of the numbers found in the previous two steps.</li>
  1037 				<li>If the flexbox is multi-line, has a definite cross size, 
  1038 				&#39;flex-line-pack&#39; is set to &#39;stretch&#39; and sum of cross size of 
  1039 				all lines is less than cross size of content box, increase cross 
  1040 				size of each line by equal amount to exactly match the cross 
  1041 				size of content box.</li>
  1042 			</ol>
  1043 		</li>
  1044 
  1045 		<li>If the flexbox doesn't have a definite cross size, the cross size of its content box is the sum of the cross sizes of all of its lines.</li>
  1046 
  1047 		<li>Determine the final cross size of each flexbox item.  For each flexbox item, if it has ''flex-item-align:stretch'', its final cross size is 
  1048 		set so that its margin-box cross size is equal to cross-size of its flexbox line.  For all other flexbox items, its final cross size is its hypothetical cross size. 
  1049 		If the result conflicts with &#39;min-width&#39;, &#39;max-width&#39;, &#39;min-height&#39; or 
  1050 		&#39;max-height&#39;, correct the result to comply the restrictions.</li>
  1051 		<p class="issue">'auto' height must only grow, not shrink here</p>
  1052 
  1053 		<li>Resolve &#39;auto&#39; margins on cross axis. For each flexbox item, if its 
  1054 		margin box cross size is smaller than cross size of its line and it has 
  1055 		any margins in cross direction set to &#39;auto&#39;, distribute the difference 
  1056 		equally to the auto margins. </li>
  1057 		<li>For each flexbox line, align the flexbox items per 'flex-item-align'.</li>
  1058 
  1059 		<li>Align the flexbox lines per 'flex-line-pack'.  The leftover free-space is calculated by subtracting the sum of the flexbox line's cross sizes from the cross size of the flexbox's content box.</li>
  1060 		<li>Layout all items again in their final size and position. 
  1061 		Implementations can determine when this step is needed and how it can be 
  1062 		optimized, but for precise results it has to be assumed that this step 
  1063 		is performed, because the layout of each item&#39;s content may change when 
  1064 		ea</li>
  1065 	</ol>
  1066 
  1067 	<p>To <dfn>resolve the flexible lengths</dfn> of the items within a flexbox line:</p>
  1068 
  1069 	<ol>
  1070 		<li>
  1071 			Determine sign of flexibility
  1072 			
  1073 			<ol>
  1074 				<li>Add sizes of all items, <i>adjusted for min/max</i>.
  1075 			
  1076 				<li>If sum of preferred sizes is less than available space, use positive flexibility, otherwise use negative flexibility.</li>
  1077 			</ol>
  1078 
  1079 			<p>for the rest of the algorithm use flexibility with the sign determined in this step</p>
  1080 		</li>
  1081 
  1082 		<li>Reset all flexible sizes to their preferred size</li>
  1083 
  1084 		<li>Find free space by subtracting sum of item sizes from available space.</li>
  1085 
  1086 		<li>
  1087 			Distribute free space proportional to flex:
  1088 			
  1089 			<ol>
  1090 				<li>If the free space is positive, but step 1 chose negative flexibility, do nothing</li>
  1091 
  1092 				<li>If the free space is negative, but step 1 chose positive flexibility, do nothing</li>
  1093 
  1094 				<li>Add a portion of free space to each flexible item, proportinal to item's flexibility</li>
  1095 			</ol>
  1096 		</li>
  1097 
  1098 		<li>
  1099 			Fix min/max violations:
  1100 			
  1101 			<ol>
  1102 				<li>Adjust each flexible item for min/max. </li>
  1103 				
  1104 				<li>If the size has changed, it is a violation. </li>
  1105 				
  1106 				<li>The violation may be positive (min violation) or negative (max violation). Accumulate the difference.</li>
  1107 			</ol>
  1108 		</li>
  1109 
  1110 		<li>
  1111 			If the sum of all violations is:
  1112 			
  1113 			<dl>
  1114 				<dt>Zero</dt>
  1115 				<dd>Exit the algorithm.</dd>
  1116 
  1117 				<dt>Positive</dt>
  1118 				<dd>Freeze items with max violations, and return to step 2.</dd>
  1119 
  1120 				<dt>Negative</dt>
  1121 				<dd>Freeze items with min violations, and return to step 2.</dd>
  1122 			</dl>
  1123 		</li>
  1124 	</ol>
  1125 	
  1126 <h2 id="pagination">
  1127 Page breaks in flexbox</h2>
  1128 
  1129 	<p>Flexboxes can break across pages between items, between lines of items (in multi-line mode) and inside items, as long as 'break-' property allow that. All 'break-' properties are supported on flexbox, on flexbox items and inside flexbox items.</p>
  1130 
  1131 	<p>The following breaking rules refer to fragmentation container as “page”. The same rules apply to any other fragmenters. Change “page” to the appropriate fragmenter type as needed.</p>
  1132 
  1133 	<p>Breaks in and around flexbox are determined as follows:</p>
  1134 
  1135 	<ol>
  1136 		<li> Break-before, break-after, break-inside properties on flexbox itself have effect as normal. If breaks inside flexbox are allowed, break points are determined using following rules.</li>
  1137 
  1138 		<li>When flexbox is continued after a break, flexbox’s available space in block direction is reduced by space consumed in previous pages. Consumed space before page break is:
  1139 
  1140 			<ol>
  1141 				<li>If flexbox starts on that page: the distance between start of content box of the flexbox and the end of available space</li>
  1142 
  1143 				<li>If flexbox continues from previous page: the size of available space.</li>
  1144 			</ol>
  1145 
  1146 			<p>If as a result of this adjustment block-direction size of flexbox becomes negative, it is set to zero.</p>
  1147 		</li>
  1148 	
  1149 		<li>
  1150 			<dfn>Forced breaks on flexbox items</dfn>
  1151 			
  1152 			<ol>
  1153 				<li>In a row-direction single-line flexbox, breaks before and after items apply to the flexbox</li>
  1154 				<p class="issue">"break-inside:avoid" in single-line horizontal should probably be applied to flexbox too.</p>
  1155 
  1156 				<li>In column-direction single-line flexbox and all multi-line flexboxes:
  1157 				
  1158 					<ol>
  1159 						<li>Forced break before the first item is applied to the flexbox</li>
  1160 						
  1161 						<li>Forced break after the last item is applied to the flexbox</li>
  1162 						
  1163 						<li>Forced break before or after any other item terminates the set of items to be laid out on this page</li>
  1164 					</ol>
  1165 				</li>
  1166 			</ol>
  1167 			
  1168 		</li>
  1169 		
  1170 		<li>When necessary, a <dfn>break inside a flexbox item is considered</dfn> as follows:
  1171 
  1172 			<ol>
  1173 				<li>If the item has “break-inside:avoid”, it is pushed to the next page</li>
  1174 
  1175 				<li>Otherwise, it may be broken, according to breaking rules and algorithms applicable to its display type. </li>
  1176 
  1177 				<li>Forced breaks inside flex item content are handled normally, but in row-direction flexbox they don&#39;t affect layout of sibling flex items (e.g. a forced break inside an item doesn&#39;t prevent its next sibling from appearing on the same page)</li>
  1178 			</ol>
  1179 		</li>
  1180 
  1181 		<li>
  1182 			<b>Column-direction flexbox: single-line</b>
  1183 
  1184 			<ol>
  1185 				<li>
  1186 					<p>If flexbox main-axis size is definite, flexbox algorithm must be run first, without constraints of available space on page and using all content of the flexbox. Otherwise items use the preferred size, adjusted for min/max.</p>
  1187 
  1188 					<p>If page size varies and flexbox size depends on page size, this step has to be repeated on each page, again with the whole content of the flexbox.</p>
  1189 				</li>
  1190 
  1191 				<li>Set of items that will fit on current page is determined by adding main-axis sizes of items until total size exceeds availalble space or a forced break is encountered.</li>
  1192 
  1193 				<li>If border box of an item doesn’t fit in available space, a break inside the item is considered</li>
  1194 
  1195 				<li>
  1196 					<p>Items that fit on a page completely or partially are aligned according to ‘flex-pack’ property, independently from the rest of flexbox content.</p>
  1197 
  1198 					<p>Note that flexible lengths are not recalculated on each page, even if there is additional free space.</p>
  1199 				</li>
  1200 			</ol>
  1201 
  1202 			<p class="note">It is the intent of this spec that column-direction single-line flexbox paginates very simlarly to block flow. As a test of the intent, a flexbox with "flex-pack:start" and no flexible items should paginate identically to a block with non-floating children with same content, same used size and same used margins. This rule is simplified and not normative, but if there is any difference it should be noted here.</p>
  1203 		</li>
  1204 
  1205 		<li>
  1206 			<b>Column-direction flexbox: multi-line</b>
  1207 
  1208 			<ol>
  1209 				<li>Items are collected in lines and laid out as usual, but in available space on current page.</li>
  1210 
  1211 				<li>If border box of an item doesn’t fit on main-axis and it is 
  1212 				the only item in its line:
  1213 					<ol>
  1214 						<li>If the flexbox is not at the top of the page, it is moved to the next page.</li>
  1215 
  1216 						<li>If the flexbox is already at the top of the page, the item may produce overflow. If 'overflow' property of the flexbox is set to 'visible', it is paginated using same rules as visible overflow of blocks in normal flow.
  1217 
  1218 							<p class="note">After the break, continuation of overflow items may overlap with other items and/or content after the flexbox. It is undesirable but there is no good resolution for this spacial conflict and this outcome is similar to effect of "overflow:visible" elsewhere.</p>
  1219 						</li>
  1220 							</ol>
  1221 						</li>
  1222 					</ol>
  1223 				</li>
  1224 				<li>Breaks inside items (forced or unforced) are not allowed</li>
  1225 			</ol>
  1226 		</li>
  1227 
  1228 		<li>
  1229 			<b>Row-direction flexbox: single-line</b>
  1230 
  1231 			<ol>
  1232 				<li>
  1233 					<p>Main-axis space distribution in each line is done based on complete content of the flexbox and without space constraint in block direction.</p>
  1234 
  1235 					<p>If page size varies and flexbox size depends on page size, this step needs to be repeated on each page.</p>
  1236 				</li>
  1237 
  1238 				<li>Items are sized and positioned as usual, but in block-direction available space of min(remaining available space in flexbox, remaining space on the page).</li>
  1239 
  1240 				<li>Any items with baseline alignment must be aligned before considering breaks inside items.</li>
  1241 
  1242 				<li>If border box of an item doesn’t fit in current page, a break inside the item is considered</li>
  1243 
  1244 				<li>Items that have fit completely on a previous page and items that are pushed to next page don’t have any rendering, leaving empty space as needed.</li>
  1245 
  1246 				<li>Items that fit completely or partially on current page are aligned on cross-axis:
  1247 		
  1248 					<ol>
  1249 						<li>For the purposes of cross-axis alignment, minimum of remaining available space in flexbox and available space at current page is used.</li>
  1250 
  1251 						<li>If an item is broken in the previous step and its alignment is not baseline, its cross-axis size is set to available space. If item alignment is baseline, its cross-axis size is adjusted so that it extends to exactly the end of available space.</li>
  1252 					</ol>
  1253 				</li>
  1254 			</ol>
  1255 		</li>
  1256 
  1257 		<li>
  1258 			<b>Row-direction flexbox:multi-line</b>
  1259 
  1260 			<ol>
  1261 				<li>Collect items into lines and determine line heights:
  1262 					
  1263 					<ol>
  1264 						<li>
  1265 							<p>If 'flex-line-pack' is 'stretch' and flexbox size in block direction is definite, layout of the complete flexbox has to be done first to determine line heights. Layout is done as usual for non-paginated case, but accounting for forced breaks.</p>
  1266 			
  1267 							<p>If page size varies, this step may have to be redone, again with complete content; special consideration should be to be given to line breaks to ensure that item at the start of current page is also at the start of a line in this hypothetical layout. This specification currently doesn't define how exactly to achieve that.</p>
  1268 						</li>
  1269 
  1270 						<li>Otherwise, items are collected into lines and each line is laid out as a single-line flexbox to determine block-direction size of each line.</li>
  1271 					</ol>
  1272 				</li>	
  1273 
  1274 				<li>Lines are added one at a time, until out of available space or a forced break is encountered</li>
  1275 
  1276 				<li>Breaks inside items (forced or unforced) are not allowed.</li>
  1277 				<li>If the first flexbox line on a page doesn&#39;t fit in 
  1278 				cross-axis,
  1279 				<ol>
  1280 					<li>If the flexbox is not at the top of the page, it is moved to the next page.</li>
  1281 					<li>If the flexbox is already at the top of the page, the item may produce overflow. If 'overflow' property of the flexbox is set to 'visible', it is paginated using same rules as visible overflow of blocks in normal flow.</li>
  1282 				</ol>
  1283 				</li>
  1284 
  1285 				<li>If size of flexbox in inline direction is not definite, multi-line layout algorithm is run using the set of items 
  1286 				that have fit on the current page.</li>
  1287 
  1288 				<li>Line packing is done on each page, with content on the page</li>
  1289 				<p class="issue">TODO: combine breaking algorigthms for multiline in both directions. If breaks inside items are not allowed the algorithm is almost same.</p>
  1290 			</ol>
  1291 		</li>
  1292 	</ol>
  1293 
  1294 <h2 id="conformance">
  1295 Conformance</h2>
  1296 
  1297 <h3 id="conventions">
  1298 Document conventions</h3>
  1299 
  1300   <p>Conformance requirements are expressed with a combination of
  1301   descriptive assertions and RFC 2119 terminology. The key words “MUST”,
  1302   “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
  1303   “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
  1304   document are to be interpreted as described in RFC 2119.
  1305   However, for readability, these words do not appear in all uppercase
  1306   letters in this specification.
  1307   
  1308   <p>All of the text of this specification is normative except sections
  1309   explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
  1310   
  1311   <p>Examples in this specification are introduced with the words “for example”
  1312   or are set apart from the normative text with <code>class="example"</code>,
  1313   like this:
  1314   
  1315   <div class="example">
  1316     <p>This is an example of an informative example.</p>
  1317   </div>
  1318   
  1319   <p>Informative notes begin with the word “Note” and are set apart from the
  1320   normative text with <code>class="note"</code>, like this:
  1321   
  1322   <p class="note">Note, this is an informative note.</p>
  1323 
  1324 <h3 id="conformance-classes">
  1325 Conformance classes</h3>
  1326 
  1327   <p>Conformance to CSS Flexbox Layout Module
  1328   is defined for three conformance classes:
  1329   <dl>
  1330     <dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
  1331       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
  1332       style sheet</a>.
  1333     <dt><dfn>renderer</dfn></dt>
  1334       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
  1335       that interprets the semantics of a style sheet and renders
  1336       documents that use them.
  1337     <dt><dfn id="authoring-tool">authoring tool</dfn></dt>
  1338       <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
  1339       that writes a style sheet.
  1340   </dl>
  1341   
  1342   <p>A style sheet is conformant to CSS Flexbox Layout Module
  1343   if all of its statements that use syntax defined in this module are valid
  1344   according to the generic CSS grammar and the individual grammars of each
  1345   feature defined in this module.
  1346   
  1347   <p>A renderer is conformant to CSS Flexbox Layout Module
  1348   if, in addition to interpreting the style sheet as defined by the
  1349   appropriate specifications, it supports all the features defined
  1350   by CSS Flexbox Layout Module by parsing them correctly
  1351   and rendering the document accordingly. However, the inability of a
  1352   UA to correctly render a document due to limitations of the device
  1353   does not make the UA non-conformant. (For example, a UA is not
  1354   required to render color on a monochrome monitor.)
  1355   
  1356   <p>An authoring tool is conformant to CSS Flexbox Layout Module
  1357   if it writes style sheets that are syntactically correct according to the
  1358   generic CSS grammar and the individual grammars of each feature in
  1359   this module, and meet all other conformance requirements of style sheets
  1360   as described in this module.
  1361 
  1362 <h3 id="partial">
  1363 Partial implementations</h3>
  1364 
  1365   <p>So that authors can exploit the forward-compatible parsing rules to
  1366   assign fallback values, CSS renderers <strong>must</strong>
  1367   treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
  1368   as appropriate</a>) any at-rules, properties, property values, keywords,
  1369   and other syntactic constructs for which they have no usable level of
  1370   support. In particular, user agents <strong>must not</strong> selectively
  1371   ignore unsupported component values and honor supported values in a single
  1372   multi-value property declaration: if any value is considered invalid
  1373   (as unsupported values must be), CSS requires that the entire declaration
  1374   be ignored.</p>
  1375   
  1376 <h3 id="experimental">
  1377 Experimental implementations</h3>
  1378 
  1379   <p>To avoid clashes with future CSS features, the CSS2.1 specification
  1380   reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
  1381   syntax</a> for proprietary and experimental extensions to CSS.
  1382   
  1383   <p>Prior to a specification reaching the Candidate Recommendation stage
  1384   in the W3C process, all implementations of a CSS feature are considered
  1385   experimental. The CSS Working Group recommends that implementations
  1386   use a vendor-prefixed syntax for such features, including those in
  1387   W3C Working Drafts. This avoids incompatibilities with future changes
  1388   in the draft.
  1389   </p>
  1390  
  1391 <h3 id="testing">
  1392 Non-experimental implementations</h3>
  1393 
  1394   <p>Once a specification reaches the Candidate Recommendation stage,
  1395   non-experimental implementations are possible, and implementors should
  1396   release an unprefixed implementation of any CR-level feature they
  1397   can demonstrate to be correctly implemented according to spec.
  1398   
  1399   <p>To establish and maintain the interoperability of CSS across
  1400   implementations, the CSS Working Group requests that non-experimental
  1401   CSS renderers submit an implementation report (and, if necessary, the
  1402   testcases used for that implementation report) to the W3C before
  1403   releasing an unprefixed implementation of any CSS features. Testcases
  1404   submitted to W3C are subject to review and correction by the CSS
  1405   Working Group.
  1406   
  1407   <p>Further information on submitting testcases and implementation reports
  1408   can be found from on the CSS Working Group's website at
  1409   <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
  1410   Questions should be directed to the
  1411   <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
  1412   mailing list.
  1413 
  1414 <h3 id="cr-exit-criteria">
  1415 CR exit criteria</h3>
  1416 
  1417   <p>
  1418   For this specification to be advanced to Proposed Recommendation,
  1419   there must be at least two independent, interoperable implementations
  1420   of each feature. Each feature may be implemented by a different set of
  1421   products, there is no requirement that all features be implemented by
  1422   a single product. For the purposes of this criterion, we define the
  1423   following terms:
  1424   
  1425   <dl>
  1426     <dt>independent <dd>each implementation must be developed by a
  1427     different party and cannot share, reuse, or derive from code
  1428     used by another qualifying implementation. Sections of code that
  1429     have no bearing on the implementation of this specification are
  1430     exempt from this requirement.
  1431   
  1432     <dt>interoperable <dd>passing the respective test case(s) in the
  1433     official CSS test suite, or, if the implementation is not a Web
  1434     browser, an equivalent test. Every relevant test in the test
  1435     suite should have an equivalent test created if such a user
  1436     agent (UA) is to be used to claim interoperability. In addition
  1437     if such a UA is to be used to claim interoperability, then there
  1438     must one or more additional UAs which can also pass those
  1439     equivalent tests in the same way for the purpose of
  1440     interoperability. The equivalent tests must be made publicly
  1441     available for the purposes of peer review.
  1442   
  1443     <dt>implementation <dd>a user agent which:
  1444   
  1445     <ol class=inline>
  1446       <li>implements the specification.
  1447   
  1448       <li>is available to the general public. The implementation may
  1449       be a shipping product or other publicly available version
  1450       (i.e., beta version, preview release, or “nightly build”). 
  1451       Non-shipping product releases must have implemented the
  1452       feature(s) for a period of at least one month in order to
  1453       demonstrate stability.
  1454   
  1455       <li>is not experimental (i.e., a version specifically designed
  1456       to pass the test suite and is not intended for normal usage
  1457       going forward).
  1458     </ol>
  1459   </dl>
  1460   
  1461   <p>The specification will remain Candidate Recommendation for at least
  1462   six months.
  1463 
  1464 
  1465 <hr title="Separator from footer">
  1466 
  1467 <h2 class="no-num" id="acknowledgments">Acknowledgments</h2>
  1468 
  1469 	<p>[This section will contain further acknowledgments.]</p>
  1470 
  1471 	<p>Thanks for feedback from James Elmore and Shinichiro Hamaji.</p>
  1472 
  1473 <h2 class="no-num" id="references">References</h2>
  1474 
  1475 <h3 class="no-num" id="normative">Normative references</h3>
  1476 <!--normative-->
  1477 
  1478 <h3 class="no-num" id="informative">Other references</h3>
  1479 <!--informative-->
  1480 
  1481 <h2 id="property" class="no-num">Property index</h2>
  1482 <!--properties-->
  1483 
  1484 <h2 class="no-num" id="index">Index</h2>
  1485 <!--index-->
  1486 
  1487 </body></html>
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