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<h1>CSS Box Alignment Module Level 3</h1>
<pre class='metadata'>
Shortname: css-align
Level: 3
Group: csswg
Status: ED
ED: http://dev.w3.org/csswg/css-align/
TR: http://www.w3.org/TR/css3-align/
Editor: fantasai, Invited Expert, http://fantasai.inkedblade.net/contact
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
Previous Version: http://www.w3.org/TR/2013/WD-css3-align-20130514/
Previous Version: http://www.w3.org/TR/2012/WD-css3-align-20120612/
Abstract: This module contains the features of <a href="http://www.w3.org/TR/CSS/">CSS</a> relating to the alignment of boxes within their containers in the various CSS box layout models: block layout, table layout, flex layout, and grid layout. (The alignment of text and inline-level content is defined in [[CSS3TEXT]] and [[CSS3LINE]].)
!Issue Tracking: <a href="http://www.w3.org/Style/CSS/Tracker/products/39">http://www.w3.org/Style/CSS/Tracker/products/39</a>
Link Defaults: selectors-3 (dfn) first formatted line, css-flexbox-1 (dfn) flex line, css-multicol-1 (dfn) multi-column element, css-fonts-3 (dfn) first available font
Ignored Terms: orthogonal, block container, table cell
At Risk: ''last-baseline''
</pre>
<style type="text/css">
.issue th:first-child { text-align: left !important; }
[rowspan] > img { float: right; }
[rowspan] { vertical-align: middle; }
small { display: block; }
</style>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
CSS Levels 1 and 2 allowed for the alignment of text via 'text-align' and
the alignment of blocks by balancing ''margin/auto'' margins. However, except in
table cells, vertical alignment was not possible. As CSS3 adds further
capabilities, the ability to align boxes in various dimensions becomes more
critical. This module attempts to create a cohesive and common box alignment
model to share among all of CSS.
Note: The alignment of text and inline-level content is defined in [[CSS3TEXT]] and [[CSS3LINE]].
Inspiration for this document:
<ul>
<li><a href="http://wiki.csswg.org/ideas/centering#alignment-property">summary of a discussion for implementing <CENTER></a>
<li><a href="https://lists.w3.org/Archives/Member/w3c-css-wg/2008AprJun/0015.html">Minutes from March 2008 F2F</a>
<li><a href="http://lists.w3.org/Archives/Public/www-style/2012Feb/0743.html">fantasai's attempt to merge all alignment properties</a>
</ul>
<h3 id="placement">
Module interactions</h3>
This module adds some new alignment capabilities to the block layout
model described in [[!CSS21]] chapters 9 and 10 and defines the interaction
of these properties with the alignment of table cell content using
'vertical-align', as defined in [[!CSS21]] chapter 17.
The interaction of these properties with
Grid Layout [[!CSS3-GRID-LAYOUT]]
and Flexible Box Layout [[!CSS3-FLEXBOX]]
is defined in their respective modules.
No properties in this module apply to the <code>::first-line</code> or
<code>::first-letter</code> pseudo-elements.
<h3 id="values">
Values</h3>
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 [[CSS3VAL]], when combined with this module, adds the
''initial'' keyword as a possible property value.
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="overview">
Overview of Alignment Properties</h2>
The alignment properties in CSS can be described along two axes:
<ul>
<li>which dimension they apply to (inline vs. stacking), and
<li>whether they control the position of the box within its parent, or the box's content within itself.
</ul>
Note: This proposal uses the terms “justify” and “align” to distinguish
between alignment in the inline and stacking dimensions, respectively.
The choice is somewhat arbitrary, but having the two terms allows for
a consistent naming scheme that works across all of CSS's layout models.
The following table summarizes the proposed alignment properties
and the display types they can apply to.
<table class=data>
<colgroup span=1></colgroup>
<colgroup span=2></colgroup>
<colgroup span=1></colgroup>
<thead>
<tr>
<th>Common
<th>Axis
<th>Aligns
<th>Applies to
</thead>
<tbody>
<tr>
<th>'justify-content'
<td>inline
<td rowspan=2>
<img src="images/content-example.svg" width=106 height=106 alt="">
content within element
<small>(effectively adjusts padding)</small>
<td>block containers and flex containers
<tr>
<th>'align-content'
<td>stacking
<td>block containers and flex containers
<tbody>
<tr>
<th>'justify-self'
<td>inline
<td rowspan=2>
<img src="images/self-example.svg" width=106 height=106 alt="">
element within parent
<small>(effectively adjusts margins)</small>
<td>block-level elements and grid items
<tr>
<th>'align-self'
<td>stacking
<td>flex items and grid items
<tbody>
<tr>
<th>'justify-items'
<td>inline
<td rowspan=2>
<img src="images/items-example.svg" width=106 height=106 alt="">
items inside element
<small>(controls child items’ ''align/justify-self: auto'')</small>
<td>grid containers
<tr>
<th>'align-items'
<td>stacking
<td>flex containers and grid containers
</table>
<p class=issue>
The exact definition of these properties
outside what's <a href="http://www.w3.org/TR/css3-flexbox">what's defined in Flexbox</a>
is still <strong>being worked out</strong>.
This is a <strong>second Working Draft</strong>
and is <strong>NOT STABLE</strong>.
Note: The '*-items' values don't affect the element itself.
When set on a flex container or grid container,
they specify the interpretation of any ''align/justify-self: auto'' used on the items in the container element.
<h2 id='alignment-values'>
Alignment Values</h2>
All of the alignment properties use a common set of values,
defined below.
<dl>
<dt><dfn export>alignment subject</dfn>
<dd>
The <a>alignment subject</a> is the thing or things being aligned by the property.
For 'justify-self' and 'align-self',
the <a>alignment subject</a> is the margin box of the box the property is set on.
For 'justify-content' and 'align-content',
the <a>alignment subject</a> is defined by the layout mode.
<dt><dfn export>alignment container</dfn>
<dd>
The <a>alignment container</a> is the rectangle that the <a>alignment subject</a> is aligned within.
This is defined by the layout mode,
but is usually the <a>alignment subject</a>’s containing block.
</dl>
<h3 id='positional-values'>
Positional Alignment: the ''center'', ''stretch'', ''start'', ''end'', ''self-start'', ''self-end'', ''flex-start'', ''flex-end'', ''left'', and ''right'' keywords</h3>
The positional alignment keywords specify a position for an <a>alignment subject</a>
with respect to its <a>alignment container</a>.
The <<item-position>> set of values is used by 'justify-self' and 'align-self'
to align the box within its <a>alignment container</a>,
and also by 'justify-items' and 'align-items'
(to specify default values for 'justify-self' and 'align-self').
The <<content-position>> set of values is used by 'justify-content' and 'align-content'
to align the box's contents within itself.
<pre class='prod'>
<dfn><item-position></dfn> = center | start | end | self-start | self-end |
flex-start | flex-end | left | right;
<dfn><content-position></dfn> = center | start | end | flex-start | flex-end | left | right;
</pre>
Values have the following meanings:
<dl dfn-type="value">
<dt><dfn for="<item-position>, <content-position>">center</dfn>
<dd>Centers the <a>alignment subject</a> within its <a>alignment container</a>.
<!--
<dt><dfn>stretch</dfn>
<dd>If the 'width' or 'height' (as appropriate) of the <a>alignment subject</a> is ''auto'',
its used value is the length necessary to make the <a>alignment subject</a>’s outer size
as close to the size of the <a>alignment container</a> as possible,
while still respecting the constraints imposed by 'min/max-width/height'.
Otherwise, this is equivalent to ''start''.
-->
<dt><dfn for="<item-position>, <content-position>">start</dfn>
<dd>Aligns the <a>alignment subject</a> to be flush with the <a>alignment container</a>’s start edge.
<dt><dfn for="<item-position>, <content-position>">end</dfn>
<dd>Aligns the <a>alignment subject</a> to be flush with the <a>alignment container</a>’s end edge.
<dt><dfn for="<item-position>">self-start</dfn>
<dd>Aligns the <a>alignment subject</a> to be flush with the edge of the <a>alignment container</a>
corresponding to the <a>alignment subject</a>’s start side.
If the <a>writing modes</a> of the <a>alignment subject</a> and the <a>alignment container</a> are <a>orthogonal</a>,
this value computes to ''start''.
<dt><dfn for="<item-position>">self-end</dfn>
<dd>Aligns the <a>alignment subject</a> to be flush with the edge of the <a>alignment container</a>
corresponding to the <a>alignment subject</a>’s end side.
If the <a>writing modes</a> of the <a>alignment subject</a> and the <a>alignment container</a> are <a>orthogonal</a>,
this value computes to ''end''.
<dt><dfn for="<item-position>, <content-position>">flex-start</dfn>
<dd><strong>Only used in flex layout.</strong> [[!CSS3-FLEXBOX]]
Aligns the <a>alignment subject</a> to be flush with the edge of the <a>alignment container</a>
corresponding to the <a>flex container</a>’s main-start or cross-start side, as appropriate.
When used in layout modes other than Flexbox,
this value computes to ''start''.
<dt><dfn for="<item-position>, <content-position>">flex-end</dfn>
<dd><strong>Only used in flex layout.</strong>
Aligns the <a>alignment subject</a> to be flush with the edge of the <a>alignment container</a>
corresponding to the <a>flex container</a>’s main-end or cross-end side, as appropriate.
When used in layout modes other than Flexbox,
this value computes to ''end''.
<dt><dfn for="<item-position>, <content-position>">left</dfn>
<dd>Aligns the <a>alignment subject</a> to be flush with the <a>alignment container</a>’s <a href="http://www.w3.org/TR/css3-writing-modes/#line-left">line-left</a> edge.
If the property's axis is not parallel with the inline axis,
this value computes to ''start''.
<dt><dfn for="<item-position>, <content-position>">right</dfn>
<dd>Aligns the <a>alignment subject</a> to be flush with the <a>alignment container</a>’s <a href="http://www.w3.org/TR/css3-writing-modes/#line-right">line-right</a> edge.
If the property's axis is not parallel with the inline axis,
this value computes to ''start''.
</dl>
<p class='issue'>
Add example images.
<h3 id='baseline'>
Baseline Alignment: the ''baseline'' and ''last-baseline'' keywords</h3>
<dfn>Baseline alignment</dfn> is a form of positional alignment
that aligns multiple <a>alignment subjects</a> within a <a>shared alignment context</a> (such as cells within a row or column)
by matching up their <a>alignment baselines</a>.
The baseline alignment keywords are:
<pre class='prod'>
<dfn><baseline-position></dfn> = baseline | last-baseline
</pre>
and are defined below:
<dl dfn-type=value dfn-for="justify-content, justify-self, justify-items, align-content, align-self, align-items">
<dt><dfn>baseline</dfn>
<dd>
Indicates <dfn>first-baseline alignment</dfn>:
aligns one of the box's <a>first baselines</a>
with the corresponding <a>first baselines</a>
of all the boxes in its <a>baseline-sharing group</a>.
If the <a>alignment subject</a>’s position is not fully determined by <a>baseline alignment</a>,
the content is ''self-start''-aligned insofar as possible while preserving the baseline alignment.
(Content that has no <a>first baselines</a> is thus start-aligned.)
<dt><dfn>last-baseline</dfn>
<dd>
Indicates <dfn>last-baseline alignment</dfn>:
aligns one of the box's <a>last baselines</a>
with th
831E
e corresponding <a>last baselines</a>
of all the boxes in its <a>baseline-sharing group</a>.
If the <a>alignment subject</a>’s position is not fully determined by <a>baseline alignment</a>,
the content is ''self-end''-aligned insofar as possible while preserving the baseline alignment.
(Content that has no <a>last baselines</a> is thus end-aligned.)
</dl>
When specified for 'align-content'/'justify-content',
these values trigger <a>baseline content-alignment</a>,
shifting the content of the box within the box,
and may also affect the sizing of the box itself.
See <a section href="#baseline-align-content"></a>.
When specified for 'align-self'/'justify-self',
these values trigger <a>baseline self-alignment</a>,
shifting the entire box within its container,
which may affect the sizing of its container.
See <a section href="#baseline-align-self"></a>.
<p class='issue'>Add example images.
Note: The 'vertical-align' property will also need a ''first-baseline'' value
because per CSS2.1 for inline blocks, ''baseline'' is equivalent to ''last-baseline''.
<hr>
A <dfn>baseline-sharing group</dfn>
is composed of boxes that participate in baseline alignment together.
This is possible only if they
<ul>
<li>share an <a>alignment context</a> whose axis is parallel to their <i>inline axis</i>
<li>have the same <a>block flow direction</a> and <a>line orientation</a>,
<li>participate in the same type of baseline alignment (first- vs. last-line, content- vs. self-alignment)
</ul>
Boxes <dfn title="shared alignment context|alignment context">share an alignment context</dfn> along a particular axis when they are:
<ul>
<li>table cells in the same row, along the table's row (inline) axis
<li>table cells in the same column, along the table's column (block) axis
<li>grid items in the same row, along the grid's row (inline) axis
<li>grid items in the same column, along the grid's colum (block) axis
<li>flex items in the same flex line, along the flex container's main axis
</ul>
Boxes in a <a>baseline-sharing group</a> are aligned to each other
using their <dfn>alignment baseline</dfn>.
This is the dominant baseline associated with
the box that generates their <a>shared alignment context</a>
on the first (last, for ''last-baseline'' alignment) <a title="first formatted line">formatted line</a>.
For example, in horizontal writing modes,
specifying ''align-content: baseline'' on table cells in the same row
will align the alphabetic baselines of their <a>first formatted lines</a>.
<h3 id='distribution-values'>
Distributed Alignment: the ''space-between'', ''space-around'', and ''space-evenly'' keywords</h3>
The distribution values are used by 'justify-content' and 'align-content'
to distribute the items in the <a>alignment subject</a>
evenly between the start and end edges of the <a>alignment container</a>.
When the <a>alignment subject</a> cannot be distributed in this way,
they behave as their <dfn export>fallback alignment</dfn>.
Each distribution value has an associated <<content-position>> as a <a>fallback alignment</a>,
but one can alternatively be explicitly specified in the property.
<pre class='prod'> <dfn><content-distribution></dfn> = space-between | space-around | space-evenly | stretch</pre>
<dl dfn-type="value" dfn-for="<content-distribution>">
<dt><dfn>space-between</dfn>
<dd>
The items are evenly distributed in the <a>alignment container</a>.
The first item is placed flush with the start edge of the <a>alignment container</a>,
the last item is placed flush with the end edge of the <a>alignment container</a>,
and the remaining items are distributed so that the spacing between any two adjacent items is the same.
Unless otherwise specified,
this value falls back to ''start''.
<dt><dfn>space-around</dfn>
<dd>
The items are evenly distributed in the <a>alignment container</a>,
with a half-size space on either end.
The items are distributed so that the spacing between any two adjacent items is the same,
and the spacing before the first and after the last item is half the size of the other spacing.
Unless otherwise specified,
this value falls back to ''center''.
<dt><dfn>space-evenly</dfn>
<dd>
The items are evenly distributed in the <a>alignment container</a>,
with a full-size space on either end.
The items are distributed so that the spacing between any two adjacent items,
before the first item,
and after the last item
is the same.
Unless otherwise specified,
this value falls back to ''center''.
<dt><dfn>stretch</dfn>
<dd>
If the combined size of the items is less than the size of the <a>alignment container</a>,
any ''width/auto''-sized items have their size increased equally so that the combined size exactly fills the <a>alignment container</a>.
Otherwise,
or if there are no ''width/auto''-sized items,
this value is identical to ''flex-start''.
<span class='note'>(For layout modes other than flex layout, ''flex-start'' is identical to ''start''.)</span>
</dl>
<p class='issue'>
Add example images.
<h3 id='overflow-values'>
Overflow Alignment: the ''safe'' and ''true'' keywords</h3>
When the <a>alignment subject</a> is larger than the <a>alignment container</a>,
it will overflow.
Some alignment modes, if honored in this situation,
may cause data loss:
for example, if the contents of a sidebar are centered,
when they overflow they may send part of their boxes past the viewport's start edge,
which can't be scrolled to.
To help combat this problem,
an <dfn export>overflow alignment</dfn> mode can be explicitly specified.
"True" alignment honors the specified alignment mode in overflow situations, even if it causes data loss,
while "safe" alignment changes the alignment mode in overflow situations in an attempt to avoid data loss.
If the <a>overflow alignment</a> isn't explicitly specified,
the default <a>overflow alignment</a> is determined by the layout mode.
Document-centric layout modes, such as block layout, default to "safe" <a>overflow alignment</a>,
while design-centric layout modes, such as flex layout, default to "true" <a>overflow alignment</a>.
<pre class='prod'><dfn><overflow-position></dfn> = true | safe</pre>
<dl dfn-type="value" dfn-for="<overflow-position>">
<dt><dfn>safe</dfn>
<dd>
If the size of the <a>alignment subject</a> overflows the <a>alignment container</a>,
the <a>alignment subject</a> is instead aligned as if the alignment mode were ''start''.
<dt><dfn>true</dfn>
<dd>
Regardless of the relative sizes of the <a>alignment subject</a> and <a>alignment container</a>,
the given alignment value is honored.
</dl>
<p class='issue'>
Transplant example 10 from flexbox.
<h2 id='content-distribution'>
Content Distribution: the 'justify-content' and 'align-content' properties</h2>
The 'justify-content' and 'align-content' properties control alignment of the box's content within the box.
<div class="figure">
<img alt="Diagram showing that the alignment of the content within the element is affected." src="images/content-example.svg" width=212 height=212>
</div>
<pre class="propdef">
Name: justify-content, align-content
Value: auto | <<baseline-position>> | [ <<item-position>> <<content-position>>? | <<content-position>> ] && <<overflow-position>>?
Initial: auto
Applies to: block containers, flex containers, and grid containers
Inherited: no
Percentages: n/a
Media: visual
Computed value: specified value
Animatable: no
</pre>
Aligns the contents of the box as a whole along the box's inline/row/main axis.
Values other than <dfn value for="justify-content, align-content">auto</dfn> are <a href="#alignment-values">defined above</a>.
If both a <<item-position>> and <<content-position>> are given,
the second value represents an explicit <a>fallback alignment</a>.
<hr>
<dl>
<dt>Block Containers:
<dd>
The <a>alignment container</a> is the <a>block container</a>’s content box.
The <a>alignment subject</a> is the entire contents of the block.
The 'align-content' property applies along the <a>block axis</a>,
but if a <var><content-distribution></var> is specified
the <a>fallback alignment</a> is used instead.
The 'justify-content' property does not apply to and has no effect on block containers.
All values other than ''justify-content/auto'' force the block container to establish a new formatting context.
For table cells, the behavior of the ''justify-content/auto'' depends on the computed value of 'vertical-align':
''top'' makes it behave as ''start'',
''middle'' makes it behave as ''center'',
''bottom'' makes it behave as ''end'',
and all other values make it behave as ''baseline''.
''justify-content/auto'' otherwise behaves as ''start''.
<dt>Multicol Layout:
<dd>
The <a>alignment container</a> is the <a>multi-column element</a>’s content box.
The <a>alignment subject</a> is the column boxes, as a unit.
The 'align-content' property applies along the <a>block axis</a>,
but if a <var><content-distribution></var> is specified
the <a>fallback alignment</a> is used instead.
The 'justify-content' property does not apply to and has no effect on multi-column elements.
''justify-content/auto'' behaves as ''start''.
<dt>Flex Containers:
<dd>
''justify-content/auto'' computes to ''stretch''.
The <a>alignment container</a> is the <a>flex container</a>’s content box.
For 'justify-content', the <a>alignment subject</a> is the <a>flex items</a> in each <a>flex line</a>;
for 'align-content', the <a>alignment subject</a> is the <a>flex lines</a>.
The 'align-content' property applies along the <a>cross axis</a>.
The 'justify-content' property applies along the <a>main axis</a>,
but ''stretch'' behaves as ''start''.
See [[!CSS3-FLEXBOX]] for details.
<dt>Grid Containers:
<dd>
''justify-content/auto'' computes to ''start'', and ''stretch'' behaves like ''start''.
The <a>alignment container</a> is the <a>grid container</a>’s content box.
The <a>alignment subject</a> is the bounds of the <a>grid</a>.
<span class='issue'>Need to dfn a better term for this in Grid.</span>
The 'align-content' property applies along the <a title="block axis">block (column) axis</a>.
The 'justify-content' property applies along the <a title="inline axis">inline (row) axis</a>.
In both properties, if a <var><content-distribution></var> is specified,
the <a>fallback alignment</a> is used instead.
</dl>
<h3 id=&quo
80C
t;baseline-align-content">
Baseline Content-Alignment</h3>
The content of boxes participating in row-like layout contexts (<a>shared alignment contexts</a>)
can be baseline-aligned to each other.
This effectively increases the padding on the box
to align the <a>alignment baseline</a> of its contents
with that of other baseline-aligned boxes in its <a title="baseline-sharing group">group</a>.
The set of boxes that participate in <dfn export title="baseline content-alignment|first-baseline content-alignment|last-baseline content-alignment">baseline content-alignment</dfn>
depends on the layout model:
<dl>
<dt>Table Cells:
<dd>
A <a>table cell</a> participates in first (last) <a>baseline content-alignment</a>
in either its row or column (whichever matches its <a>inline axis</a>)
if its computed 'align-content' is ''baseline'' (''last-baseline'').
<dt>Flex Items:
<dd>
A <a>flex item</a> participates in first (last) <a>baseline content-alignment</a>
in its flex line if
its computed 'align-content' is ''baseline'' (''last-baseline'')
and its computed 'align-self' is ''stretch'' or ''start'' (''end'').
<dt>Grid Items:
<dd>
A <a>grid item</a> participates in first (last) <a>baseline content-alignment</a>
in either its row or column (whichever matches its <a>inline axis</a>)
if its computed 'align-content' is ''baseline'' (''last-baseline''),
and its computed 'align-self' or 'justify-self' (whichever affects its <a>block axis</a>) is ''stretch'' or ''start'' (''end'').
</dl>
If a box spans multiple <a>shared alignment contexts</a>,
it participates in first (last) <a>baseline content-alignment</a>
within its start-most (end-most) <a>shared alignment context</a> along that axis.
When a box participates in first (last) <a>baseline content-alignment</a>
its <a>alignment subject</a> is aligned to the start (end) edge of its <a>alignment container</a>
and the minimum necessary extra space is added between its start (end) edge and the <a>alignment subject</a>
to match its <a>alignment baseline</a> in that axis up to that of its <a>baseline-sharing group</a>.
This increases the intrinsic size of the box.
<h2 id='self-alignment'>
Self-Alignment: Aligning the Box within its Parent</h2>
The 'justify-self' and 'align-self' properties control alignment of the box within its containing block.
<div class="figure">
<img alt="Diagram showing that the alignment of the element within its containing block is affected." src="images/self-example.svg" width=212 height=212>
</div>
<h3 id='justify-self-property'>
Inline/Main-Axis Alignment: the 'justify-self' property</h3>
<pre class="propdef">
Name: justify-self
Value: auto | ''stretch'' | <<baseline-position>> | [ <<item-position>> && <<overflow-position>>? ]
Initial: auto
Applies to: block-level boxes, absolutely-positioned boxes, and grid items
Inherited: no
Percentages: n/a
Media: visual
Computed value: specified value
Animatable: no
</pre>
Justifies the box within its parent along the inline/row/main axis:
the box's outer edges are aligned within its <a>alignment container</a>
<a href="#alignment-values">as described by its alignment value</a>.
The <dfn value for="justify-self">auto</dfn> keyword computes to
''stretch'' on absolutely-positioned elements,
and to the computed value of 'justify-items' on the parent
(minus any ''legacy'' keywords) on all other boxes.
<hr>
<dl>
<dt>Block-level Boxes:
<dd>
The 'justify-self' property applies along its containing block's <a>inline axis</a>.
The <a>alignment container</a> is the block's containing block
except that for block-level elements that establish a block formatting context
and are placed next to a float,
the <a>alignment container</a> is reduced by the space taken up by the float.
(Note: This is the legacy behavior of HTML <code>align</code>.)
The <a>alignment subject</a> is the block's margin box.
The default <a>overflow alignment</a> is ''safe''.
In terms of CSS2.1 block-level formatting [[!CSS21]],
the rules for "over-constrained" computations in <a href="http://www.w3.org/TR/CSS21/visudet.html#blockwidth">section 10.3.3</a>
are ignored in favor of alignment as specified here
and the used value of the offset properties are not adjusted to correct for the over-constraint.
The ''stretch'' value is equivalent to ''start'' on block-level boxes.
This property does not apply to floats.
<dt>Absolutely-positioned Boxes:
<dd>
The 'justify-self' property applies along its containing block's <a>inline axis</a>.
When neither margin in this dimension is ''margin/auto''
and neither offset property in this dimension is ''top/auto'',
values other than ''stretch'' cause <a href="http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-width">non-replaced absolutely-positioned boxes</a>
to use shrink-to-fit sizing for calculating ''width/auto'' measures,
and 'justify-self' dictates alignment as follows:
The <a>alignment container</a> is the box's containing block
as modified by the offset properties ('top'/'right'/'bottom'/'left').
The <a>alignment subject</a> is the box's margin box.
The default <a>overflow alignment</a> is ''safe''.
In terms of CSS2.1 formatting [[!CSS21]],
the rules for "over-constrained" computations in <a href="http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-width">section 10.3.7</a>
are ignored in favor of alignment as specified here
and the used value of the offset properties are not adjusted to correct for the over-constraint.
The ''stretch'' keyword is equivalent to ''start'' on replaced absolutely-positioned boxes.
(This is because CSS 2.1 does not stretch replaced elements to fit into fixed offsets.)
<dt>Table Cells:
<dd>
This property does not apply to table cells,
because their position and size is fully constrained by table layout.
<dt>Flex Items:
<dd>
This property does not apply to <a>flex items</a>,
because there is more than one item in the <a>main axis</a>.
See 'flex' for stretching and 'justify-content' for <a>main-axis</a> alignment. [[!CSS3-FLEXBOX]]
<dt>Grid Items:
<dd>
The 'justify-self' property applies along the grid's <a>row axis</a>.
The <a>alignment container</a> is the <a>grid cell</a>.
The <a>alignment subject</a> is the <a>grid item</a>’s margin box.
The default <a>overflow alignment</a> is ''true''.
</dl>
<p class='issue'>
Should we make ''stretch'' actually work on block-level/abspos replaced boxes?
To maintain legacy compat, this requires adding a new value (named ''normal''?)
which has the current behavior of stretching non-replaced boxes and start-aligning replaced ones.
<div class="example">
The effect of these rules is that an auto-sized block-level table,
for example, can be aligned while still having side margins.
If the table's max-content size is narrower than its containing block,
then it is shrink-wrapped to that size and aligned as specified.
If the table's max-content size is wider, then it fills its containing block,
and the margins provide appropriate spacing from the containing block edges.
</div>
<h3 id='align-self-property'>
Block/Cross-Axis Alignment: the 'align-self' property</h3>
<pre class="propdef">
Name: align-self
Value: auto | ''stretch'' | <<baseline-position>> | [ <<item-position>> && <<overflow-position>>? ]
Initial: auto
Applies to: block-level boxes
Inherited: no
Percentages: n/a
Media: visual
Computed value: specified value
Animatable: no
</pre>
Aligns the box within its parent along the block/column/cross axis:
the box's outer edges are aligned within its <a>alignment container</a>
<a href="#alignment-values">as described by its alignment value</a>.
The <dfn value for="align-self">auto</dfn> keyword computes to
''stretch'' on absolutely-positioned elements,
and to the computed value of 'align-items' on the parent
(minus any ''legacy'' keywords) on all other boxes.
<hr>
<dl>
<dt>Block-level Boxes:
<dd>
The 'align-self' property does not apply to block-level boxes
(including floats),
because there is more than one item in the <a>block axis</a>.
<dt>Absolutely-positioned Boxes:
<dd>
The 'justify-self' property applies along its containing block's <a>inline axis</a>.
When neither margin in this dimension is ''margin/auto''
and neither offset property in this dimension is ''top/auto'',
values other than ''stretch'' cause <a href="http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-height">non-replaced absolutely-positioned boxes</a>
to use shrink-to-fit sizing for calculating ''width/auto'' measures,
and 'align-self' dictates alignment as follows:
The <a>alignment container</a> is the box's containing block
as modified by the offset properties ('top'/'right'/'bottom'/'left').
The <a>alignment subject</a> is the box's margin box.
The default <a>overflow alignment</a> is ''safe''.
In terms of CSS2.1 formatting [[!CSS21]],
the rules for "over-constrained" computations in <a href="http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-height">section 10.6.4</a>
are ignored in favor of alignment as specified here
and the used value of the offset properties are not adjusted to correct for the over-constraint.
The ''stretch'' keyword is equivalent to ''start'' on replaced absolutely-positioned boxes.
(This is because CSS 2.1 does not stretch replaced elements to fit into fixed offsets.)
<dt>Table Cells:
<dd>
This property does not apply to table cells,
because their position and size is fully constrained by table layout.
<dt>Flex Items:
<dd>
The 'align-self' property applies along the flexbox's <a>cross axis</a>.
The <a>alignment container</a> is the <a>flex line</a> the item is in.
The <a>alignment subject</a> is the <a>flex item</a>’s margin box.
The default <a>overflow alignment</a> is ''true''.
See [[!CSS3-FLEXBOX]] for details.
<dt>Grid Items:
<dd>
The 'align-self' property applies along the grid's <a>column axis</a>.
The <a>alignment container</a> is the <a>grid cell</a>.
The <a>alignment subject</a> is the <a>grid item</a>’s margin box.
The default <a>overflow alignment</a> is ''true''.
</dl>
<h3 id="baseline-align-self">
Baseline Self-Alignment</h3>
Boxes participating in row-like layout contexts (<a>shared alignment contexts</a>)
can be baseline-aligned to each other.
This effectively increases the margins on the box
to align its <a>alignment baseline</a>
with other baseline-aligned boxes in its <a title="baseline-sharing group">group</a>.
The set of boxes that participate in <dfn export title="baseline self-alignment|first-baseline self-alignment|last-baseline self-alignment">baseline self-alignment</dfn>
depends on the layout model:
<dl>
<dt>Flex Items:
<dd>
A <a>flex item</a> participates in first (last) <a>baseline self-alignment</a> in its flex line
if its computed 'align-self' is ''baseline'' (''last-baseline'').
See [[!CSS3-FLEXBOX]] for details.
<dt>Grid Items:
<dd>
A <a>grid item</a> participates in first (last) <a>baseline content-alignment</a>
in either its row or column (whichever matches its <a>inline axis</a>)
if its 'justify-self' or 'align-self' property (whichever matches its <a>inline axis</a>)
computes to ''baseline'' (''last-baseline'').
</dl>
If a box spans multiple <a>shared alignment contexts</a>,
it participates in first (last) <a>baseline self-alignment</a>
within its start-most (end-most) <a>shared alignment context</a> along that axis.
When a box participates in first (last) <a>baseline self-alignment</a>
its <a>alignment subject</a> is aligned to the start (end) edge of its <a>alignment container</a>
and the minimum necessary extra space is added between its start (end) edge and the <a>alignment container</a>
to match its <a>alignment baseline</a> in that axis up to that of its <a>baseline-sharing group</a>.
This may increase the intrinsic size contribution of the <a>alignment subject</a>.
<h2 id='default-alignment'>
Default Alignment</h2>
The 'align-items' and 'justify-items' properties set the default 'align-self' and 'justify-self' behavior of the items contained by the element.
<div class="figure">
<img alt="Diagram showing that the alignment of grid items within the element is affected." src="images/items-example.svg" width=212 height=212>
</div>
<h3 id='justify-items-property'>
Inline/Main-Axis Alignment: the 'justify-items' property</h3>
<pre class="propdef">
Name: justify-items
Value: auto | ''stretch'' | <<baseline-position>> | [ <<item-position>> && <<overflow-position>>? ] | [ legacy && [ ''left'' | ''right'' | ''center'' ] ]
Initial: auto
Applies to: block containers, flex containers, and grid containers
Inherited: no
Percentages: n/a
Media: visual
Computed value: specified value
Animatable: no
</pre>
This property specifies the default 'justify-self' for all of the boxes
(including anonymous boxes)
participating in this box's formatting context.
Values have the following meanings:
<dl dfn-type="value" dfn-for="justify-items">
<dt><dfn>auto</dfn>
<dd>
If the <a>inherited value</a> of ''justify-items'' includes the ''legacy'' keyword,
''justify-items/auto'' computes to the the <a>inherited value</a>.
Otherwise, ''justify-items/auto'' computes to:
<ul>
<li>''stretch'' for flex containers and grid containers
<li>''start'' for everything else
</ul>
<dt><dfn>legacy</dfn>
<dd>
This keyword causes the value to effectively inherit into descendants.
It can only be combined with the ''center'', ''left'', and ''right'' positions.
The alignment keyword, but not the ''legacy'' keyword, is passed to 'justify-self'.
It exists to implement the legacy alignment behavior of HTML's <code><center></code> element and <code>align</code> attribute.
</dl>
Other values have no special handling and are merely passed to 'justify-self'.
<h3 id='align-items-property'>
Block/Cross-Axis Alignment: the 'align-items' property</h3>
<pre class="propdef">
Name: align-items
Value: auto | ''stretch'' | <<baseline-position>> | [ <<item-position>> && <<overflow-position>>? ]
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: n/a
Media: visual
Computed value: specified value
Animatable: no
</pre>
This property specifies the default 'align-self' for all of the boxes
(including anonymous boxes)
participating in this box's formatting context.
Values have the following meanings:
<dl dfn-type="value" dfn-for="align-items">
<dt><dfn>auto</dfn>
<dd>
Computes to:
<ul>
<li>''stretch'' for flex containers and grid containers
<li>''start'' for everything else
</ul>
</dl>
Other values have no special handling and are merely passed to 'align-self'.
<h2 id='baseline-rules'>
Determining the Baseline of a Box</h2>
The <dfn title="baselines of a box"></dfn><dfn>first baselines</dfn> (and <dfn>last baselines</dfn>) of a box
for a given axis
are a set of baselines (alphabetic, central, etc.)
nominally associated with the first (last) line of text within the box.
The <a>alignment baseline</a> is one of these,
usually the dominant baseline of the <a>alignment container</a>.
(See 'alignment-baseline'.)
Note that boxes might not have baselines in a particular axis.
The first and last <i>baselines of a box</i>
are determined differently based on the layout model, as follows:
<dl>
<dt>block containers
<dd>
The first (last) inline-axis baselines of a block container
are generated from the dominant baseline of the first (last) in-flow line box in the block container,
or are taken from the first (last) in-flow block-level child in the block container that contributes a set of baselines,
whichever comes first (last).
If there is no such line box or child,
then the block container has no baselines.
For the purposes of finding the baselines,
in-flow boxes with a scrolling mechanisms (see the 'overflow' property)
must be considered as if scrolled to their origin (final) position.
A block container has no block-axis baselines.
<dt>tables
<dd>
The first (last) inline-axis baselines of a table box are
the baselines of its first (last) row.
When finding the baselines of an inline-block,
any baselines contributed by table boxes must be skipped.
(This quirk is a legacy behavior from [[CSS21]].)
The block-axis baselines of a table box are the baselines of its first (last) column.
<dt>table rows
<dd>
If any cells in the row participate in ''baseline'' (''last-baseline'') alignment along the row axis,
the first (last) inline-axis baselines of the row
are generated from their shared <a>alignment baseline</a>
and the row's <a>first available font</a>,
after alignment has been performed.
Otherwise, the first (last) inline-axis baselines of the row
are synthesized from the lowest and highest content edges of the cells in the row.
[[!CSS21]]
A table row has no block-axis baselines.
<dt>table columns
<dd>
If any cells in the column participate in ''baseline'' (''last-baseline'') alignment
along the column axis (by having a writing mode perpendicular to that of the table),
the first (last) block-axis baselines of the column
are generated from their shared <a>alignment baseline</a>
and the column's <a>first available font</a>,
after alignment has been performed.
Otherwise, the the first (last) inline-axis baselines of the row
are synthesized from the extreme content edges of the cells in the row.
[[!CSS21]]
A table column has no inline-axis baselines.
<dt>flex containers
<dd>
See <a href="http://www.w3.org/TR/css3-flexbox/#flex-baselines">Flex Baselines</a> in [[!CSS3-FLEXBOX]].
<dt>grid containers
<dd>
See <a href="http://www.w3.org/TR/css3-grid-layout/#grid-baselines">Grid Baselines</a> in [[!CSS3-GRID-LAYOUT]].
</dl>
To <dfn>generate baselines</dfn> for a box from a single baseline,
use the baseline table from the font settings and <a>first available font</a> of that box.
To <dfn>synthesize baselines</dfn> from a rectangle (or two parallel lines),
synthesize the alphabetic baseline from the lower line
and the central baseline by averaging the positions of the upper and lower lines.
Note: The forthcoming Inline Layout Module will define synthesis rules for baselines other than alphabetic and central.
<p class='issue'>
Maybe these things are wrong?
CSS 2.1 is really weird about baseline alignment.
<div class="issue">
We're still working on handling multiple baseline types (alphabetc, central, etc.)
and rows with multiple baseline-sharing groups (e.g. contains both first and last-aligned cells, or contains opposing writing modes).
Consider this section to be Under Construction.
<img src="http://dev.w3.org/csswg/construction.gif" style="display: block; margin: 1em auto;">
</div>
<h2 id="changes">
Changes</h2>
Changes since the <a href="http://www.w3.org/TR/2013/WD-css3-align-20130514/">previous Working Draft</a> include:
<ul>
<li>
Added ''last-baseline'' keyword.