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<h1>CSS Intrinsic & Extrinsic Sizing Module Level 3</h1>
<pre class='metadata'>
Shortname: css-sizing
Level: 3
Status: ED
Work Status: Exploring
Group: csswg
ED: http://dev.w3.org/csswg/css-sizing
TR: http://www.w3.org/TR/css3-sizing/
Editor: Tab Atkins, Google, http://xanthir.com/contact/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact/
!Issue Tracking: <a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&component=Sizing&resolution=---">W3C Bugzilla</a>
Previous version: http://www.w3.org/TR/2012/WD-css3-sizing-20120927/
Abstract: This module extends the CSS sizing properties with keywords that represent content-based "intrinsic" sizes and context-based "extrinsic" sizes, allowing CSS to more easily describe boxes that fit their content or fit into a particular layout context.
Link Defaults: css21 (property) min-width/min-height/max-width/max-height, css21 (dfn) initial containing block/block container box
Ignored Terms: block-level box
</pre>
<pre class='anchors'>
url: http://dev.w3.org/csswg/css2/visuren.html#x3; type: dfn; text: containing block
</pre>
<h2 id="intro">
Introduction</h2>
<p><em>This section is not normative.</em>
<p>
CSS layout has several different concepts of automatic sizing that are used in various layout calculations.
This section defines some more precise terminology
to help connect the layout behaviors of this spec to the calculations used in other modules,
and some new keywords for the 'width' and 'height' properties
to allow authors to assign elements the dimensions resulting from these size calculations.
<h3 id="placement">
Module interactions</h3>
<p>This module extends the 'width', 'height', 'min-width', 'min-height', 'max-width', 'max-height', and 'column-width'
features defined in [[!CSS21]] chapter 10 and in [[!CSS3COL]]
<h3 id="values">
Values</h3>
<p>This specification follows the
<a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
definition conventions</a> from [[!CSS21]]. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
Other CSS modules may expand the definitions of these value types: for
example [[CSS3COLOR]], when combined with this module, expands the
definition of the <color> value type as used in this specification.</p>
<p>In addition to the property-specific values listed in their definitions,
all properties defined in this specification also accept the
<a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
keyword as their property value. For readability it has not been repeated
explicitly.
<h2 id="terms">
Terminology</h2>
<dl export>
<dt><dfn>size</dfn>
<dd>
A one- or two-dimensional measurement:
a <a>block size</a> and/or <a>inline size</a>;
alternatively a width and/or height.
<dt><dfn id="definite" lt="definite|definite size">definite size</dfn>
<dd>
A size that can be determined without measuring content;
that is, a <var><length></var>,
a size of the <a>initial containing block</a>,
or a <var><percentage></var> that is resolved against a <a>definite</a> size.
Additionally, the size of the <a>containing block</a> of an absolutely positioned element is always <a>definite</a>
with respect to that element.
<dt><dfn id="indefinite" lt="indefinite|indefinite size">indefinite size</dfn>
<dd>
A size that is not <a>definite</a>.
An <a>indefinite</a> <a>available size</a> is essentially infinite.
<dt><dfn id="available" local-lt="available" lt="available size|available inline size|available block size|available space">available size</dfn>
<dd>
The space into which a box is laid out.
Unless otherwise specified, this is
either a measurement of its <a>containing block</a> (if that is <a>definite</a>)
or an infinite size (when it is <a>indefinite</a>).
An <a>available size</a> can alternatively be either a
<a>min-content constraint</a> or
a <a>max-content constraint</a>.
<!--
<p>
The space <a>available</a> to a box is determined by the formatting context in which it participates.
In block, table, and inline formatting contexts [[!CSS21]],
the <a>available size</a> is infinite in the block-axis dimension.
<span class="issue">What, exactly, is the available size in the inline axis??? What about other formatting models?</span>
-->
<dt><dfn>fill-available fit</dfn>
<dd>
The <a>fill-available fit</a> into a given size
is that size,
minus the element's computed margins (not collapsed, treating ''margin/auto'' as zero),
border, and padding in the given dimension.
<p class='note'>
Note: This is the formula used to calculate the ''width/auto'' widths
of non-replaced blocks in normal flow in <a href="http://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
<dt><dfn lt="fallback|fallback size">fallback size</dfn>
<dd>
Some sizing algorithms do not work well with an infinite size.
In these cases, the <a>fallback size</a> is used instead.
Unless otherwise specified,
this is the size of the <a>initial containing block</a>.
</dl>
<h3 id='auto-box-sizes'>
Auto Box Sizes</h3>
<div export>
: <dfn>fill-available size</dfn>
: <dfn>fill-available inline size</dfn>
: <dfn>fill-available block size</dfn>
:: Roughly, the <a>size</a> a box would take
if it filled its <a>available size</a>
in the given axis.
(See [[#extrinsic]].)
Note: For the <a>inline axis</a>, this is called the “available width” in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and computed by the rules in <a href="http://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
: <dfn>max-content size</dfn>
:: A box’s “ideal” <a>size</a> in a given axis when given infinite available space.
Usually this is the smallest <a>size</a> the box could take in that axis
while still fitting around its contents,
i.e. minimizing unfilled space while avoiding overflow.
: <dfn>max-content inline size</dfn>
:: The box's “ideal” <a>size</a> in the <a>inline axis</a>.
Usually the narrowest <a>inline size</a> it could take while fitting around its contents
if <em>none</em> of the soft wrap opportunities within the box were taken.
(See [[#intrinsic]].)
Note: This is called the “preferred width” in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and the “maximum cell width” in <a href="http://www.w3.org/TR/CSS21/tables.html#auto-table-layout">CSS2.1§17.5.2.2</a>.
: <dfn>max-content block size</dfn>
:: The box's “ideal” <a>size</a> in the <a>block axis</a>.
Usually the <a>block size</a> of the content after layout.
: <dfn>min-content size</dfn>
:: The smallest <a>size</a> a box could take
that doesn't lead to overflow
that could be avoided by choosing a larger <a>size</a>.
(See [[#intrinsic]].)
: <dfn>min-content inline size</dfn>
:: The narrowest <a>inline size</a> a box could take
that doesn't lead to inline-dimension overflow
that could be avoided by choosing a larger <a>inline size</a>.
Roughly, the <a>inline size</a> that would fit around its contents
if <em>all</em> soft wrap opportunities within the box were taken.
Note: This is called the “preferred minimum width” in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and the “minimum content width” in <a href="http://www.w3.org/TR/CSS21/tables.html#auto-table-layout">CSS2.1§17.5.2.2</a>.
: <dfn>min-content block size</dfn>
:: Equivalent to the <a>max-content block size</a>.
Issue: Or should this be the minimum between allowed break points?
It might make sense in multi-col contexts
to have min-content and max-content block-sizes be different,
even if they are the same elsewhere.
: <dfn>fit-content size</dfn>
: <dfn>fit-content inline size</dfn>
: <dfn>fit-content block size</dfn>
:: If the <a>available size</a> in a given axis is finite,
equal to <code>min(<a>max-content size</a>, max(<a>min-content size</a>, <a>fill-available size</a>))</code>.
Otherwise, equal to the <a>max-content size</a> in that axis.
Note: This is called the “shrink-to-fit” width in <a href="http://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and <a href="http://www.w3.org/TR/css3-multicol/#pseudo-algorithm">CSS Multi-column Layout § 3.4</a>.
</div>
<h3 id='contributions'>
Intrinsic Size Contributions</h3>
<dl export>
<dt><dfn lt="max-content contribution|max-content inline-size contribution|max-content block-size contribution">max-content contribution</dfn>
<dd>
The size that a box contributes to its <a>containing block</a>'s <a>max-content size</a>.
<dt><dfn lt="min-content contribution|min-content inline-size contribution|min-content block-size contribution">min-content contribution</dfn>
<dd>
The size that a box contributes to its <a>containing block</a>'s <a>min-content size</a>.
</dl>
<h3 id='constraints'>
Intrinsic Size Constraints</h3>
<dl export>
<dt><dfn>max-content constraint</dfn>
<dd>
A sizing constraint imposed by the box's <a>containing block</a>
that causes it to produce its <a>max-content contribution</a>.
<dt><dfn>min-content constraint</dfn>
<dd>
A sizing constraint imposed by the box's <a>containing block</a>
that causes it to produce its <a>min-content contribution</a>.
</dl>
<h2 id='size-keywords'>
New Sizing Keywords</h2>
<h3 id='width-height-keywords'>
New Keywords for 'width' and 'height'</h3>
<pre class=propdef partial>
Name: width, min-width, max-width, height, min-height, max-height
New values: fill | max-content | min-content | fit-content
</pre>
<p>There are four types of automatically-determined sizes in CSS (which
are represented in the width and height properties by the keywords
defined above):
<dl dfn-type=value dfn-for="width, min-width, max-width, height, min-height, max-height">
<dt><dfn>fill</dfn>
<dd>
Use the <a>fill-available inline size</a> or <a>fill-available block size</a>,
as appropriate to the writing mode.
<dt><dfn>max-content</dfn>
<dd>
Use the <a>max-content inline size</a> or <a>max-content block size</a>,
as appropriate to the writing mode.
<dt><dfn>min-content</dfn>
<dd>
Use the <a>min-content inline size</a> or <a>min-content block size</a>,
as appropriate to the writing mode.
<dt><dfn>fit-content</dfn>
<dd>
Use the <a>fit-content inline size</a> or <a>fit-content block size</a>,
as appropriate to the writing mode.
<dt><dfn>repudiate-floats</dfn> <span class='issue'>Less stupid name?</span>
<dd>
In the <a>inline axis</a>,
use the larger of the <a>min-content inline size</a>,
and the <a>fill-available inline size</a> minus the size of any non-descendant floats in the same formatting context.
(This is similar to the behavior of a box that establishes a formatting context next to a float.)
In the <a>block axis</a>,
this is identical to ''width/auto''.
<p class='issue'>
Does this value work?
Is it needed?
<a href="http://lists.w3.org/Archives/Public/www-style/2013Feb/0212.html">Feedback from dbaron</a>
</dl>
<p class="issue">
Right now all of these except ''width/fill'' mean the same thing
for block-sizes. This may or may not be ideal.
<p class='issue'>
If the inline-size is ''width/auto'',
we could have min-content block-size imply a max-content inline-size,
and vice versa.
<p class='note'>
Note that percentages resolved against the intrinsic sizes
(''width/max-content'', ''width/min-content'', ''width/fit-content'', ''width/repudiate-floats'')
will compute to ''width/auto'',
<a href="http://www.w3.org/TR/CSS21/visudet.html#the-height-property">as defined by CSS 2.1</a>. [[!CSS21]]
<h3 id='the-contain-floats-value'>
Containing Floats</h3>
<pre class="propdef partial">
Name: min-width, min-height
New values: contain-floats
</pre>
<dl>
<dt><dfn for="min-width, min-height" value>contain-floats</dfn>
<dd>
Equivalent to ''min-width/min-content''
except that when applied to the block-size of a block box
it forces the inner block-size to be large enough to contain
the margin boxes of any floats that originate inside the block
and that participate in the same block formatting context as the block's immediate contents.
</dl>
<h3 id='column-sizing'>
Column Sizing Keywords</h3>
<pre class="propdef partial">
Name: column-width
New values: fill | max-content | min-content | fit-content
</pre>
<p>When used as values for 'column-width',
the new keywords specify the optimal column width:
<dl dfn-type=value dfn-for="column-width">
<dt><dfn>fill</dfn>
<dd>Specifies the optimal column width as the <a>fill-available inline size</a>
of the multi-column element.
<dt><dfn>max-content</dfn>
<dd>Specifies the optimal column width as the <a>max-content inline size</a>
of the multi-column element's contents.
<dt><dfn>min-content</dfn>
<dd>Specifies the optimal column width as the <a>min-content inline size</a>
of the multi-column element's contents.
<dt><dfn>fit-content</dfn>
<dd>Specifies the optimal column width as
<code>min(<a>max-content inline size</a>, max(<a>min-content inline size</a>, <a>fill-available inline size</a>))</code>.
</dl>
<h2 id='intrinsic'>
Intrinsic Size Determination</h2>
<p>
<dfn export>Intrinsic sizing</dfn> determines sizes based on the contents of an element,
without regard for its context.
<h3 id='replaced-intrinsic'>
Intrinsic Sizes of Replaced Elements</h3>
<p>
For replaced elements,
the <a>min-content size</a> and <a>max-content size</a> are equivalent
and correspond to the appropriate dimension of the concrete object size returned by
the <a href="http://www.w3.org/TR/css3-images/#default-sizing">default sizing algorithm</a> [[!CSS3-IMAGES]]
of the element,
calculated with an unconstrained specified size.
The <a>min-content contribution</a> and <a>max-content contribution</a> in each axis
is the element's specified outer size in that axis, if definite;
otherwise, they are the <a>min-content size</a>, as specified above,
plus the element's margin/border/padding in that axis,
clamped by the element's min and max size properties in that axis.
<h3 id='inline-intrinsic'>
Intrinsic Sizes of Non-Replaced Inlines</h3>
<p>
The <a>min-content inline size</a> of an inline box
is the length of the largest unbreakable sequence of inline content.
The <a>min-content inline-size contribution</a> of an inline box
is its <a>min-content inline size</a>,
plus any <a>inline-axis</a> margin, border, and padding
adjacent to that sequence.
<p>
The <a>max-content inline size</a> of an inline box
is the length of the largest sequence of inline content
on a single line
when only forced line breaks are taken.
The <a>max-content inline-size contribution</a> of an inline box
is its <a>max-content inline size</a>,
plus any <a>inline-axis</a> margin, border, and padding
adjacent to that sequence.
<p>
The <a>min-content block size</a>, <a>max-content block size</a>, <a>min-content block-size contribution</a>, and <a>max-content block-size contribution</a> of an inline box
are the distance from the head edge of the first line box
to the foot edge of the last line box
on which the inline appears.
<h3 id='block-intrinsic'>
Intrinsic Sizes of Non-Replaced Blocks</h3>
<p>
The <a>min-content inline size</a> of a <a>block container box</a>
is the largest <a>min-content inline-size contribution</a>
of its in-flow or floated children.
<p>
The <a>max-content inline size</a> of a <a>block container box</a>
is the inline-size of the box after layout,
if all children are sized under a <a>max-content constraint</a>.
<p>
If the computed inline-size of a <a>block-level box</a>
is ''width/min-content'', ''width/max-content'', or a <a>definite size</a>,
its <a>min-content inline-size contribution</a>
is that size
plus any <a>inline-axis</a> margin, border, and padding.
Otherwise, if the computed inline-size of the block is ''width/fit-content'', ''width/auto'', or ''width/fill'',
its <a>min-content inline-size contribution</a>
is its <a>min-content inline size</a>
plus any <a>inline-axis</a> margin, border, and padding.
<p>
If the computed inline-size of a <a>block-level box</a>
is ''width/min-content'', ''width/max-content'', or a <a>definite size</a>,
its <a>max-content inline-size contribution</a>
is that size
plus any <a>inline-axis</a> margin, border, and padding.
Otherwise, if the computed inline-size of the block is ''width/fit-content'', ''width/auto'', or ''width/fill'',
its <a>max-content inline-size contribution</a>
is its <a>max-content inline size</a>
plus any <a>inline-axis</a> margin, border, and padding.
<p>
The <a>min-content block size</a> and <a>max-content block size</a> of a <a>block container box</a>
is the content block-size as defined
(for horizontal writing modes)
in <a href="http://www.w3.org/TR/CSS21/visudet.html#normal-block">CSS2.1§10.6.3</a>
and <a href="http://www.w3.org/TR/CSS21/tables.html#height-layout">CSS2.1§17.5.3</a>
for elements with ''height: auto'',
and analogously for vertical writing modes.
<p>
The <a>min-content block-size contribution</a> and <a>max-content block-size contribution</a> of a <a>block-level box</a>
is the block-size of the block after layout,
plus any <a>block-axis</a> margin, border, and padding.
Issue: Need to handle floats.
See <a href="https://lists.w3.org/Archives/Public/www-style/2014Oct/0061.html">Greg's issue</a>
and <a href="https://lists.w3.org/Archives/Public/www-style/2014Nov/0085.html">dbaron's response</a>.
<h3 id='table-intrinsic'>
Intrinsic Sizes in Table Layout</h3>
<p class='issue'>
???
<h3 id="multicol-intrinsic">
Intrinsic Sizes in Multi-column Layout</h3>
<h4 id="multicol-min-content">
Min-content Sizes in Multi-column Layout</h3>
<p>
The <a>min-content inline size</a> of a multi-column element
with a computed 'column-width' not ''column-width/auto''
is the smaller of
its 'column-width'
and the largest <a>min-content inline-size contribution</a> of its contents.
<p>
The <a>min-content inline size</a> of a multi-column element
with a computed 'column-width' of ''column-width/auto''
is the largest <a>min-content inline-size contribution</a> of its contents
multiplied by its 'column-count'
(treating ''column-count/auto'' as ''1'').
<h4 id="multicol-max-content">
Max-content Sizes in Unconstrained-height Multi-column Layout</h3>
<p>
The <a>max-content inline size</a> of a multi-column element
with unrestrained column heights
and a computed 'column-count' not ''column-count/auto''
is its 'column-count'
multiplied by the larger of
its 'column-width' (treating ''column-width/auto'' as zero)
and the largest <a>min-content inline-size contribution</a> of its contents.
<p class="note">
Note that the contents of the multi-column element
can still grow to be wider and shorter
if the resulting column width is still smaller
than the largest <a>max-content inline-size contribution</a> of its contents.
<p>
The <a>max-content inline size</a> of a multi-column element
with unrestrained column heights
and a computed 'column-count' of ''column-count/auto''
is its 'column-width'
multiplied by the number of columns obtained by taking all allowed column breaks [[CSS3-BREAK]].
<!--
The above was decided based on dholbert's example in http://lists.w3.org/Archives/Public/www-style/2012Oct/0017.html .
These definitions make multicols work well in flexbox,
both in ''stretch'' and non-''stretch'' cases,
without wasted space or needless overflow.
-->
<h4 id="multicol-max-content-restrained">
Max-content Sizes in Constrained-height Multi-column Layout</h3>
<p>
The <a>max-content inline size</a> of a multi-column element
with restrained-height columns (i.e. a specified 'height' or 'max-height', or whichever properties map to the <a>block size</a> of the element)
is the <a>inline size</a> that would exactly include all of its columns.
It may be approximated by:
<ul>
<li>
Laying out the element with column-spanning elements given ''display: none'',
and taking a inline-size that includes all the columns.
<li>
Laying out all of the column-spanning elements into that resulting inline-size,
and taking the resulting block-size.
<li>
Subtracting that resulting block-size from the specified restraint,
laying out the element without column-spanning elements again into this adjusted block-size,
and taking the inline-size of all its columns as its <a>max-content inline size</a>.
</ul>
<p>
or by some more accurate method.
<p class='note'>
This approximation can result in some slack,
but avoids overflow in the most common cases,
where the balanced height of the columns above spanning elements are approximately equal.
<p class='note'>
In the common case of no column-spanning elements,
this approximation collapses to simply doing a layout,
and measuring the resulting columns.
<h2 id='extrinsic'>
Extrinsic Size Determination</h2>
<p>
<dfn export>Extrinsic sizing</dfn> determines sizes based on the context of an element,
without regard for its contents.
<h3 id="fill-available-sizing">
Fill-available Sizing</h3>
<p>
The inner <a>fill-available inline size</a> of a box is…
<ul>
<li>
If the box is the root or is absolutely-positioned, the inline-size of its containing block, else
<li>
<code>max(<a>min-content inline size</a>|0, min(<a>max-content inline size</a>|infinity, <a>inline size</a>|<a>fill-available inline size</a>))</code>
where the sizes are inner inline-sizes of the element establishing the box's <a>containing block</a>,
and where the first value is used if it is <a>definite</a> and the second value otherwise.
</ul>
<p>
…less the box's <a>inline-axis</a> margins
(after any <a href="http://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin collapsing</a>, and treating ''margin/auto'' margins as zero),
borders, and padding.
<p>
The <a>fill-available block size</a> of a box is defined analogously,
but in the other dimension.
<p class='issue'>
This definition might end up skipping further up the ancestor chain than we'd like in some cases.
<a href="http://lists.w3.org/Archives/Public/www-style/2012Oct/0204.html">Example.</a>
Maybe it should stop at each formatting root, or something similar?
<h3 id="percentage-sizing">
Percentage Sizing</h3>
Issue: Write something useful here. Like about when to fall back to auto or whatever.
In order to prevent cyclic sizing,
the ''min-height/auto'' value of 'min-height' and 'max-height'
does not factor into the percentage size resolution of the box’s contents.
For example, a percentage-height block
whose <a>flex item</a> parent has
''height: 120em; min-height: auto'' will size itself against ''height: 120em''
regardless of the impact that 'min-height' might have
on the used size of the <a>flex item</a>.
<h2 class=no-num id="changes">
Changes</h2>
<p>
Changes since the <a href="http://www.w3.org/TR/2012/WD-css3-sizing-20120927/">September 2012 Working Draft</a> include:
<ul>
<li>Changed <code>fill-available</code> keyword to <code>fill</code>.
<li>Hooked up intrinsic sizes of replaced elements to the <a>default sizing algorithm</a> in [[!CSS3-IMAGES]].
<li>Specified that <a>extrinsic sizing</a> treates ''margin/auto'' margins as zero.
<li>Clarified definition of <a>available size</a>.
<li>Specified that percentages resolved against an intrinsic size of the parent computes falls back to auto sizing.
<li>Fixed bad wording wrt containing blocks in <a>fill-available inline size</a> definition,
and specified that it is calculated <em>after</em> margin collapsing.
<li>Specified that an absolutely-positioned box's containing block is always considered definite.
<li>Improved section on intrinsic sizing of multi-column elements.
<li>Cleaned up some terminology (“min-size” to “min-content size”, “measure” to “inline-size, “extent” to “block-size”).
</ul>
<h2 class=no-num id="acknowledgments">
Acknowledgments</h2>
<p>Special thanks go to Aaron Gustafson, L. David Baron
for their contributions to this module.