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<h1>CSS Intrinsic & Extrinsic Sizing Module Level 3</h1>
<pre class='metadata'>
Shortname: css-sizing
Level: 3
Status: WD
Work Status: Revising
Group: csswg
ED: https://drafts.csswg.org/css-sizing
TR: https://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, w3cid 35400
Previous version: https://www.w3.org/TR/2016/WD-css-sizing-3-20160512/
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.
Ignored Terms: block-level box
</pre>
<pre class='link-defaults'>
spec:css-display-3; type:property; text:display
spec:css-display-3; type:dfn; text:box
spec:css21; type: property
text: min-width
text: min-height
text: max-width
text: max-height
</pre>
<!-- Notes on stuff to do... [copy/pasted from etherpad, probably out-of-date, evaluate later]
Swap definition of preferred size in for max-content.
Define current max-content as super-max-content.
Mark an isssue about whether it's a necessary concept;
I'm unsure, but I think it will show up in orthogonal flow sizing.
-->
<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="https://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="https://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>inner size</dfn>
<dd>
The <a href="https://www.w3.org/TR/css2/box.html#box-dimensions">content-box</a> size of a <a>box</a>.
<dt><dfn>outer size</dfn>
<dd>
The <a href="https://www.w3.org/TR/css2/box.html#box-dimensions">margin-box</a> size of a <a>box</a>.
<dt><dfn id="definite" lt="definite|definite size">definite size</dfn>
<dd>
A size that can be determined without measuring content;
that is, a <<length>>,
a measure of text (without consideration of line-wrapping),
a size of the <a>initial containing block</a>,
or a <<percentage>> or other formula
(such the <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">“stretch-fit” sizing of non-replaced blocks</a> [[CSS21]])
that is resolved solely against <a>definite</a> sizes.
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>.
<a>Indefinite</a> <a>available space</a> is essentially infinite.
<dt><dfn id="available" local-lt="available" lt="available space|available inline space|available block space">available space</dfn>
<dd>
A size representing the space into which a box is laid out,
as determined by
the rules of the formatting context in which it participates.
The space available to a box is usually
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>).
<a>Available space</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 space</a> is infinite in the block-axis dimension.
<span class="issue">What, exactly, is the available space in the inline axis??? What about other formatting models?</span>
-->
<dt><dfn>stretch fit</dfn>
<dd>
The <a>stretch 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
(such that the outer size is a perfect fit),
and flooring at zero
(so that the inner size is not negative).
<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="https://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>stretch-fit size</dfn>
: <dfn>stretch-fit inline size</dfn>
: <dfn>stretch-fit block size</dfn>
:: Roughly, the <a>size</a> a box would take
if it filled its <a>available space</a>
in the given axis.
(See [[#extrinsic]].)
Note: For the <a>inline axis</a>, this is called the “available width” in <a href="https://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and computed by the rules in <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
: <dfn lt="max-content | max-content size">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="https://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and the “maximum cell width” in <a href="https://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 lt="min-content | min-content size">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="https://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and the “minimum content width” in <a href="https://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 space</a> in a given axis is finite,
equal to <code>min(<a>max-content size</a>, max(<a>min-content size</a>, <a>stretch-fit 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="https://www.w3.org/TR/CSS21/visudet.html#float-width">CSS2.1§10.3.5</a>
and <a href="https://www.w3.org/TR/css3-multicol/#pseudo-algorithm">CSS Multi-column Layout § 3.4</a>.
</div>
<h3 id='contributions' dfn export lt="intrinsic size contribution">
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>
Intrinsic size contributions are based on the <a>outer size</a> of the box;
for this purpose ''margin/auto'' margins are treated as zero.
<h3 id='constraints' dfn export lt="intrinsic size constraint">
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: stretch | max-content | min-content | fit-content | fit-content(<<length-percentage>>)
</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>stretch</dfn>
<dd>
Use the <a>stretch-fit inline size</a> or <a>stretch-fit block size</a>,
as appropriate to the writing mode.
NOTE: This is the formula used to calculate ''width/auto'' widths
for non-replaced blocks in normal flow, see <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">CSS2.1§10.3.3</a>.
It allows re-using this formula for boxes that are otherwise shrink-wrapped,
like tables.
<dt><dfn>max-content</dfn>
<dd>
If specified for the inline axis,
use the <a>max-content inline size</a>;
otherwise compute to ''width/auto''.
<dt><dfn>min-content</dfn>
<dd>
If specified for the inline axis,
use the <a>min-content inline size</a>;
otherwise compute to ''width/auto''.
<dt><dfn>fit-content</dfn>
<dd>
If specified for the inline axis,
use the <a>max-content inline size</a>
i.e.
<code>min(<a>max-content size</a>, max(<a>min-content size</a>, <a>stretch-fit size</a>))</code>;
otherwise compute to ''width/auto''.
<dt><dfn>fit-content(<<length-percentage>>)</dfn>
<dd>
If specified for the inline axis,
use the fit-content formula
with the <a>available space</a> replaced by the specified argument,
i.e.
<code>min(<a>max-content size</a>, max(<a>min-content size</a>, <<length-percentage>>))</code>;
otherwise compute to ''width/auto''.
</dl>
Issue: Is ''width/stretch'' stable enough or should we defer to L4?
Note: To size an element such that it avoids overlapping sibling floats,
make sure it's a <a>formatting context</a> root.
For some layout modes, such as Grid and Flexbox,
this is true automatically.
For Block layout, this means using ''display: flow-root;''.
<p class='note'>
Note that percentages resolved against the intrinsic sizes
(''width/max-content'', ''width/min-content'', ''width/fit-content'')
will compute to ''width/auto'',
<a href="https://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>
Note: To ensure that a container sizes itself to contain any descendant floats,
make sure it's a <a>formatting context</a>.
For some layout modes, such as Grid and Flexbox,
this is true automatically.
For Block layout, this means using ''display: flow-root;''.
<h3 id='column-sizing'>
Column Sizing Keywords</h3>
<pre class="propdef partial">
Name: column-width
New values: stretch | max-content | min-content | fit-content | fit-content(<<length-percentage>>)
</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>stretch</dfn>
<dd>Specifies the optimal column width as the <a>stretch-fit 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>stretch-fit inline size</a>))</code>.
<dt><dfn>fit-content(<<length-percentage>>)</dfn>
<dd>
Specifies the optimal column width as
<code>min(<a>max-content size</a>, max(<a>min-content size</a>, <<length-percentage>>))</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='intrinsic-sizes'>
Intrinsic Sizes</h3>
The <a>min-content size</a> of a box in each axis
is the size it would have as an
''width/auto''-sized (in that axis) float in a zero-sized containing block.
(In other words, the minimum size it has when sized as “shrink-to-fit”.)
The <a>max-content size</a> of a box in each axis
is the size it would have as an
''width/auto''-sized (in that axis) float in an infinitely-sized containing block.
(In other words, the maximum size it has when sized as “shrink-to-fit”.)
Note: This specification does not define how to determine these sizes.
Please refer to [[CSS21]],
the relevant CSS specification for that display type,
and/or existing implementations
for further details.
Issue: Intrinsic sizes of images without an intrinsic size are weird and undefined, and should probably get some love
<h3 id="intrinsic-contribution">
Intrinsic Contributions</h3>
A box’s <a>min-content contribution</a>/<a>max-content contribution</a> in each axis
is the size of the content box
of a hypothetical ''width/auto''-sized float
that contains only that box,
if that hypothetical float's containing block is zero-sized/infinitely-sized.
Note: This specification does not define how to determine these sizes.
Please refer to [[CSS21]],
the relevant CSS specification for that display type,
and/or existing implementations
for further details.
<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="stretch-fit-sizing">
Stretch-fit Sizing</h3>
<p>
The inner <a>stretch-fit 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>stretch-fit 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="https://www.w3.org/TR/CSS21/box.html#collapsing-margins">margin collapsing</a>, and treating ''margin/auto'' margins as zero),
borders, and padding,
flooring at zero.
Issue: Shouldn't this be ignoring margin collapsing?
<p>
The <a>stretch-fit 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>
Percentages specify sizing of a box with respect to the box’s <a>containing block</a>.
Although this may require an additional layout pass to re-resolve percentages in some cases,
the ''min-width/auto'', ''min-width/min-content'', ''min-width/max-content'', and ''min-width/fit-content'' values
of 'min-width' and 'min-height'
do not prevent the resolution of percentage sizes of the box’s contents.
However, in order to prevent cyclic sizing in the general case,
percentages do not otherwise resolve against <a>indefinite</a> sizes,
and instead are treated as <css>auto</css>.
Note: See definition of percentage
<a href="https://www.w3.org/TR/CSS2/visudet.html#the-width-property"><css>width</css></a>
and <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property"><css>height</css></a>
in [[CSS21]].
<h2 class=no-num id="changes">
Changes</h2>
<p>
Changes since the <a href="https://www.w3.org/TR/2016/WD-css-sizing-3-20160512/">12 May 2016 Working Draft</a>:
<ul>
<li>Renamed <css>fill</css> keyword to ''width/stretch''
and updated terminology to match.
<li>Deferred intrinsic sizing of replaced elements to CSS2.1.
<li>Updated definition of <a>definite</a> to allow measurements against text content, per WG resolution.
</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.
<h2 class=no-num id=priv-sec>
Privacy and Security Considerations</h2>
This specification introduces no new privacy or security considerations.