Overview.bs

<pre class='metadata'>
Title: CSS Intrinsic &amp; Extrinsic Sizing Module Level 3
Shortname: css-sizing
Level: 3
Status: ED
Work Status: Refining
Group: csswg
ED: https://drafts.csswg.org/css-sizing-3/
TR: https://www.w3.org/TR/css-sizing-3/
Editor: Tab Atkins, Google, http://xanthir.com/contact/, w3cid 42199
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: content box, border box, non-replaced
</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
spec:css-align-3; type:value; text:stretch; for:align-self
</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 [copy/pasted from etherpad, probably out-of-date, evaluate later]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.


		Issue: This spec needs illustrations! See <a href="https://github.com/w3c/csswg-drafts/issues/1938">issue</a>.

<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 [[!CSS2]] chapter 10 and in [[!CSS3COL]]

<h3 id="values">
Values</h3>

	This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]].
	Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
	Other CSS modules may expand the definitions of these value types.

	In addition to the property-specific values listed in their definitions,
	all properties defined in this specification
	also accept the <a>CSS-wide keywords</a> keywords as their property value.
	For readability they have not been repeated explicitly.


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<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 performing layout;
			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/CSS2/visudet.html#blockwidth">“stretch-fit” sizing of non-replaced blocks</a> [[CSS2]])
			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 [[!CSS2]],
				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/CSS2/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>

	There are four types of automatically-determined sizes in CSS
	(sizes resulting fromy ''width/auto'' sizing rules, depending on context):

	<dl export>
		: <dfn>stretch-fit size</dfn>
		: <dfn>stretch-fit inline size</dfn>
		: <dfn>stretch-fit block size</dfn>
		:: The <a>size</a> a box would take
			if its <a>outer size</a> filled the <a>available space</a>
			in the given axis;
			in other words, the <a>stretch fit</a> into the <a>available space</a>,
			if that is <a>definite</a>.
			Undefined if the <a>available space</a> is <a>indefinite</a>.

			Note: For the <a>inline axis</a>, this is called the “available width” in <a href="https://www.w3.org/TR/CSS2/visudet.html#float-width">CSS2.1§10.3.5</a>
			and computed by the rules in <a href="https://www.w3.org/TR/CSS2/visudet.html#blockwidth">CSS2.1§10.3.3</a>.

			Note: Calculations involving this size need to specify a fallback behavior
			for when the <a>available space</a> is <a>indefinite</a>
			if that happens to be possible.

		: <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/CSS2/visudet.html#float-width">CSS2.1§10.3.5</a>
				and the “maximum cell width” in <a href="https://www.w3.org/TR/CSS2/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/CSS2/visudet.html#float-width">CSS2.1§10.3.5</a>
				and the “minimum content width” in <a href="https://www.w3.org/TR/CSS2/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/CSS2/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>.
	</dl>

<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>

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<h2 id="specifying-sizes" oldid='size-keywords'>
Specifying Box Sizes</h2>

<h3 id="sizing-properties">
Sizing Properties: the 'width', 'height', 'min-width', 'min-height', 'max-width', and 'max-height' properties</h3>

	This section defines the <dfn lt="sizing property">sizing properties</dfn>
	'width', 'height', 'min-width', 'min-height', 'max-width', and 'max-height'.

	Note: Additional <a>flow-relative</a> aliases to these properties are defined in [[CSS-LOGICAL-1]].

	<pre class=propdef>
	Name: width, height
	Value: auto | <<length-percentage>> | min-content | max-content | <nobr>fit-content(<<length-percentage>>)</nobr>
	Initial: auto
	Applies to: all elements except <a>non-replaced</a> <a>inlines</a>
	Inherited: no
	Percentages: relative to width/height of <a>containing block</a>
	Computed Value: as specified, with lengths made absolute
	Animatable: As <<length-percentage>>, or keyword, or as ''fit-content()'' with the <<length-percentage>> argument interpolable
	</pre>

	The 'width' and 'height' properties specify
	the preferred (<a>physical</a>) <dfn>width</dfn> and <dfn>height</dfn> of the box, respectively.

	<pre class=propdef>
	Name: min-width, min-height
	Value: auto | <<length-percentage>> | min-content | max-content | <nobr>fit-content(<<length-percentage>>)</nobr>
	Initial: auto
	Applies to: all elements that accept 'width' or 'height'
	Inherited: no
	Percentages: relative to width/height of <a>containing block</a>
	Computed Value: as specified, with lengths made absolute
	Animatable: As <<length-percentage>>, or keyword, or as ''fit-content()'' with the <<length-percentage>> argument interpolable
	</pre>

	The 'min-width' and 'min-height' properties specify
	the <dfn lt="min width | minimum width | min size | minimum size">minimum width</dfn> (or “min width”)
	and <dfn lt="min height | minimum height">minimum height</dfn> (or “min height”)
	of the box, respectively.

	Note: The initial value of ''min-width/auto'' is new;
	in [[CSS2]] the initial value was zero.

	<pre class=propdef>
	Name: max-width, max-height
	Value: none | <<length-percentage>> | min-content | max-content | <nobr>fit-content(<<length-percentage>>)</nobr>
	Initial: none
	Applies to: all elements that accept 'width' or 'height'
	Inherited: no
	Percentages: relative to width/height of <a>containing block</a>
	Computed Value: as specified, with lengths made absolute
	Animatable: As <<length-percentage>>, or keyword, or as ''fit-content()'' with the <<length-percentage>> argument interpolable
	</pre>

	The 'max-width' and 'max-height' properties specify
	the <dfn lt="max width | maximum width | max size | maximum size">maximum width</dfn> (or “max width”)
	and <dfn lt="max height | maximum height">maximum height</dfn> (or “max height”)
	of the box, respectively.


<h3 id="sizing-values" oldids='width-height-keywords'>
Sizing Values: the <<length-percentage>>, ''width/auto'' | ''max-width/none'', ''width/min-content'', ''width/max-content'', and ''width/fit-content()'' values</h3>

	<dl dfn-type=value dfn-for="width, min-width, max-width, height, min-height, max-height">
		<dt><dfn><<length-percentage>></dfn>
		<dd>
			Specifies the size of the box using <<length>> and/or <<percentage>>.
			The 'box-sizing' property indicates whether the <a>content box</a> or <a>border box</a> is measured.

			Percentages are resolved against the width/height, as appropriate,
			of the box's <a>containing block</a>.
			If, in a particular axis,
			the <a>containing block’s</a> size depends on the box’s size,
			see the relevant layout module
			for special rules on how to resolve percentages.

			Negative values are invalid.

		<dt><dfn for="width, height, min-width, min-height">auto</dfn>
		<dd>
			For 'width'/'height', specifies an <dfn dfn for>automatic size</dfn>.
			See the relevant layout module for how to calculate this.

			For 'min-width'/'min-height',
			specifies an <dfn dfn for>automatic minimum size</dfn>.
			Unless otherwise defined by the relevant layout module,
			this computes to zero.

		<dt><dfn for="max-width, max-height">none</dfn>
		<dd>
			No limit on the size of the box.

<!--
		<dt><dfn>stretch</dfn>
		<dd>
			If 'align-self' or 'justify-self' (as appropriate to the relevant axis)
			is defined to apply,
			size the box as if ''align-self/stretch'' were specified for that property.
			(If the box does not exactly fill its <a>alignment container</a>,
			the <a>box alignment properties</a> will dictate how the size differential is handled.)
			Otherwise,
			this value behaves as the initial value.

			NOTE: For the <a>inline size</a> of a <a>block-level box</a>,
			this is exactly the formula used to calculate ''width/auto'' widths
			for non-replaced blocks in normal flow, see <a href="https://www.w3.org/TR/CSS2/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>min-content</dfn>
		<dd>
			If specified for the inline axis,
			use the <a>min-content inline size</a>;
			otherwise compute to the property’s <a>initial value</a>.

		<dt><dfn>max-content</dfn>
		<dd>
			If specified for the inline axis,
			use the <a>max-content inline size</a>;
			otherwise compute to the property’s <a>initial value</a>.
<!--
		<dt><dfn>fit-content</dfn>
		<dd>
			If specified for the inline axis,
			use the <a>fit-content inline size</a>
			i.e.
			<code>min(''width/max-content'', max(''width/min-content'', ''width/stretch''))</code>;
			otherwise compute to the property’s <a>initial value</a>.
-->
		<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(''width/max-content'', max(''width/min-content'', <<length-percentage>>))</code>;
			otherwise compute to the property’s <a>initial value</a>.

			Negative values are invalid.
	</dl>

	In all cases,
	the used value is floored to preserve a non-negative <a>inner size</a>.

	Note: The ''width/min-content'', ''width/max-content'', and ''width/fit-content()'' values
	are new in Level 3.

	Note: The 'flex-basis' property hereby also gains these new keywords,
	as its values are defined by reference to <'width'>.

	Note: This section previously defined ''width/stretch'' and ''width/fit-content''
	as keywords representing the <a>stretch-fit size</a> and <a>fit-content size</a>,
	respectively.
	These keywords have been deferred to Level 4
	(along with an additional ''width/contain'' keyword that
	behaves similarly to ''width/stretch''
	but preserves the intrinsic aspect ratio, if any)
	to better work out the implications in situations with
	<a>indefinite</a> <a>available space</a>.

<h4 id="behave-auto">
“Behaving as ''width/auto''”</h4>

<!-- THIS NEED SOME THINKING THROUGH, WE DID WEIRD THINGS WITH FLEXBOX/GRID
	Note: Percentages resolved against the intrinsic sizes
	(''width/max-content'', ''width/min-content'', ''width/fit-content'')
	will behave as ''width/auto''.
-->
	<!-- <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property">as defined by CSS 2</a>. [[!CSS2]] -->
	<!-- Restore the above when 2.1 is updated and published with the new "behaves as" text we resolved on. -->

	To have a common term for both when
	'width'/'height' computes to ''width/auto''
	and when it is defined to behave as if ''width/auto'' were specified
	(as in the case of <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property">block percentage heights</a>
	resolving against an <a>indefinite</a> size,
	see <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property">CSS2&sect;10.5</a>),
	the property is said to <dfn export lt="behave as auto|behaves as auto|behaving as auto">behave as auto</dfn>
	in both of these cases.

	Note: Legacy spec prose defining layout behavior
	might explicitly refer to 'width'/'height' having a computed value of ''width/auto'' as a condition;
	these should be interpreted as meaning <a>behaves as auto</a>,
	and reported to the CSSWG for updating.

	ISSUE: Replace this section with references to the new term <a>automatic size</a>.

<h4 id='the-contain-floats-value'>
Containing or Excluding Floats</h4>

	<em>This section is non-normative.</em>

	Although <a>block box</a> boundaries are typically pervious to floats,
	sometimes an author needs them to contain their own (descendant) floats
	or to exclude floats from outside.
	For Block layout,
	specifying ''display: flow-root''
	will make the box a <a>formatting context</a> root,
	which has this behavior.

	Note: Boxes participating in Flex, Grid, or Table layout will automatically have this behavior.

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<h3 id="box-sizing">
Inner vs Outer Sizes: the 'box-sizing' property</h3>


	<pre class="propdef">
	Name: box-sizing
	Value: content-box | border-box
	Initial: content-box
	Applies to: all elements that accept 'width' or 'height'
	Inherited: no
	Percentages: N/A
	Animation type: discrete
	</pre>

	The 'box-sizing' property switches the box model
	between asssigning finite sizes such as <<length>>s and <<percentage>>s
	to either the <a>content box</a> or <a>border box</a>.
	It affects the interpretation of all <a>sizing properties</a>,
	including 'flex-basis'.

	Values have the following meanings:

	<dl dfn-type=value dfn-for=box-sizing>
		<dt><dfn>content-box</dfn>
		<dd>
			Sizes specified on <a>sizing properties</a>
			as <<length-percentage>>
			represent <a>inner sizes</a>:
			they are applied to the <a>content box</a>.
			The padding and border of the box
			are laid out and drawn <em>outside</em> the specified 'width' and 'height'.

			Note: This is the behavior of width and height as specified by CSS2.1,
			and is thus the default.

		<dt><dfn>border-box</dfn>
		<dd>
			Sizes specified on <a>sizing properties</a>
			as <<length-percentage>>
			represent <a>outer sizes</a>:
			they are applied to the <a>border box</a>.
			The padding and border of the box
			are laid out and drawn <em>inside</em> the specified 'width' and 'height',
			with the <a>content box</a> sized to fill the remaining space,
			floored at zero.

			The content width and height are calculated
			by subtracting the border and padding widths of the respective sides
			from the specified <<length-percentage>>.
			As the content width and height
			[[#sizing-values|cannot be negative]],
			this computation is floored at zero.

			Used values, as exposed for instance through {{getComputedStyle()}},
			also refer to the border box.
	</dl>

	Values affected by 'box-sizing' include
	both raw <<length-percentage>> values
	and those used in functional notations such as ''width/fit-content()''.
	In contrast,
	non-quantitative values such as ''width/auto'' and ''width/min-content''
	are not influenced by the 'box-sizing' property
	(unless otherwise specified).

	<div class=example>
		For example, the following properties set the <a>inner size</a> of the box to ''100px'',
		and the <a>outer size</a> to ''120px'':

		<pre class=lang-css>
		.box {
			box-sizing:   content-box; /* default */
			width:        100px;
			padding-left: 10px;
			border-left:  10px solid;
		}
		</pre>

		On the other hand, by changing to ''border-box'',
		the <a>inner size</a> is now ''80px'',
		while the <a>outer size</a> is ''100px'':

		<pre class=lang-css>
		.box {
			box-sizing:   border-box;
			width:        100px;
			padding-left: 10px;
			border-left:  10px solid;
		}
		</pre>

		The <a>inner size</a> can't be less than zero,
		so if the 'padding' + 'border' is greater than the specified <a>outer size</a>,
		the box will end up larger than specified.
		In this case, the <a>inner size</a> will be ''0px''
		while the <a>outer size</a> is ''120px'',
		even though ''width: 100px'' is specified:

		<pre class=lang-css>
		.box {
			box-sizing:   border-box;
			width:        100px;
			padding-left: 60px;
			border-left:  60px solid;
			/* padding + border = 120px */
		}
		</pre>
	</div>

	Note: Certain HTML elements,
	such as <{button}>,
	default to ''border-box'' behavior.
	See HTML for details on which elements have this behavior.

	In the CSS specifications,
	the terms <a>width</a>, <a>height</a>,
	<a lt="min width">minimum (min) width</a>,
	<a lt="min height">minimum (min) height</a>,
	<a lt="max width">maximum (max) width</a>, and
	<a lt="max height">maximum (max) height</a>
	refer to the <a lt="inner size">inner</a> size
	(<a lt="content box">content-box</a> size)
	of a <a>box</a>
	unless otherwise indicated.


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<h3 id='column-sizing'>
New Column Sizing Values: the ''column-width/min-content'', ''column-width/max-content'', and ''column-width/fit-content()'' values</h3>

	<pre class="propdef partial">
	Name: column-width
	New values: min-content | max-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>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>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>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>

	Note: The column width never varies by column.
	When the column width is informed by the multi-column element's contents
	(as in the keywords above),
	all of its contents are taken under consideration
	and the calculated width is shared by all the columns.


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<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”.)
	For replaced elements,
	if this size would depend on the size of the containing block
	(as it does for images with an intrinsic aspect ratio but no intrinsic size,
	see <a href="https://www.w3.org/TR/CSS2/visudet.html#inline-replaced-width">CSS2&sect;10.3.2</a>)
	and therefore can't be calculated against an infinitely-sized containing block
	then:

	<dl>
		<dt>For boxes with an intrinsic aspect ratio, but no intrinsic size:
		<dd>
			* If the <a>available space</a> is <a>definite</a>
				in the <a>inline axis</a>,
				use the <a>stretch fit</a> into that size for the inline size
				and calculate the block size using the aspect ratio.
			* Otherwise
				if the box has a <a>definite</a> non-zero 'min-width' or 'min-height',
				use that size and calculate the other dimension using the aspect ratio;
				if both dimensions have a definite minimum,
				choose the one that results in the larger overall size.

				Note: This case was previous calculated from a 300x150 default size,
				rather than the box's min size.
				This is believed to be a better behavior,
				and likely to be web compatible,
				but please send feedback to the CSSWG if there are any problems.
			* Otherwise
				use a width of ''300px''
				(height of ''150px'' in <a>vertical writing modes</a>)
				and calculate the other dimension using the aspect ratio.
		<dt>For boxes without an intrinsic aspect ratio:
		<dd>
			* If the <a>available space</a> is <a>definite</a>
				in the appropriate dimension,
				use the <a>stretch fit</a> into that size in that dimension.
			* Otherwise,
				if the box has a <a>definite</a> non-zero minimum size ('min-width'/'min-height') in that dimension,
				use that size.
			* Otherwise,
				use ''300px'' for the width
				and/or ''150px'' for the height
				as needed.
	</dl>

	Since a block-level or inline-level replaced element
	whose 'height' or 'width' <a>behaves as auto</a>
	is effectively defined to use its <a>max-content size</a>
	(<a href="https://www.w3.org/TR/CSS2/visudet.html#inline-replaced-width">CSS2&sect;10.3.2</a>),
	this specification applies the rules above
	to the undefined case of a replaced element
	whose 'height' and 'width' both <a>behave as auto</a>.

	Note: This specification does not define how to determine
	the size of a float.
	Please refer to [[CSS2]],
	the relevant CSS specification for that display type,
	and/or existing implementations
	for further details.
	A future specification will define this in detail,
	replacing the CSS2 “definition”,
	such as it is.

	Note: The intrinsic sizing keywords might be re-used
	in a future level of this module
	to specify content-based sizes for replaced elements
	such as <{textarea}> and <{iframe}>
	whose ''height/auto'' size and intrinsic size contributions
	are currently arbitrarily fixed.
	See <a href="https://github.com/w3c/csswg-drafts/issues/1771">discussion</a>.

<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.

	However, in the case of a [=replaced element|replaced=] box with a percentage-based 'width'/'max-width'/'height'/'max-height',
	the percentage is resolved to zero
	when calculating the <a>min-content contribution</a> in the corresponding axis.
	(See [[#min-content-zero]] for a list of which elements in HTML this applies to.)
	The UA may additionally floor this size based on UI considerations,
	such as ensuring certain UI elements remain visible
	(for example, the dropdown arrow on a <{select}>).

	<div class=example>
		For example,
		an <{input}> assigned ''width: calc(50% + 50px)''
		has a <a>min-content contribution</a> of ''50px'',
		plus any horizontal margin/border/padding.
	</div>

	Note: We are not 100% sure if zeroing out a percentage 'max-width' on form controls is web-compatible.
	See <a href="https://github.com/w3c/csswg-drafts/issues/765">Issue 765</a>.

	Note: This specification does not define how to determine these sizes.
	Please refer to [[CSS2]],
	the relevant CSS specification for that display type,
	and/or existing implementations
	for further details.


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<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="percentage-sizing">
	Percentage Sizing</h3>

	Percentages specify sizing of a box with respect to the box’s <a>containing block</a>.

	<div class="example">
		For example, in the following markup:

		<xmp class='lang-html'>
			<article style="height: 60em">
				<aside style="height: 50%;">
				</aside>
			</article>
		</xmp>

		the <code>&lt;aside></code> would be 30em tall.
	</div>

	Sometimes the size of a percentage-sized box's containing block
	depends on the intrinsic size contribution of the box itself,
	creating a cyclic dependency.
	When calculating the containing block's size,
	the percentage <a>behaves as auto</a>.
	Then, unless otherwise specified,
	when calculating the used sizes and positions of the containing block’s <em>contents</em>:

	* If the cyclic dependency was introduced due to
		a 'block-size' or 'max-block-size' on the containing block
		that causes it to depend on the size of its contents,
		the box’s percentage is not resolved and instead <a>behaves as auto</a>.

		Note: <a href="https://www.w3.org/TR/css-grid/">Grid containers</a> (<a href="https://github.com/w3c/csswg-drafts/issues/1679">and flex items?</a>) do allow percentages to resolve in this case.

	* Otherwise, the percentage is resolved against the containing block’s size.
		(The containing block’s size is not re-resolved based on the resulting size of the box;
		the contents might thus overflow or underflow the containing block).

	Note: These rules specify the previously-undefined behavior of this cyclic case in <a href="https://www.w3.org/TR/CSS2/visudet.html#the-width-property">CSS2&sect;10.2</a>.
	Note also, the behavior in <a href="https://www.w3.org/TR/CSS2/visudet.html#the-height-property">CSS2&sect;10.5</a>
	is superseded in their respective specifications for layout modes
	(such as <a href="http://www.w3.org/TR/css-flexbox/">flex layout</a>)
	not described in CSS2.

	Similarly, percentage margins and padding behave as zero in such cyclic cases
	when calculating the containing block's size,
	and then resolve when calculating the used sizes and positions of its content.
	(This defines the previously-undefined behavior of this cyclic case in
	<a href="https://www.w3.org/TR/CSS2/box.html#margin-properties">CSS2&sect;8.3</a>
	and
	<a href="https://www.w3.org/TR/CSS2/box.padding-properties">CSS2&sect;8.4</a>.

	<div class="example">
		For example, in the following markup:

		<xmp class='lang-html'>
			<article style="width: min-content">
				<aside style="width: 50%;">
				LOOOOOOOOOOOOOOOOOOOONG
				</aside>
			</article>
		</xmp>

		When calculating the width of the outer <code>&lt;article></code>,
		the inner <code>&lt;aside></code> behaves as ''width: auto'',
		so the <code>&lt;article></code> sets itself to the width of the long word.
		Since the <code>&lt;article></code>’s width didn't depend on "real" layout,
		though, it's treated as <a>definite</a>
		for resolving the <code>&lt;aside></code>,
		whose width resolves to half that of the <code>&lt;article></code>.
	</div>

	<div class="example">
		In this example,

		<xmp class=lang-html>
			<article style="height:auto">
				<aside style="height: 50%;">
					<div class=block style="height: 150px;"></div>
				</aside>
				<section style="height: 30px;"></section>
			</article>
		</xmp>

		because the percentage <a>block size</a> ('height', in this case) on block-level elements
		is defined to not resolve inside content-sized containing blocks,
		the percentage height on the <code>&lt;aside></code> is ignored,
		that is, it behaves exactly as if ''height/auto'' were specified.
	</div>

	<div class=example>
		Issue: Letting %s still resolve against a definite 'height'
		when the min-height is intrinsic is an open issue.
		(CSS2 has a general statement about "height depending on contents",
		which this technically is,
		even though CSS2 didn't have content-dependent keywords for 'min-height'.
		Since this is new, we think we could have this different behavior.)

		The following examples illustrate how block-axis percentages resolve against a containing block whose size depends on its contents.

		<xmp class=lang-html>
			<article style="height:100px; min-height: min-content;">
				<aside style="height: 50%;">
					<div style="height: 150px;"></div>
				</aside>
				<section style="height: 30px;"></section>
			</article>
		</xmp>

		The initial height of the <code>&lt;article></code> is 100px, as specified,
		which would make the <code>&lt;aside></code> 50px tall
		when it resolved its percentage.
		However, we must calculate the min-height,
		by substituting it in for 'height'.
		This causes the percentage on the <code>&lt;aside></code> to <a>behave as auto</a>,
		so the <code>&lt;aside></code> ends up 150px tall.
		The total height of the contents is thus 180px.
		This is larger than the specified 100px height,
		so the <code>&lt;article></code> gets adjusted to 180px tall.

		Then, since the percentage could <em>originally</em> resolve against the (100px) height,
		it now resolves against the 180px height,
		so the <code>&lt;aside></code> ends up being 90px tall.

		<xmp class=lang-html>
			<article style="height:auto; min-height: min-content;">
				<aside style="height: 50%;">
					<div class=block style="height: 150px;"></div>
				</aside>
				<section style="height: 30px;"></section>
			</article>
		</xmp>

		In this case, the percentage on the <code>&lt;aside></code> won't normally resolve,
		because the containing block's height is ''height/auto''
		(and thus depends on the size of its contents).
		Instead it <a>behaves as auto</a>,
		resulting in a height of 150px for the <code>&lt;aside></code>,
		and an initial height of 180px for the <code>&lt;article></code>
		The 'min-height' doesn't change this;
		''height: min-content;'' acts similarly to ''height: auto;''
		and results in the same sizes.

		<xmp class=lang-html>
			<article style="height:100px; min-height: min-content;">
				<aside style="height: 200%;">
					<div style="height: 150px;"></div>
				</aside>
				<section style="height: 30px;"></section>
			</article>
		</xmp>

		This is a variation on the first code block,
		and follows a similar path;
		the <code>&lt;aside></code> initially wants to compute to 200px tall
		(200% of the 100px containing block height).
		When we calculate the effects of 'min-height',
		the percentage <a>behaves as auto</a>,
		causing it to become 150px tall,
		and the total ''height/min-content'' height of the containing block
		to be 180px tall.
		Since this is larger than 100px,
		the <code>&lt;article></code> gets clamped to 180px,
		the percentage resolves against this new height,
		and the <code>&lt;aside></code> ends up being 360px tall,
		overflowing the <code>&lt;article></code>
	</div>

<h2 class=no-num id="min-content-zero">
Compressible Replaced Elements</h2>

	The following elements can have their <a>min-content size</a> compressed
	when their 'width'/'height' or 'max-width'/'max-height' is expressed with a percentage size.
	See [[#intrinsic-contribution]].

	* All <a>replaced elements</a> that aren't form controls.
		See <a href="https://html.spec.whatwg.org/multipage/rendering.html#replaced-elements">HTML&sect;14.4</a>. [[!HTML]]
	* <{input}> with any <{input/type}> other than <code>"button"</code>, <code>"submit"</code>, or <code>"reset"</code>.
	* <{select}>, <{textarea}>, <{progress}>, <{meter}>.

<h2 class=no-num id="changes">
Changes</h2>

	<p>
		Significant changes since the <a href="https://www.w3.org/TR/2017/WD-css-sizing-3-20170207/">7 February 2017 Working Draft</a> include:

	<ul>
		<li>More accurate definition of min-content and max-content sizes for replaced elements.
		<li>Compute new keywords to the initial value, not to a potentially non-existent ''width/auto'', when applied to the block axis.
		<li>Specify that percent sizes on replaced elements zero out their min-content contribution.
		<li>Fix confusing/wrong definition of percentage sizes resolved against a dependent containing block.
		<li>Deferred the ''width/stretch'' and ''width/fit-content'' keywords to Level 4
			to allow for further consideration of their behavior in <a>indefinite</a> containing blocks.
	</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.