CSS Intrinsic & Extrinsic Sizing Module Level 3
Shortname: css-sizing
Level: 3
Status: WD
Work Status: Revising
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/
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
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
Introduction
This section is not normative.
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.
Module interactions
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]]
Values
This specification follows the CSS property definition conventions 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 CSS-wide keywords keywords as their property value.
For readability they have not been repeated explicitly.
Terminology
- size
-
A one- or two-dimensional measurement:
a block size and/or inline size;
alternatively a width and/or height.
- inner size
-
The content-box size of a box.
- outer size
-
The margin-box size of a box.
- definite size
-
A size that can be determined without performing layout;
that is, a <>,
a measure of text (without consideration of line-wrapping),
a size of the initial containing block,
or a <> or other formula
(such the “stretch-fit” sizing of non-replaced blocks [[CSS2]])
that is resolved solely against definite sizes.
Additionally, the size of the containing block of an absolutely positioned element is always definite
with respect to that element.
- indefinite size
-
A size that is not definite.
Indefinite available space is essentially infinite.
- available space
-
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 containing block (if that is definite)
or an infinite size (when it is indefinite).
Available space can alternatively be either a
min-content constraint or
a max-content constraint.
- stretch fit
-
The stretch fit 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).
Note: This is the formula used to calculate the ''width/auto'' widths
of non-replaced blocks in normal flow in CSS2.1§10.3.3.
- fallback size
-
Some sizing algorithms do not work well with an infinite size.
In these cases, the fallback size is used instead.
Unless otherwise specified,
this is the size of the initial containing block.
Auto Box Sizes
:
stretch-fit size
:
stretch-fit inline size
:
stretch-fit block size
:: Roughly, the
size a box would take
if it filled the
available space
in the given axis.
(See [[#extrinsic]].)
Note: For the
inline axis, this is called the “available width” in
CSS2.1§10.3.5
and computed by the rules in
CSS2.1§10.3.3.
:
max-content size
:: A box’s “ideal”
size in a given axis when given infinite available space.
Usually this is the smallest
size the box could take in that axis
while still fitting around its contents,
i.e. minimizing unfilled space while avoiding overflow.
:
max-content inline size
:: The box's “ideal”
size in the
inline axis.
Usually the narrowest
inline size it could take while fitting around its contents
if
none of the soft wrap opportunities within the box were taken.
(See [[#intrinsic]].)
Note: This is called the “preferred width” in
CSS2.1§10.3.5
and the “maximum cell width” in
CSS2.1§17.5.2.2.
:
max-content block size
:: The box's “ideal”
size in the
block axis.
Usually the
block size of the content after layout.
:
min-content size
:: The smallest
size a box could take
that doesn't lead to overflow
that could be avoided by choosing a larger
size.
(See [[#intrinsic]].)
:
min-content inline size
:: The narrowest
inline size a box could take
that doesn't lead to inline-dimension overflow
that could be avoided by choosing a larger
inline size.
Roughly, the
inline size that would fit around its contents
if
all soft wrap opportunities within the box were taken.
Note: This is called the “preferred minimum width” in
CSS2.1§10.3.5
and the “minimum content width” in
CSS2.1§17.5.2.2.
:
min-content block size
:: Equivalent to the
max-content block size.
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.
:
fit-content size
:
fit-content inline size
:
fit-content block size
:: If the
available space in a given axis is finite,
equal to
min(max-content size, max(min-content size, stretch-fit size)).
Otherwise, equal to the
max-content size in that axis.
Note: This is called the “shrink-to-fit” width in
CSS2.1§10.3.5
and
CSS Multi-column Layout § 3.4.
Intrinsic Size Contributions
- max-content contribution
-
The size that a box contributes to its containing block's max-content size.
- min-content contribution
-
The size that a box contributes to its containing block's min-content size.
Intrinsic size contributions are based on the outer size of the box;
for this purpose ''margin/auto'' margins are treated as zero.
Intrinsic Size Constraints
- max-content constraint
-
A sizing constraint imposed by the box's containing block
that causes it to produce its max-content contribution.
- min-content constraint
-
A sizing constraint imposed by the box's containing block
that causes it to produce its min-content contribution.
New Sizing Keywords
New Keywords for 'width' and 'height'
Name: width, min-width, max-width, height, min-height, max-height
New values: stretch | max-content | min-content | fit-content | fit-content(<>)
There are four types of automatically-determined sizes in CSS (which
are represented in the width and height properties by the keywords
defined above):
- stretch
-
Use the stretch-fit inline size or stretch-fit block size,
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 CSS2.1§10.3.3.
It allows re-using this formula for boxes that are otherwise shrink-wrapped,
like tables.
- max-content
-
If specified for the inline axis,
use the max-content inline size;
otherwise compute to the property’s initial value.
- min-content
-
If specified for the inline axis,
use the min-content inline size;
otherwise compute to the property’s initial value.
- fit-content
-
If specified for the inline axis,
use the max-content inline size
i.e.
min(max-content size, max(min-content size, stretch-fit size));
otherwise compute to the property’s initial value.
- fit-content(<>)
-
If specified for the inline axis,
use the fit-content formula
with the available space replaced by the specified argument,
i.e.
min(max-content size, max(min-content size, <>));
otherwise compute to the property’s initial value.
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 formatting context root.
For some layout modes, such as Grid and Flexbox,
this is true automatically.
For Block layout, this means using ''display: flow-root;''.
Note: Percentages resolved against the intrinsic sizes
(''width/max-content'', ''width/min-content'', ''width/fit-content'')
will behave as ''width/auto''.
To have a common term for both when
'width'/'height' computes to ''width/auto''
and when it is defined to behave as ''width/auto''
(as in the case above),
the property is said to behave as auto
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 behaves as auto,
and reported to the CSSWG for updating.
Containing Floats
Note: To ensure that a container sizes itself to contain any descendant floats,
make sure it's a formatting context root.
For some layout modes, such as Grid and Flexbox,
this is true automatically.
For Block layout, this means using ''display: flow-root;''.
Column Sizing Keywords
Name: column-width
New values: stretch | max-content | min-content | fit-content | fit-content(<>)
When used as values for 'column-width',
the new keywords specify the optimal column width:
- stretch
- Specifies the optimal column width as the stretch-fit inline size
of the multi-column element.
- max-content
- Specifies the optimal column width as the max-content inline size
of the multi-column element's contents.
- min-content
- Specifies the optimal column width as the min-content inline size
of the multi-column element's contents.
- fit-content
- Specifies the optimal column width as
min(max-content inline size, max(min-content inline size, stretch-fit inline size)).
- fit-content(<>)
-
Specifies the optimal column width as
min(max-content size, max(min-content size, <>))
Intrinsic Size Determination
Intrinsic sizing determines sizes based on the contents of an element,
without regard for its context.
Intrinsic Sizes
The min-content size 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 max-content size 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”.)
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)
and therefore can't be calculated against an infinitely-sized containing block
then:
- For boxes with an intrinsic aspect ratio, but no intrinsic size:
-
* If the available space is definite
in the inline axis,
use the stretch fit into that size for the inline size
and calculate the block size using the aspect ratio.
* Otherwise
use a width of ''300px''
(height of ''150px'' in vertical writing modes)
and calculate the other dimension using the aspect ratio.
- For boxes without an intrinsic aspect ratio:
-
* If the available space is definite
in the appropriate dimension,
use the stretch fit into that size in that dimension.
* Otherwise,
use ''300px'' for the width
and/or ''150px'' for the height
as needed.
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.
Intrinsic Contributions
A box’s min-content contribution/max-content contribution 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 min-content contribution in the corresponding axis.
For example,
an <{input}> assigned ''width: calc(50% + 50px)''
has a
min-content contribution of ''50px'',
plus any horizontal margin/border/padding.
Extrinsic Size Determination
Extrinsic sizing determines sizes based on the context of an element,
without regard for its contents.
Stretch-fit Sizing
The inner stretch-fit inline size of a box is…
-
If the box is the root or is absolutely-positioned, the inline-size of its containing block, else
-
max('min-inline-size'|0, min('max-inline-size'|infinity, 'inline-size'|stretch-fit inline size))
where the sizes are inner inline-sizes of the element establishing the box's containing block,
and where the first value is used if it is definite and the second value otherwise.
…less the box's inline-axis margins
(after any margin collapsing, and treating ''margin/auto'' margins as zero),
borders, and padding,
flooring at zero.
Issue: Shouldn't this be ignoring margin collapsing?
The stretch-fit block size of a box is defined analogously,
but in the other dimension.
This definition might end up skipping further up the ancestor chain than we'd like in some cases.
Example.
Maybe it should stop at each formatting root, or something similar?
Percentage Sizing
Percentages specify sizing of a box with respect to the box’s containing block.
For example, in the following markup:
<![CDATA[
<article style="height: 60em">
<aside style="height: 50%;">
</aside>
</article>
]]>
the
<aside> would be 30em tall.
For example, in the following markup:
<![CDATA[
<article style="width: min-content">
<aside style="width: 50%;">
LOOOOOOOOOOOOOOOOOOOONG
</aside>
</article>
]]>
When calculating the width of the outer
<article>,
the inner
<aside> behaves as ''width: auto'',
so the
<article> sets itself to the width of the long word.
Since the
<article>’s width didn't depend on "real" layout,
though, it's treated as
definite
for resolving the
<aside>,
whose width resolves to half that of the
<article>.
In this example,
<![CDATA[
<article style="height:auto">
<aside style="height: 50%;">
<div class=block style="height: 150px;"></div>
</aside>
<section style="height: 30px;"></section>
</article>
]]>
because the percentage
block size ('height', in this case) on block-level elements
is defined to not resolve inside content-sized containing blocks,
the percentage height on the
<aside> is ignored,
that is, it behaves exactly as if ''height/auto'' were specified.
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.
<![CDATA[
<article style="height:100px; min-height: min-content;">
<aside style="height: 50%;">
<div style="height: 150px;"></div>
</aside>
<section style="height: 30px;"></section>
</article>
]]>
The initial height of the
<article> is 100px, as specified,
which would make the
<aside> 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
<aside> to
behave as auto,
so the
<aside> ends up 150px tall.
The total height of the contents is thus 180px.
This is larger than the specified 100px height,
so the
<article> gets adjusted to 180px tall.
Then, since the percentage could
originally resolve against the (100px) height,
it now resolves against the 180px height,
so the
<aside> ends up being 90px tall.
<![CDATA[
<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>
]]>
In this case, the percentage on the
<aside> won't normally resolve,
because the containing block's height is ''height/auto''
(and thus depends on the size of its contents).
Instead it
behaves as auto,
resulting in a height of 150px for the
<aside>,
and an initial height of 180px for the
<article>
The 'min-height' doesn't change this;
''height: min-content;'' acts similarly to ''height: auto;''
and results in the same sizes.
<![CDATA[
<article style="height:100px; min-height: min-content;">
<aside style="height: 200%;">
<div style="height: 150px;"></div>
</aside>
<section style="height: 30px;"></section>
</article>
]]>
This is a variation on the first code block,
and follows a similar path;
the
<aside> initially wants to compute to 200px tall
(200% of the 100px containing block height).
When we calculate the effects of 'min-height',
the percentage
behaves as auto,
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
<article> gets clamped to 180px,
the percentage resolves against this new height,
and the
<aside> ends up being 360px tall,
overflowing the
<article>
Changes
Significant changes since the 7 February 2017 Working Draft include:
- More accurate definition of min-content and max-content sizes for replaced elements.
- Compute new keywords to the initial value, not to a potentially non-existent ''width/auto'', when applied to the block axis.
- Specify that percent sizes on replaced elements zero out their min-content contribution.
- Fix confusing/wrong definition of percentage sizes resolved against a dependent containing block.
-
ISSUE: Drop ''width/stretch'' for now?
Acknowledgments
Special thanks go to Aaron Gustafson, L. David Baron
for their contributions to this module.
Privacy and Security Considerations
This specification introduces no new privacy or security considerations.