CSS Intrinsic & Extrinsic Sizing Module Level 3
Shortname: css-sizing
Level: 3
Status: ED
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
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: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 [[!CSS21]] chapter 10 and in [[!CSS3COL]]
Values
This specification follows the
CSS property
definition conventions 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.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification also accept the
inherit
keyword as their property value. For readability it has 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 measuring content;
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 “fill-available” sizing of non-replaced blocks [[CSS21]])
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.
An indefinite available size is essentially infinite.
- available space
-
The space into which a box is laid out.
Unless otherwise specified, this is
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.
- fill-available fit
-
The fill-available 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.
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
:
fill-available size
:
fill-available inline size
:
fill-available block size
:: Roughly, the
size a box would take
if it filled its
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, fill-available 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: fill | 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):
- fill
-
Use the fill-available inline size or fill-available 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
-
Use the max-content inline size or max-content block size,
as appropriate to the writing mode.
- min-content
-
Use the min-content inline size or min-content block size,
as appropriate to the writing mode.
- fit-content
-
Use the fit-content inline size or fit-content block size,
as appropriate to the writing mode,
i.e.
min(max-content size, max(min-content size, fill-available size))
- fit-content(<>)
-
Use the fit-content formula
with the available size replaced by the specified argument,
i.e.
min(max-content size, max(min-content size, <>))
Issue: Is ''width/fill'' stable enough or should we defer to L4?
Is the name clear enough, or should it be reverted to ''fill-available''
(or changed to something else)?
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;''.
Right now all of these except ''width/fill'' mean the same thing
for block-sizes. This may or may not be ideal.
If the inline-size is ''width/auto'',
we could have min-content block-size imply a max-content inline-size,
and vice versa.
Note that percentages resolved against the intrinsic sizes
(''width/max-content'', ''width/min-content'', ''width/fit-content'')
will compute to ''width/auto'',
as defined by CSS 2.1. [[!CSS21]]
Containing Floats
Note: To ensure that a container sizes itself to contain any descendant floats,
make sure it's a formatting context.
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: fill | max-content | min-content | fit-content | fit-content(<>)
When used as values for 'column-width',
the new keywords specify the optimal column width:
- fill
- Specifies the optimal column width as the fill-available 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, fill-available 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 of Replaced Elements
For replaced elements,
the min-content size and max-content size are equivalent
and correspond to the appropriate dimension of the concrete object size returned by
the default sizing algorithm [[!CSS3-IMAGES]]
of the element,
calculated with an unconstrained specified size.
Intrinsic Sizes of Other Boxes
The min-content size of a box is the size it would have as an
''width/auto''-sized 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 is the size it would have as an
''width/auto''-sized 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.
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.
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.
Extrinsic Size Determination
Extrinsic sizing determines sizes based on the context of an element,
without regard for its contents.
Fill-available Sizing
The inner fill-available inline size of a box is…
…less the box's inline-axis margins
(after any margin collapsing, and treating ''margin/auto'' margins as zero),
borders, and padding.
The fill-available 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.
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 indefinite sizes,
and instead are treated as auto.
Note: See definition of percentage
width
and height
in [[CSS21]].
Changes
Changes since the 12 May 2016 Working Draft:
- Updated definition of definite to allow measurements against text content, per WG resolution.
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.