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.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.
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Table of Contents
1
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.
1.1
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]
1.2
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.
2
Terminology
- size
-
A one- or two-dimensional measurement:
a measure
and/or extent;
alternatively a width and/or height.
- definite size
-
A size that can be determined without measuring content;
that is, a <length>,
a size of the initial containing block,
or a <percentage> that is resolved against a definite size.
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 size
-
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).
An available size 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 auto as zero),
border, and padding in the given dimension.
Note: This is the formula used to calculate the 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.
- fill-available measure
-
Roughly, the measure a box would take
if it filled its available measure.
(See Extrinsic Size Determination.)
Note: 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 measure
-
Roughly, the narrowest measure a box could take while fitting around its contents
if none of the soft wrap opportunities within the box were taken.
(See Intrinsic Size Determination.)
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.
- min-content measure
-
The narrowest measure a box could take
that doesn’t lead to inline-dimension overflow
that could be avoided by choosing a larger measure.
Roughly, the measure that would fit around its contents
if all soft wrap opportunities within the box were taken.
(See Intrinsic Size Determination.)
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.
- fit-content measure
-
If the available measure is finite,
equal to
min(preferred measure, max(min-content measure, fill-available measure)).
Otherwise, equal to the max-content measure.
Note: This is called the “shrink-to-fit” width in CSS2.1§10.3.5
and CSS Multi-column Layout § 3.4.
- preferred measure
-
A box’s “ideal” measure.
Unless otherwise specified, this is the max-content measure.
- fill-available extent
-
Roughly, the extent a box would take
if it filled its available extent.
(See Intrinsic Size Determination.)
- max-content extent
-
Roughly, the extent of the content after layout.
- min-content extent
-
Equivalent to the max-content extent.
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 extents be different,
even if they are the same elsewhere.
- fit-content extent
-
Analogous to the fit-content measure.
- preferred extent
-
Equivalent to the max-content extent.
Same issue as min-content extent.
- max-size contribution
-
The size that a box contributes to its containing block’s max-content size.
- min-size contribution
-
The size that a box contributes to its containing block’s min-content size.
- preferred-size contribution
-
If the box’s layout mode has a defined preferred-size contribution, that.
Otherwise, the box’s max-size contribution.
- max-size constraint
-
A sizing constraint imposed by the box’s containing block
that causes it to produce its max-size contribution.
- min-size constraint
-
A sizing constraint imposed by the box’s containing block
that causes it to produce its min-size contribution.
- preferred-size constraint
-
A sizing constraint imposed by the box’s containing block
that causes it to produce its preferred-size contribution.
3
New Sizing Keywords
3.1
New Keywords for width and height
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 measure or fill-available extent,
as appropriate to the writing mode.
- max-content
-
Use the max-content measure or max-content extent,
as appropriate to the writing mode.
- min-content
-
Use the min-content measure or min-content extent,
as appropriate to the writing mode.
- fit-content
-
Use the fit-content measure or fit-content extent,
as appropriate to the writing mode.
- repudiate-floats Less stupid name?
-
In the inline axis,
use the larger of the min-content measure,
and the fill-available measure 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 block axis,
this is identical to auto.
Does this value work?
Is it needed?
Right now all of these except fill mean the same thing
for extents. This may or may not be ideal.
If the measure is auto,
we could have min-content extent imply a max-content measure,
and vice versa.
Note that percentages resolved against the intrinsic sizes
(max-content, min-content, fit-content, repudiate-floats)
will compute to auto,
as defined by CSS 2.1. [CSS21]
3.2
Containing Floats
- contain-floats
-
Equivalent to min-content
except that when applied to the extent of a block box
it forces the inner extent 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.
3.3
Column Sizing Keywords
| Name: | column-width |
|---|
| New values: | fill | max-content | min-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 measure
of the multi-column element.
- max-content
- Specifies the optimal column width as the max-content measure
of the multi-column element’s contents.
- min-content
- Specifies the optimal column width as the min-content measure
of the multi-column element’s contents.
- fit-content
- Specifies the optimal column width as
min(max-content measure, max(min-content measure, fill-available measure)).
4
Intrinsic Size Determination
Intrinsic sizing determines sizes based on the contents of an element,
without regard for its context.
4.1
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.
4.2
Intrinsic Sizes of Non-Replaced Inlines
The min-content measure of an inline box
is the length of the largest unbreakable sequence of inline content.
The min-measure contribution of an inline box
is its min-content measure,
plus any inline-axis margin, border, and padding
adjacent to that sequence.
The max-content measure 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 max-measure contribution of an inline box
is its max-content measure,
plus any inline-axis margin, border, and padding
adjacent to that sequence.
The min-content extent, max-content extent, min-extent contribution, and max-extent contribution 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.
4.3
Intrinsic Sizes of Non-Replaced Blocks
The min-content measure of a block container box
is the largest min-measure contribution
of its in-flow or floated children.
The max-content measure of a block container box
is the measure of the box after layout,
if all children are sized under a max-size constraint.
If the computed measure of a block-level box
is min-content, max-content, or a definite size,
its min-measure contribution
is that size
plus any inline-axis margin, border, and padding.
Otherwise, if the computed measure of the block is fit-content, auto, or fill,
its min-measure contribution
is its min-content measure
plus any inline-axis margin, border, and padding.
If the computed measure of a block-level box
is min-content, max-content, or a definite size,
its max-measure contribution
is that size
plus any inline-axis margin, border, and padding.
Otherwise, if the computed measure of the block is fit-content, auto, or fill,
its max-measure contribution
is its max-content measure
plus any inline-axis margin, border, and padding.
The min-content extent and max-content extent of a block container box
is the content extent as defined
(for horizontal writing modes)
in CSS2.1§10.6.3
and CSS2.1§17.5.3
for elements with height: auto,
and analogously for vertical writing modes.
The min-extent contribution and max-extent contribution of a block-level box
is the extent of the block after layout,
plus any block-axis margin, border, and padding.
4.4
Intrinsic Sizes in Table Layout
???
4.5
Intrinsic Sizes in Multi-column Layout
The min-content measure of a multi-column element
with a computed column-width not auto
is the smaller of
its column-width
and the largest min-measure contribution of its contents.
The min-content measure of a multi-column element
with a computed column-width of auto
is the largest min-measure contribution of its contents
multiplied by its column-count
(treating auto as 1).
The max-content measure of a multi-column element
with unrestrained column heights
and a computed column-count not auto
is its column-count
multiplied by the larger of
its column-width (treating auto as 0)
and the largest max-measure contribution of its contents.
The max-content measure of a multi-column element
with unrestrained column heights
and a computed column-count of auto
is its column-width
multiplied by the number of columns obtained by taking all allowed column breaks [CSS3-BREAK].
The max-content measure of a multi-column element
with restrained-height columns (i.e. a specified height or max-height, or whichever properties map to the extent of the element)
is the measure that would exactly include all of its columns.
It may be approximated by:
-
Laying out the element with column-spanning elements given 'display: none',
and taking a measure that includes all the columns.
-
Laying out all of the column-spanning elements into that resulting measure,
and taking the resulting extent.
-
Subtracting that resulting extent from the specified restraint,
laying out the element without column-spanning elements again into this adjusted extent,
and taking the measure of all its columns as its max-content measure.
or by some more accurate method.
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.
In the common case of no column-spanning elements,
this approximation collapses to simply doing a layout,
and measuring the resulting columns.
The preferred measure of a multi-column element
with non-auto column-width and column-count
is the product of the column-width and column-count,
plus the appropriate amount of column gaps.
In all other cases,
it is the max-content measure.
5
Extrinsic Size Determination
Extrinsic sizing determines sizes based on the context of an element,
without regard for its contents.
The inner fill-available measure of a box is…
…less the box’s inline-axis margins
(after any margin collapsing, and treating auto margins as zero),
borders, and padding.
The fill-available extent 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?
Acknowledgments
Special thanks go to Aaron Gustafson, L. David Baron
for their contributions to this module.
Document conventions
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words "for example"
or are set apart from the normative text with class="example",
like this:
This is an example of an informative example.
Informative notes begin with the word "Note" and are set apart from the
normative text with class="note", like this:
Note, this is an informative note.
Conformance to this specification
is defined for three conformance classes:
- style sheet
- A CSS
style sheet.
- renderer
- A UA
that interprets the semantics of a style sheet and renders
documents that use them.
- authoring tool
- A UA
that writes a style sheet.
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Partial implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must
treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.
Experimental implementations
To avoid clashes with future CSS features, the CSS2.1 specification
reserves a prefixed
syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage
in the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations
use a vendor-prefixed syntax for such features, including those in
W3C Working Drafts. This avoids incompatibilities with future changes
in the draft.
Non-experimental implementations
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group’s website at
http://www.w3.org/Style/CSS/Test/.
Questions should be directed to the
public-css-testsuite@w3.org
mailing list.
References
Normative References
Index
- available, 2
- available extent, 2
- available measure, 2
- available size, 2
- available space, 2
- contain-floats, 3.2
- definite, 2
- definite size, 2
- Extrinsic sizing, 5
- fallback, 2
- fallback size, 2
- fill
- value for width, min-width, max-width, height, min-height, max-height, 3.1
- value for column-width, 3.3
- fill-available extent, 2
- fill-available fit, 2
- fill-available measure, 2
- fit-content
- value for width, min-width, max-width, height, min-height, max-height, 3.1
- value for column-width, 3.3
- fit-content extent, 2
- fit-content measure, 2
- indefinite, 2
- indefinite size, 2
- Intrinsic sizing, 4
- max-content
- value for width, min-width, max-width, height, min-height, max-height, 3.1
- value for column-width, 3.3
- max-content constraint, 2
- max-content extent, 2
- max-content measure, 2
- max-content size, 2
- max-extent contribution, 2
- max-measure, 2
- max-measure contribution, 2
- max-size constraint, 2
- max-size contribution, 2
- min-content
- value for width, min-width, max-width, height, min-height, max-height, 3.1
- value for column-width, 3.3
- min-content constraint, 2
- min-content extent, 2
- min-content measure, 2
- min-content size, 2
- min-extent contribution, 2
- min-measure, 2
- min-measure contribution, 2
- min-size constraint, 2
- min-size contribution, 2
- preferred extent, 2
- preferred measure, 2
- preferred-size constraint, 2
- preferred-size contribution, 2
- repudiate-floats, 3.1
- size, 2
Property index
Issues Index
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 extents be different,
even if they are the same elsewhere.
↵
Does this value work?
Is it needed?
↵
Right now all of these except
fill mean the same thing
for extents. This may or may not be ideal.
↵
If the measure is
auto,
we could have min-content extent imply a max-content measure,
and vice versa.
↵
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?
↵