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.
Status of this document
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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, typically either a
measurement of its containing block or an infinite size. 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 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(max-content, max(min-content, fill-available)). 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.
- 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.
- 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.
- 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.
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-available’
- 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-available’ 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
When used as values for ‘column-width’, the new keywords specify the
optimal column width:
- ‘
min-content’
- Specifies the optimal column width as the min-content measure of the
multi-column element's contents.
- ‘
max-content’
- Specifies the optimal column width as the max-content measure of the
multi-column element's contents.
- ‘
fill-available’
- Specifies the optimal column width as the fill-available measure of the
multi-column element.
- ‘
fit-content’
- Specifies the optimal column width as
min(max-content, max(min-content, fill-available)).
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 and max-content sizes 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-content 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-available’, 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-available’, 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
‘extent’ or ‘max-extent’) 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.
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…
- If the box is the root or is absolutely-positioned, the measure of
its containing block, else
-
max(min-measure|0, min(max-measure|infinity,
measure|fill-available)) where the sizes
are inner measures 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 ‘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?
6.1. 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 CSS Intrinsic & Extrinsic Sizing Level 3 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 CSS Intrinsic & Extrinsic Sizing
Level 3 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 CSS Intrinsic & Extrinsic Sizing Level 3
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined by CSS
Intrinsic & Extrinsic Sizing Level 3 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 CSS Intrinsic & Extrinsic Sizing
Level 3 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.
6.3. 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.
6.4. 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.
6.5. 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.
6.6. CR exit criteria
For this specification to be advanced to Proposed Recommendation, there
must be at least two independent, interoperable implementations of each
feature. Each feature may be implemented by a different set of products,
there is no requirement that all features be implemented by a single
product. For the purposes of this criterion, we define the following
terms:
- independent
- each implementation must be developed by a different party and cannot
share, reuse, or derive from code used by another qualifying
implementation. Sections of code that have no bearing on the
implementation of this specification are exempt from this requirement.
- interoperable
- passing the respective test case(s) in the official CSS test suite,
or, if the implementation is not a Web browser, an equivalent test. Every
relevant test in the test suite should have an equivalent test created if
such a user agent (UA) is to be used to claim interoperability. In
addition if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those equivalent
tests in the same way for the purpose of interoperability. The equivalent
tests must be made publicly available for the purposes of peer review.
- implementation
- a user agent which:
- implements the specification.
- is available to the general public. The implementation may be a
shipping product or other publicly available version (i.e., beta
version, preview release, or “nightly build”). Non-shipping product
releases must have implemented the feature(s) for a period of at least
one month in order to demonstrate stability.
- is not experimental (i.e., a version specifically designed to pass
the test suite and is not intended for normal usage going forward).
The specification will remain Candidate Recommendation for at least six
months.
Acknowledgments
Special thanks go to Aaron Gustafson, L. David Baron for their
contributions to this module.
References
Normative references
-
- [CSS21]
- Bert Bos; et al. Cascading Style
Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June
2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607/
- [CSS3-FLEXBOX]
- Tab Atkins Jr.; Elika J. Etemad; Alex Mogilevsky. CSS
Flexible Box Layout Module. 18 September 2012. W3C Candidate
Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-flexbox-20120918/
- [CSS3-IMAGES]
- Elika J. Etemad; Tab Atkins Jr. CSS Image
Values and Replaced Content Module Level 3. 17 April 2012. W3C
Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-images-20120417/
- [CSS3COL]
- Håkon Wium Lie. CSS
Multi-column Layout Module. 12 April 2011. W3C Candidate
Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2011/CR-css3-multicol-20110412
- [RFC2119]
- S. Bradner. Key
words for use in RFCs to Indicate Requirement Levels. Internet
RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
Other references
-
- [CSS3-BREAK]
- Rossen Atanassov; Elika J. Etemad. CSS
Fragmentation Module Level 3. 23 August 2012. W3C Working
Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-break-20120823/
- [CSS3COLOR]
- Tantek Çelik; Chris Lilley; L. David Baron. CSS Color
Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607
Index
- authoring tool, 6.2.
- 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-available’, 3.1.
- fill-available extent, 2.
- fill-available fit, 2.
- fill-available measure, 2.
- ‘
fit-content’,
3.1.
- fit-content extent, 2.
- fit-content measure, 2.
- indefinite, 2.
- indefinite size, 2.
- Intrinsic sizing, 4.
- ‘
max-content’,
3.1.
- max-content constraint, 2.
- max-content extent, 2.
- max-content measure, 2.
- max-size constraint, 2.
- max-size contribution, 2.
- ‘
min-content’,
3.1.
- min-content constraint, 2.
- min-content extent, 2.
- min-content measure, 2.
- min-size constraint, 2.
- min-size contribution, 2.
- renderer, 6.2.
- ‘
repudiate-floats’, 3.1.
- size, 2.
- style sheet
- as conformance class, 6.2.
Property index
| Property
| Values
| Initial
| Applies to
| Inh.
| Percentages
| Media
|