CSS Intrinsic & Extrinsic Sizing Module Level 3

Editor’s Draft, 24 July 2014

This version:
http://dev.w3.org/csswg/css-sizing
Latest version:
http://www.w3.org/TR/css3-sizing/
Previous Versions:
http://www.w3.org/TR/2012/WD-css3-sizing-20120927/
Feedback:
www-style@w3.org with subject line “[css-sizing] … message topic …”(archives)
Editors:
Tab Atkins (Google)
fantasai (Mozilla)
Issue Tracking:
W3C Bugzilla

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

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css-sizing” in the subject, preferably like this: “[css-sizing] …summary of comment…

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

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 block-size and/or inline-size; 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 inline-size
Roughly, the inline-size a box would take if it filled its available inline-size. (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 inline-size
Roughly, the narrowest inline-size 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 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. (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 inline-size
If the available inline-size is finite, equal to min(preferred inline-size, max(min-content inline-size, fill-available inline-size)). Otherwise, equal to the max-content inline-size.

Note: This is called the “shrink-to-fit” width in CSS2.1§10.3.5 and CSS Multi-column Layout § 3.4.

preferred inline-size
A box’s “ideal” inline-size. Unless otherwise specified, this is the max-content inline-size.
fill-available block-size
Roughly, the block-size a box would take if it filled its available block-size. (See Intrinsic Size Determination.)
max-content block-size
Roughly, the block-size of the content after layout.
min-content block-size
Equivalent to the max-content block-size.

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 block-size
Analogous to the fit-content inline-size.
preferred block-size
Equivalent to the max-content block-size.

Same issue as min-content block-size.

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.
preferred-size contribution
If the box’s layout mode has a defined preferred-size contribution, that. Otherwise, the box’s max-content contribution.
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.
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

Name:width, min-width, max-width, height, min-height, max-height
New values:fill | max-content | min-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.
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.
repudiate-floats Less stupid name?
In the inline axis, use the larger of the min-content inline-size, and the fill-available inline-size 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? Feedback from dbaron

Right now all of these except fill mean the same thing for block-sizes. This may or may not be ideal.

If the inline-size is 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 (max-content, min-content, fit-content, repudiate-floats) will compute to auto, as defined by CSS 2.1. [CSS21]

3.2 Containing Floats

Name:min-width, min-height
New values:contain-floats
contain-floats
Equivalent to min-content except that when applied to the block-size of a block box it forces the inner block-size 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 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)).

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 inline-size of an inline box is the length of the largest unbreakable sequence of inline content. The min-content inline-size contribution of an inline box is its min-content inline-size, plus any inline-axis margin, border, and padding adjacent to that sequence.

The max-content inline-size 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-content inline-size contribution of an inline box is its max-content inline-size, plus any inline-axis margin, border, and padding adjacent to that sequence.

The min-content block-size, max-content block-size, min-content block-size contribution, and max-content block-size 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 inline-size of a block container box is the largest min-content inline-size contribution of its in-flow or floated children.

The max-content inline-size of a block container box is the inline-size of the box after layout, if all children are sized under a max-content constraint.

If the computed inline-size of a block-level box is min-content, max-content, or a definite size, its min-content inline-size contribution is that size plus any inline-axis margin, border, and padding. Otherwise, if the computed inline-size of the block is fit-content, auto, or fill, its min-content inline-size contribution is its min-content inline-size plus any inline-axis margin, border, and padding.

If the computed inline-size of a block-level box is min-content, max-content, or a definite size, its max-content inline-size contribution is that size plus any inline-axis margin, border, and padding. Otherwise, if the computed inline-size of the block is fit-content, auto, or fill, its max-content inline-size contribution is its max-content inline-size plus any inline-axis margin, border, and padding.

The min-content block-size and max-content block-size of a block container box is the content block-size 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-content block-size contribution and max-content block-size contribution of a block-level box is the block-size 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 inline-size of a multi-column element with a computed column-width not auto is the smaller of its column-width and the largest min-content inline-size contribution of its contents.

The min-content inline-size of a multi-column element with a computed column-width of auto is the largest min-content inline-size contribution of its contents multiplied by its column-count (treating auto as 1).

The max-content inline-size 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-content inline-size contribution of its contents.

The max-content inline-size 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 inline-size of a multi-column element with restrained-height columns (i.e. a specified height or max-height, or whichever properties map to the block-size of the element) is the inline-size that would exactly include all of its columns. It may be approximated by:

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 inline-size 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 inline-size.

5 Extrinsic Size Determination

Extrinsic sizing determines sizes based on the context of an element, without regard for its contents.

The inner fill-available inline-size 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 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?

Acknowledgments

Special thanks go to Aaron Gustafson, L. David Baron for their contributions to this module.

Conformance

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 classes

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

[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-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. URL: http://www.ietf.org/rfc/rfc2119.txt

Informative 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

Property index

No properties defined.

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 block-sizes be different, even if they are the same elsewhere.
Same issue as min-content block-size.
Less stupid name?
Does this value work? Is it needed? Feedback from dbaron
Right now all of these except fill mean the same thing for block-sizes. This may or may not be ideal.
If the inline-size is auto, we could have min-content block-size imply a max-content inline-size, 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?