CSS Intrinsic & Extrinsic Sizing Module Level 3

Shortname: css-sizing
Level: 3
Status: ED
Work Status: Exploring
Group: csswg
ED: http://dev.w3.org/csswg/css-sizing
TR: http://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/
!Issue Tracking: W3C Bugzilla
Previous version: http://www.w3.org/TR/2012/WD-css3-sizing-20120927/
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.
Link Defaults: css21 (property) min-width/min-height/max-width/max-height, css21 (dfn) initial containing block/block container box
Ignored Terms: block-level box
url: http://dev.w3.org/csswg/css2/visuren.html#x3; type: dfn; text: containing block

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 <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 ''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 size 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 size 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 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
	

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 ''width/auto''.

Does this value work? Is it needed? Feedback from dbaron

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'', ''width/repudiate-floats'') will compute to ''width/auto'', as defined by CSS 2.1. [[!CSS21]]

Containing Floats

	Name: min-width, min-height
	New values: contain-floats
	
contain-floats
Equivalent to ''min-width/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.

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)).

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. The min-content contribution and max-content contribution in each axis is the element's specified outer size in that axis, if definite; otherwise, they are the min-content size, as specified above, plus the element's margin/border/padding in that axis, clamped by the element's min and max size properties in that axis.

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.

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 ''width/min-content'', ''width/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 ''width/fit-content'', ''width/auto'', or ''width/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 ''width/min-content'', ''width/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 ''width/fit-content'', ''width/auto'', or ''width/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. Issue: Need to handle floats. See Greg's issue and dbaron's response.

Intrinsic Sizes in Table Layout

???

Intrinsic Sizes in Multi-column Layout

Min-content Sizes in Multi-column Layout

The min-content inline size of a multi-column element with a computed 'column-width' not ''column-width/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 ''column-width/auto'' is the largest min-content inline-size contribution of its contents multiplied by its 'column-count' (treating ''column-count/auto'' as ''1'').

Max-content Sizes in Unconstrained-height Multi-column Layout

The max-content inline size of a multi-column element with unrestrained column heights and a computed 'column-count' not ''column-count/auto'' is its 'column-count' multiplied by the larger of its 'column-width' (treating ''column-width/auto'' as zero) and the largest min-content inline-size contribution of its contents.

Note that the contents of the multi-column element can still grow to be wider and shorter if the resulting column width is still smaller than 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 ''column-count/auto'' is its 'column-width' multiplied by the number of columns obtained by taking all allowed column breaks [[CSS3-BREAK]].

Max-content Sizes in Constrained-height Multi-column Layout

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.

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

Issue: Write something useful here. Like about when to fall back to auto or whatever. In order to prevent cyclic sizing, the ''min-height/auto'' value of 'min-height' and 'max-height' does not factor into the percentage size resolution of the box’s contents. For example, a percentage-height block whose flex item parent has ''height: 120em; min-height: auto'' will size itself against ''height: 120em'' regardless of the impact that 'min-height' might have on the used size of the flex item.

Changes

Changes since the September 2012 Working Draft include:

Acknowledgments

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