CSS Intrinsic & Extrinsic Sizing Module Level 3

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#sizeReferenced in:2.1. Auto Box Sizes (2) (3) (4) (5) (6) (7): 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#outer-sizeReferenced in:2.2. Intrinsic Size Contributions: The margin-box size of a box.
definite size#definiteReferenced in:2. Terminology (2) (3) (4)4.3. Intrinsic Sizes of Non-Replaced Blocks (2)5.1. Fill-available Sizing: A size that can be determined without measuring content; that is, a <length>, a size of the initial containing block, or a <percentage> 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#indefiniteReferenced in:2. Terminology (2)5.2. Percentage Sizing: A size that is not definite. An indefinite available size is essentially infinite.
available space#availableReferenced in:2. Terminology (2)2.1. Auto Box Sizes (2) Changes: 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#fill-available-fitReferenced in:2. Terminology: 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#fallbackReferenced in:2. Terminology: 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.

2.1. Auto Box Sizes

fill-available size#fill-available-sizeReferenced in:2.1. Auto Box Sizes

fill-available inline size#fill-available-inline-sizeReferenced in:3.1. New Keywords for width and height3.3. Column Sizing Keywords (2)5.1. Fill-available Sizing (2) Changes

fill-available block size#fill-available-block-sizeReferenced in:3.1. New Keywords for width and height5.1. Fill-available Sizing

Roughly, the size a box would take if it filled its available space in the given axis. (See §5 Extrinsic Size Determination.)

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#max-contentReferenced in:2.1. Auto Box Sizes (2)2.2. Intrinsic Size Contributions4.1. Intrinsic Sizes of Replaced Elements

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#max-content-inline-sizeReferenced in:3.1. New Keywords for width and height3.3. Column Sizing Keywords (2)4.2. Intrinsic Sizes of Non-Replaced Inlines (2)4.3. Intrinsic Sizes of Non-Replaced Blocks (2)4.5.2. Max-content Sizes in Unconstrained-height Multi-column Layout (2)4.5.3. Max-content Sizes in Constrained-height Multi-column Layout (2)5.1. Fill-available Sizing

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 §4 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.

max-content block size#max-content-block-sizeReferenced in:2.1. Auto Box Sizes3.1. New Keywords for width and height4.2. Intrinsic Sizes of Non-Replaced Inlines4.3. Intrinsic Sizes of Non-Replaced Blocks

The box’s “ideal” size in the block axis. Usually the block size of the content after layout.

min-content size#min-contentReferenced in:2.1. Auto Box Sizes2.2. Intrinsic Size Contributions4.1. Intrinsic Sizes of Replaced Elements (2)

The smallest size a box could take that doesn’t lead to overflow that could be avoided by choosing a larger size. (See §4 Intrinsic Size Determination.)

min-content inline size#min-content-inline-sizeReferenced in:3.1. New Keywords for width and height3.3. Column Sizing Keywords (2)4.2. Intrinsic Sizes of Non-Replaced Inlines (2)4.3. Intrinsic Sizes of Non-Replaced Blocks (2)4.5.1. Min-content Sizes in Multi-column Layout (2)5.1. Fill-available Sizing

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#min-content-block-sizeReferenced in:3.1. New Keywords for width and height4.2. Intrinsic Sizes of Non-Replaced Inlines4.3. Intrinsic Sizes of Non-Replaced Blocks

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 size

fit-content inline size#fit-content-inline-sizeReferenced in:3.1. New Keywords for width and height

fit-content block size#fit-content-block-sizeReferenced in:3.1. New Keywords for width and height

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.

2.2. Intrinsic Size Contributions

max-content contribution#max-content-contributionReferenced in:2.3. Intrinsic Size Constraints4.1. Intrinsic Sizes of Replaced Elements4.2. Intrinsic Sizes of Non-Replaced Inlines (2)4.3. Intrinsic Sizes of Non-Replaced Blocks (2) (3)4.5.2. Max-content Sizes in Unconstrained-height Multi-column Layout: The size that a box contributes to its containing block’s max-content size.
min-content contribution#min-content-contributionReferenced in:2.3. Intrinsic Size Constraints4.1. Intrinsic Sizes of Replaced Elements4.2. Intrinsic Sizes of Non-Replaced Inlines (2)4.3. Intrinsic Sizes of Non-Replaced Blocks (2) (3) (4)4.5.1. Min-content Sizes in Multi-column Layout (2)4.5.2. Max-content Sizes in Unconstrained-height Multi-column Layout: 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 auto margins are treated as zero.

2.3. Intrinsic Size Constraints

max-content constraint#max-content-constraintReferenced in:2. Terminology4.3. Intrinsic Sizes of Non-Replaced Blocks: A sizing constraint imposed by the box’s containing block that causes it to produce its max-content contribution.
min-content constraint#min-content-constraintReferenced in:2. Terminology: A sizing constraint imposed by the box’s containing block that causes it to produce its min-content 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#valdef-width-fillReferenced in:3.1. New Keywords for width and height4.3. Intrinsic Sizes of Non-Replaced Blocks (2): Use the fill-available inline size or fill-available block size, as appropriate to the writing mode.
max-content#valdef-width-max-contentReferenced in:3.1. New Keywords for width and height4.3. Intrinsic Sizes of Non-Replaced Blocks (2)5.2. Percentage Sizing: Use the max-content inline size or max-content block size, as appropriate to the writing mode.
min-content#valdef-width-min-contentReferenced in:3.1. New Keywords for width and height4.3. Intrinsic Sizes of Non-Replaced Blocks (2)5.2. Percentage Sizing: Use the min-content inline size or min-content block size, as appropriate to the writing mode.
fit-content#valdef-width-fit-contentReferenced in:3.1. New Keywords for width and height4.3. Intrinsic Sizes of Non-Replaced Blocks (2)5.2. Percentage Sizing: Use the fit-content inline size or fit-content block size, as appropriate to the writing mode.

Note: To size an element such that it avoids overlapping sibling 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;.

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) will compute to auto, as defined by CSS 2.1. [CSS21]

3.2. 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;.

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.

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.

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.

Need to handle floats. See Greg’s issue and dbaron’s response.

4.4. Intrinsic Sizes in Table Layout

???

4.5. Intrinsic Sizes in Multi-column Layout

4.5.1. Min-content 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).

4.5.2. 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 auto is its column-count multiplied by the larger of its column-width (treating 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 auto is its column-width multiplied by the number of columns obtained by taking all allowed column breaks [CSS3-BREAK].

4.5.3. 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:

Laying out the element with column-spanning elements given display: none, and taking a inline-size that includes all the columns.
Laying out all of the column-spanning elements into that resulting inline-size, and taking the resulting block-size.
Subtracting that resulting block-size from the specified restraint, laying out the element without column-spanning elements again into this adjusted block-size, and taking the inline-size of all its columns as its max-content inline size.

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#extrinsic-sizingReferenced in: Changes determines sizes based on the context of an element, without regard for its contents.

5.1. Fill-available Sizing

The inner fill-available inline size of a box is…

If the box is the root or is absolutely-positioned, the inline-size of its containing block, else
max(min-content inline size|0, min(max-content inline size|infinity, inline size|fill-available inline size)) where the sizes are inner inline-sizes 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 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?

5.2. 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 auto, min-content, max-content, and 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 September 2012 Working Draft include:

Changed fill-available keyword to fill.
Hooked up intrinsic sizes of replaced elements to the default sizing algorithm in [CSS3-IMAGES].
Specified that extrinsic sizing treates auto margins as zero.
Clarified definition of available space.
Specified that percentages resolved against an intrinsic size of the parent computes falls back to auto sizing.
Fixed bad wording wrt containing blocks in fill-available inline size definition, and specified that it is calculated after margin collapsing.
Specified that an absolutely-positioned box’s containing block is always considered definite.
Improved section on intrinsic sizing of multi-column elements.
Cleaned up some terminology (“min-size” to “min-content size”, “measure” to “inline-size, “extent” to “block-size”).

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:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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.

Implementations of Unstable and Proprietary Features

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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.