1. Introduction
This is a diff spec from CSS Sizing Level 3. It is currently an Exploratory Working Draft: if you are implementing anything, please use Level 3 as a reference. We will merge the Level 3 text into this draft once it reaches CR.
1.1. Module interactions
This module extends the width, height, min-width, min-height, max-width, max-height, and column-width features defined in [CSS2] chapter 10 and in [CSS3COL]
1.2. Values
This specification follows the CSS property definition conventions from [CSS2]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords keywords as their property value. For readability they have not been repeated explicitly.
2. Terminology
2.1. Auto Box Sizes
2.2. Intrinsic Size Contributions
2.3. Intrinsic Size Constraints
3. Specifying Box Sizes
3.1. Sizing Properties
3.1.1. Preferred Size Properties: the width and height properties
3.1.2. Minimum Size Properties: the min-width and min-height properties
3.1.3. Maximum Size Properties: the max-width and max-height properties
3.2. Sizing Values: the <length-percentage>, auto | none, min-content, max-content, and fit-content() values
3.2.1. “Behaving as auto”
3.2.2. Containing or Excluding Floats
3.3. Box Edges for Sizing: the box-sizing property
3.4. New Column Sizing Values: the min-content, max-content, and fit-content() values
4. Aspect Ratios
This section is a rough draft proposal. See discussion in Issue 333 and Issue 1173.
4.1. Intrinsic Aspect Ratios: the aspect-ratio property
Name: | aspect-ratio |
---|---|
Value: | auto | <ratio> |
Initial: | auto |
Applies to: | all elements except inline boxes and internal ruby or table boxes |
Inherited: | no |
Percentages: | n/a |
Computed value: | specified keyword or a pair of numbers |
Canonical order: | per grammar |
Animation type: | discrete |
This property sets a preferred aspect ratio for the box, which will be used in the calculation of auto sizes and some other layout functions. The box’s automatic sizes are then calculated the same as for a replaced element with an intrinsic aspect ratio, see e.g. CSS2 § 10 and CSS Flexible Box Model Level 1 § 9.2. The axis in which the preferred size calculation depends on this aspect ratio is called the ratio-dependent axis, and the resulting size is definite if its input sizes are also definite.
- auto
- Replaced elements with an intrinsic aspect ratio use that aspect ratio; otherwise the box has no preferred aspect ratio. Size calculations involving intrinsic aspect ratio work with the content box dimensions always.
- <ratio>
- The box’s preferred aspect ratio is the specified ratio of width / height. Size calculations involving preferred aspect ratio work with the dimensions of the box specified by box-sizing.
Note: If a box has both a width and a height that are not automatic (or, in the case of a flex item, both a content flex basis and a non-automatic preferred cross size), then the preferred aspect ratio has no effect: at least one of these sizes must be auto (/content) for the preferred aspect ratio to have an impact on sizing.
< ul > < li > …< li > …< li > …< li > …</ ul >
ul { display : grid; grid-template-columns : repeat ( auto-fill, minmax ( 12 em , 1 fr )); } li { aspect-ratio : 1 / 1 ; overflow : auto; }
iframe
element’s width
and height
attributes
to set the aspect-ratio property,
giving the iframe an aspect ratio to use for sizing
so that it behaves exactly like an image with that aspect ratio.
< iframe src = "https://www.youtube.com/embed/0Gr1XSyxZy0" width = 560 height = 315 >
@supports ( aspect-ratio: attr(width) / 1) { iframe { aspect-ratio: attr(width) / attr(height); width: 100%; height: auto; } }
Can we just slip this aspect-ratio rule into the UA default style sheet for images, so that they have an aspect ratio while they load? This would avoid the need for extra reflows after loading.
On non-replaced elements with an auto inline size (or any replaced element whose intrinsic block size depends on its inline size), the intrinsic block sizes of the box are assumed to be zero when applying min-block-size to the aspect ratio calculations, and assumed to be infinity when applying max-block-size to the aspect ratio calculations. These limits take effect as usual for non-replaced elements once the inline size is resolved.
width: auto; min-height: min-content; aspect-ratio: 1/1;
using the aspect ratio to resolve the width would require knowing the min-content height of the element.
Since for a non-replaced element, this value is not known until the width is resolved, we assume the min-height value is zero while resolving the width.
Then, once the width is known, we resolve the height, taking into account the min-content minimum.
4.1.1. Automatic Content-based Minimum Sizes
In order to avoid unintentional overflow, the automatic minimum size in the ratio-dependent axis of a box with a preferred aspect ratio that is neither a replaced element nor a scroll container is the minimum of its min-content size and its maximum size.
div { aspect-ratio: 1/1; /* 'width' and 'height' both default to 'auto' */ }
+----------+ +----------+ +----------+ | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | | | ~~~ | | ~~~~~~~~ | +----------+ +----------+ | ~~~~~~~~ | | ~~~~~~ | +----------+
When overflow: auto is specified, however, even the box with excess content maintains the 1:1 aspect ratio.
div { overflow: auto; aspect-ratio: 1/1; }
+----------+ +----------+ +----------+ | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~^| | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | | | ~~~ | | ~~~~~~~~v| +----------+ +----------+ +----------+
Overriding the min-height property also maintains the 1:1 aspect ratio, but will result in content overflowing the box if it is not otherwise handled.
div { aspect-ratio: 1/1; min-height: 0; }
+----------+ +----------+ +----------+ | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~ | | ~~~~~~~~ | | ~~~~~~~~ | | | | ~~~ | | ~~~~~~~~ | +----------+ +----------+ +-~~~~~~~~-+ ~~~~~~
5. Intrinsic Size Determination
Need to Sync This Section With Level 3 It is Super Out Of Date
Intrinsic sizing determines sizes based on the contents of a box, without regard for the context in which it is placed.
Precisely how to calculate the intrinsic size of a box depends on its layout mode; the following subsections define intrinsic sizing for several types of layouts. Individual layout modes, such as Flexbox or Grid, can define their own intrinsic sizing rules.
If a box has an explicit intrinsic inner size in an axis, then that is all of its intrinsic sizes in that axis. If the other axis does not have an explicit intrinsic inner size, then its intrinsic sizes are determined normally, without regard for the explicit intrinsic inner size in the first axis.
5.1. 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.
5.2. 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.
5.3. 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 stretch, 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 stretch, 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.
5.4. Intrinsic Sizes in Table Layout
5.5. Intrinsic Sizes in Multi-column Layout
5.5.1. Min-content Sizes in Multi-column Layout
The min-content inline size of a multi-column container 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 container 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), plus its column-gap multiplied by column-count minus 1.
5.5.2. Max-content Sizes in Unconstrained-height Multi-column Layout
The max-content inline size of a multi-column container 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, plus its column-gap multiplied by column-count minus 1.
Note that the contents of the multi-column container 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 container 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], plus its column-gap multiplied by that same number of columns minus 1.
5.5.3. Max-content Sizes in Constrained-height Multi-column Layout
The max-content inline size of a multi-column container 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.
5.6. Overriding Intrinsic Sizes: the intrinsic-* properties
Name: | intrinsic-block-size, intrinsic-inline-size, intrinsic-height, intrinsic-width |
---|---|
Value: | legacy | auto | <length> |
Initial: | legacy |
Applies to: | all elements except non-replaced inlines |
Inherited: | no |
Percentages: | n/a |
Computed value: | as specified, with <length-percentage> values computed |
Canonical order: | per grammar |
Animation type: | by computed value type |
The intrinsic-* properties determine whether the intrinsic sizes of a box are calculated from sizes derived from its content (as defined by the rest of this chapter), or from a specified explicit intrinsic inner size. Values are defined as follows:
- legacy
-
Does not specify an explicit intrinsic inner size of the box in the relevant axis. (Intrinsic sizes are thus determined in various ways based on examining the content of the box as usual. See § 5 Intrinsic Size Determination, for example.)
- auto
-
If overflow is not visible in the relevant axis, specifies a zero explicit intrinsic inner size in that axis. Otherwise same as legacy.
- <length>
-
Specifies an explicit intrinsic inner size in the relevant axis.
Note: The value of intrinsic-* in one axis has no affect on the other axis.
Name: | intrinsic-size |
---|---|
Value: | <'intrinsic-width'> <'intrinsic-height'>? |
Initial: | see individual properties |
Applies to: | see individual properties |
Inherited: | see individual properties |
Percentages: | see individual properties |
Computed value: | see individual properties |
Animation type: | see individual properties |
Canonical order: | per grammar |
The intrinsic-size property is a shorthand for setting all the intrinsic-* properties in one declaration.
The first value sets intrinsic-width, and the second value sets intrinsic-height. If the second value is omitted, it defaults to the first.
When we figure out how to make other physical x/y shorthands, like background-size, work with logical sizes, we’ll apply the same logic here.
6. Intrinsic Size Contributions
A box’s min-content contribution/max-content contribution in each axis is the size of the content box of a hypothetical auto-sized float that contains only that box, if that hypothetical float’s containing block is zero-sized/infinitely-sized.
Note: This specification does not define precisely how to determine these sizes. Please refer to [CSS2], the relevant CSS specification for that display type, the rules for handling percentages (below), and/or existing implementations for further details.
7. Extrinsic Size Determination
…
7.1. Stretch-fit Sizing
…
7.2. Contain-fit Sizing: stretching while maintaining an aspect ratio
Contain-fit sizing essentially applies stretch-fit sizing, but reduces the size of the box in one axis to maintain the box’s intrinsic aspect ratio, similar to the contain keyword of the object-fit and background-size properties.
First, a target rectangle is determined:
- The initial target rectangle is the size of the box’s containing block, with any indefinite size assumed as infinity. If both dimensions are indefinite, the initial target rectangle is set to match the outer edges of the box were it stretch-fit sized.
- Next, if the box has a non-none max-width or max-height, the target rectangle is clamped in the affected dimension to less than or equal to the “maximum size” of the box’s margin box, i.e. the size its margin box would be if the box was sized at its max-width/height. (Note that, consistent with normal box-sizing rules, this “maximum size” is floored by the effects of the box’s min-width/min-height.)
- Last, the target rectangle is reduced in one dimension by the minimum necessary for it to match the box’s intrinsic aspect ratio.
The contain-fit size in each dimension is the size that would result from stretch-fitting into the target rectangle.
Copy whatever stretch-fit ends up doing wrt margin collapsing.
If there is a minimum size in one dimension that would cause overflow of the target rectangle if the aspect ratio were honored, do we honor the aspect ratio or skew the image? If the former, we need a step similar to #2 that applies the relevant minimums.
7.3. Percentage Sizing
…
Changes
Changes since the September 2012 Working Draft include:
- Changed
fill
keyword tostretch-fit
. - 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 stretch-fit 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 containers.
- 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.
Privacy and Security Considerations
This specification introduces no new privacy or security considerations.