CSS Box Sizing Module Level 4

Editor’s Draft,

More details about this document
This version:
https://drafts.csswg.org/css-sizing-4/
Latest published version:
https://www.w3.org/TR/css-sizing-4/
Previous Versions:
Feedback:
CSSWG Issues Repository
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Inline In Spec
Editors:
Tab Atkins (Google)
Elika J. Etemad / fantasai (Invited Expert)
Jen Simmons (Apple)
Suggest an Edit for this Spec:
GitHub Editor

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. This is a delta spec over CSS Sizing Level 3.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, 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.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-sizing” in the title, like this: “[css-sizing] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 2 November 2021 W3C Process Document.

1. Introduction

This is a diff spec over 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. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with 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 as their property value. For readability they have not been repeated explicitly.

2. Terminology

CSS Sizing 3 § 2 Terminology

3. Specifying Box Sizes

CSS Sizing 3 § 3 Specifying Box Sizes

3.1. Sizing Properties

Add shorthands. [Issue #820]

3.2. New Sizing Values: the stretch, fit-content, and contain keywords

fit-content

In all current engines.

Firefox52+Safari10.1+Chrome57+
Opera44+Edge79+
Edge (Legacy)16+IENone
Firefox for Android52+iOS Safari10.3+Chrome for Android57+Android WebView57+Samsung Internet6.0+Opera Mobile43+

fit-content

In all current engines.

Firefox52+Safari10.1+Chrome57+
Opera44+Edge79+
Edge (Legacy)16+IENone
Firefox for Android52+iOS Safari10.3+Chrome for Android57+Android WebView57+Samsung Internet6.0+Opera Mobile43+

max-height

In all current engines.

Firefox1+Safari1.3+Chrome18+
Opera7+Edge79+
Edge (Legacy)12+IE7+
Firefox for Android4+iOS Safari1+Chrome for Android18+Android WebView37+Samsung Internet1.0+Opera Mobile14+

max-width

In all current engines.

Firefox1+Safari1+Chrome1+
Opera4+Edge79+
Edge (Legacy)12+IE7+
Firefox for Android4+iOS Safari1+Chrome for Android18+Android WebView4.4+Samsung Internet1.0+Opera Mobile14+

width

In all current engines.

Firefox1+Safari1+Chrome1+
Opera3.5+Edge79+
Edge (Legacy)12+IE4+
Firefox for Android4+iOS Safari1+Chrome for Android18+Android WebView1+Samsung Internet1.0+Opera Mobile10.1+
Name: width, height, inline-size, block-size, min-width, min-height, min-inline-size, min-block-size, max-width, max-height, max-inline-size, max-block-size
New values: stretch | fit-content | contain
stretch
Applies stretch-fit sizing, attempting to match the size of the box’s margin box to the size of its containing block. See § 6.1 Stretch-fit Sizing: filling the containing block.
fit-content
Essentially fit-content(stretch) i.e. min(max-content, max(min-content, stretch)).
contain
If the box has a preferred aspect ratio, applies contain-fit sizing, attempting to fit into the box’s constraints while maintaining its preferred aspect ratio insofar as possible. See § 6.2 Contain-fit Sizing: stretching while maintaining an aspect ratio.

If the box has no preferred aspect ratio, applies stretch-fit sizing.

4. Aspect Ratios

Images often have a natural aspect ratio, which the CSS layout algorithms attempt to preserve as they resize the element.

The aspect-ratio property allows specifying this behavior for non-replaced elements, and for altering the effective aspect ratio of replaced elements.

We are still working through the details of this section. If there is any behavior specified here that would cause replaced elements with a preferred aspect ratio to behave differently than they would under the requirements of the CSS2, Flex Layout, and Grid Layout specs combined (without this specification in effect), this is an error and should be reported to the CSSWG.

4.1. Preferred Aspect Ratios: the aspect-ratio property

aspect-ratio

In all current engines.

Firefox89+Safari15+Chrome88+
Opera74+Edge88+
Edge (Legacy)NoneIENone
Firefox for Android89+iOS Safari15+Chrome for Android88+Android WebView88+Samsung Internet15.0+Opera Mobile63+
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: by computed value

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.

auto
Replaced elements with a natural aspect ratio use that aspect ratio; otherwise the box has no preferred aspect ratio. Size calculations involving the 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 the aspect ratio work with the dimensions of the box specified by box-sizing.

If the <ratio> is degenerate, the property instead behaves as auto.

auto && <ratio>
If both auto and a <ratio> are specified together, the preferred aspect ratio is the specified ratio of width / height unless it is a replaced element with a natural aspect ratio, in which case that aspect ratio is used instead. In all cases, size calculations involving the aspect ratio work with the content box dimensions always.

If the <ratio> is degenerate, the property instead behaves as auto.

Note: Having a preferred aspect ratio does not make a box into a replaced element; layout rules specific to replaced elements do not generally apply to non-replaced boxes with a preferred aspect ratio. For example, a non-replaced absolutely-positioned box treats justify-self: normal as stretch, not as start (CSS Box Alignment 3 § 6.1.2 Absolutely-Positioned Boxes), even if it has a preferred aspect ratio

CSS2.1 does not cleanly differentiate between replaced elements vs. elements with an aspect ratio; need to figure out specific cases that are unclear and define them, either in the appropriate Level 3 spec or here.

This example sets each item in the grid to render as a square, determining the number of items and their widths by the available space.
<ul>
  <li><li><li><li></ul>
ul {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(12em, 1fr));
}
li {
  aspect-ratio: 1/1;
  overflow: auto;
}
This example uses the 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 number) / 1) {
  iframe {
    aspect-ratio: attr(width number) / attr(height number);
    width: 100%;
    height: auto;
  }
}

If a replaced element’s only natural dimension is a natural width or a natural height, giving it a preferred aspect ratio also gives it an natural height or width, whichever was missing, by transferring the existing size through the preferred aspect ratio.

4.2. Effects of Preferred Aspect Ratio on Automatic Sizes

When a box has a preferred aspect ratio, its automatic sizes are calculated the same as for a replaced element with a natural aspect ratio and no natural size in that axis, 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. The opposite axis (on which the ratio-dependent axis size depends) is the ratio-determining axis.

Note: A preferred aspect ratio only ever has an effect if at least one of the box’s sizes is automatic. If neither width nor height is an automatic size, it can have no effect on its preferred sizes.

When we move all the sizing information here, rather than crowbar-ing our way into 2.1, then the core principle here is just: the resolved preferred size in the ratio-determining axis (before applying min/max) gets transferred thru the ratio. Min/max constraints get transferred afterwards, and then applied to each axis independently without regards to aspect-ratio.

4.2.1. Margin-collapsing

For the purpose of margin collapsing (CSS 2 § 8.3.1 Collapsing margins), if the block axis is the ratio-dependent axis, it is not considered to have a computed block-size of auto.

4.3. 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 its min-content size capped by its maximum size.

In the following example, the box is as wide as the container (as usual), and its height is as tall as needed to contain its content but at least as tall as it is wide.
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 (and handles overflow by becoming scrollable instead, as usual).

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;
}
+----------+  +----------+  +----------+
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~ |
| ~~~~~~~~ |  | ~~~~~~~~ |  | ~~~~~~~~ |
| ~~~~~~~  |  | ~~~~~~~~ |  | ~~~~~~~~ |
|          |  | ~~~      |  | ~~~~~~~~ |
+----------+  +----------+  +-~~~~~~~~-+
                              ~~~~~~    
This automatic minimum operates in both axes. Consider this example:
<div style="height: 100px; aspect-ratio: 1/1;">
  <span style="display: inline-block; width: 50px;"></span>
  <span style="display: inline-block; width: 150px;"></span>
</div>

The width of the container, being auto, resolves through the aspect ratio to 100px. However, its min-width, being auto, resolves to 150px. The resulting width of the container is thus 150px. To ignore the contents when sizing the container, min-width: 0 can be specified.

4.4. Min/Max Size Transfers

Sizing constraints in either axis (the origin axis) are transferred through the preferred aspect ratio and applied to any indefinite minimum, maximum, or preferred size in the other axis (the destination axis) as follows:

Note: Thus, any definite sizes are completely unaffected by a transferred constraint; and a transferred minimum will never cause an element to exceed a definite preferred/maximum size, nor will a transferred maximum cause an element to violate its preferred/minimum size.

Note: The basic principle is that sizing constraints transfer through the aspect-ratio to the other side to preserve the aspect ratio to the extent that they can without violating any sizes specified explicitly on that affected axis. (This is the principle that drove the contents of the constraint table in CSS2 Section 10.4.)

In the following example:
<div id=container style="height: 100px; float: left;">
  <div id=item style="height: 100%; aspect-ratio: 1/1;">content</div>
</div>

Since the height of the #item is a percentage that resolves against a definite container, the width of the item resolves to 100px for both its intrinsic size contributions as well as for final layout, so the container also sizes to a width of 100px.

<div id=container style="height: auto; float: left;">
  <div id=item style="height: 100%; aspect-ratio: 1/1;">content</div>
</div>

In this next example, the percentage height of the item cannot be resolved and behaves as auto (see CSS 2 § 10.5 Content height: the 'height' property). Since both axes now have an automatic size, the height becomes the ratio-dependent axis. Calculating the intrinsic size contributions of the box produces a width derived from its content, and a height calculated from that width and the aspect ratio, yielding a square box (and a container) sized to the width of the word “content”.

This section might not be written correctly. [Issue #6071]

5. Intrinsic Size Determination

5.1. Intrinsic Sizes

CSS Sizing 3 § 5.1 Intrinsic Sizes

5.2. Overriding Contained Intrinsic Sizes: the contain-intrinsic-* properties

Name: contain-intrinsic-width, contain-intrinsic-height, contain-intrinsic-block-size, contain-intrinsic-inline-size
Value: none | <length> | auto <length>
Initial: none
Applies to: elements with size containment
Inherited: no
Percentages: n/a
Computed value: as specified, with <length> values computed
Canonical order: per grammar
Animation type: by computed value type

These properties allow elements with size containment to specify an explicit intrinsic inner size, causing the box to size as if its in-flow content totals to a width and height matching the specified explicit intrinsic inner size (rather than sizing as if it were empty).

Note: This is not always equivalent to laying out as if the element had one child of the specified explicit intrinsic inner size. For example, a grid container with one child of the specified size would still size according to the specified grid, usually ending up with a larger content size than specified.

Values are defined as:

none

The corresponding axis does not have an explicit intrinsic inner size.

<length>

The corresponding axis has an explicit intrinsic inner size of the specified <length>.

auto <length>

If the element has a last remembered size, is currently skipping its contents, and is not relevant to the user, its explicit intrinsic inner size is the last remembered size.

Note: This occurs, for example, when an element with content-visibility: auto is off-screen.

Otherwise, its explicit intrinsic inner size is the specified <length>.

If an element has an explicit intrinsic inner size in an axis, then after laying out the element as normal for size containment, the size of the contents in that axis are instead treated as being the explicit intrinsic inner size instead of what was calculated in layout, and layout is performed again if necessary. (If it has an explicit intrinsic inner size in both axises, this implies the first layout can be skipped.)

These four properties are part of a logical property group.

Note: An element with size containment is laid out as if it had no contents [CSS-CONTAIN-1], which in many cases this will cause the element to collapse to zero inner height. This can be corrected with an explicit height chosen to show the expected contents, but that can have unintended effects in some layout systems, such as Flex and Grid Layout, which treat an explicit height as a stronger command than an implicit content-based height. The element thus might lay out substantially differently than it would have were it simply filled with content up to that height. Providing an explicit intrinsic inner size for the element preserves the performance benefits of ignoring its contents for layout while still allowing it to size as if it had content.

Name:

contain-intrinsic-size

In only one current engine.

FirefoxNoneSafariNoneChrome83+
Opera69+Edge83+
Edge (Legacy)NoneIENone
Firefox for AndroidNoneiOS SafariNoneChrome for Android83+Android WebView83+Samsung Internet13.0+Opera Mobile59+
contain-intrinsic-size
Value: [ none | <length> | auto <length> ]{1,2}
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

contain-intrinsic-size is a shorthand property that sets the contain-intrinsic-width and contain-intrinsic-height properties.

The first value represents the contain-intrinsic-width value, and the second represents the contain-intrinsic-height value. If only one value is given, it applies to both properties.

5.2.1. Last Remembered Size

Size containment is very valuable for ensuring a page can render efficiently, restricting the scope of layout work that can happen as a result of an element changing its rendering. However, it’s also very restrictive for the author, requiring them to correctly predict what the size of the element will be; if this guess is incorrect, even slightly, it can cause unsightly scrollbars or accidentally-hidden content.

The contain-intrinsic-size: auto value allows a middle-ground: if an element is ever not size-contained, this value causes the element to remember its size (calculated as normal by layout); then, if the element gains size containment later, it will use the remembered size, offering the performance benefits of size containment while probably sizing accurately to its contents.

Only elements capable of being ResizeObserver targets can have a last remembered size.

The last remembered size of an element is determined by:

Note: The last remembered size is state attached to the element, not any particular box generated by the element. So long as the element retains contain-intrinsic-size: auto, it will remember its last remembered size even across changes such as going to/from display: none.

5.2.2. Interaction With overflow: auto

The contain-intrinsic-size property provides an estimate of how large the author expects the content of an element to be, but this estimate is not actual content and does not represent anything that needs to be shown to the user. Therefore, an element with overflow: auto must not generate scrollbars as a consequence of contain-intrinsic-size.

However, if contain-intrinsic-size indicates a size large enough that the element would generate scrollbars if it contained actual content of that size, then the element must be sized as if it generated those scrollbar(s) in accordance with such hypothetical content.

In the following example code:
div {
  width: max-content;
  contain-intrinsic-size: 100px 100px;
  overflow: auto;
}

The element ends up being 100px wide and 100px tall: contain-intrinsic-size provides the max-content width, and also the height.

If the element then ended up with content that was 150px tall, it would show a vertical scrollbar; if the scrollbar is not overlay, it will take up some of that 100px width, leaving a smaller amount (roughly 84px, typically) for the content to flow into. (See CSS Overflow 3 § 3.2 Scrollbars and Layout.)

Even though there’s now less than 100px of horizontal space available for the content, it will not generate a horizontal scrollbar just because contain-intrinsic-size indicates a 100px width; that would only happen if the actual content had something unbreakable and wider than the remaining space.

In contrast, in the following example code:
div {
  width: max-content;
  contain-intrinsic-size: 100px 100px;
  height: 50px;
  overflow: auto;
}

The element has a fixed 50px height, but contain-intrinsic-size indicates a 100px “estimated content height”. The element thus assumes that it will need a vertical scrollbar when it’s filled with actual content, resulting in a max-content width a little more than 100px (roughly 116px, typically), to accommodate the estimated 100px of max-content width from contain-intrinsic-size, and as well as the vertical scrollbar width (roughly 16px, typically).

However, even though the element reserves space on the assumption of needing a scrollbar, it will not actually generate one unless the actual content overflows: if it ends up containing content that’s less than 50px tall, no vertical scrollbar will be generated at all, but the element will still be 116px wide.

5.3. Intrinsic Size Contributions

CSS Sizing 3 § 5.2 Intrinsic Contributions

5.4. Zeroing Min-Content Size Contributions: the min-intrinsic-sizing property

Name: min-intrinsic-sizing
Value: legacy | zero-if-scroll || zero-if-extrinsic
Initial: legacy
Applies to: all elements except inline boxes
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete

This property seriously needs some name bikeshedding.

This property defines whether the min-content contribution of a non-replaced box is “compressed” under certain circumstances. Values have the following meanings:

legacy
The box’s min-content contribution is handled as normal.
zero-if-scroll
The box’s min-content contribution is “compressed” if it is a scroll container.
zero-if-extrinsic
The box’s min-content contribution is “compressed” if has an extrinsic preferred or maximum size.

Note: This is the default behavior of most replaced elements.

The following rule will make all scroll containers essentially ignore their contents when passing up their size contributions (unless they specifically requested a content-based size):
*, ::before, ::after { min-intrinsic-size: zero-if-scroll; }

This prevents the scroll container from blowing up the size of its ancestors if it contains large items such as a table or long lines of unbreakable text. Meanwhile, it allows boxes that are not scroll containers to continue influencing the min-content size of their ancestors.

Note: The behavior of zero-if-scroll would have been a better default, but due to Web-compat, it cannot be the initial value. :(

The “compressed” min-content contributions is calculated by pretending the box were empty, except when factoring in sizing constraints imposed by explicit min-content, max-content, and fit-content values of the sizing properties.

6. Extrinsic Size Determination

CSS Sizing 3 § 4 Extrinsic Size Determination

6.1. Stretch-fit Sizing: filling the containing block

Stretch-fit sizing tries to set the box’s used size to the length necessary to make its outer size as close to filling the containing block as possible while still respecting the constraints imposed by min-height/min-width/max-height/max-width.

Formally, its behavior is the same as specifying an automatic size together with a self-alignment property value of stretch (in the relevant axis), except that the resulting box, which can end up not exactly fitting its alignment container, can be subsequently aligned by its actual self-alignment property value.

Additionally, in formatting contexts and axes in which the relevant self-alignment property does not apply (such as the block axis in Block Layout, or the main axis in Flex Layout), in cases where a percentage size in that axis would resolve to a definite value, a stretch-fit size causes the box to attempt to fill its containing block—behaving as 100% but applying the resulting size to its margin box instead of the box indicated by box-sizing. For this purpose, auto margins are treated as zero, and furthermore, for block-level boxes in particular, if its block-start/block-end margin would be adjoining to its parent’s block-start/block-end margin if its parent’s sizing properties all had their initial values, then its block-start/block-end margin is treated as zero.

Note: Consequently, if neither stretch alignment applies nor percentage sizing can resolve, then the box will resolve to its automatic size.

For example, given the following HTML representing two block boxes:
<div class="outer">
  <div class="inner"></div>
</div>

In the following case, the outer height of the inner box will exactly match the height of the outer box (200px), but its inner height will be 20px less, to account for its margins.

.outer { height: 200px; border: solid; }
.inner { height: stretch; margin: 10px; }

In the following case, the height of the inner box will exactly match the height of the outer box (200px). The top margins will collapse, but the bottom margins do not collapse (because the bottom margin of a box is not adjoining to the bottom margin of a parent with a non-auto height, see CSS 2 § 8.3.1 Collapsing margins), and therefore the inner box’s bottom margin will be truncated.

.outer { height: 200px; margin: 0; }
.inner { height: stretch; margin: 10px; }
Similarly, width: stretch causes the box to fill its container, being 20px narrower than the width of "some more text" (due to the 10px margin):
<div class="outer">
  <div class="inner">text</div>
</div>
some more text
.outer { float: left; margin: 0; }
.inner { width: stretch; margin: 10px; }
On the other hand, in this example the container’s height is indefinite, which would cause a percentage height on the child to behave as auto, so height: stretch behaves as auto as well.
.outer { height: auto; margin: 0; }
.inner { height: stretch; margin: 10px; }

6.2. 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 preferred aspect ratio, similar to the contain keyword of the object-fit and background-size properties.

First, a target rectangle is determined:

  1. 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.
  2. 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.)
  3. Last, the target rectangle is reduced in one dimension by the minimum necessary for it to match the box’s preferred 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.

6.3. Percentage Sizing

Changes

Recent Changes

Significant changes since the 20 October 2020 Working Draft include:

Significant changes since the 26 May 2020 First Public Working Draft include:

Additions Since Level 3

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.

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.

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.

Tests

Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.


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.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ALIGN-3]
Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. 24 December 2021. WD. URL: https://www.w3.org/TR/css-align-3/
[CSS-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 26 July 2021. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS-BOX-4]
Elika Etemad. CSS Box Model Module Level 4. 21 April 2020. WD. URL: https://www.w3.org/TR/css-box-4/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 13 January 2022. CR. URL: https://www.w3.org/TR/css-cascade-5/
[CSS-CONTAIN-1]
Tab Atkins Jr.; Florian Rivoal. CSS Containment Module Level 1. 22 December 2020. REC. URL: https://www.w3.org/TR/css-contain-1/
[CSS-CONTAIN-2]
Tab Atkins Jr.; Florian Rivoal; Vladimir Levin. CSS Containment Module Level 2. 16 December 2020. WD. URL: https://www.w3.org/TR/css-contain-2/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 3 September 2021. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-IMAGES-3]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Images Module Level 3. 17 December 2020. CR. URL: https://www.w3.org/TR/css-images-3/
[CSS-IMAGES-4]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Image Values and Replaced Content Module Level 4. 13 April 2017. WD. URL: https://www.w3.org/TR/css-images-4/
[CSS-LOGICAL-1]
Rossen Atanassov; Elika Etemad. CSS Logical Properties and Values Level 1. 27 August 2018. WD. URL: https://www.w3.org/TR/css-logical-1/
[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 23 December 2021. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-SIZING-3]
Tab Atkins Jr.; Elika Etemad. CSS Box Sizing Module Level 3. 17 December 2021. WD. URL: https://www.w3.org/TR/css-sizing-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 16 December 2021. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 30 July 2019. CR. URL: https://www.w3.org/TR/css-writing-modes-4/
[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
[CSS3COL]
Florian Rivoal; Rachel Andrew. CSS Multi-column Layout Module Level 1. 12 October 2021. CR. URL: https://www.w3.org/TR/css-multicol-1/
[RESIZE-OBSERVER-1]
Aleks Totic; Greg Whitworth. Resize Observer. 11 February 2020. WD. URL: https://www.w3.org/TR/resize-observer-1/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119

Informative References

[CSS-GRID-2]
Tab Atkins Jr.; Elika Etemad; Rossen Atanassov. CSS Grid Layout Module Level 2. 18 December 2020. CR. URL: https://www.w3.org/TR/css-grid-2/
[CSS-POSITION-3]
Elika Etemad; Tab Atkins Jr.. CSS Positioned Layout Module Level 3. 16 December 2021. WD. URL: https://www.w3.org/TR/css-position-3/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
aspect-ratio auto || <ratio> auto all elements except inline boxes and internal ruby or table boxes no n/a by computed value per grammar specified keyword or a pair of numbers
contain-intrinsic-block-size none | <length> | auto <length> none elements with size containment no n/a by computed value type per grammar as specified, with <length> values computed
contain-intrinsic-height none | <length> | auto <length> none elements with size containment no n/a by computed value type per grammar as specified, with <length> values computed
contain-intrinsic-inline-size none | <length> | auto <length> none elements with size containment no n/a by computed value type per grammar as specified, with <length> values computed
contain-intrinsic-size [ none | <length> | auto <length> ]{1,2} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
contain-intrinsic-width none | <length> | auto <length> none elements with size containment no n/a by computed value type per grammar as specified, with <length> values computed
min-intrinsic-sizing legacy | zero-if-scroll || zero-if-extrinsic legacy all elements except inline boxes no n/a discrete per grammar as specified

Issues Index

This is a diff spec over 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.
CSS Sizing 3 § 2 Terminology
CSS Sizing 3 § 3 Specifying Box Sizes
Add shorthands. [Issue #820]
We are still working through the details of this section. If there is any behavior specified here that would cause replaced elements with a preferred aspect ratio to behave differently than they would under the requirements of the CSS2, Flex Layout, and Grid Layout specs combined (without this specification in effect), this is an error and should be reported to the CSSWG.
CSS2.1 does not cleanly differentiate between replaced elements vs. elements with an aspect ratio; need to figure out specific cases that are unclear and define them, either in the appropriate Level 3 spec or here.
When we move all the sizing information here, rather than crowbar-ing our way into 2.1, then the core principle here is just: the resolved preferred size in the ratio-determining axis (before applying min/max) gets transferred thru the ratio. Min/max constraints get transferred afterwards, and then applied to each axis independently without regards to aspect-ratio.
This section might not be written correctly. [Issue #6071]
CSS Sizing 3 § 5.1 Intrinsic Sizes
CSS Sizing 3 § 5.2 Intrinsic Contributions
This property seriously needs some name bikeshedding.
CSS Sizing 3 § 4 Extrinsic Size Determination
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.