CSS Box Sizing Module Level 3

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.

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This document was published by the CSS Working Group as a Working Draft using the Recommendation track . Publication as a Working Draft does not imply endorsement by W3C and its Members.

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The following features are at-risk, and may be dropped during the CR period:

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.

This spec needs illustrations! See issue.

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]

The definition of the box-sizing property in this module supersedes the one in [CSS-UI-3].

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

Some key terminology related to coordinate axises and dimensions
is defined in CSS Writing Modes 3 § 6 Abstract Box Terminology.

2.1.

Auto Box Sizes

There are four types of automatically-determined sizes in CSS
(sizes resulting from auto sizing rules, depending on context):

2.2.

Intrinsic Size Contributions

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

A sizing constraint imposed by the box’s containing block that causes it to produce its max-content contribution.

min-content constraint

A sizing constraint imposed by the box’s containing block that causes it to produce its min-content contribution.

preferred aspect ratio

A width:height ratio inherent to a box,
which biases various sizing algorithms
to produce a size consistent with that aspect ratio
insofar as possible while honoring other sizing inputs.
Unless otherwise specified,
a box’s preferred aspect ratio is its natural aspect ratio if it has one
and is applied to its content box.
Most boxes do not have a

preferred aspect ratio

.

3.

Specifying Box Sizes

3.1.

Sizing Properties

This section defines the sizing properties width, height, min-width, min-height, max-width, and max-height.
Their potential values are defined in the next section, § 3.2 Sizing Values: the <length-percentage>, auto | none, min-content, max-content, and fit-content() values.

Note: Additional flow-relative aliases to these properties are defined in [CSS-LOGICAL-1].

We would like to define shorthands for each pair of sizing properties
(e.g. width and height)
but there is a naming conflict with the @page size descriptor [CSS-PAGE-3],
so this has been deferred to Level 4.
Suggestions on how to resolve this problem are welcome,
see discussion.

3.1.1.

Preferred Size Properties: the width and height properties

The width and height (physical) properties specify
the preferred width and height of the box, respectively.

The min-width and min-height properties specify
the minimum width (or “min width”)
and minimum height (or “min height”)
of the box, respectively.

Note: The initial value of auto is new;
in [CSS2] the initial value was zero.

The max-width and max-height properties specify
the maximum width (or “max width”)
and maximum height (or “max height”)
of the box, respectively.

In all cases,
the used value is floored to preserve a non-negative inner size.

Note: The min-content, max-content, and fit-content() values
are new in Level 3.

Note: The flex-basis property hereby also gains these new keywords,
as its values are defined by reference to <width>.

Note: This section previously defined stretch and fit-content as keywords representing the stretch-fit size and fit-content size,
respectively.
These keywords have been deferred to Level 4
(along with an additional contain keyword that
behaves similarly to stretch but preserves the preferred aspect ratio, if any)
to better work out the implications in situations with indefinite available space.

To have a common term for both when width/height computes to auto and when it is defined to behave as if auto were specified
(as in the case of block percentage heights resolving against an indefinite size,
see CSS2§10.5),
the property is said to behave as auto in both of these cases.

Note: Legacy spec prose defining layout behavior, particularly in [CSS2],
might explicitly refer to width/height having a computed value of auto as a condition;
some of these cases should be interpreted as meaning behaves as auto,
and reported to the CSSWG for updating.

Replace this section with references to the new term automatic size.

3.2.2.

Containing or Excluding Floats

This section is non-normative.

Although block box boundaries are typically pervious to floats,
sometimes an author needs them to contain their own (descendant) floats
or to exclude floats from outside.
For Block layout,
specifying display: flow-root will make the box a formatting context root,
which has this behavior.

Note: Boxes participating in Flex, Grid, or Table layout will automatically have this behavior.

3.3.

Box Edges for Sizing: the box-sizing property

The box-sizing property defines whether
fixed sizes (such as <length>s and <percentage>s)
are assigned to the content box or to the border box.
It affects the interpretation of all sizing properties,
including flex-basis.

Values have the following meanings:

content-box

Sizes specified on sizing properties as <length-percentage> represent the box’s inner sizes,
excluding the margins/border/padding:
they are applied to the content box.
The padding and border of the box
are laid out and drawn outside the specified width and height.

Note: This is the behavior of width and height as specified by CSS2.1,
and is thus the default.

border-box

Sizes specified on sizing properties as <length-percentage> represent the box’s visually-apparent sizes,
including the borders/padding (but not margin):
they are applied to the border box.
The padding and border of the box
are laid out and drawn inside the specified width and height,
with the content box sized to fill the remaining space,
floored at zero.

The content width and height are calculated
by subtracting the border and padding widths of the respective sides
from the specified <length-percentage>.
As the content width and height cannot be negative,
this computation is floored at zero.

Used values, as exposed for instance through getComputedStyle(),
also refer to the border box.

Values affected by box-sizing include
both raw <length-percentage> values
and those used in functional notations such as fit-content().
In contrast,
non-quantitative values such as auto and min-content are not influenced by the box-sizing property
(unless otherwise specified).

100px

,
with the border-box size calculating to

120px

:

.box {
  box-sizing:   content-box; /* default */
  width:        100px;
  padding-left: 10px;
  border-left:  10px solid;
}

For example, the following properties set the content-box size of the box to, with the border-box size calculating to

On the other hand, by changing to border-box,
the border-box is set to 100px,
with the content-box size calculating to 80px:

.box {
  box-sizing:   border-box;
  width:        100px;
  padding-left: 10px;
  border-left:  10px solid;
}

The inner size can’t be less than zero,
so if the padding + border is greater than the specified border-box size,
the box will end up larger than specified.
In this case, the content-box size will floor at 0px so the border-box size ends up at 120px,
even though width: 100px is specified for the border box:

.box {
  box-sizing:   border-box;
  width:        100px;
  padding-left: 60px;
  border-left:  60px solid;
  /* padding + border = 120px */
}

This example uses box-sizing to evenly horizontally split two divs with fixed size borders inside a div container, which would otherwise require additional markup.

sample CSS:

div.container {
  width:38em;
  border:1em solid black;
}

div.split {
  box-sizing:border-box;
  width:50%;
  border:1em silver ridge;
  float:left;
}

sample HTML fragment:

<div class="container">
<div class="split">This div occupies the left half.</div>
<div class="split">This div occupies the right half.</div>
</div>

demonstration of sample CSS and HTML:

This div should occupy the left half.

This div should occupy the right half.

The two divs above should appear side by side, each (including borders) 50% of the content width of their container. If instead they are stacked one on top of the other then your browser does not support

The two divs above should appear side by side, each (including borders) 50% of the content width of their container. If instead they are stacked one on top of the other then your browser does not support box-sizing

Note: Certain HTML elements,
such as button,
default to border-box behavior.
See HTML for details on which elements have this behavior.

In legacy CSS specifications,
the terms width, height, minimum (min) width, minimum (min) height, maximum (max) width, and maximum (max) height generally refer to the inner size
(content-box size)
of a box unless otherwise indicated.

Refer to CSS User Interface 3 § 3.1 Changing the Box Model: the box-sizing property for an explicit disambiguation of these terms
for the Visual formatting model details section of [CSS2].

To avoid ambiguities, specification authors should avoid ambiguous uses of terms such as width or height without further qualification, and should explicitly refer and link to the inner size, the outer size, the size of the border-box , the computed value of the sizing properties , etc, as appropriate for each case.

When used as values for column-width,
the new keywords specify the optimal column width:

Note: The column width never varies by column.
When the column width is informed by the multi-column container’s contents
(as in the keywords above),
all of its contents are taken under consideration
and the calculated width is shared by all the columns.

4.

Extrinsic Size Determination

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

4.1.

Percentage Sizing

Percentages specify sizing of a box with respect to the box’s containing block.

<article style="height: 60em">
  <figure style="height: 50%;">
    <img style="height: 50%;">
  </figure>
</article>

• the <figure> would be 30em tall
  = 50% of the definite 60em height of the <article>

• the <img> would be 15em tall
  = 50% of the <figure>’s height
  (which is itself definite because it’s a percentage resolved against a definite length)

For example, in the following markup:

See § 5.2.1 Intrinsic Contributions of Percentage-Sized Boxes for details on how to resolve percentages
when the size of the containing block depends
on the size of its content.

5.

Intrinsic Size Determination

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

5.1.

Intrinsic Sizes

The min-content size of a box in each axis
is the size it would have if it was
a float given an auto size in that axis
(and no minimum or maximum size in that axis)
and if its containing block was zero-sized in that axis.
(In other words, the minimum size it has when sized as “shrink-to-fit”.)

The max-content size of a box in each axis
is the size it would have if it was
a float given an auto size in that axis
(and no minimum or maximum size in that axis),
and if its containing block was infinitely-sized in that axis.
(In other words, the maximum size it has when sized as “shrink-to-fit”.)

The min-content size and max-content size are collectively referred to
as the intrinsic sizes.

Note: When the box has a preferred aspect ratio,
size constraints in the opposite dimension will transfer through
and can affect the auto size in the considered one.
See CSS2§10.

This specification does not define how to determine the sizes of floats.
Please refer to [CSS2].
However, the intrinsic sizes of replaced elements without natural sizes are defined below:

Since a block-level or inline-level replaced element
whose height or width behaves as auto is effectively defined to use its max-content size (CSS2§10.3.2),
this specification applies the rules above
to the undefined case of a replaced element
whose height and width both behave as auto.

Note: This specification does not define how to determine
the size of a float.
Please refer to [CSS2],
the relevant CSS specification for that display type,
and/or existing implementations
for further details.
A future specification will define this in detail,
replacing the CSS2 “definition”,
such as it is.

Although the auto size of text input controls
such as HTML’s <input type=text> and <textarea> elements
is typically a fixed size,
the contents of such elements can be used to determine a content-based intrinsic size,
as for non-replaced block containers.
The min-content and max-content keywords of the sizing properties thus represent content-based sizes
for form controls which render their value
as text contained within their box,
allowing such controls to size to fit their visible contents
similarly to regular non-replaced elements.

The content in this case is defined to be the input control’s values
(the raw value in the case of textarea,
or the value in the case of input),
possibly transformed to a more human-readable and/or localized display format,
which is then treated as child text runs of the input control,
allowing soft wrap opportunities only where the input control would actually allow wrapping
(whether keyed off of CSS properties or other, UA-internal constraints).
If the input control has designated placeholder text
to be overlaid in its value display area,
then that text is also measured for the purpose of calculating the content-based size—whether or not the placeholder text is visible at the moment.
(Thus the content-based intrinsic size of the input control
is the larger of the size to fit the placeholder text and the size to fit the value.)

The UA may enforce a minimum
(such as the size required to contain a single zero-width character,
or the smallest usable size of a touch target)
on the form control’s min-content and max-content sizes to ensure sufficient space for the caret
and otherwise maintain usability of the form control.

Note: This might be extended to iframe or other content-containing replaced elements
(see discussion),
but text inputs are a major use-case;
and being document-internal,
have the least additional complications.

5.2.

Intrinsic 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.

5.2.1.

Intrinsic Contributions of Percentage-Sized Boxes

Sometimes the size of a percentage-sized box’s containing block depends on the intrinsic size contribution of the box itself,
creating a cyclic dependency.
When calculating the intrinsic size contribution of such a box
(including any calculations for a content-based automatic minimum size),
a percentage value that resolves against a size
in the same axis as the intrinsic size contribution (a cyclic percentage size)
is resolved specially:

Then, unless otherwise specified,
when calculating the used sizes and positions of the containing block’s contents:

• If the cyclic dependency was introduced due to
  a block-size or max-block-size on the containing block
  that causes it to depend on the size of its contents,
  the box’s percentage is not resolved and instead behaves as auto.

  Note: Grid items and flex items do allow percentages to resolve in this case.

• Otherwise, the percentage is resolved against the containing block’s size.
  (The containing block’s size is not re-resolved based on the resulting size of the box;
  the contents might thus overflow or underflow the containing block).

Note: These rules specify the previously-undefined behavior of this cyclic case
in CSS2§10.2, CSS2§8.3,
and CSS2§8.4.
Note also, the behavior in CSS2§10.5 is superseded in their respective specifications for layout modes
(such as flex layout)
not described in CSS2.

<article style="width: min-content">
  <aside style="width: 50%;">
  LOOOOOOOOOOOOOOOOOOOONG
  </aside>
</article>

For example, in the following markup:

When calculating the width of the outer <article>,
the inner <aside> behaves as width: auto,
so the <article> sets itself to the width of the long word.
Since the <article>’s width didn’t depend on “real” layout,
though, it’s treated as definite for resolving the <aside>,
whose width resolves to half that of the <article>.

<article style="height:auto">
  <aside style="height: 50%;">
    <div class=block style="height: 150px;"></div>
  </aside>
  <section style="height: 30px;"></section>
</article>

In this example,

because the percentage block size (height, in this case) on block-level elements
is defined to not resolve inside content-sized containing blocks,
the percentage height on the <aside> is ignored,
that is, it behaves exactly as if auto were specified.

Letting percentages still resolve against a definite height when the min-height is intrinsic is an open issue.
(CSS2 has a general statement about “height depending on contents”,
which this technically is,
even though CSS2 didn’t have content-dependent keywords for min-height.
Since this is new, we think we could have this different behavior.)

The following examples illustrate how block-axis percentages resolve against a containing block whose size depends on its contents.

<article style="height:100px; min-height: min-content;">
  <aside style="height: 50%;">
    <div style="height: 150px;"></div>
  </aside>
  <section style="height: 30px;"></section>
</article>

The initial height of the <article> is 100px, as specified,
which would make the <aside> 50px tall
when it resolved its percentage.
However, we must calculate the min-height,
by substituting it in for height.
This causes the percentage on the <aside> to behave as auto,
so the <aside> ends up 150px tall.
The total height of the contents is thus 180px.
This is larger than the specified 100px height,
so the <article> gets adjusted to 180px tall.

Then, since the percentage could originally resolve against the (100px) height,
it now resolves against the 180px height,
so the <aside> ends up being 90px tall.

<article style="height:auto; min-height: min-content;">
  <aside style="height: 50%;">
    <div class=block style="height: 150px;"></div>
  </aside>
  <section style="height: 30px;"></section>
</article>

In this case, the percentage on the <aside> won’t normally resolve,
because the containing block’s height is auto (and thus depends on the size of its contents).
Instead it behaves as auto,
resulting in a height of 150px for the <aside>,
and an initial height of 180px for the <article> The min-height doesn’t change this; height: min-content; acts similarly to height: auto; and results in the same sizes.

<article style="height:100px; min-height: min-content;">
  <aside style="height: 200%;">
    <div style="height: 150px;"></div>
  </aside>
  <section style="height: 30px;"></section>
</article>

This is a variation on the first code block,
and follows a similar path;
the <aside> initially wants to compute to 200px tall
(200% of the 100px containing block height).
When we calculate the effects of min-height,
the percentage behaves as auto,
causing it to become 150px tall,
and the total min-content height of the containing block
to be 180px tall.
Since this is larger than 100px,
the <article> gets clamped to 180px,
the percentage resolves against this new height,
and the <aside> ends up being 360px tall,
overflowing the <article>

5.2.2.

Compressible Replaced Elements

In addition to the replaced elements listed
in HTML§14.4 [HTML],
the following HTML elements are also considered to be replaced elements for the purpose of the percentage-sized replaced element rule above,
and can have their min-content contribution compressed
when their width/height or max-width/max-height is expressed with a cyclic percentage size:

• input with any type that is not “button-like”;
  this can vary depending on the UA.

  A type is “button-like” in a particular UA if it displays similar to a button element,
  where it can contains actual content that determines the layout of the element.
  In most UAs, the “button”, “reset”, “submit”, and “color” types are button-like;
  the “file” type is also partially button-like in some UAs,
  when it’s displayed as a combination of a text input (shrinkable)
  and a button (button-like, and thus not shrinkable).

• select, textarea, progress, meter, marquee.

Tracking web-compat & implementation progress of applying this to max-width/height in Issue 6348. [Issue #6348]

Changes

Recent Changes

Changes since the 18 December 2020 Working Draft include:

Major changes since the 22 May 2019 Working Draft include:

Major changes since the 4 March 2018 Working Draft include:

Major changes since the 7 February 2017 Working Draft include:

• More accurate definition of min-content and max-content sizes for replaced elements.
• Compute new keywords to the initial value, not to a potentially non-existent auto, when applied to the block axis.
• Specify that percent sizes on replaced elements zero out their min-content contribution.
• Fix confusing/wrong definition of percentage sizes resolved against a dependent containing block.
  (This may require further work.)
• Deferred the stretch and fit-content keywords to Level 4
  to allow for further consideration of their behavior in indefinite containing blocks.
• Pulled in full definitions for all of the sizing properties (rather than diffing them): width, height, min-width, min-height, max-width’, max-height, and box-sizing.

Additions since CSS Level 2

In addition to substantially more detail
to the various automatic and content-based sizing algorithms,
the following new features have been added since [CSS2]:

• The box-sizing property (originally defined in [CSS-UI-3], then moved here).
• The min-content, max-content, and fit-content() values of the sizing properties.
• The auto initial value of the min-width and min-height properties (originally defined in [CSS-FLEXBOX-1], then moved here).

Acknowledgments

Special thanks go to
L. David Baron,
Aaron Gustafson,
Daniel Holbert,
and
Mats Palmgren
for their contributions to this module.

Privacy and Security Considerations

In order to support automatic layout,
CSS sizes boxes to fit their contents.
In conjunction with various [DOM] and [CSSOM] APIs
which can return the size of those boxes to script,
this can expose information about those contents.
However, this information is more directly and easily available
by inspecting the DOM for the contents,
rather than indirecting through the box’s size.
Containers that can’t have their contents inspected
(such as cross-origin iframes)
also do not expose sizing information to the outer page,
except insofar as replaced elements such as images
expose their natural size and/or aspect ratio.