CSS Box 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.

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.

size

A one- or two-dimensional measurement: a block size and/or inline size; alternatively a width and/or height.

inner size

The content-box size of a box.

outer size

The margin-box size of a box.

definite size

A size that can be determined without performing layout; that is, a <length>, a measure of text (without consideration of line-wrapping), a size of the initial containing block, or a <percentage> or other formula (such the “stretch-fit” sizing of non-replaced blocks [CSS2]) 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

A size that is not definite. Indefinite available space is essentially infinite.

available space

A size representing the space into which a box is laid out, as determined by the rules of the formatting context in which it participates. The space available to a box is usually 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, which forces boxes laid into it to be laid out under that constraint.

stretch fit

The stretch fit into a given size is that size, minus the box’s computed margins (not collapsed, treating auto as zero), border, and padding in the given dimension (such that the outer size is a perfect fit), and flooring at zero (so that the inner size is not negative).

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

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

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

stretch-fit size

stretch-fit inline size

stretch-fit block size

The size a box would take if its outer size filled the available space in the given axis; in other words, the stretch fit into the available space, if that is definite. Undefined if the available space is indefinite.

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.

Note: Calculations involving this size need to specify a fallback behavior for when the available space is indefinite if that happens to be possible.

max-content size

A box’s “ideal” size in a given axis when given infinite available space. Usually this is the smallest size the box could take in that axis while still fitting around its contents, i.e. minimizing unfilled space while avoiding overflow.

max-content inline size

The box’s “ideal” size in the inline axis. Usually the narrowest inline size it could take while fitting around its contents if none of the soft wrap opportunities within the box were taken. (See § 5 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

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

min-content size

Nominally, the smallest size a box could take that doesn’t lead to overflow that could be avoided by choosing a larger size. Formally, the size of the box when sized under a min-content constraint, see § 5 Intrinsic Size Determination.

min-content inline size

The min-content size in the inline axis. Typically, 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

The min-content size in the block axis. For block containers, tables, and inline boxes, this is equivalent to the max-content block size.

fit-content size

fit-content inline size

fit-content block size

If the available space in a given axis is definite, equal to clamp(min-content size, stretch-fit size, max-content size) (i.e. max(min-content size, min(max-content size, stretch-fit size))). When sizing under a min-content constraint, equal to the min-content size. Otherwise, equal to the max-content size in that axis.

Note: This is called the “shrink-to-fit” width in CSS2.1§10.3.5 and CSS Multi-column Layout § 3.4.

intrinsic size

A max-content size or min-content size, i.e. a size arising primarily from the size of the content. (Some uses of this term may refer also to sizes derived primarily from one of these two sizes.)

Replaced elements frequently derive their intrinsic size from their natural dimensions.

2.2. Intrinsic Size Contributions

max-content contribution

The size that a box contributes to its containing block’s max-content size.

min-content contribution

The size that a box contributes to its containing block’s min-content size.

intrinsic size contribution

A max-content contribution, min-content contribution, or similarly-calculated content-based size contribution.

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.

3.1.2. Minimum Size Properties: the min-width and min-height properties

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.

3.1.3. Maximum Size Properties: the max-width and max-height properties

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

3.2. Sizing Values: the <length-percentage>, auto | none, min-content, max-content, and fit-content() values

<length-percentage>

Specifies the size of the box using <length> and/or <percentage>. The box-sizing property indicates whether the content box or border box is measured.

Percentages are resolved against the width/height, as appropriate, of the box’s containing block. If, in a particular axis, the containing block’s size depends on the box’s size, see the relevant layout module for special rules on how to resolve percentages.

Negative values are invalid.

auto

For width/height, specifies an automatic size (automatic block size/automatic inline size). See the relevant layout module for how to calculate this.

For min-width/min-height, specifies an automatic minimum size. Unless otherwise defined by the relevant layout module, however, it resolves to a used value of 0. For backwards-compatibility, the resolved value of this keyword is zero for boxes of all [CSS2] display types: block and inline boxes, inline blocks, and all the table layout boxes. It also resolves to zero when no box is generated.

none

No limit on the size of the box.

min-content

Use the min-content size in the relevant axis; for a box’s block size, unless otherwise specified, this is equivalent to its automatic size.

max-content

Use the max-content size in the relevant axis; for a box’s block size, unless otherwise specified, this is equivalent to its automatic size.

fit-content(<length-percentage>)

Use the fit-content formula with the available space replaced by the specified argument, i.e. min(max-content, max(min-content, <length-percentage>)), where the <length-percentage> argument is resolved exactly as for <length-percentage> values standing alone.

Negative <length-percentage> values are invalid.

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.

3.2.1. “Behaving as auto”

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).

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

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

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

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

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

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

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].

3.4. New Column Sizing Values: the min-content, max-content, and fit-content() values

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

min-content

Specifies the optimal column width as the min-content inline size of the multi-column container’s contents.

max-content

Specifies the optimal column width as the max-content inline size of the multi-column container’s contents.

fit-content(<length-percentage>)

Specifies the optimal column width as min(max-content size, max(min-content size, <length-percentage>))

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>

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:

If it has a preferred aspect ratio:

For the min-content size, use zero.

For the max-content size:

• If the available space is definite in the inline axis, use the stretch fit into that size for the inline size and calculate the block size using the aspect ratio.

• Otherwise if the box has a <length> as its computed value for min-width or min-height, use that size and calculate the other dimension using the aspect ratio; if both dimensions have a <length> minimum, choose the one that results in the larger overall size.

  Note: This case was previous calculated from a 300x150 default size, rather than the box’s min size. This is believed to be a better behavior, and likely to be Web-compatible, but please send feedback to the CSSWG if there are any problems.

• Otherwise use an inline size matching the corresponding dimension of the initial containing block and calculate the other dimension using the aspect ratio.

If it has no preferred aspect ratio:

For both the min-content size and max-content size:

• If the box has a <length> as its computed minimum size (min-width/min-height) in that dimension, use that size.

  Note: This author-controllable behavior is made possible by the new auto value for the min size properties. This is believed to be a better behavior, but it is not yet clear if it is Web-compatible, so please send feedback to the CSSWG if there are any problems.

• Otherwise, use 300px for the width and/or 150px for the height as needed.

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:

1. If the box is non-replaced, then the entire value of any max size property or preferred size property (width/max-width/height/max-height) specified as an expression containing a percentage (such as 10% or calc(10px + 0%)) that is cyclic is treated for the purpose of calculating the box’s intrinsic size contributions only as that property’s initial value. For example, given a box with width: calc(20px + 50%), its max-content contribution is calculated as if its width were auto. (The percentage is honored as usual, however, during the actual sizing of the box itself; see below.)
2. Likewise, if the box is replaced, then the entire value of any max size property or preferred size property specified as an expression containing a percentage that is cyclic is treated for the purpose of calculating the box’s max-content contributions only as that property’s initial value.
3. If the box is replaced, a cyclic percentage in the value of any max size property or preferred size property (width/max-width/height/max-height), is resolved against zero when calculating the min-content contribution in the corresponding axis. (See § 5.2.2 Compressible Replaced Elements for a list of which elements in HTML this applies to.) The UA may additionally floor this size based on UI considerations, such as ensuring certain UI elements remain visible (for example, the dropdown arrow on a select). This rule also applies when calculating a content-based automatic minimum size or its corresponding size contribution, yielding a definite “specified size suggestion”.
   For example, an input assigned width: calc(50% + 50px) has a min-content contribution of 50px, plus any horizontal margin/border/padding.
4. For the min size properties, as well as for margins and paddings (and gutters), a cyclic percentage is resolved against zero for determining intrinsic size contributions.

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>

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

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

<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>

<article style="height:100px; min-height: min-content;">
  <aside style="height: 200%;">
    <div style="height: 150px;"></div>
  </aside>
  <section style="height: 30px;"></section>
</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. <https://github.com/w3c/csswg-drafts/issues/6348>

Changes

Recent Changes

Changes since the 18 December 2020 Working Draft include:

• Fixed various errors in definition of max-content sizes of replaced elements in § 5.1 Intrinsic Sizes. (Issue 6072)
• Added missing statement handling min-content constraint to definition of fit-content size.
• Renamed replaced element “intrinsic” dimensions to “natural” dimensions in order to avoid confusion with intrinsic sizes (see Issue 4961).
• Various other minor editorial fixes and improvements.

Major changes since the 22 May 2019 Working Draft include:

• Defined that min-content and max-content do not necessarily behave the same as the property’s initial value if otherwise specified (by the relevant layout module). (Issue 3973)
• Switched intrinsic contribution of fit-content() to treat its argument as that argument would be treated alone for intrinsic contribution calculations and resolve the fit-content formula accordingly, rather than having special behavior for fit-content() resolution when calculating intrinsic contributions. (Issue 3731)
• Changed the max-content size of replaced boxes without an intrinsic size to use their minimum size in place of ICB or 300px×150px only when it is a <length>, see § 5.1 Intrinsic Sizes. (Issue 4217)
• Switched default sizing of an object with a natural aspect ratio to use the ICB size instead of 300px×150px. (Issue 4218)
• Defined preferred aspect ratio and used it in place of “intrinsic aspect ratio” where appropriate.
• Miscellaneous minor / editorial fixes.

Major changes since the 4 March 2018 Working Draft include:

• Imported the box-sizing definition from CSS UI Level 3.
• More rigorously specified handling of cyclic percentages. (#1132, #2384, #2297, #2674)
• Changed the *-content values applied to the bock axis to not compute to the property’s initial value, but to rather “behave as” the property’s initial value. (#2708)
• Fixed miscellaneous trivial errors.

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.