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 interactionsThis 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 DefinitionsThis 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. TerminologySome key terminology related to coordinate axises and dimensions is defined in CSS Writing Modes 3 § 6 Abstract Box Terminology.
Additionally, the size of the containing block of an absolutely positioned element is always definite with respect to that element.
Note: intrinsic sizing keywords such as max-content are indefinite, even if they can be determined without laying out the children e.g. due to size containment or lack of children.
Note: This is the formula used to calculate the auto widths of non-replaced blocks in normal flow in CSS2.1§10.3.3.
There are four types of automatically-determined sizes in CSS (sizes resulting from auto sizing rules, depending on context):
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.
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.
The box’s “ideal” size in the block axis. Usually the block size of the content after layout.
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.
The min-content size in the block axis. For block containers, tables, and inline boxes, this is equivalent to the max-content block size.
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.
Replaced elements frequently derive their intrinsic size from their natural dimensions.
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 ConstraintsThis 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 [0,∞]>, 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 propertiesThe 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 propertiesThe 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 propertiesThe 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 [0,∞]>, auto | none, min-content, max-content, and fit-content() valuesPercentages 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.
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.
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.
Note: The none keyword is not usable within calc-size().
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 FloatsThis 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 propertyThe 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:
Note: This is the behavior of width and height as specified by CSS2.1, and is thus the default.
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).
For example, the following properties set the content-box size of the box to
100px, with the border-box size calculating to
120px:
.box {
box-sizing: content-box; /* default */
width: 100px;
padding-left: 10px;
border-left: 10px solid;
}
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
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 § 4.1 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
innersize, the
outersize, the size of the
border-box, the
computed valueof the
sizing properties, etc, as appropriate for each case.
3.4. New Column Sizing Values: the min-content, max-content, and fit-content() valuesWhen used as values for column-width, the new keywords specify the optimal column width:
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 DeterminationExtrinsic sizing determines sizes based on the context of an element, without regard for its contents.
4.1. Percentage SizingPercentages specify sizing of a box with respect to the box’s containing block.
For example, in the following markup:
<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)
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 DeterminationIntrinsic sizing determines sizes based on the contents of an element, without regard for its context.
5.1. Intrinsic SizesThe min-content size of a box in each axis is the size it would have if it was a float given an auto preferred 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 preferred 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:
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 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 sequences 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.
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 BoxesSometimes 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:
Should we resolve transferred percentages against their containing block instead of zero before transferring them? See discussion.
The UA may additionally floor the min-content contribution based on UI considerations, such as ensuring certain UI elements remain visible (for example, the dropdown arrow on a select).
Note: The min-content contribution is, as always, also floored by the minimum size in its own axis.
This rule also applies when calculating a content-based automatic minimum size or its corresponding size contribution, yielding a definite “specified size suggestion”.
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-axis size other than a minimum size on the containing block (i.e. a block-size or max-block-size in most layout modes, or a flex-basis in flex layout) 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.
For example, in the following markup:
<article style="width: min-content"> <aside style="width: 50%;"> LOOOOOOOOOOOOOOOOOOOONG </aside> </article>
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>.
In this example,
<article style="height:auto">
<aside style="height: 50%;">
<div class=block style="height: 150px;"></div>
</aside>
<section style="height: 30px;"></section>
</article>
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>
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).
Tracking web-compat & implementation progress of applying this to max-width/height in Issue 6348. [Issue #6348]
Changes Recent ChangesChanges since the 18 December 2020 Working Draft include:
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:
In addition to substantially more detail to the various automatic and content-based sizing algorithms, the following new features have been added since [CSS2]:
Special thanks go to L. David Baron, Aaron Gustafson, Daniel Holbert, and Mats Palmgren for their contributions to this module.
Privacy and Security ConsiderationsIn 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.
Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:
Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:
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.
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.
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.
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.
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.3