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CSS Box Model Module Level 3

W3C Working Draft,

This version:
https://www.w3.org/TR/2018/WD-css-box-3-20180802/
Latest published version:
https://www.w3.org/TR/css-box-3/
Editor's Draft:
https://drafts.csswg.org/css-box-3/
Previous Versions:
Editor:
Elika J. Etemad / fantasai (Invited Expert)
Suggest an Edit for this Spec:
GitHub Editor
Issue Tracking:
GitHub Issues

Abstract

This specification describes the margin and padding properties, which create spacing in and around a CSS box. It may later be extended to include borders (currently described in [css-backgrounds-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 section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-box” in the title, preferably like this: “[css-box] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

1. Introduction

This subsection is not normative.

CSS describes how each element and each string of text in a source document is laid out by transforming the document tree into a set of boxes, whose size, position, and stacking level on the canvas depend on the values of their CSS properties.

Note: CSS Cascading and Inheritance describes how properties are assigned to elements in the box tree, while CSS Display 3 §1 Introduction describes how the document tree is transformed into the box tree.

Each CSS box has a rectangular content area, a band of padding around the content, a border around the padding, and a margin outside the border. The sizing properties [css-sizing-3], together with various other properties that control layout, define the size of the content area. The box styling properties—padding and its longhands, border and its longhands, and margin and its longhands—define the sizes of these other areas. Margins and padding are defined in this module; borders are defined in [css-backgrounds-3].

Note: This module originally contained the CSS Level 3 specification prose relating to box generation (now defined in [[css-display-3]), the box model (defined here), as well as block layout (now only defined in [CSS2] Chapters 9 and 10). Since its maintenance was put aside during the development of CSS2.1, its prose was severely outdated by the time CSS2 Revision 1 was finally completed. Therefore, the block layout portion of the prose has been retired, to be re-synched to CSS2 and updated as input to a new Block Layout module at some point in the future. It is being split apart from this module and from the CSS Display Module both because of the practical concern that it would be a huge amount of work and also in recognition that CSS now has multiple layout models (Flex Layout, Grid Layout, Positioned Layout, and Table Layout, in addition to Block Layout) which each deserve their own parallel module.

1.1. Values

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]. Other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords keywords as their property value. For readability they have not been repeated explicitly.

1.2. Module Interactions

This module replaces the definitions of the margin and padding properties defined in [CSS2] sections 8.1, 8.2, 8.3 (but not 8.3.1), and 8.4.

All properties in this module apply to the ::first-line and ::first-letter pseudo-elements.

2. The CSS Box Model

Each box has a content area (which contains its content—text, descendant boxes, an image or other replaced element content, etc.) and optional surrounding padding, border, and margin areas; the size of each area is specified by corresponding properties, and can be zero (or in the case of margins, negative). The following diagram shows how these areas relate and the terminology used to refer to the various parts of the box:

Diagram of a typical box, showing the
		content, padding, border and margin areas

The various areas and edges of a typical box.

The margin, border, and padding can be broken down into top, right, bottom, and left segments, each of which can be controlled independently by its corresponding property.

The perimeter of each of the four areas (content, padding, border, and margin) is called an “edge”, and each edge can be broken down into a top, right, bottom, and left side. Thus each box has four edges each composed of four sides:

content edge or inner edge
The content edge surrounds the rectangle given by the width and height of the box, which often depend on the element’s content and/or its containing block size. The four sides of the content edge together define the box’s content box.
padding edge
The padding edge surrounds the box’s padding. If the padding has zero width on a given side, the padding edge coincides with the content edge on that side. The four sides of the padding edge together define the box’s padding box, which contains both the content and padding areas.
border edge
The border edge surrounds the box’s border. If the border has zero width on a given side, the border edge coincides with the padding edge on that side. The four sides of the border edge together define the box’s border box, which contains the box’s content, padding, and border areas.
margin edge or outer edge
The margin edge surrounds the box’s margin. If the margin has zero width on a given side, the margin edge coincides with the border edge on that side. The four sides of the margin edge together define the box’s margin box, which contains the all of the box’s content, padding, border, and margin areas.

The background of the content, padding, and border areas of a box is specified by its background properties. The border area can additionally be painted with a border style using the border properties. Margins are always transparent. See [css-backgrounds-3].

When a box fragmentsis broken, as across lines or across pages, into separate box fragmentseach of its boxes (content box, padding box, border box, margin box) also fragments. How the content/padding/border/margin areas react to fragmentation is specified in [css-break-3] and controlled by the box-decoration-break property.

3. Margins

Margins surround the border edge of a box, providing spacing between boxes. The margin properties specify the thickness of the margin area of a box. The margin shorthand property sets the margin for all four sides while the margin longhand properties only set their respective side. This specification defines the physical margin longhands; [css-logical-1] additionally defines flow-relative margin longhands. Both sets of properties control the same set of margins: they are just different ways of indexing each side.

Note: Adjoining margins in block layout collapse. See CSS2§8.3.1 Collapsing Margins for details. Also, margins adjoining a fragmentation break are sometimes truncated. See CSS Fragmentation 3 §5.2 Adjoining Margins at Breaks for details.

3.1. Page-relative (Physical) Margin Properties: the margin-top, margin-right, margin-bottom, and margin-left properties

Name: margin-top, margin-right, margin-bottom, margin-left
Value: <length-percentage> | auto
Initial: 0
Applies to: all elements except internal table elements
Inherited: no
Percentages: refer to logical width of containing block
Computed value: the keyword auto or a computed <length-percentage> value
Canonical order: per grammar
Animation type: by computed value type

These properties set the top, right, bottom, and left margin of a box, respectively.

Negative values for margin properties are allowed, but there may be implementation-specific limits.

3.2. Margin Shorthand: the margin property

Name: margin
Value: <‘margin-top’>{1,4}
Initial: 0
Applies to: all elements except internal table elements
Inherited: no
Percentages: refer to logical width of containing block
Computed value: see individual properties
Canonical order: per grammar
Animation type: by computed value type

The margin property is a shorthand property for setting margin-top, margin-right, margin-bottom, and margin-left in a single declaration.

If there is only one component value, it applies to all sides. If there are two values, the top and bottom margins are set to the first value and the right and left margins are set to the second. If there are three values, the top is set to the first value, the left and right are set to the second, and the bottom is set to the third. If there are four values they apply to the top, right, bottom, and left, respectively.

The following code demonstrates some possible margin declarations.
body { margin: 2em }         /* all margins set to 2em */
body { margin: 1em 2em }     /* top & bottom = 1em, right & left = 2em */
body { margin: 1em 2em 3em } /* top=1em, right=2em, bottom=3em, left=2em */

The last rule of the example above is equivalent to the example below:

body {
  margin-top: 1em;
  margin-right: 2em;
  margin-bottom: 3em;
  margin-left: 2em; /* copied from opposite side (right) */
}

4. Padding

Padding is inserted between the content edge and the padding edge of a box, providing spacing between the content and the border. The padding properties specify the thickness of the padding area of a box. The padding shorthand property sets the padding for all four sides while the padding longhand properties only set their respective side. This specification defines the physical padding longhands; [css-logical-1] additionally defines flow-relative padding longhands. Both sets of properties control the same set of padding: they are just different ways of indexing each side.

Note: Backgrounds specified on the box are by default laid out and painted within the padding edges. (They are additionally painted underneath the border, in the border area.) This behavior can be adjusted using the background-origin and background-clip properties.

4.1. Page-relative (Physical) Padding Properties: the padding-top, padding-right, padding-bottom, and padding-left properties

Name: padding-top, padding-right, padding-bottom, padding-left
Value: <length-percentage>
Initial: 0
Applies to: all elements except internal table elements
Inherited: no
Percentages: refer to logical width of containing block
Computed value: a computed <length-percentage> value
Canonical order: per grammar
Animation type: by computed value type

These properties set the top, right, bottom, and left padding of a box, respectively.

Negative values for padding properties are invalid.

4.2. Padding Shorthand: the padding property

Name: padding
Value: <‘padding-top’>{1,4}
Initial: 0
Applies to: all elements except internal table elements
Inherited: no
Percentages: refer to logical width of containing block
Computed value: see individual properties
Canonical order: per grammar
Animation type: by computed value type

The padding property is a shorthand property for setting padding-top, padding-right, padding-bottom, and padding-left in a single declaration.

If there is only one component value, it applies to all sides. If there are two values, the top and bottom padding are set to the first value and the right and left padding are set to the second. If there are three values, the top is set to the first value, the left and right are set to the second, and the bottom is set to the third.

The following code demonstrates some possible padding declarations.
body { padding: 2em }         /* all padding set to 2em */
body { padding: 1em 2em }     /* top & bottom = 1em, right & left = 2em */
body { padding: 1em 2em 3em } /* top=1em, right=2em, bottom=3em, left=2em */

The last rule of the example above is equivalent to the example below:

body {
  padding-top: 1em;
  padding-right: 2em;
  padding-bottom: 3em;
  padding-left: 2em; /* copied from opposite side (right) */
}

5. Changes Since CSS Level 2

There are no non-editorial changes since CSS Level 2, other than adapting the prose slightly to account for vertical writing modes.

6. Privacy and Security Considerations

Box Model introduces no new privacy leaks, or security considerations beyond "implement it correctly".

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.

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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 17 October 2017. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS-BREAK-3]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 3. 9 February 2017. CR. URL: https://www.w3.org/TR/css-break-3/
[CSS-CASCADE-4]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 28 August 2018. CR. URL: https://www.w3.org/TR/css-cascade-4/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 28 August 2018. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-PSEUDO-4]
Daniel Glazman; Elika Etemad; Alan Stearns. CSS Pseudo-Elements Module Level 4. 7 June 2016. WD. URL: https://www.w3.org/TR/css-pseudo-4/
[CSS-SCROLL-SNAP-1]
Matt Rakow; et al. CSS Scroll Snap Module Level 1. 14 August 2018. CR. URL: https://www.w3.org/TR/css-scroll-snap-1/
[CSS-SIZING-3]
Tab Atkins Jr.; Elika Etemad. CSS Intrinsic & Extrinsic Sizing Module Level 3. 4 March 2018. 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. 14 August 2018. 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. 10 October 2018. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 24 May 2018. 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/CSS2/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[SVG2]
Amelia Bellamy-Royds; et al. Scalable Vector Graphics (SVG) 2. 4 October 2018. CR. URL: https://www.w3.org/TR/SVG2/

Informative References

[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/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
margin <‘margin-top’>{1,4} 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar see individual properties
margin-bottom <length-percentage> | auto 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar the keyword auto or a computed <length-percentage> value
margin-left <length-percentage> | auto 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar the keyword auto or a computed <length-percentage> value
margin-right <length-percentage> | auto 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar the keyword auto or a computed <length-percentage> value
margin-top <length-percentage> | auto 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar the keyword auto or a computed <length-percentage> value
padding <‘padding-top’>{1,4} 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar see individual properties
padding-bottom <length-percentage> 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar a computed <length-percentage> value
padding-left <length-percentage> 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar a computed <length-percentage> value
padding-right <length-percentage> 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar a computed <length-percentage> value
padding-top <length-percentage> 0 all elements except internal table elements no refer to logical width of containing block by computed value type per grammar a computed <length-percentage> value