CSS Box Model Module Level 3

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 element 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 element 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 similarly defined in [css-backgrounds-3].

Note: This module only defines the physical per-side longhand properties. Additional, flow-relative longhand properties are defined in [css-logical-1]. Note that both sets of longhand properties control the same margins/padding: they are just different ways of indexing each side.

History of CSS Box module and the definition of Block Layout

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 Since its maintenance was put aside during the development of CSS2.1, its prose was severely outdated by the time CSS2 Revision 1 was finally completed.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. 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.

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 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].

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.

2.1. Boxes and Edges

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.

2.2. Fragmentation

When a box fragments—is broken, as across lines or across pages, into separate box fragments—each 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.

2.3. Box-edge Keywords

The following <box> CSS keywords are defined for use in properties (such as transform-box and background-clip) that need to refer to various box edges:

content-box: Refers to the content box or content edge. (In an SVG context, treated as fill-box.)
padding-box: Refers to the padding box or padding edge. (In an SVG context, treated as fill-box.)
border-box: Refers to the border box or border edge. (In an SVG context, treated as stroke-box.)
margin-box: Refers to the margin box or margin edge. (In an SVG context, treated as stroke-box.)
fill-box: Refers to the object bounding box or its edges. (In a CSS box context, treated as content-box.)
stroke-box: Refers to the stroke bounding box or its edges. (In a CSS box context, treated as border-box.)
view-box: Refers to the nearest SVG viewport element’s origin box, which is a rectangle with the width and height of the initial SVG user coordinate system established by the viewBox attribute for that element, positioned such that its top left corner is anchored at the coordinate system origin. (In a CSS box context, treated as border-box.)
Note: When the viewBox includes non-zero min-x or min-y offsets, this origin box does not actually correspond to the visible region rectangle defined by the viewBox!

For convenience, the following value types are defined to represents commonly used subsets of <box>:

<visual-box> = content-box | padding-box | border-box
<layout-box> = <visual-box> | margin-box
<paint-box> = <visual-box> | fill-box | stroke-box
<coord-box> = <paint-box> | view-box

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 section defines the physical margin longhands. (Additional flow-relative margin longhands are defined in [css-logical-1].)

Note: Adjoining margins in block layout can 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
Logical property group:	margin

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.

Note: Application to internal ruby boxes [CSS-RUBY-1] is not defined in this specification.

3.2. Margin Shorthand: the margin property

Name:	margin
Value:	<'margin-top'>{1,4}
Initial:	0
Applies to:	see individual properties
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 section defines the physical padding longhands. (Additional flow-relative padding longhands are defined in [css-logical-1].)

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 [0,∞]>
Initial:	0
Applies to:	all elements except: internal table elements other than table cells
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
Logical property group:	padding

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

Negative values for padding properties are invalid.

Note: Application to internal ruby boxes [CSS-RUBY-1] is not defined in this specification.

4.2. Padding Shorthand: the padding property

Name:	padding
Value:	<'padding-top'>{1,4}
Initial:	0
Applies to:	see individual properties
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. Borders

Borders fill the border area, to visually delineate the edges of the box, The border properties specify the thickness of the border area of a box, as well as its drawing style and color. See [css-backgrounds-3] for the definition of these properties, including their physical longhands. (Additional flow-relative border longhands are defined in [css-logical-1].)

6. Changes

6.1. Recent Changes

There have only been minor editorial adjustments since the 6 April 2023 Recommendation:

Adding some subheadings to § 2 The CSS Box Model and moving a paragraph accordingly.
Changing “document tree” to “element tree” in the § 1 Introduction to align with the terminology used in [css-display-3].
Adding the “Logical Property Group” line to the property definition tables.

6.2. Changes Since CSS Level 2

The following changes have been made to this module since CSS Level 2:

Marked application to internal ruby boxes as undefined in this level.
Adapting the prose slightly to account for vertical writing modes.
Cross-linking to relevant concepts in [css-break-3] and [css-backgrounds-3].
Providing a centralized common definition of keywords that reference the various box edges for use in properties defined outside this spec (e.g. background-clip) to avoid repetition (and the resulting inevitable synchronization errors).

7. Privacy Considerations

No privacy considerations have been reported on this module.

8. Security Considerations

No security considerations have been reported on this module.

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:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

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

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.