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 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 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.

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:

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 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.

2.1. Box-edge Keywords

The following 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:

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

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

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.

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 */

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

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

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.

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 */

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 Since CSS Level 2

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

• Adapting the prose slightly to account for vertical writing modes.
• Cross-linking to relevant concepts in [css-break-3] and [css-backgrounds-3].
• Defining the non-applicability of margin, padding and their longhands to ruby base containers and ruby annotation containers (at risk).
• 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 and Security Considerations

Box Model introduces no new privacy leaks or security considerations.