When laying out a document, the browser's rendering engine represents each element as a rectangular box according to the standard CSS box model. CSS determines the size, position, and properties (color, background, border size, etc.) of these boxes.
Every box is composed of four parts (or areas), defined by their respective edges: the content edge, padding edge, border edge, and margin edge.
The content area, bounded by the content edge, contains the "real" content of the element, such as text, an image, or a video player. Its dimensions are the content width (or content-box width) and the content height (or content-box height). It often has a background color or background image.
The padding area, bounded by the padding edge, extends the content area to include the element's padding. Its dimensions are the padding-box width and the padding-box height. When the content area has a background, it extends into the padding.
The thickness of the borders are determined by the
border-width and shorthand
border properties. If the
box-sizing property is set to
border-box, the border area's size can be explicitly defined with the
The margin area, bounded by the margin edge, extends the border area to include an empty area used to separate the element from its neighbors. Its dimensions are the margin-box width and the margin-box height.
The size of the margin area is determined by the
margin-left, and shorthand
margin properties. When margin collapsing occurs, the margin area is not clearly defined since margins are shared between boxes.
Finally, note that for non-replaced inline elements, the amount of space taken up (the contribution to the height of the line) is determined by the
line-height property, even though the borders and padding are still displayed around the content.
|CSS Level 2 (Revision 1)||Recommendation||Though more precisely worded, there is no practical change.|
|CSS Level 1||Recommendation||Initial definition.|
- CSS Reference
- CSS Key Concepts: CSS syntax, at-rule, comments, specificity and inheritance, the box, layout modes and visual formatting models, and margin collapsing, or the initial, computed, resolved, specified, used, and actual values. Definitions of value syntax, shorthand properties and replaced elements.
- Related CSS properties: