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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.

CSS Box model

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.

If the box-sizing property is set to content-box (default), the content size is determined by the width, min-width, max-width, height, min-height, and max-height properties.

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 size of the padding is determined by the padding-top, padding-right, padding-bottom, padding-left, and shorthand padding properties.

The border area, bounded by the border edge, extends the padding area to include the element's borders. Its dimensions are the border-box width and the border-box height.

The size of the borders are determined by the border-width and shorthand border properties.

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-top, margin-right, margin-bottom, margin-left, and shorthand margin properties.

When margin collapsing happens, 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 border and padding appear visually around the content.

Specification

Specification Status Comment
CSS Level 2 (Revision 1) Recommendation Though more precisely worded, there is no practical change.
CSS Level 1 Recommendation Initial definition.

See also

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 Last updated by: mfluehr,