When laying out a document, the browser's rendering engine represents each element as a rectangular box according to the standard CSS basic 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 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.