CSS Box model

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.

Credit: MDN

Every box is composed of four parts (or areas), defined by their respective edges: the content edge, padding edge, border edge, and margin edge.

Explanation of the different parts:

  • Content Edge — The content edge of the box where text and images appear.

If the box-sizing property is set to content-box (default) and if the element is a block element, the content area's size can be explicitly defined with 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.

If the box-sizing property is set to border-box, the border area's size can be explicitly defined with the width, min-width, max-width, height, min-height, and max-height properties. When there is a background (background-color or background-image) set on a box, it extends to the outer edge of the border (i.e. extends underneath the border in z-ordering). This default behavior can be altered with the background-clip css property.

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.

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Developer with 3 yrs of industrial experience in developing scalable web applications.

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Abhay Jain

Developer with 3 yrs of industrial experience in developing scalable web applications.