The width property of the model is based on the sum of the content, padding, and border spaces and any space taken up by the padding and border is subtracted from space given to the content.

Border box

Explanation:

The width property of border-box model that tells the browser to account for any content, border and padding space in the values specify for an element's width and height. For example If an element's width is 200 pixels, that 200 pixels will include any content, border or padding added and the content box will shrink to absorb the extra and any space taken up by the padding and border is subtracted from space given to the content.

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