box-sizing :
The box-sizing property defines how the width and height of an element are calculated – should they include padding and borders, or not.

There are four possible value of box-sizing :
content-box –
This is Default. The width and height properties include only the content. Border and padding are not included. border-box -
The width and height properties (and min/max properties) include content, padding, and border. initial
Sets this property to its default value. inherit
Inherits this property from its parent element.