content-box     Default. The width and height properties (and min/max properties)
                includes only the content.
                Border, padding, or margin are not included
border-box      The width and height properties (and min/max properties) 
                includes content, padding and border,
                but not the margin

ex>
div {
    width: 300px;
    height: 100px;
    border: 1px solid blue;
    box-sizing: border-box;
}