HTML and CSS Reference
■ border-box : The width and height dimensions contain the border and
padding as well as the content of the element, while an element's
margins continue to fall outside of this area.
Although using box-sizing: border-box may sometimes seem
appealing (such as when mixing a percentage-based width and pixel-
based border), this box-sizing model may cause confusion and difficulty when
mixing with code written by others or working with a team of developers accus-
tomed to the standard content-box behavior.