css box model

All HTML elements can be considered as boxes. 
In CSS, the term "box model" is used when talking 
about design and layout.

The CSS box model is essentially a box that 
wraps around every HTML element. 
It consists of: margins, borders, padding, and the actual content. 
MDN (Mozilla Developer Network)
Probably the best place for an in-depth explanation of
web related technologies.

See the link below regarding the CSS BOX MODELheader, ul, nav, li, a /* other elements */{

 display: block;
   -webkit-box-sizing: border-box;
   -moz-box-sizing: border-box;
   box-sizing: border-box;