box model css

CSS
MDN (Mozilla Developer Network)
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See the link below regarding the CSS BOX MODELAll 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. 
header, ul, nav, li, a /* other elements */{

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

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