https //www.w3schools.com css box model code example
Example 1: how to style rule to apply the Border Box model css
header, ul, nav, li, a /* other elements */{
display: block;
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
Example 2: css box model
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 MODEL