adding boxes in html code example
Example 1: how do i make a boxin html
<div style="width:500px;height:100px;border:1px solid #000;">This is a rectangle!</div>
Example 2: how to style rule to apply the Border Box model css
header, ul, nav, li, a {
display: block;
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
Example 3: 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