display a box in html code example
Example 1: how to make a box in 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 /* other elements */{
display: block;
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;