how to make a box in html

<div style="width:500px;height:100px;border:1px solid #000;">This is a rectangle!</div>

header, ul, nav, li, a /* other elements */{

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

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 MODEL