div css design 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: what is a div element in html

The <div> tag is nothing more than a container unit that encapsulates other page elements and divides the HTML document into sections.
 The div can be used for reading purposes or to nest all the information inside the div then to style the div through css as a background card.

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