css create box code example

Example 1: border-box css

#example1 {
  box-sizing: border-box;
}

Example 2: how to make a box in html

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

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