how to make a box in css 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: border-box css

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

Example 3: how to make a circle in html

<svg xmlns="http://www.w3.org/2000/svg">
    <circle cx="50" cy="50" r="50" fill="red" />
</svg>

Example 4: 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;

Example 5: 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