how to make a circle div in css code example

Example 1: circle div

border-radius: 50%;
width: 200px;
height: 200px; 
/* width and height can be anything, as long as they're equal */

Example 2: how to create a circle with css

<div id="circle">
</div>

#circle {
    width: 100px;
    height: 100px;
    background: red;
    border-radius: 50%
}