With CSS3, you can give any element "rounded corners" by using the border-radius property.
border-radius: 20px;
background-color: green;
color: white;
padding: 50px;
border-radius: 0 0 20px 20px;
A rectangle can be turned into a circle using only CSS. To create a circle, the border radius should be half of the height and the width.
The rectangle in the example below has a width and height of 200px. By setting the border radius to 100px, the corners will be rounded to form a circle.
width: 200px;
height: 200px;
border-radius: 100px;
background-color: limegreen;
color: white;