User guide
border-radius takes 1 to 4 values (top-left, top-right, bottom-right, bottom-left). In percentages you get ellipses and fluid shapes.
- 4 equal values = uniform rounding.
- 50% on a square = a perfect circle.
FAQ
How do I make a circle in CSS?
Apply border-radius: 50% to a square element (same width and height).
px or percentage?
px gives a fixed radius; percentage scales with the element size, ideal for responsive shapes.