border-radius accepts different horizontal and vertical radii, if you use a slash to separate them:

  
  border-radius: 100px / 75px;

border-radius also accepts percentages - resolving to width and height, therefore same percentage can compute to different values!

border-radius: 50% / 50%; = border-radius: 50%;

Half Ellipses

border-radius is shorthand for: border-top-left-radius/ border-top-right-radius/ border-bottom-right-radius/ border-bottom-left-radius

Quarter Ellipses

One corner is 100% rounded, the other three not:

border-radius: 100% 0 0 0;