border-radius: <haut+droite+bas+gauche>;
border-radius: <top-left+bottom-right> <top-right+bottom-left>;
border-radius: <top-left> <top-right+bottom-left> <bottom-right>;
border-radius: <top-left> <top-right> <bottom-left> <bottom-right>;
border-<top|bottom>-<left|right>: <value>
border-radius: <horizontal-radius>/<vertical-radius>;

1. w3.org/TR/css3-background
2. css3generator.com