The property takes in one or two values that define the radii of a quarter ellipse that defines the shape of the corner of the outer border edge (see the diagram below). The first value is the horizontal radius, the second the vertical radius. If the second value is omitted it is copied from the first. If either length is zero, the corner is square, not rounded. Percentages for the horizontal radius refer to the width of the border box, whereas percentages for the vertical radius refer to the height of the border box. Negative values are not allowed.
The border-top-left-radius property defines the radius of the top-left corner.
Note: If you set two values, the first one is for the top border, and the second one for the left border. If the second value is omitted, it is copied from the first. If either length is zero, the corner is square, not rounded.