CSS border radius generator

The slash syntax is the whole trick: border-radius: h values / v values gives each corner an ellipse instead of a circle, which is where blob shapes come from. Two honest caveats the output says out loud: when adjacent radii on one side add up past that side's length, CSS does not overflow, it scales every radius by the same factor so the curve still fits; and the 9999px pill works because the browser clamps each radius to the box, not because 9999 is meaningful. Percent values are per axis, so 50% is only round on a square box.