a {
    border-color: rgba(255, 255, 255, 0) rgba(255, 255, 255, 0) rgba(255, 255, 255, 0) #fd0;
}
b {
    border-color: rgba(0, 0, 0, .2) rgba(0, 0, 0, .1) rgba(0, 0, 0, .2) rgba(0, 0, 0, .1);
}
c {
    border-color: hsl(0, 0%, 10%) rgb(0, 0, 0) rgba(127, 127, 127, 0.5) hsla(0, 10%, 10%, 0.1);
}