How do I create a radial css3 border gradient shadow

body {
    margin: 30px;
}

#b {
    width: 50%;
    padding: 30px 0px;
    text-align: center;
    border: 1px solid #999; color: #333;
    border-radius: 4px;
}
#a {
    width: 50%;
    height: 5px;
    background: -webkit-radial-gradient(50% 0%, 50% 5px, #aaa 0%, white 100%);
}

HELLO!!