CSS3 nav-up 属性

button#b1 {
            top: 20%;
            left: 25%;
            nav-index: 1;
            nav-right: #b2;
            nav-left: #b4;
            nav-down: #b2;
            nav-up: #b4;
        }

        button#b2 {
            top: 40%;
            left: 50%;
            nav-index: 2;
            nav-right: #b3;
            nav-left: #b1;
            nav-down: #b3;
            nav-up: #b1;
        }

        button#b3 {
            top: 70%;
            left: 25%;
            nav-index: 3;
            nav-right: #b4;
            nav-left: #b2;
            nav-down: #b4;
            nav-up: #b2;
        }

        button#b4 {
            top: 40%;
            left: 0%;
            nav-index: 4;
            nav-right: #b1;
            nav-left: #b3;
            nav-down: #b1;
            nav-up: #b3;
        }