CSS3 nav-down 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;
}