指定两个按钮的Tab键顺序：

    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;

	}