Horizontal {
    padding: 1 2;     /* (1)! */
    background: white;
    height: 1fr;
}

#top {}               /* (2)! */

#middle {             /* (3)! */
    visibility: hidden;
}

#bot {                /* (4)! */
    visibility: hidden;
}

#bot > Placeholder {  /* (5)! */
    visibility: visible;
}

Placeholder {
    width: 1fr;
}