Horizontal {
    height: 100%;
    width: 100%;
}

Placeholder {
    width: 100%;
    height: 1fr;
}

#p1 {
    max-width: 50h;
}

#p2 {
    max-width: 999;  /* (1)! */
}

#p3 {
    max-width: 50%;
}

#p4 {
    max-width: 30;
}