html, body { padding: 0; margin: 0;}
background{
    display: flex;
    flex-flow: column;
    position: absolute;
    height: 100%;
    width: 100%;
    z-index: -1;
}
red { 
    background-color: rgba(255, 0, 0, 1);
}
white {
    background-color: rgba(255, 255, 255, 1);
}
blue {
    background-color: rgba(0, 0, 255, 1);
}

background red, background white, background blue {
    display: flex;
    flex-grow: 1;
}