.square {
    width: 400px;
    height: 400px;
    float: left;
    display: inline-block;
}


#red{
    background-color: red;
    position: relative;
}

#blue{
    background-color: blue;
    position: relative;
}

#black{
    background-color: black;
}

#container {
    margin: 0 auto;
    width: 800px;
    height: 800px;
    position: relative;
    
}

.rectangle1 {
    width: 200px;
    height: 40px;
position: relative;
    left: 650px;
    bottom: 40px;
}

.rectangle2 {
    width: 200px;
    height: 40px;
position: relative;
    left: 1050px;
    bottom: 840px;
}