#container {
    position: relative;   
    width: 400px;
    height: 400px;
    background-color: blue;
}

.square {
    position: absolute;   
    width: 50%;         
    height: 50%;          
    background-color: red;
}

#blockRed {
    top: 0;
    left: 0;
}

#blockRed2 {
    bottom: 0;
    right: 0;
}