#container {
    background-color: brown;
    margin:0px auto;
    padding: 0px;
    width: 400px;
    height: 400px;
}
.square, .circle {
    width: 100px; 
    height: 100px;
    display: block;
    float: left;
}
.circle {
    border-radius: 50%;
}
#blockRed {
    background-color: red;
}
#blockYellow {
    background-color: yellow;
}