.square, .circle {
    width: 100px;
    height: 100px;
    display:inline-block;
    float:left;
}
.circle {
    border-radius: 50%;
}

#blockBlue {
    background-color:deepskyblue;
    position:relative;
}
#blockYellow {
    background-color: yellow;
    position: relative;
}

#container {
    background-color:hotpink;
    margin: 0px auto;
    width: 400px;
    height: 400px;
}