.square, .circle {
    width: 100px;
    height: 100px;
}

.circle {
    border-radius: 50%;
}

#blockRed {
    background-color: red;
    position: relative;
    top:0px;
    left: 0px;
}

#blockYellow {
    background-color:yellow;
    position: absolute; /*relative to blockRed*/
    left: 100px;
    top: auto;
}

.square, .circle {
    width: 100px;
    height: 100px;
    display: block;
}


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