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

#blockWood {
background-image: url('wood.jpeg');
}