#container {
 background-color: antiquewhite;
 margin: 0px auto;
 padding: 0px;
 width: 400px;
 height: 400px;
 display: flex;
 flex-wrap: wrap;

}
.square{
width: 200px;
height: 200px;
display: inline-block;
float: none;
}
#blockgreen {
background-color: green;
position: relative;
}
#blockyellow{
background-color: yellow;
position: relative;
}
.small-rectangle {
    width: 90px;
    height: 20px;
    background-color: lightgreen;
    border: 2px solid black;
    position: absolute;
     top: 0;
    left: 50%;            
    transform: translateX(-50%);
 
}
.small-rectangle2 {
 width: 90px;
height: 20px;
background-color: lightgreen;
border: 2px solid black;
position: absolute;
bottom: 0;
left: 50%;            
transform: translateX(-50%);

}