Square div pure CSS

square.css

.wrapper{
    position: relative;
    width: 50%;
    overflow: hidden;
    background: #4679BD;
}
.wrapper:before{
    content: "";
    display: block;
    padding-top: 100%;
}
.inner{
    position:  absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
}