*{
  margin: 0;
  padding: 0;
  list-style: none;
}

ul{
  display: flex;
  flex-wrap: wrap;
  justify-content: space-around;
}

li{
  box-sizing:border-box;
  flex:1;
  min-width: 100px;
  margin:2%;
  
  border: 1px solid #eee;
}

li:after{
  content: '';
  display:block;
  padding-bottom:100%;
}