table {
  margin: auto;
  margin-top: 20px;
  width: 80%;
  border: 10px solid transparent;
}
td {
  width: 25%;
  position: relative;
  border: 10px solid transparent;
}
td:after {
  content: '';
  display: block;
  margin-top: 100%;
}
td .content {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  background-color: #CCC;
}
td .valid {
  background-color: #65a3d0;
}