.matrix {
  margin: 40px 0 0 240px;
  width: 200px;
  height: 200px;
  border: 1px solid #ccc;
}

.singlecell {
  width: 10px;
  height: 10px;
  display: block;
  color: white;
  background-color: white;
  float: left;
}

.singlecell em {
  position: relative;
  width: 10px;
  height: 10px;
  right: 200px;
  display: block;
  background: yellow;
  text-indent: -9999px;
}

.singlecell:hover {
  background: red;
}

.singlecell:hover em {
  background: blue;
}