Menu in CSS

div.menu {
  float: left;
  width: 25%;
  background-color: lightcyan;
}

div.menu a {
  position: relative;
  display: block;
  margin: 0;
  padding: 1em;
  border: 1px solid darkblue;
}

div.menu a:hover {
  background-color: lightred;
  color: darkred;
}

div.menu a span {
  display: none;
}

div.menu a:hover span {
  position: absolute;
  top: 0;
  left: 100%;
  width: 200px;
  border: 1px solid red;
  background-color: lightgreen;
  display: block;
  z-index: 100
}

div.contenuto {
  float: left;
  width: 70%;
}