div {
  padding: 10px;
  margin: auto;
  border: 2px solid darkviolet;
  border-radius: 10px;
  font-family: "Trebuchet MS", "Lucida Sans Unicode", "Lucida Grande",
    "Lucida Sans", Arial, sans-serif;

  color: darkviolet;
  width: 200px;
  
  transition-property: width;
  transition-duration: 2s;
}

div:hover {
  color: white;
  background-color: darkviolet;
  width: 400px;
}