css opacity transition

/* Answer to: "css transitions" */

div {
  width: 100px;
  height: 100px;
  background: red;
  transition: width 2s;
  /* transition: width 2s, height 4s; */
}
 
div:hover {
  width: 300px;
  height: 1000px;
}div {
  transition: opacity seconds;
}