CSS

give transition on box shadow

div {
  height: 200px;
  width: 200px;
  box-shadow: 0;
  transition: box-shadow 1s;
  border: 1px solid #eee;
}

div:hover {
  box-shadow: 0 0 3px #515151;
  ;
}

box shadow css transition

/* Scale up the box */
.box:hover {
  transform: scale(1.2, 1.2);
}

/* Fade in the pseudo-element with the bigger shadow */
.box:hover::after {
  opacity: 1;
}