how to add animation to a button hover

1/* Changes an element's color on hover */
2
3.selector {
4	background-color: black;
5}
6
7.selector:hover {
8	background-color: blue;
9}

1.a:hover{background-color: black;}

1.btn {
2  background-color: #ddd;
3  border: none;
4  color: black;
5  padding: 16px 32px;
6  text-align: center;
7  font-size: 16px;
8  margin: 4px 2px;
9  transition: 0.3s;
10}
11
12.btn:hover {
13  background-color: #3e8e41;
14  color: white;
15}